Selected Publications

For more recent publications please see the Publications List of Juliet C Pickering

The datasets associated with the publications below are published as electronic machine readable tables with the papers, and are accessible via the publishing Journals. (Some datasets are also available via international databases).

2022

• Wavelengths and Energy Levels of Singly Ionized Nickel (Ni II) Measured Using Fourier Transform Spectroscopy
  Clear CP, Pickering JC, Nave G, Uylings P, Raassen T, ApJS 261, 35 (2022) Publication   abstract  
• Comment on: “Hyperfine structure measurements of Co I and Co  II with Fourier transform spectroscopy” by Fu et al. [JQSRT 2021, 107590]
  Ding M & Pickering JC , JQSRT, 288, 108240 (2022) Publication   Download Arxiv PDF abstract  
• The Gaia-ESO Public Spectroscopic Survey: Implementation, data products, open cluster survey, science, and legacy
  Randich S et al, A & A, in press (2022) Download Arxiv PDF abstract  

2021

• New Ritz Wavelengths and Transition Probabilities of Parity-forbidden [Mn II] Lines of Astrophysical Interest
  Liggins FS, Pickering JC, Nave G, Kramida A, Gamrath S, Quinet P, ApJ 907, 69 (2021) Publication   abstract  
• Reference wavelengths of Si II, C II, Fe II, and Ni II for quasar absorption spectroscopy
  Clear C & Nave G, Mon. Not. Royal Astronomical Soc, 502 (4), 5679-5685 (2021) Publication   Download Arxiv PDF abstract  
• Emission spectroscopic study of an analytical glow discharge with plane and hollow cathodes: Titanium and iron in argon discharge
  Weiss Z, Concepcion-Mairey F, Pickering JC, Smid P, Spectrochim. Acta B - Atomic Spectroscopy, 180,106208 (2021) Publication  
• Atomic data for the Gaia-ESO Survey
  Heiter U et al, Astronomy & Astrophysics, 645, A106 (2021) Publication   Download Arxiv PDF Data tables
• The Spectrum and Term Analysis of Singly Ionized Manganese
  Liggins FS, Pickering JC, Nave G, Ward JW, Tchang-Brillet WUL, Astrophysical J Supp, 252 (1), 10 (2021) Publication   Download PDF Abstract

2020

• Measurements of the Hyperfine Structure of Atomic Energy Levels in Co II
  Ding ML & Pickering JC, Astrophysical J Supp, 251 (2), 24 (2020) Publication   Download PDF Abstract
• VizieR Online Data Catalog: Atomic data for the Gaia-ESO Survey
  Heiter et al (2020) Source Paper   Abstract VizieR Catalogue link
• Recent advances in experimental laboratory astrophysics for stellar astrophysics applications and future data needs
  Pickering JC, Belmonte MT, Clear CP, Liggins F, Concepcion-Mairey F, Proceedings of the International Astronomical Union, Vol 15, Symposium S350: Laboratory Astrophysics: From Observations to Interpretation, April 2019 , pp. 220 - 228 (2020) Publication   Download PDF Abstract
• Charge transfer from doubly charged ions of transition elements in a neon glow discharge: evidence based on emission spectra
  Weiss Z & Pickering JC, Plasma Sources Science & Technology, 29 (4), 045025 (2020) Publication   Download PDF

2015-2019

for details of publications 2015-2019 please see Publications List of Juliet C Pickering

2015

• Atomic and molecular data for optical stellar spectroscopy
  Heiter U, Lind K, Asplund M, Barklem P, Bergemann M, Magrini L, Masseron T, Mikolaitis Š, Pickering J, Ruffoni M, Physica Scripta, 90 (5) Art. No. 054010 (2015) (more info PDF)
  
  

  Authors: Heiter U, Lind K, Asplund M, Barklem P, Bergemann M, Magrini L, Masseron T, Mikolaitis Š, Pickering J, Ruffoni M Abstract: High-precision spectroscopy of large stellar samples plays a crucial role for several topical issues in astrophysics. Examples include studying the chemical structure and evolution of the Milky Way galaxy, tracing the origin of chemical elements, and characterizing planetary host stars. Data are accumulating from instruments that obtain high-quality spectra of stars in the ultraviolet, optical and infrared wavelength regions on a routine basis. These instruments are located at ground-based 2- to 10-m class telescopes around the world, in addition to the spectrographs with unique capabilities available at the Hubble Space Telescope. The interpretation of these spectra requires high-quality transition data for numerous species, in particular neutral and singly ionized atoms, and di- or triatomic molecules. We rely heavily on the continuous efforts of laboratory astrophysics groups that produce and improve the relevant experimental and theoretical atomic and molecular data. The compilation of the best available data is facilitated by databases and electronic infrastructures such as the NIST Atomic Spectra Database, the VALD database, or the Virtual Atomic and Molecular Data Centre (VAMDC). We illustrate the current status of atomic data for optical stellar spectra with the example of the Gaia-ESO Public Spectroscopic Survey. Data sources for 35 chemical elements were reviewed in an effort to construct a line list for a homogeneous abundance analysis of up to 105 stars. Copyright (2015) The American Astronomical Society. Links: Publication   Download PDF

2014

• Fe I Oscillator Strengths for Transitions from High-Lying Even-Parity Levels
  Den Hartog EA, Ruffoni MP, Lawler JE, Pickering JC, Lind K, Brewer NR, ApJS 215(2), 23 (2014) (more info PDF)
  
  

  Authors: Den Hartog EA, Ruffoni MP, Lawler JE, Pickering JC, Lind K, Brewer NR Abstract: New radiative lifetimes, measured to ±5 % accuracy, are reported for 31 even-parity levels of Fe I ranging from 45061 cm-1 to 56842 cm-1. These lifetimes have been measured using single-step and two-step time-resolved laser-induced fluorescence on a slow atomic beam of iron atoms. Branching fractions have been attempted for all of these levels, and completed for 20 levels. This set of levels represents an extension of the collaborative work reported in Ruffoni et al. (2014). The radiative lifetimes combined with the branching fractions yields new oscillator strengths for 203 lines of Fe I. Utilizing a 1D-LTE model of the solar photosphere, spectral syntheses for a subset of these lines which are unblended in the solar spectrum yields a mean iron abundance of <log[ε(Fe)]> = 7.45 ± 0.06. Copyright (2014) The American Astronomical Society. Links: Publication   Pre-Print   Download PDF

• Fe I Oscillator Strengths for the Gaia-ESO Survey
  Ruffoni MP, Den Hartog EA, Lawler JE, Brewer NR, Lind K, Nave G, Pickering JC, Mon. Not. R. Astron. Soc., 441(4), 3127 (2014) (more info PDF)
  
  

  Authors: Ruffoni MP, Den Hartog EA, Lawler JE, Brewer NR, Lind K, Nave G, Pickering JC Abstract: The Gaia-ESO Public Spectroscopic Survey (GES) is conducting a large-scale study of multi-element chemical abundances of some 100,000 stars in the Milky Way with the ultimate aim of quantifying the formation history and evolution of young, mature and ancient Galactic populations. However, in preparing for the analysis of GES spectra, it has been noted that atomic oscillator strengths of important Fe I lines required to correctly model stellar line intensities are missing from the atomic database. Here, we present new experimental oscillator strengths derived from branching fractions and level lifetimes, for 142 transitions of Fe I between 3526 Å and 10864 Å, of which at least 38 are urgently needed by GES. We also assess the impact of these new data on solar spectral synthesis and demonstrate that for 36 lines that appear unblended in the Sun, Fe abundance measurements yield a small line-by-line scatter (0.08 dex) with a mean abundance of 7.44 dex in good agreement with recent publications. (C) 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Links: Publication   Pre-Print   Download PDF

• Transition rate diagrams - A new approach to the study of selective excitation processes: The spectrum of manganese in a Grimm-type glow discharge
  Weiss Z., Steers E.B.M., Pickering J.C., Mushtaq S Spectrochim. Ac. B - At. Spect., 92, 70 (2014) (more info coming soon PDF)
  
  

  Authors: Weiss Z., Steers E.B.M., Pickering J.C., Mushtaq S. Abstract: Awaiting information Copyright (2014) Links: Awaiting Download PDF

• Excitation of higher levels of singly charged copper ions in argon and neon glow discharges
  Weiss Z., Steers E.B.M., Pickering J.C., Hoffmann V., and Mushtaq S., J Analyt. Atomic Spectrometry, 29 (12), 2256 - 2261 (2014) (more info coming soon PDF)
  
  

  Authors: Weiss Z., Steers E.B.M., Pickering J.C., Hoffmann V., and Mushtaq S. Abstract: Awaiting information Copyright (2014) JAAS Links: Awaiting Download PDF

• Enhancement of analyte atomic lines with excitation energies of about 5 eV in the presence of molecular gases in analytical glow discharges
  Mushtaq S., Steers E.B.M., Pickering J.C. & Šmid P. J Analytical Atomic Spectrom., 29, 2022-2026 (2014) (more info coming soon PDF)
  
  

  Authors:Mushtaq S., Steers E.B.M., Pickering J.C. & Šmid P. Abstract: Awaiting information Copyright (2014) JAAS Links: Awaiting Download PDF

• Effect of small quantities of oxygen in a neon glow discharge
  Mushtaq S., Steers E.B.M., Pickering J.C. & Weinstein V. J. Analytical Atomic Spectrom., 29, 2027-2041 (2014) (more info coming soon PDF)
  
  

  Authors: Mushtaq S., Steers E.B.M., Pickering J.C. & Weinstein V. Abstract: Awaiting information Copyright (2014) JAAS Links: Awaiting Download PDF

• Excitation and transition rate diagrams of singly ionized iron in analytical glow discharges in argon, neon and an argon–hydrogen mixture
  Weiss Z., Steers E.B.M., Pickering J.C. & Mushtaq S. J. Analytical. At. Spectrom., 29, 2078-2090 (2014) (more info coming soon PDF)
  
  

  Authors: Weiss Z., Steers E.B.M., Pickering J.C. & Mushtaq S. Abstract: Awaiting information Copyright (2014) JAAS Links: Awaiting Download PDF

2013

• Infrared Laboratory Oscillator Strengths of Fe I in the H-Band
  Ruffoni MP, Allende Prieto C, Nave G, Pickering JC Astrophysical Journal, 779 pp17 (2013) (more info PDF)
  
  

  Authors: Ruffoni MP, Allende Prieto C, Nave G, Pickering JC Abstract: We report experimental oscillator strengths for 28 infrared Fe I transitions, for which no previous experimental values exist. These transitions were selected to address an urgent need for oscillator strengths of lines in the H-band (between 1.4 μm and 1.7 μm) required for the analysis of spectra obtained from the Sloan Digital Sky Survey (SDSS-III) Apache Point Galactic Evolution Experiment (APOGEE). Upper limits have been placed on the oscillator strengths of an additional 7 transitions, predicted to be significant by published semi-empirical calculations, but not observed to be so. Copyright (2013) The American Astronomical Society Links: Publication   Download PDF

• The Spectrum and Term Analysis of V II
  Thorne AP, Pickering JC and Semeniuk JI, Astrophysical Journal Supplement Series, 207 (1) Art UNSP 13 (2013) (more info PDF)
  
  

  Authors: Thorne AP, Pickering JC and Semeniuk JI Abstract: The spectrum and extended term analysis of V II are presented. Fourier transform spectrometry was used to record high resolution spectra of singly ionized vanadium in the region 1492-5800 angstrom (67020-17260 cm(-1)) with vanadium-neon and vanadium-argon hollow cathode lamps as sources. The wavenumber uncertainty for the center of gravity of the strongest lines is typically 0.002 cm(-1), an improvement of an order of magnitude over previous measurements. Most of the lines exhibit partly resolved hyperfine structure. The V II energy levels in the 1985 compilation of Sugar and Corliss have been confirmed and revised, with the exception of the high-lying 4f levels and eight of the lower levels. Thirty-nine of the additional eighty-five high levels published by Iglesias et al. have also been confirmed and revised, and three of their missing levels have been found. The energy uncertainty of the revised levels has been reduced by about an order of magnitude. In total, 176 even levels and 233 odd levels are presented. Wavenumbers and classifications are given for 1242 V II lines. Copyright (2013) The American Astronomical Society Links: Publication   Download PDF

• Atomic data for iron-group elements of astrophysical interest
  Nave G, Sansonetti CJ, Pickering JC, Ruffoni MP, Thorne AP, Liggins FS, AIP Conf. Proc., 1545 pp. 27 (2013) (more info Publication)
  
  

  Authors: Nave G, Sansonetti CJ, Pickering JC, Ruffoni MP, Thorne AP, Liggins FS Abstract: The joint project between NIST and Imperial College London to measure spectra of singly-ionized iron-group elements using Fourier transform and high-resolution grating spectroscopy is described. Copyright (2013) AIP Publishing LLC. Links: Publication

• Accurate Ritz wavelengths of parity-forbidden [Co II] and [V II] lines of astrophysical interest
  Ruffoni MP and Pickering JC, Astrophysical Journal Supplement, 207 pp20 (2013) (more info PDF)
  
  

  Authors: Ruffoni MP and Pickering JC Abstract: We report a comprehensive list of accurate Ritz wavelengths for parity-forbidden [Co II] and [V II] lines obtained from the analysis of energy levels measured in the laboratory with Fourier transform emission spectroscopy. Such lines, particularly those in the infrared, are in demand for the analysis of low-density astrophysical plasmas in and around objects such as planetary nebulae, star-forming regions, and active galactic nuclei. Transitions between all known metastable levels of Co II and V II are included in our analysis, producing wavelengths for 1477 [V II] lines and 782 [Co II] lines. Of these, 170 [V II] lines and 171 [Co II] lines arise from transitions with calculated transition probabilities greater than 1 x 10-2 s-1 and upper level excitations of less than 5 eV, and thus are likely to be observed in astrophysical spectra. Copyright (2013) The American Astronomical Society Links: Publication   Download PDF

2012

• Recent advances in measurement of the water vapour continuum in the far-infrared spectral region
  PD Green, SM Newman, RJ Beeby, JE Murray, JC Pickering & JE Harries, Phil. Trans. Roy. Soc. A, 370(1968), 2637-2655 (2012) (more info PDF)
  
  

  Authors: PD Green, SM Newman, RJ Beeby, JE Murray, JC Pickering & JE Harries Abstract: We present a new derivation of the foreign-broadened water vapour continuum in the far-infrared (far-IR) pure rotation band between 24 μm and 120 μm (85–420 cm−1) from field data collected in flight campaigns of the Continuum Absorption by Visible and IR radiation and Atmospheric Relevance (CAVIAR) project with Imperial College's Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) far-IR spectro-radiometer instrument onboard the Facility for Airborne Atmospheric Measurement (FAAM) BAe-146 research aircraft; and compare this new derivation with those recently published in the literature in this spectral band. This new dataset validates the current Mlawer–Tobin-Clough–Kneizys–Davies (MT-CKD) 2.5 model parametrization above 300 cm−1, but indicates the need to strengthen the parametrization below 300 cm−1, by up to 50 per cent at 100 cm−1. Data recorded at a number of flight altitudes have allowed measurements within a wide range of column water vapour environments, greatly increasing the sensitivity of this analysis to the continuum strength. Copyright (2012) The Royal Society. Links: Publication   Download PDF

2011

• Effects of traces of oxygen on Grimm-type glow discharges in argon
  Mushtaq S, Steers EBM, Pickering JC, et al., JAAS 26 766 (2011) (more info PDF)
  
  

  Authors: Mushtaq S, Steers EBM, Pickering JC, Gusarova T, Smid P, Weinstein V Abstract: We report results of studies using Fourier Transform Optical Emission Spectroscopy (FT-OES) to investigate the effects of added oxygen (0.04–0.8% v/v) on observed spectra from a Grimm-type glow discharge, generated in argon plasma with a pure iron sample. Significant changes in the sputter rate and the intensities of atomic lines of the carrier gas and sample in the presence of oxygen are discussed in detail; these changes are greater than those observed with Ar/H2 and Ar/N2 mixtures. A detailed study of spectral line profiles shows changes in self-absorption of Ar I lines due to O2 and H2 traces. The sputter rate for a given Ar/O2 gas mixture is found not to be proportional to current. Copyright (2011) Royal Society of Chemistry. Links: Publication   Download PDF

• High-resolution photoabsorption cross section measurements of SO2 at 198 K from 213 to 325 nm
  Blackie D, Blackwell-Whitehead R, Stark G, Pickering JC, Smith PL. Rufus J,Thorne AP JGR VOL. 116, E03006 (2011) (more info PDF)
  
  

  Authors: Blackie D, Blackwell-Whitehead R, Stark G, Pickering JC, Smith PL. Rufus J,Thorne AP Abstract: SO2 plays an important role in the atmospheric chemistry of the Earth, Venus, and Io. This paper presents photoabsorption cross sections of SO2 from 213 to 325 nm at 198 K, encompassing the C1B2 − X1A1 and B1B1 − X1A1 electronic bands. These measurements are part of a series of measurements over the 160 to 300 K temperature range between 190 and 325 nm. The cross sections have been measured at high resolution (lambda/delta lambda ≈ 450,000) using Fourier transform spectrometry and are compared to other high‐resolution measurements in the literature. Copyright (2011) American Geophysical Union. Links: Publication   Download PDF

• The Spectrum and Term Analysis of V I
  Thorne AP, Pickering JC, Semeniuk J, ApJS 192 11 (2011) doi: 10.1088/0067-0049/192/1/11 (more info PDF)
  
  

  Authors: Thorne AP, Pickering JC, Semeniuk J Abstract: High-resolution spectra of neutral vanadium have been recorded by Fourier transform spectrometry in the region 1492-28600 Å (67020-3496 cm–1) with vanadium-neon and vanadium-argon hollow cathode lamps as sources. Most of the lines exhibit fully or partly resolved hyperfine structure, but the wavenumber uncertainty for the center of gravity of the strongest lines is 0.002 cm–1. Of the previously published energy levels of V I, 72 have been revised and 37 discarded. In addition, 89 new levels have been found. In total 346 odd levels and 198 even levels are presented. Wavenumbers and classifications are given for 3130 V I lines. Copyright (2011) The American Astronomical Society. Links: Publication   Download PDF

• Infrared Mn I laboratory oscillator strengths for the study of late type stars and ultracool dwarfs
  Blackwell-Whitehead R., Pavlenko Y,V, Nave G., Pickering J.C., Jones H.R.A., Lyubchik Y. and Nilsson H.,Astronomy & Astrophysics,A&A 525 A44 (2011) (more info PDF)
  
  

  Authors: Blackwell-Whitehead R., Pavlenko Y,V, Nave G., Pickering J.C., Jones H.R.A., Lyubchik Y. and Nilsson H. Abstract: Aims. The aim of our new laboratory measurements is to measure accurate absolute oscillator strengths for neutral manganese transitions in the infrared needed for the study of late-type stars and ultracool dwarfs. Methods. Branching fractions have been measured by high resolution Fourier transform spectroscopy and combined with radiative level lifetimes in the literature to yield oscillator strengths. Results. We present experimental oscillator strengths for 20 Mn I transitions in the wavelength range 3216 to 13997 Å, 15 of which are in the infrared. The transitions at 12899 Å and 12975 Å are observed as strong features in the spectra of late-type stars and ultracool dwarfs. We have fitted our calculated spectra to the observed Mn I lines in spectra of late-type stars. Using the new experimentally measured Mn I log(gf) values together with existing data for Mn I hyperfine structure splitting factors we determined the manganese abundance to be log N(Mn) = −6.65 ± 0.05 in the atmosphere of the Sun, log N(Mn) = 6.95 ± 0.20 in the atmosphere of Arcturus, and log N(Mn) = −6.70 ± 0.20 in the atmosphere of M 9.5 dwarf 2MASSW 0140026+270150. Copyright (2010) ESO Links: Download PDF

• Laboratory measurements of oscillator strengths and their astrophysical applications
  Pickering JC, Blackwell-Whitehead R, Thorne AP, et al., Can. J. Phys 89 pp. 387 (2011) (more info)
  
  

  Authors: Pickering JC, Blackwell-Whitehead R, Thorne AP, Ruffoni MP, Holmes CE Abstract: We present an overview of current needs for accurate laboratory atomic transition probabilities (loggfs) for astrophysical applications, particularly for iron group element spectra in the IR, optical, UV and VUV spectral regions. Examples are given of our recent measurements, undertaken using the combination of high resolution Fourier transform spectrometry and time resolved laser induced fluorescence. Laboratory measured loggfs are particularly important for determination of elemental abundances in astrophysical objects. With new facilities, telescopes and spectrographs, access to underexplored regions (IR, vacuum UV,VUV), and new stellar atmosphere models (e.g. 3D, NLTE), astronomers are tackling problems ranging from determining accurate elemental abundances in the Sun, studying Galactic chemical evolution, to low mass stars and exoplanets. Such advances require improved accuracy and completeness of the atomic database for meaningful analyses of astrophysical spectra. Links: Publication

2010

• New atomic data for astrophysics by high-resolution spectroscopy
  Ruffoni MP, Pickering JC, Thorne AP, et al., Proceedings of the NASA laboratory astrophysics workshop (2010) (more info)
  
  

  Authors: Ruffoni MP, Pickering JC, Thorne AP, Holmes CE, Smith PL Abstract: We report a summary of recent significant improvements to atomic and molecular data obtained through high resolution Fourier transform spectrometry at Imperial College London. These primarily include wavelengths, energy levels, and oscillator strengths of neutral, singly, and doubly ionised iron-group atoms of astrophysical significance. Copyright (2010) Links:

• Measurement and simulation of mid- and far-infrared spectra in the presence of cirrus
  C. V. Cox, J. E. Harries, J. P. Taylor, P. D. Green, A. J. Baran, J. C. Pickering, A. E. Last and J.E. Murray, Q. J. R. Meteorol. Soc. 136: 718–739, April 2010 Part A (more info)
  
  

  Authors: C. V. Cox, J. E. Harries, J. P. Taylor, P. D. Green, A. J. Baran, J. C. Pickering, A. E. Last and J.E. Murray Abstract: Observed far-infrared and mid-infrared radiance spectra measured in situ above cirrus with both the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) and the Airborne Research Interferometer Evaluation System (ARIES) are presented. Two sets of upwelling spectra between wavenumbers 100 and 1415cm−1 taken over ice cloud of visible optical thickness 0.1 and 0.3 are shown. These observations address the need for more measurements of radiance from cirrus, particularly in the far infrared, where experimental data are lacking. A test of consistency in modelling in both the far infrared and mid infrared is performed by fitting a model calculation of radiances in the infrared window region, and comparing with the measured radiances. In this consistency check, the sampling of the surrounding atmosphere and the cloud is given extensive consideration. The model was not able to consistently reproduce the observed radiance across the entire region, and was particularly poor in the 330–600cm-1 region of the far infrared. Possible causes are thought to be primarily model input uncertainties arising from inadequate sampling of cloud and atmosphere. RoyalMeteorological Society and Crown Copyright (2010) Links: Publication   Download PDF

• Accurate laboratory wavelengths of the 1910 Å Ti II resonance transitions relevant to studies of possible variations of the fine structure constant
  Ruffoni MP, Pickering JC, ApJ. 725 pp. 424 (2010) (more info PDF)
  
  

  Authors: Ruffoni MP, Pickering JC Abstract: In recent years, the analysis of absorption lines in quasi-stellar object (QSO) spectra, using the many-multiplet (MM) method, has provided evidence for space-time variations in the fine-structure constant, α. Future studies aim to reduce systematic errors in these measurements by considering a greater number of transitions, but this is only possible for lines where high-precision laboratory standards exist. Two transitions of high importance for future MM analyses, but which currently lack accurately measured wavelengths, are the Ti II transitions observed at 1910 Å. We report accurate measurements of these transitions by high-resolution Fourier transform spectroscopy, giving line wavenumbers of 52329.889 ± 0.001 cm-1 and 52339.240 ± 0.001 cm-1. Lines from other important Ti II, Mg I, Mg II, and Zn II transitions were measured simultaneously; minimising their relative wavenumber uncertainties, and permitting the newly measured 1910 Å Ti II line wavenumbers to be linked directly to lines from other studies. Copyright (2010) The American Astronomical Society. Links: Publication   Download PDF

• NLTE analysis of Co I/Co II lines in spectra of cool stars with new laboratory hyperfine splitting constants
  Maria Bergemann, Juliet C. Pickering, and Thomas Gehren, MNRAS, 401 (2) 1334-1346 (2010) (more info PDF)
  
  

  Authors: Maria Bergemann, Juliet C. Pickering, and Thomas Gehren Abstract: The analysis of stellar abundances for odd-Z Fe-peak elements requires accurate non-local thermodynamic equilibrium (NLTE) modelling of spectral lines fully taking into account the hyperfine structure (HFS) splitting of lines. Here, we investigate the statistical equilibrium of Co in the atmospheres of cool stars and the influence of NLTE and HFS on the formation of Co lines and abundances. Significant departures from LTE level populations are found for Co I; number densities of excited states in Co II also differ from LTE at low metallicity. The NLTE level populations are used to determine the abundance of Co in solar photosphere, log ε = 4.95 ± 0.04 dex, which is in agreement with that in C I meteorites within the combined uncertainties. The spectral lines of Co I were calculated using the results of recent measurements of hyperfine interaction constants by UV Fourier transform spectrometry. For Co II, the first laboratory measurements of HFS A and B factors were performed. These highly accurate A factor measurements (errors of the order of 3–7 per cent) allow, for the first time, reliable modelling of Co II lines in the solar and stellar spectra and, thus, a test of the Co I/Co II ionization equilibrium in stellar atmospheres. A differential abundance analysis of Co is carried out for 18 stars in the metallicity range −3.12 < [Fe/H] < 0. The abundances are derived by the method of spectrum synthesis. At low [Fe/H], NLTE abundance corrections for Co I lines are as large as +0.6, . . . , + 0.8 dex. Thus, LTE abundances of Co in metal-poor stars are severely underestimated. The stellar NLTE abundances determined from the single UV line of Co II are lower by 0.5–0.6 dex. The discrepancy might be attributed to possible blends that have not been accounted for in the solar Co II line and its erroneous oscillator strength. The increasing [Co/Fe] trend in metal-poor stars, as calculated from the Co I lines under NLTE, can be explained if Co is overproduced relative to Fe in massive stars. The models of Galactic chemical evolution are wholly inadequate to describe this trend suggesting that the problem is in supernova yields. Copyright (2009) RAS Links: Publication   Download PDF

• A detailed comparison of spectral line intensities with plane and hollow cathodes in a Grimm type glow discharge source
  Weinstein V, Steers EBM, Smid P, Pickering JC, Mushtaq S., J. Analytical Atomic Spectrometry (JAAS), 25 (8) 1283-1289 (2010) (more info PDF)
  
  

  Authors: Weinstein V, Steers EBM, Smid P, Pickering JC, Mushtaq S. Abstract: The hollow cathode discharge was investigated and compared with the plane cathode discharge in a Grimm-type source studying a large number of lines with iron and titanium as cathode materials in argon plasma. The high resolution Fourier Transform Spectrometer (FTS) at Imperial College (IC) was used for recording spectra. The intensity changes for atomic and ionic lines of iron, titanium and argon are discussed, whereby clear trends in intensity increase and reduction according to the excitation energies of the emission lines are identified. Differences in line profiles in spectra using the hollow and plane cathodes were detected. The comparison between the hollow (10-15 mm deep) and shallow (2 mm deep) cathodes gives additional information about the depth influence on the hollow cathode effect. The extended study presented in this paper is a further step towards a better understanding of the discharge processes in the glow discharge. Copyright (2010) Royal Society of Chemistry. Links: Publication   Download PDF

• Laboratory measurements of oscillator strengths and their astrophysical applications
  Pickering JC, Blackwell-Whitehead R, Thorne AP, et al., Can. J. Phys (ASOS10 Ed.) submitted (2010) (more info)
  
  

  Authors: Pickering JC, Blackwell-Whitehead R, Thorne AP, Ruffoni MP, Holmes CE Abstract: We present an overview of current needs for accurate laboratory atomic transition probabilities (loggfs) for astrophysical applications, particularly for iron group element spectra in the IR, optical, UV and VUV spectral regions. Examples are given of our recent measurements, undertaken using the combination of high resolution Fourier transform spectrometry and time resolved laser induced fluorescence. Laboratory measured loggfs are particularly important for determination of elemental abundances in astrophysical objects. With new facilities, telescopes and spectrographs, access to underexplored regions (IR, vacuum UV,VUV), and new stellar atmosphere models (e.g. 3D, NLTE), astronomers are tackling problems ranging from determining accurate elemental abundances in the Sun, studying Galactic chemical evolution, to low mass stars and exoplanets. Such advances require improved accuracy and completeness of the atomic database for meaningful analyses of astrophysical spectra. Links: Publication

2009

• High-resolution photoabsorption cross-section measurements of SO₂ at 160 K between 199 and 220 nm
  J. Rufus, G. Stark, A. P. Thorne, J. C. Pickering, R. J. Blackwell-Whitehead, D. Blackie and Peter L. Smith, Journal of Geophysical Research, 14, E06003, doi:10.1029/2008JE003319 (2009) (more info PDF)
  
  

  Authors: Rufus J, Stark G, Thorne AP, Pickering JC, Blackwell-Whitehead RJ, Blackie D and Smith PL Abstract: Photoabsorption cross sections of sulfur dioxide over a range of temperatures are required to interpret observations and to support models of the atmospheres of Io and Venus. We report high-resolution (lambda/Delta lambda approximate to 450,000) photoabsorption cross-section measurements by Fourier transform spectrometry of SO2 at 160 K in the wavelength region 199 to 220 nm, which encompasses the strongest features in the prominent (C) over tilde (1) B2 -(X) over tilde (1)A1 system. Our results are compared with literature values obtained at lower resolutions and with 295 K cross sections recorded earlier with the same instrument. Copyright (2010) Amer Geophysical Union Links: Publication   Download PDF

2008

• Cross Sections and Reaction Rates for Comparative Planetary Aeronomy
  Huestis DL, Bougher SW, Fox JL, Galand M, Johnson RE, Moses J, Pickering JC, Space Science Reviews, 139 (1-4), 63-105, (2008) (more info PDF)
  
  

  Authors: Huestis DL, Bougher SW, Fox JL, Johnson RE, Moses J, Pickering JC, Abstract: In this chapter we describe the current knowledge of a selection of collision processes and chemical reactions of importance to planetary aeronomy. Emphasis is placed on critical evaluation of what we know and what we wish we knew about fundamental processes required for interpretation, explanation, and modeling of atmospheric observations. Copyright (2008) Links: Publication   Download PDF

• Reference ultraviolet wavelengths of Cr III measured by Fourier transform spectrometry
  Smillie DG, Pickering JC & Smith PL, Monthly Notices of the Royal Astronomical Society, 390 (2), 733-740 (2008) (more info PDF)
  
  

  Authors: Smillie DG, Pickering JC & Smith PL Abstract: We report Cr III ultraviolet (UV) transition wavelengths measured using a high-resolution Fourier transform spectrometer (FTS), for the first time, available for use as wavelength standards. The doubly ionized iron group element spectra dominate the observed opacity of hot B stars in the UV, and improved, accurate, wavelengths are required for the analysis of astronomical spectra. The spectrum was excited using a chromium-neon Penning discharge lamp and measured with the Imperial College vacuum ultraviolet FTS. 140 classified 3d34s-3d34p Cr III transition lines, in the spectral range 38 000 to 49 000 cm-1 (2632 to 2041 angstrom), the strongest having wavelength uncertainties less than one part in 107, are presented. Copyright (2008) RAS. Links: Publication   Download PDF

• The effect of hydrogen and nitrogen on emission spectra of iron and titanium atomic lines in analytical glow discharges
  Smid P, Steers E, Weiss Z, Pickering J & Hoffmann V,JAAS, J Analytical Atomic Spectrometry, 23 (9), 1223-1233 (2008) (more info PDF)
  
  

  Authors: Smid P, Steers E, Weiss Z, Pickering J & Hoffmann V Abstract: It is now well known that traces of molecular gases such as hydrogen or nitrogen can affect significantly the electrical characteristics, sputtering rates and relative intensities of emission lines in glow discharge optical emission spectrometry (GD-OES). These changes, caused by the molecular gases which are very often present in the discharge for various reasons, have a serious impact on the accuracy of analytical results. Therefore, it is important to describe these effects in detail and to try to understand the processes involved. The results presented in this paper focus on the effects of hydrogen and nitrogen on intensities of atomic emission lines of iron and titanium. In the case of hydrogen, when intensity ratios (intensities measured in argon–hydrogen relative to those measured in pure argon) are plotted against the excitation energies of the lines, these intensity ratios increase with the excitation energy between approx. 3 and 5 eV for both elements studied. Furthermore, in the case of iron, several emission lines with the excitation energy between 5.3 and 5.6 eV are strongly enhanced in the presence of hydrogen. It has been also observed that this effect is more pronounced at lower currents. On the other hand, it has been found that nitrogen does not have any similar effect: the gradient of the intensity ratios of both elements is negligible or even negative and no emission lines are enhanced in the presence of nitrogen. A comparison was also made between direct current and radiofrequency powered glow discharges and very similar trends have been obtained. Copyright (2008)The Royal Society of Chemistry. Links: Publication   Download PDF

2007

• A revision of the solar manganese abundance using new and remeasured laboratory oscillator strengths
  Blackwell-Whitehead RJ, Bergemann M, A&A 472, L43–L46 (2007) (more info PDF)
  
  

  Authors: Blackwell-Whitehead RJ, Bergemann M Abstract: Context. The solar photospheric element abundances are generally in good agreement with the meteoritic CI chondrite abundances, with the exception of a small number of elements including manganese. The solar photospheric abundances, determined using model atmospheres, include laboratory oscillator strengths where available. However, the current laboratory database for Mn I oscillator strengths is derived from several different laboratory observations determined from several different laboratory techniques. The uncertainty in the solar photospheric manganese abundance and the difference between it and the meteoritic CI chondrite abundance may just be an artefact of inaccurate laboratory data. Aims. The aim of our new laboratory measurements is to measure a self consistent set of accurate absolute oscillator strengths and use the new laboratory data to re-evaluate the solar manganese abundance. Methods. New and more accurate oscillator strengths have been determined by combining branching fractions with previously measured energy level lifetimes. Using the new laboratory data, the solar photospheric abundance of manganese has been determined with theoretical and semi-empirical model atmospheres, MAFAGS-ODF and Holweger & Müller, respectively. Results. We present experimental oscillator strengths for 94 Mn I transitions covering the wavelength range 2384 to 17 744 Å. Using 22 relatively un-blended solar Mni transitions, we determine the photospheric abundance of manganese to be log ε = 5.37±0.05 dex. Conclusions. The new value is in good agreement with previous photospheric abundance determinations. The implications for the solar photospheric and meteoritic CI chondrite abundance is discussed. Copyright (2007) ESO Links: Publication   Download PDF

2006

• Experimental Ti I oscillator strengths and their application to cool star analysis
  Blackwell-Whitehead RJ, Lundberg H, Nave G, Pickering JC, Jones HRA, Lyubchik Y, Pavlenko YV and Viti S, Mon. Not. R. Astron. Soc. 373, 1603-1609 (2006) (more info PDF)
  
  

  Authors: Blackwell-Whitehead RJ, Lundberg H, Nave G, Pickering JC, Jones HRA, Lyubchik Y, Pavlenko YV, Viti S Abstract: We report experimental oscillator strengths for 88 Ti I transitions covering the wavelength range 465-3892 nm, 67 of which had no previous experimental values. Radiative lifetimes for 13 energy levels, including the low energy levels 3d2 (3F) 4s4p (3)P) z5Dj degrees have been measured using time-resolved laser-induced fluorescence. Intensity-calibrated Ti I spectra have been measured using Fourier transform spectroscopy to determine branching fractions for the decay channels of these levels. The branching fractions are combined with the radiative lifetimes to yield absolute transition probabilities and oscillator strengths. Our measurements include 50 transitions in the previously unobserved infrared region lambda > 1.0 mu m, a region of particular interest to the analysis of cool stars and brown dwarfs. Copyright (2006) RAS Links: Publication   Download PDF

• Investigation of the hyperfine structure of Ta I lines (X)
  Jaritz N, Windholz L, Zaheer U, Farooq M, Arcimowicz B, Engleman Jr. R, Pickering JC, J¨ager H & Guth¨ohrlein GH, Physica Scripta 74, 211-217 (2006) (more info PDF)
  
  

  Authors: Jaritz N, Windholz L, Zaheer U, Farooq M, Arcimowicz B, Engleman Jr. R, Pickering JC, J¨ager H & Guth¨ohrlein GH Abstract: We report the discovery of 23 new energy levels of even parity and 21 new energy levels of odd parity of the tantalum atom. The results given here are based on investigations of the hyperfine structure of 221 new spectral lines of the tantalum atom (Ta I) by means of laser spectroscopic methods, detecting laser-induced fluorescence. The excitation wavelengths were extracted from high-resolution Fourier transform spectra Copyright (2006) R Swedish Ac Sciences Links: Publication   Download PDF

2005

• Hyperfine structure measurements of neutral manganese with Fourier transform spectroscopy
  Blackwell-Whitehead RJ, Pickering JC, Pearse O, Nave G, Astrophysical Journal Suppl. Series 157 (2): 402-409 (2005) (more info PDF)
  
  

  Authors: Blackwell-Whitehead RJ, Pickering JC, Pearse O, Nave G Abstract: We report experimental hyperfine structure constants of levels in the neutral manganese atom, measured using Fourier transform spectroscopy of hollow cathode discharges. In total, 208 spectral lines of astrophysical interest have been analyzed to obtain hyperfine structure constants for 106 levels in Mn I, of which 67 have no previous hyperfine structure measurements. The uncertainties in the magnetic dipole constants, A, are between 1 x 10-4 and 5 x 10-4 cm-1. Hyperfine structure constants for an additional 18 levels compiled from the literature are also given. Copyright (2005) AAS Links: Publication   Download PDF

• Laboratory oscillator strength measurements for the spectrum of neutral manganese
  Blackwell-Whitehead R, Xu HL, Pickering JC, Nave G, Lundberg H. MNRAS, 361, 1281 (2005) (more info PDF)
  
  

  Authors: Blackwell-Whitehead R, Xu HL, Pickering JC, Nave G, Lundberg H. Abstract: We report laboratory measurements of oscillator strengths for 44 transitions in the Mn I spectrum covering the wavelength range 209-2780 nm. Nine energy level lifetimes have been measured using time resolved laser-induced fluorescence (LIF). The lifetimes have been combined with branching fractions measured by Fourier transform spectroscopy to obtain absolute oscillator strengths. In total, 24 of these oscillator strengths are measured for the first time, including transitions in the previously unobserved infrared region above 2.0 micro m. Copyright (2005) RAS Links: Publication   Download PDF

2004

• Atomic lines in infrared spectra for ultracool dwarfs
  Y. Lyubchik, H.R.A. Jones, Y.V. Pavlenko, et al., A&A 416(2) pp. 655-659 (2004) (more info)
  
  

  Authors: Y. Lyubchik, H.R.A. Jones, Y.V. Pavlenko, S. Viti, J.C. Pickering, R. Blackwell-Whitehead Abstract: We provide a set of atomic lines which are suitable for the description of ultracool dwarf spectra from 10 000 to 25 000 Å. This atomic linelist was made using both synthetic spectra calculations and existing atlases of infrared spectra of Arcturus and Sunspot umbra. We present plots which show the comparison of synthetic spectra and observed Arcturus and Sunspot umbral spectra for all atomic lines likely to be observable in high resolution infrared spectra. doi:10.1051/0004-6361:20034419 Copyright (2004) Links: Publication

• Atomic lines in infrared spectra for ultracool dwarfs
  N. Jaritz, G.H. Guthoehrlein, L. Windholz, et al., Physica Scripta 69 pp. 441-450 (2004) (more info)
  
  

  Authors: N. Jaritz, G.H. Guthoehrlein, L. Windholz, D. Messnarz, R. Engleman, J.C. Pickering, H. Jaeger Abstract: The hyperfine structures of 57 new spectral lines of the tantalum atom were investigated by means of laser excitation. From the hyperfine patterns we have determined the angular momenta J, the magnetic hyperfine interaction constants A and the electric quadrupole interaction constants B of 15 new energy levels of even parity and three new levels of odd parity. The observation of laser-induced fluorescence led to the classification of further 156 spectral lines for the first time. Additionally 92 lines, observed in high resolution Fourier transform spectra, were classified using a combination of their wavenumbers and their characteristic hyperfine patterns. doi:10.1238/Physica.Regular.069a00441 Copyright (2004) Links: Publication

2003

• High resolution photoabsorption cross section measurements of SO₂ at 295 K between 220 and 325 nm
  J.Rufus, G.Stark, P.L.Smith, et al., J. Geophys. Res.:Planets 108 (E2) art no. 5011 (2003) (more info)
  
  

  Authors: J.Rufus, G.Stark, P.L.Smith, J.C.Pickering, A.P.Thorne Abstract: High-resolution, ultraviolet SO2 photoabsorption cross section data are required for modeling density and temperature profiles of SO2 in solar system atmospheres. We report measurements of such cross sections on SO2 at 295 K for the inline equation1B1 - inline equation1A1 and C tilde1B2 - inline equation1A1 bands over the wavelength range 220 to 325 nm. Resolving powers of up to 550,000 were employed in order to study the congested spectrum of sharp SO2 features in parts of this region. Our data are compared to earlier values obtained with lower resolving power. doi:10.1029/2002JE001931 Copyright (2003) Links: Publication

• High resolution absorption cross-section measurements of the Schumann-Runge bands of O₂ by VUV Fourier transform spectroscopy
  T. Matsui, A. S.-C. Cheung, K. W. S. Leung, et al., J. Molecular Spectroscopy 219 (1) pp. 45-57 (2003) (more info PDF)
  
  

  Authors: T. Matsui, A. S.-C. Cheung, K. W. S. Leung, K. Yoshino, W. H. Parkinson, A. P. Thorne, J. E. Murray, K. Ito and T Imajo Abstract: The photoabsorption spectrum of the O2 Schumann–Runge bands was measured with resolution comparable to the Doppler widths by using the VUV Fourier transform spectrometer from Imperial College, London, combined with synchrotron radiation as a continuum light source at the Photon Factory, KEK, Japan. The analysis of the (12,0)–(17,0) bands of the Schumann–Runge system provides accurate rotational line positions as well as the line intensities from 185 to 175 nm. Molecular constants of the v′=12 to 17 levels of the B3Σ-u state have been determined. The (v′,0) band oscillator strengths were determined as 2.38, 2.62, 2.70, 2.66, 2.40, and 2.12×10−5 for the bands from v′=12 to 17, respectively. doi:10.1016/S0022-2852(03)00009-2 Copyright (2003) Elsevier Links: Publication   Download PDF

• Investigation of the hyperfine structure of Ta I lines (VII)
  D. Messnarz, N. Jaritz, B. Arcimowics, et al., Physica Scripta 68 (3) pp. 170-191 (2003) (more info)
  
  

  Authors: D. Messnarz, N. Jaritz, B. Arcimowics, R. Engleman, J.C. Pickering, H. Jaeger, G.H. Guthoehrlein, L. Windholz, V.O. Zilio Abstract: We have classified about 400 new spectral lines of neutral tantalum via laser excitation or observation of laser-induced fluorescence. 37 energy levels with even parity and seven levels with odd parity were found by means of systematic hyperfine structure investigations. For the new levels angular momentum, parity, magnetic hyperfine interaction constant A and electric quadrupole interaction constant B were deduced. In addition, we have classified 230 new lines observed in high resolution Fourier transform spectra by analysing their observed hyperfine structure patterns. doi:10.1238/Physica.Regular.068a00170 Copyright (2003) Links: Publication

• Extended term analysis of Mo II
  H. Nilsson & J. C. Pickering, Physica Scripta 67 (3) pp. 223-233 (2003) (more info)
  
  

  Authors: Abstract: The spectrum of singly ionized molybdenum has been recorded with Fourier transform spectrometers in the wavelength interval 1500–7000 Å. We report improved energy values for a total of 330 levels, 153 even and 177 odd, which belong to the 4d5, 4d45s, 4d35s2, 4d46s, 4d45d, 4d45p and 4d35s5p configurations. 110 of the levels have been found in the present analysis. We have also present calculated eigenvector components and oscillator strengths. doi:10.1238/Physica.Regular.067a00223 Copyright (2003) Links: Publication

• Application of a VUV Fourier transform spectrometer and synchrotron radiation source to measurements of. VI. The epsilon(0,0) band of NO
  A. S-C. Cheung, A. L. Wong, D. H-Y. Lo, et al., Journal of Chemical Physics 119(16) pp. 8373-8378 (2003) (more info)
  
  

  Authors: A. S-C. Cheung, A. L. Wong, D. H-Y. Lo, K. W-S. Leung, K. Yoshino, A. P. Thorne and J. E. Murray, T. Imajo, K. Ito and T. Matsui Abstract: The ϵ(0,0) (D2Σ+ –X2Πr) band of NO has been recorded by using a vacuum ultraviolet Fourier transform spectrometer from Imperial College, London, with synchrotron radiation at the Photon Factory, KEK, Japan, as a continuum light source. Analysis of the ϵ(0,0) band provides accurate rotational line positions and term values as well as the photoabsorption cross sections. Molecular constants of the v = 0 level of the D2Σ state have been determined as T0 = 53291.10 ± 0.10 cm-1, B0 = 1.99107±0.00005 cm-1 , and D0 = (6.6±0.1)×10−6 cm−1. Accurate rotational line strengths have also been obtained and the sum of the line strengths for all rotational lines is determined as 2.18×10−15 cm2 cm−1. The band oscillator strength of the ϵ(0,0) band is determined to be (2.47±0.12)×10−3. Copyright (2003) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in (Journal of Chemical Physics vol. 119 pages 8373-8378) and may be found at (http://jcp.aip.org/resource/1/jcpsa6/v119/i16/p8373_s1). Links: Publication   Download PDF

2002

• High Resolution Fourier transform spectroscopy with the Imperial College UV FT spectrometer, and its applications to astrophysics and atmospheric physics: a review
  J. C. Pickering, Vibrational Spectroscopy 29(1-2) pp. 27-43 (2002) (more info PDF)
  
  

  Authors: Abstract: The improvements in astronomical telescopes and their spectrographs, and in spectrographs used for studies of the earth’s atmosphere have created a requirement for matching improvements in the laboratory atomic and molecular database needed in the analysis of the astrophysical and atmospheric spectra. The technique of Fourier transform spectroscopy (FTS) is described and its suitability in improving the atomic and molecular databases is discussed. The extension of the wavelength range of the Fourier transform (FT) spectrometry into the vacuum ultraviolet to 140 nm is described. Examples are given of recent atomic and molecular spectroscopy research using the Imperial College (IC) UV-FT spectrometer and applications to astrophysics and atmospheric physics are given. doi:10.1016/S0924-2031(01)00174-6 Copyright (2002) Elsevier Links: Publication   Download PDF

• The FERRUM Project: Experimental oscillator strengths of the UV 8 multiplet and other UV transitions from the y⁶P levels of Fe II
  J. C. Pickering, M. Donnelly, H. Nilsson, et al., A&A 396(2) pp. 715-722 (2002) (more info)
  
  

  Authors: J. C. Pickering, M. Donnelly, H. Nilsson, A. Hibbert & S. Johansson Abstract: We report on experimental branching fractions (BFs) for 19 transitions from the three 3d5 (6S)4s4p(3P) y6P levels in Fe II, measured in Fourier transform spectra of a Penning discharge lamp and a hollow cathode lamp. The transition wavelengths span the interval between 1600 and 2850 Å. Absolute oscillator strengths ( f-values) have been derived by combining the BFs with experimental radiative lifetimes recently reported in the literature. Theoretical f-values have been calculated using configuration interaction wavefunctions . The new experimental and theoretical f-values are compared with data available in the literature and in databases. The strongest lines measured belong to the UV 8 multiplet of Fe II, which has one ground state transition appearing as a prominent feature at 1608 Å in interstellar spectra. The line is accompanied by a satellite at 1611 Å due to a level mixing, which is discussed in the paper. doi:10.1051/0004-6361:20021388 Copyright (2003) Links: Publication

• Measurements of hyperfine structure in Ta II
  V. Zilio & J. C. Pickering, MNRAS 334(1) pp. 48-52 (2002) (more info)
  
  

  Authors: V. Zilio & J. C. Pickering Abstract: We report the first extensive measurements of hyperfine structure in Ta II. Spectra of Ta II were recorded by high-resolution Fourier transform spectrometry in the region 10000 – 53000 cm-1 (1886–10 000 Å) and the majority of observed lines show significant hyperfine structure. Computer fits to several hundred of these line profiles have yielded values of the magnetic dipole hyperfine interaction constant A for 88 energy levels with an uncertainty of between 0.5 and 10 per cent for the majority of A factors. The A factors range from −0.078 to +0.065 cm-1 for the even levels and from −0.064 to +0.083 cm-1 for the odd levels. For the majority of these A factors no previous measurements are known. Approximate values of the electric quadrupole hyperfine interaction constant B were found for 73 levels. These measurements of A and B factors allow, for the first time, the effects of hyperfine structure in Ta II lines to be correctly accounted for both in abundance analysis and in the resolution of blended lines in astrophysical spectra. doi:10.1046/j.1365-8711.2002.05443.x Copyright (2002) Links: Publication

• The FERRUM Project: New f-value data for Fe II and astrophysical applications
  S.Johansson, A.Derkatch, M.Donnelly, et al., Physica Scripta T100 pp. 71-80 (2002) (more info)
  
  

  Authors: S.Johansson, A.Derkatch, M.Donnelly, H.Hartman, A.Hibbert, H.Karlsson, M.Kock, Z.Li, D.S.Leckrone, U.Litzen, H.Lundberg, S.Mannervik, L-O.Norlin, H.Nilsson, J.C.Pickering, T.Rassen, D.Rostohar, P.Royen, A.Schmitt, M.Schultz-Johanning, C.M.Sikstrom, P.L.Smith, S.Svanberg, G.M.Wahlgren Abstract: We present the FERRUM Project, an international collaboration aiming at a production and evaluation of oscillator strengths (transition probabilities) of selected spectral lines of singly ionized iron group elements, that are of astrophysical relevance. The results obtained include measurements and calculations of permitted and forbidden lines of Fe II. The data have been applied to both emission and absorption lines in astrophysical spectra. We make comparisons between experimental, theoretical and astrophysical f-values. We give a general review of the various measurements, and discuss the UV8 multiplet of Fe II around 1610 Å in detail. doi:10.1238/Physica.Topical.100a00071 Copyright (2002) Links: Publication

• The application of a vacuum ultraviolet Fourier transform spectrometer and synchrotron radiation source to measurements of: V. The beta(11,0) band of NO
  J. Rufus, K. Yoshino, A. P. Thorne, et al., Journal of Chemical Physics 117(23) pp. 10621-10626 (2002) (more info PDF)
  
  

  Authors: J. Rufus, K. Yoshino, A. P. Thorne, J. E. Murray, T. Imajo, K. Ito, T. Matsui Abstract: The β(11,0) band of NO was measured at high resolution (0.06 cm-1 ) by the vacuum ultraviolet Fourier transform spectrometer from Imperial College, London, using synchrotron radiation at the Photon Factory, KEK, Japan, as a continuum light source. Such resolution facilitates a line by line analysis of the NO β(11,0) band which yields accurate rotational line positions and term values as well as the photoabsorption cross sections. The molecular constants of the B(11) 2Πr level are found to be T0 = 55983.203 ± 0.017 cm-1, A = 47.977±0.024, Bv = 1.01077 ± 0.00037 cm-1, Dv = (6.7±0.8)×10-6 cm-1 and AD = 0.0438±0.0035 cm-1. The sum of the line strengths for all rotational transitions of the NO β(11,0) band is determined as 3.04×10−16 cm2 cm-1, corresponding to a band oscillator strength of (3.44±0.21)×10-4. Copyright (2002) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in (Journal of Chemical Physics vol. 117 page 10621) and may be found at (http://jcp.aip.org/resource/1/jcpsa6/v117/i23/p10621_s1). Links: Publication   Download PDF

• The application of a vacuum ultraviolet Fourier transform spectrometer and synchrotron radiation source to measurements of: IV. The beta(6,0) and gamma(3,0) bands of NO
  A. S. C. Cheung, D. H. Y. Lo, K. W. S. Leung, et al., Journal of Chemical Physics 116(1) pp. 155-161 (2002) (more info PDF)
  
  

  Authors: A. S. C. Cheung, D. H. Y. Lo, K. W. S. Leung, K. Yoshino, A. P. Thorne, J. E. Murray, K. Ito, T. Matsui, T. Imajo Abstract: The β(6,0) (B2 Πr –X2Πr ) and γ(3,0) (A2Σ+ – X 2Πr) bands of NO have been recorded using a vacuum ultraviolet Fourier transform spectrometer with synchrotron radiation as light source. The analysis of the β(6,0) and γ(3,0) bands of NO provides accurate rotational line positions and term values. Molecular constants of the v = 6 level of the B2 Πr and v = 3 level of the A2Σ+ have been determined. Accurate rotational line strengths have also been obtained. The band oscillator strength of the β(6,0) and γ(3,0) bands are determined to be 0.48×10−4 and 2.69×10−4, respectively. Copyright (2002) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in (Journal of Chemical Physics vol. 116 page 155) and may be found at (http://jcp.aip.org/resource/1/jcpsa6/v116/i1/p155_s1). Links: Publication   Download PDF

2001

• New accurate data on the spectrum of neutral silver
  J. C. Pickering, V. Zilio, Eur. Phys. J. D 13 pp. 181-185 (2001) (more info)
  
  

  Authors: J. C. Pickering, V. Zilio Abstract: High resolution spectra of neutral silver have been recorded by Fourier Transform Spectrometry in the region 2000-8280 Å(50000 - 12000 cm-1) with silver-neon and silver-argon hollow cathode lamps as sources. This work represents order-of-magnitude improvements, compared to previous work on Ag I, in accuracy of transition wavelengths and wavenumbers, and energy level values. 35 classified Ag I transitions are given, and the wavenumber uncertainty for the strongest lines is less than 0.002 cm-1. Improved values for 28 energy levels are presented. doi:10.1007/s100530170264 Copyright (2001). The final publication is available at www.epj.org. Links: Publication

• Oscillator strengths of UV transitions in Ti II
  J. C. Pickering, A. P. Thorne, R. Perez, ApJ SS 132 pp. 403-409 (2001) (more info PDF)
  
  

  Authors: J. C. Pickering, A. P. Thorne, R. Perez Abstract: The relative intensities of 694 emission lines of Ti II between 187 and 602 nm from 89 levels have been measured by high-resolution Fourier transform spectrometry, using a hollow cathode lamp as light source. The spectral response function of the instrument was determined with calibrated tungsten halogen and deuterium lamps. Errors due to self-absorption in strong lines were eliminated by running the lamp at different currents. At least 95% of the predicted transition intensity has been measured for 83 of these levels, allowing reliable branching fractions to be found. The branching fractions have been combined with 39 measured and 44 computed lifetimes to give absolute transition probabilities for 624 lines. These agree well with the most recent values for 55 of the lines obtained in a similar way from seven of the measured lifetimes. doi:10.1086/318958 Copyright (2001) The American Astronomical Society Links: Publication   Download PDF

• The FERRUM Project: Branching ratios and atomic transition probabilities of Fe II transitions from the 3d⁶ (a³F)4p subconfiguration in the visible to VUV spectral region
  J. C. Pickering, S. Johansson & P. L. Smith, A&A 377 pp. 361-367 (2001) (more info)
  
  

  Authors: J. C. Pickering, S. Johansson & P. L. Smith Abstract: We report measurements of the relative intensities of 81 emission lines of Fe II between 160 nm and 350 nm (62168 cm-1 to 28564 cm-1) from 4 levels by high resolution Fourier transform spectrometry, using a Penning discharge lamp as light source. These relative intensities have been used to determine the line branching fractions, which have then been combined with accurate experimental radiative lifetime measurements reported recently to give absolute transition probabilities and oscillator strengths for 81 lines. The accuracy of these f-values is compared with other previous experimental measurements, and with theoretical values. The new transition probabilities will allow accurate determinations of Fe II abundances in a wide variety of astrophysical objects. doi:10.1051/0004-6361:20010943 Copyright (2001) Links: Publication

• The application of a VUV Fourier transform spectrometer and synchrotron radiation source to measurements of: III. The e (1,0) band of NO
  J. Rufus, K. Yoshino, J. R. Esmond, et al., J. Chem. Phys. 115(8) pp. 3719-3723 (2001) (more info)
  
  

  Authors: J. Rufus, K. Yoshino, J. R. Esmond, A. P. Thorne, J. E. Murray, T. Imajo, K. Ito and T. Matsui Abstract: The ϵ(1,0) band of NO was measured at high resolution (0.06 cm−1) by the vacuum ultraviolet (VUV) Fourier transform spectrometer from Imperial College, London, using synchrotron radiation at the Photon Factory, KEK, Japan, as a continuum light source. Such resolution facilitates a line by line analysis of the NO ϵ(1,0) band which yields accurate rotational line positions and term values as well as the photoabsorption cross sections. The molecular constants of the D(1) 2Σ level are found to be T0 = 55 570.582±0.055 cm−1, Bv = 1.979 66±0.000 19 cm−1, Dv = (5.8±4.7)×10−5cm−1, γ = −0.127±0.008 cm−1 and γD = −(1.03±0.04)×10−3 cm−1. The sum of the line strengths for all rotational transitions of the NO ϵ(1,0) band is determined as 2.55×10−15 cm2 cm−1, corresponding to a band oscillator strength of 0.00288 ± 0.00017. Copyright (2001) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in (Journal of Chemical Physics vol. 115 page 3719) and may be found at (http://jcp.aip.org/resource/1/jcpsa6/v115/i8/p3719_s1). Links: Publication

• Atomic and molecular spectroscopy with the Imperial College VUV high resolution Fourier transform spectrometer
  in Fourier Transform Spectroscopy
  J. C. Pickering, A. P. Thorne, R. C. M. Learner, et al., OSA Technical Digest (Optical Society of America, Washington DC), pp. 5-6 (2001) (more info)
  
  

  Authors: J. C. Pickering, A. P. Thorne, R. C. M. Learner, R. Blackwell Abstract: Copyright (2001) Links: Publication

2000

• Precise Laboratory Wavelengths of importance to studies of the time variation of the fine structure constant
  Pickering JC, Thorne AP, LitzÉN, Webb JK, Zilio V, Monthly Notices of the Roy. Astron. Soc. 319, 163-167 (2000). (more info PDF)
  
  

  Authors: Pickering JC, Thorne AP, LitzÉN, Webb JK, Zilio V Abstract: The quality of astronomical spectroscopic data now available is so high that interpretation and analysis are often limited by the uncertainties of the laboratory data base. In particular, the limit with which space±time variations in the fine structure constant a can be constrained using quasar spectra depends on the availability of more accurate laboratory rest wavelengths. We recently measured some transitions in magnesium by high-resolution Fourier transform spectroscopy for this purpose, and we now report measurements on some ultraviolet resonance lines of Zn II (2062 and 2026 Å), Cr II (2066, 2062 and 2056 Å ) and Ni II (1751, 1741, 1709 and 1703 Å). Apart from the last line, which is very weak, the uncertainty of these measurements is 0.002 cm-1 (0.08 mÅ ) for the lines around 2000 Å and 0.004 cm-1 (0.12mÅ) for the lines around 1700Å. doi:10.1051/0004-6361:20010943 Copyright (2000) RAS Links: Publication   Download PDF

Selected Publications: 2004 -1987

Selected Publications: 2004 - 1987