98 results found
Weiss Z, Concepcion-Mairey F, Pickering JC, et al., 2021, Emission spectroscopic study of an analytical glow discharge with plane and hollow cathodes: Titanium and iron in argon discharge, SPECTROCHIMICA ACTA PART B-ATOMIC SPECTROSCOPY, Vol: 180, ISSN: 0584-8547
Liggins FS, Pickering JC, Nave G, et al., 2021, New Ritz Wavelengths and Transition Probabilities of Parity-forbidden [Mn II] Lines of Astrophysical Interest, ASTROPHYSICAL JOURNAL, Vol: 907, ISSN: 0004-637X
Liggins FS, Pickering JC, Nave G, et al., 2021, The Spectrum and Term Analysis of Singly Ionized Manganese, ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, Vol: 252, ISSN: 0067-0049
Ding M, Pickering JC, 2020, Measurements of the hyperfine structure of atomic energy levels in Co ii, Astrophysical Journal Supplement Series, Vol: 251, Pages: 1-7, ISSN: 0067-0049
Analysis of hyperfine structure constants of singly ionized cobalt (Co II) were performed on cobalt spectra measured by Fourier transform spectrometers in the region 3000–63,000 cm−1 (33333 – 1587 Å). Fits to over 700 spectral lines led to measurements of 292 magnetic dipole hyperfine interaction A constants, with values between −32.5 mK and 59.5 mK (1 mK = 0.001 cm−1). Uncertainties of 255 A constants were between ±0.4 mK and ±3.0 mK, the remaining 37 ranged up to ±7 mK. The electric quadrupole hyperfine interaction B constant could be estimated for only one energy level. The number of Co II levels with known A values has now increased tenfold, improving and enabling the wider, more reliable, and accurate application of Co II in astronomical chemical abundance analyses.
Murray JE, Brindley HE, Fox S, et al., 2020, Retrievals of high latitude surface emissivity across the infrared from high altitude aircraft flights, Journal of Geophysical Research: Atmospheres, Vol: 125, Pages: 1-16, ISSN: 2169-897X
We present retrievals of infrared spectral surface emissivities spanning the far and mid infrared from aircraft observations over Greenland, taken at an altitude of 9.2 km above sea level. We describe the flight campaign, available measurements and the retrieval method. The principal barriers to reducing uncertainty in the emissivity retrievals are found to be instrumental noise and our ability to simultaneously retrieve the underlying surface temperature. However, our results indicate that using the instrumentation available to us it is possible to retrieve emissivities from altitude with an uncertainty of ~ 0.02 or better across much of the infrared. They confirm that the far‐infrared emissivity of snow and ice surfaces can depart substantially from unity, reaching values as low as 0.9 between 400‐450 cm‐1. They also show good consistency with retrievals from the same flight made from near‐surface observations giving confidence in the methodology used and the results obtained for this more challenging viewing configuration. To the best of our knowledge, this is the first time that far‐infrared surface emissivity has been retrieved from altitude and demonstrates that the methodology has the potential to be extended to planned satellite far‐infrared missions.
Bantges RJ, Brindley HE, Murray JE, et al., 2020, A test of the ability of current bulk optical models to represent the radiative properties of cirrus cloud across the mid- and far-infrared, Atmospheric Chemistry and Physics, Vol: 20, Pages: 12889-12903, ISSN: 1680-7316
Measurements of mid- to far-infrared nadir radiances obtained from the UK Facility for Airborne Atmospheric Measurements (FAAM) BAe 146 aircraft during the Cirrus Coupled Cloud-Radiation Experiment (CIRCCREX) are used to assess the performance of various ice cloud bulk optical property models. Through use of a minimization approach, we find that the simulations can reproduce the observed spectra in the mid-infrared to within measurement uncertainty, but they are unable to simultaneously match the observations over the far-infrared frequency range. When both mid- and far-infrared observations are used to minimize residuals, first-order estimates of the spectral flux differences between the best-performing simulations and observations indicate a compensation effect between the mid- and far-infrared such that the absolute broadband difference is < 0.7 W m−2. However, simply matching the spectra using the mid-infrared (far-infrared) observations in isolation leads to substantially larger discrepancies, with absolute differences reaching ∼ 1.8 (3.1) W m−2. These results show that simulations using these microphysical models may give a broadly correct integrated longwave radiative impact but that this masks spectral errors, with implicit consequences for the vertical distribution of atmospheric heating. They also imply that retrievals using these models applied to mid-infrared radiances in isolation will select cirrus optical properties that are inconsistent with far-infrared radiances. As such, the results highlight the potential benefit of more extensive far-infrared observations for the assessment and, where necessary, the improvement of current ice bulk optical models.
Pickering JC, Teresa Belmonte M, Clear CP, et al., 2020, Recent advances in experimental laboratory astrophysics for stellar astrophysics applications and future data needs, Proceedings of the International Astronomical Union, Vol: 15, Pages: 220-228, ISSN: 1743-9213
Accurate atomic data for line wavelengths, energy levels, line broadening such as hyperfine structure and isotope structure, and f-values, particularly for the line rich iron group elements, are needed for stellar astrophysics applications, and examples of recent measurements are given. These atomic data are essential for determination of elemental abundances in astronomical objects. With modern facilities, telescopes and spectrographs, access to underexplored regions (IR, UV, VUV), and improved stellar atmosphere models (3D, NLTE), and extremely large datasets, astronomers are tackling problems ranging from studying Galactic chemical evolution, to low mass stars and exoplanets. Such advances require improved accuracy and completeness of the atomic database for analyses of astrophysical spectra.
Bantges R, Brindley H, Russell J, et al., 2020, A test of the ability of current bulk optical models to represent the radiative properties of cirrus cloud across the mid-and far-infrared, Publisher: Atmospheric Chemistry and Physics. Discussion. Copernicus Publications
Measurements of mid- to far-infrared nadir radiances obtained from the UK Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft during the Cirrus Coupled Cloud-Radiation Experiment (CIRCCREX) are used to assess the performance of various ice cloud bulk optical (single-scattering) property models. Through use of a minimisation approach, we find that the simulations can reproduce the observed spectra in the mid-infrared to within measurement uncertainty but are unable to simultaneously match the observations over the far-infrared frequency range. When both mid and far-infrared observations are used to minimise residuals, first order estimates of the flux differences between the best performing simulations and observations indicate a strong compensation effect between the mid and far infrared such that the absolute broadband difference is < 0.7 W m−2. However, simply matching the spectra using the mid-infrared observations in isolation leads to substantially larger discrepancies, with absolute differences reaching ~ 1.8 W m−2. These results highlight the benefit of far infrared observations for better constraining retrievals of cirrus cloud properties and their radiative impact, and provide guidance for the development of more realistic ice cloud optical models.
Weiss Z, Pickering JC, 2020, Charge transfer from doubly charged ions of transition elements in a neon glow discharge: evidence based on emission spectra, Plasma Sources Science and Technology, Vol: 29, Pages: 1-12, ISSN: 0963-0252
An extensive study of Mn II, Fe II, Ti II, Cr II and Cu II emission spectra from a Grimm-type glow discharge in neon was performed, using the formalism of transition rate (TR) diagrams. In this method, radiative depopulation rates of individual excited levels of a species under study are established based on the emission spectrum, prospective contributions from radiative decay of higher excited levels (cascade excitation) are subtracted and the resulting net depopulation rates are plotted as function of energy of the levels involved. A peak at a particular energy in such a diagram reflects a collisional process in operation, selectively populating levels in a narrow interval around that energy. By comparing net TR diagrams of ionic spectra of the elements listed above, a common pattern was found indicating that singly charged ions of these elements are created, in addition to other mechanisms, by charge transfer between doubly charged ions of the element under study and metastable neutral neon atoms. This mechanism appears to be significant and needs to be taken into account in collisional–radiative models describing excitation and ionization of some elements in neon glow discharges.
Weiss Z, Pickering JC, Hoffmann V, 2019, Sixty years of spectroscopic research: a tribute to Professor Edward B. M. Steers, 16th Czech-Slovak Spectroscopic Conference (CSSC), Publisher: SPRINGER INTERNATIONAL PUBLISHING AG, Pages: 2891-2896, ISSN: 2585-7290
Fox S, Mendrok J, Eriksson P, et al., 2019, Airborne validation of radiative transfer modelling of ice clouds at millimetre and sub-millimetre wavelengths, ATMOSPHERIC MEASUREMENT TECHNIQUES, Vol: 12, Pages: 1599-1617, ISSN: 1867-1381
Weiss Z, Pickering JC, Hoffmann V, 2018, Obituary Prof. Edward B. M. Steers (1931-2018), SPECTROCHIMICA ACTA PART B-ATOMIC SPECTROSCOPY, Vol: 149, Pages: 241-242, ISSN: 0584-8547
Belmonte MT, Pickering JC, Clear CP, et al., 2018, The laboratory astrophysics spectroscopy programme at Imperial College London, Galaxies, Vol: 6, ISSN: 2075-4434
Accurate atomic parameters, such as transition probabilities, wavelengths, and energy levels, are indispensable for the analysis of stellar spectra and the obtainment of chemical abundances. However, the quantity and quality of the existing data in many cases lie far from the current needs of astronomers, creating an acute need for laboratory measurements of matching accuracy and completeness to exploit the full potential of the very expensively acquired astrophysical spectra. The Fourier Transform Spectrometer at Imperial College London works in the vacuum ultraviolet-visible region with a resolution of 2,000,000 at 200 nm. We can acquire calibrated spectra of neutral, singly, and doubly ionized species. We collaborate with the National Institute of Standards and Technology (NIST) and the University of Lund to extend our measurements into the infrared region. The aim of this review is to explain the current capabilities of our experiment in an understandable way to bring the astronomy community closer to the field of laboratory astrophysics and encourage further dialogue between our laboratory and all those astronomers who need accurate atomic data. This exchange of ideas will help us to focus our efforts on the most urgently needed data.
Weiss Z, Steers EBM, Pickering JC, 2018, Transition rate diagrams and excitation of titanium in a glow discharge in argon and neon, SPECTROCHIMICA ACTA PART B-ATOMIC SPECTROSCOPY, Vol: 144, Pages: 20-28, ISSN: 0584-8547
Rhodin AP, Belmonte MT, Engstrom L, et al., 2017, Lifetime measurements and oscillator strengths in singly ionized scandium and the solar abundance of scandium, Monthly Notices of the Royal Astronomical Society, Vol: 472, Pages: 3337-3353, ISSN: 0035-8711
The lifetimes of 17 even-parity levels (3d5s, 3d4d, 3d6s and 4p2) in the region57 743–77 837 cm−1 of singly ionized scandium (Sc II) were measured by two-step timeresolvedlaser induced fluorescence spectroscopy. Oscillator strengths of 57 lines from thesehighly excited upper levels were derived using a hollow cathode discharge lamp and a Fouriertransform spectrometer. In addition, Hartree–Fock calculations where both the main relativisticand core-polarization effects were taken into account were carried out for both low- andhigh-excitation levels. There is a good agreement for most of the lines between our calculatedbranching fractions and the measurements of Lawler & Dakin in the region 9000–45 000 cm−1for low excitation levels and with our measurements for high excitation levels in the region23 500–63 100 cm−1. This, in turn, allowed us to combine the calculated branching fractionswith the available experimental lifetimes to determine semi-empirical oscillator strengths fora set of 380 E1 transitions in Sc II. These oscillator strengths include the weak lines that wereused previously to derive the solar abundance of scandium. The solar abundance of scandiumis now estimated to log = 3.04 ± 0.13 using these semi-empirical oscillator strengths toshift the values determined by Scott et al. The new estimated abundance value is in agreementwith the meteoritic value (logmet = 3.05 ± 0.02) of Lodders, Palme & Gail.
Bellisario C, Brindley H, Murray J, et al., 2017, Retrievals of the Far Infrared surface emissivity over the Greenland Plateau using the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS)., Journal of Geophysical Research, Vol: 122, Pages: 12152-12166, ISSN: 0148-0227
The Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) measured near surface upwelling and downwelling radiances within the far infrared (FIR) over Greenland during two flights in March 2015. Here we exploit observations from one of these flights to provide in-situ estimates of FIR surface emissivity, encompassing the range 80-535 cm-1. The flight campaign and instrumental set-up is described as well as the retrieval method, including the quality control performed on the observations. The combination of measurement and atmospheric profile uncertainties means that the retrieved surface emissivity has the smallest estimated error over the range 360-535 cm-1, (18.7-27.8 μm), lying between 0.89 and 1 with an associated error which is of the order ± 0.06. Between 80 and 360 cm-1, the increasing opacity of the atmosphere, coupled with the uncertainty in the atmospheric state, means that the associated errors are larger and the emissivity values cannot be said to be distinct from 1. These FIR surface emissivity values are, to the best of our knowledge, the first ever from aircraft-based measurements. We have compared them to a recently developed theoretical database designed to predict the infrared surface emissivity of frozen surfaces. When considering the FIR alone, we are able to match the retrievals within uncertainties. However, when we include contemporaneous retrievals from the mid infrared (MIR), no single theoretical representation is able to capture the FIR and MIR behaviour simultaneously. Our results point towards the need for model improvement and further testing, ideally including in-situ characterisation of the underlying surface conditions.
Belmonte MT, Pickering JC, Ruffoni MP, et al., 2017, Fe I Oscillator Strengths for Transitions from High-lying Odd-parity Levels, Astrophysical Journal, Vol: 848, ISSN: 0004-637X
We report new experimental Fe I oscillator strengths obtained by combining measurements of branching fractionsmeasured with a Fourier Transform spectrometer and time-resolved, laser-induced fluorescence lifetimes. Thisstudy covers the spectral region ranging from 213 to 1033 nm. A total of 120 experimental log( ) gf -values comingfrom 15 odd-parity energy levels are provided, 22 of which have not been reported previously and 63 of whichhave values with lower uncertainty than the existing data. The radiative lifetimes for 60 upper energy levels arepresented, 39 of which have no previous measurements.
Nave G, Sansonetti CJ, Townley-Smith K, et al., 2017, Comprehensive atomic wavelengths, energy levels, and hyperfine structure for singly ionized iron-group elements, CANADIAN JOURNAL OF PHYSICS, Vol: 95, Pages: 811-816, ISSN: 0008-4204
Belmonte MT, Pickering JC, Clear C, et al., 2017, Accurate atomic data for Galactic Surveys, 330th Symposium of the International-Astronomical-Union (IAU), Publisher: CAMBRIDGE UNIV PRESS, Pages: 203-205, ISSN: 1743-9213
O'Shea SJ, Choularton TW, Lloyd G, et al., 2016, Airborne observations of the microphysical structure of two contrasting cirrus clouds, Journal of Geophysical Research: Atmospheres, Vol: 121, Pages: 13510-13536, ISSN: 2169-8996
We present detailed airborne in situ measurements of cloud microphysics in two midlatitude cirrus clouds, collected as part of the Cirrus Coupled Cloud-Radiation Experiment. A new habit recognition algorithm for sorting cloud particle images using a neural network is introduced. Both flights observed clouds that were related to frontal systems, but one was actively developing while the other dissipated as it was sampled. The two clouds showed distinct differences in particle number, habit, and size. However, a number of common features were observed in the 2-D stereo data set, including a distinct bimodal size distribution within the higher-temperature regions of the clouds. This may result from a combination of local heterogeneous nucleation and large particles sedimenting from aloft. Both clouds had small ice crystals (<100 µm) present at all levels However, this small ice mode is not present in observations from a holographic probe. This raises the possibility that the small ice observed by optical array probes may at least be in part an instrument artifact due to the counting of out-of-focus large particles as small ice. The concentrations of ice crystals were a factor ~10 higher in the actively growing cloud with the stronger updrafts, with a mean concentration of 261 L−1 compared to 29 L−1 in the decaying case. Particles larger than 700 µm were largely absent from the decaying cirrus case. A comparison with ice-nucleating particle parameterizations suggests that for the developing case the ice concentrations at the lowest temperatures are best explained by homogenous nucleation.
Townley-Smith K, Nave G, Pickering JC, et al., 2016, Hyperfine structure constants for singly ionized manganese (Mn II) using Fourier transform spectroscopy, Monthly Notices of the Royal Astronomical Society, Vol: 461, Pages: 73-78, ISSN: 0035-8711
We expand on the comprehensive study of hyperfine structure (HFS) in Mn II conducted byHolt et al. (1999) by verifying hyperfine magnetic dipole constants (A) for 20 levels previouslymeasured by Holt et al. (1999) and deriving A constants for 47 previously unstudied levels. TheHFS patterns were measured in archival spectra from Fourier transform (FT) spectrometers atImperial College London and the National Institute of Standards and Technology. Analysis ofthe FT spectra was carried out in XGREMLIN. Our A constant for the ground level has a loweruncertainty by a factor of 6 than that of Blackwell-Whitehead et al.
Mushtaq S, Steers EBM, Hoffmann V, et al., 2016, Evidence for charge transfer from hydrogen molecular ions to copper atoms in a neon-hydrogen analytical glow discharge, Journal of Analytical Atomic Spectrometry, Vol: 31, Pages: 2175-2181, ISSN: 1364-5544
Extensive investigations have been carried out in recent years on the effects of the presence of small amounts of molecular gases in analytical glow discharges (GDs) using argon as the plasma gas. Complementary studies using neon have shown that, when hydrogen is added to a neon discharge with a copper sample, the intensities of lines from most of the upper energy levels in the Cu II 3d9(2D)4p sub-configuration decrease. However, the 224.700 nm spectral line and other lines from the same upper level (3Po2) increase in intensity and this effect cannot be produced by the excitation processes normally considered. We show that asymmetric charge transfer (ACT) excitation by hydrogen molecules (H2-ACT) is the most likely explanation, the first time this process has been reported for any element in analytical GDs. We also explain why a similar effect is not observed with added nitrogen, although the ionization energies of hydrogen and nitrogen molecules are very similar.
Holmes CE, Pickering JC, Ruffoni MP, et al., 2016, EXPERIMENTALLY MEASURED RADIATIVE LIFETIMES AND OSCILLATOR STRENGTHS IN NEUTRAL VANADIUM, Astrophysical Journal Supplement Series, Vol: 224, ISSN: 1538-4365
We report a new study of the V i atom using a combination of time-resolved laser-induced fluorescence and Fourier transform spectroscopy that contains newly measured radiative lifetimes for 25 levels between 24,648 cm−1 and 37,518 cm−1 and oscillator strengths for 208 lines between 3040 and 20000 Å from 39 upper energy levels. Thirteen of these oscillator strengths have not been reported previously. This work was conducted independently of the recent studies of neutral vanadium lifetimes and oscillator strengths carried out by Den Hartog et al. and Lawler et al., and thus serves as a means to verify those measurements. Where our data overlap with their data, we generally find extremely good agreement in both level lifetimes and oscillator strengths. However, we also find evidence that Lawler et al. have systematically underestimated oscillator strengths for lines in the region of 9000 ± 100 Å. We suggest a correction of 0.18 ± 0.03 dex for these values to bring them into agreement with our results and those of Whaling et al. We also report new measurements of hyperfine structure splitting factors for three odd levels of V i lying between 24,700 and 28,400 cm−1.
Smillie DG, Pickering JC, Nave G, et al., 2016, The spectrum and term analysis of Co iii measured using Fourier transforms and grating spectroscopy, Astrophysical Journal Supplement Series, Vol: 223, ISSN: 1538-4365
The spectrum of Co iii has been recorded in the region 1562–2564 Å (64,000 cm−1–39,000 cm−1) by Fourier transform (FT) spectroscopy, and in the region 1317–2500 Å (164,000 cm−1–40,000 cm−1) using a 10.7 m grating spectrograph with phosphor image plate detectors. The spectrum was excited in a cobalt–neon Penning discharge lamp. We classified 514 Co iii lines measured using FT spectroscopy, the strongest having wavenumber uncertainties approaching 0.004 cm−1 (approximately 0.2 mÅ at 2000 Å, or 1 part in 107), and 240 lines measured with grating spectroscopy with uncertainties between 5 and 10 mÅ. The wavelength calibration of 790 lines of Raassen & Ortí Ortin and 87 lines from Shenstone has been revised and combined with our measurements to optimize the values of all but one of the 288 previously reported energy levels. Order of magnitude reductions in uncertainty for almost two-thirds of the 3d64s and almost half of the 3d64p revised energy levels are obtained. Ritz wavelengths have been calculated for an additional 100 forbidden lines. Eigenvector percentage compositions for the energy levels and predicted oscillator strengths have been calculated using the Cowan code.
Weiss Z, Steers EBM, Mushtaq S, et al., 2016, The use of radiative transition rates to study the changes in the excitation of Cu ions in a Ne glow discharge caused by small additions of H-2, O-2 and N-2, Spectrochimica Acta Part B: Atomic Spectroscopy, Vol: 118, Pages: 81-89, ISSN: 1873-3565
The excitation of Cu+ ions in a Ne glow discharge with small additions of H2, O2 and N2 was studied. Ratios of radiative transition rates between different Cu II levels in a discharge in neon, with and without the molecular gas added, were calculated, and the formalism of transition rate ratio (TRR) diagrams was developed and used to study the changing excitation conditions. Virtually no changes in the excitation of Cu+ ions occur in a neon discharge if nitrogen is added. Additions of hydrogen and oxygen to neon as the discharge gas affect excitation of the 4d, 5s and some other Cu II levels in the vicinity of the ionization energy of neon (21.56 eV). Also some lower Cu II levels, excited by radiative decay of those higher energy levels, are affected. The 4p 3P2 level at 15.96 eV is enhanced by additions of hydrogen. It was suggested that this enhancement is caused by the asymmetric charge transfer reaction between neutral copper atoms and the H2+ molecular ions.
Mushtaq S, Steers EBM, Churchill G, et al., 2016, Does asymmetric charge transfer play an important role as an ionization mode in low power-low pressure glow discharge mass spectrometry?, Spectrochimica Acta Part B-Atomic Spectroscopy, Vol: 118, Pages: 56-61, ISSN: 0584-8547
Weiss Z, Steers EBM, Pickering JC, 2015, Transition rates and transition rate diagrams in atomic emission spectroscopy: A review, SPECTROCHIMICA ACTA PART B-ATOMIC SPECTROSCOPY, Vol: 110, Pages: 79-90, ISSN: 0584-8547
Heiter U, Lind K, Asplund M, et al., 2015, Atomic and molecular data for optical stellar spectroscopy, PHYSICA SCRIPTA, Vol: 90, ISSN: 0031-8949
This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.