Imperial College London

Dr. Christian Clear

Faculty of Natural SciencesDepartment of Physics

Research Fellow



+44 (0)20 7594 8184c.clear14 Website




H720BHuxley BuildingSouth Kensington Campus





Publication Type

10 results found

Clear CP, Pickering JC, Nave G, Uylings P, Raassen Tet al., 2023, Wavelengths and Energy Levels of the Upper Levels of Singly Ionized Nickel (Ni ii) from 3d<sup>8</sup>(<sup>3</sup>F)5f to 3d<sup>8</sup>(<sup>3</sup>F)9s, Astrophysical Journal, Supplement Series, Vol: 269, ISSN: 0067-0049

Using high-resolution spectra of Ni ii recorded using Fourier transform (FT) spectroscopy of continuous, nickel-helium hollow cathode discharge sources in the region 143-5555 nm (1800-70,000 cm−1, the analysis of 1016 Ni ii lines confirmed and optimized 206 previously reported energy levels of the (3 F) parent term, from 3d 8(3 F)5f to 3d 8(3 F)9s, lying between 122,060 and 138,563 cm−1. The uncertainties of these levels have been improved by at least an order of magnitude compared with their previously reported values. With the increased resolution and spectral range of the FT measurements, compared to previously published grating spectra, we were able to extend our analysis to identify and establish 33 new energy levels of Ni ii, which are reported here for the first time. Eigenvector compositions of all revised and newly established energy levels were calculated using the orthogonal operator method. In addition, an improved ionization energy of 146,541.35 ± 0.15 cm−1 for Ni ii, using highly excited levels of the 3d 8(3 F)5g, 3d 8(3 F)6g, and 3d 8(3 F)6h configurations, has been derived.

Journal article

Concepcion F, Clear CP, Ding M, Pickering JCet al., 2023, The Laboratory Astrophysics Programme at Imperial College London, EUROPEAN PHYSICAL JOURNAL D, Vol: 77, ISSN: 1434-6060

Journal article

Clear CP, Uylings P, Raassen T, Nave G, Pickering JCet al., 2023, New Ritz wavelengths and transition probabilities for parity-forbidden, singly ionized nickel [Ni II] lines of astrophysical interest, Monthly Notices of the Royal Astronomical Society, Vol: 519, Pages: 4040-4046, ISSN: 0035-8711

We report accurate Ritz wavelengths for parity-forbidden [Ni II] transitions, derived from energy levels determined using high-resolution Fourier transform spectroscopy. Transitions between the 18 lowest Ni II energy levels of even-parity produced Ritz wavelengths for 126 parity-forbidden lines. Uncertainties for the Ritz wavelengths derived in this work are up to two orders of magnitude lower than previously published values. Transition probabilities were calculated using the semi-empirical orthogonal operator method, with uncertainties ranging from approximately 1 per cent for strong M1 lines and up to 10 per cent for weak E2 lines. Accurate forbidden line wavelengths and transition probabilities, particularly for lines in the infrared, are important in the analyses of low-density astrophysical plasmas, such as supernova remnants, planetary nebulae, and active galactic nuclei.

Journal article

Clear CP, Pickering JC, Nave G, Uylings P, Raassen Tet al., 2022, Wavelengths and energy levels of singly ionized nickel (Ni ii) measured using fourier transform spectroscopy, The Astrophysical Journal Supplement Series, Vol: 261, Pages: 35-35, ISSN: 0067-0049

High-resolution spectra of singly ionized nickel (Ni ii) have been recorded using Fourier transform spectroscopy in the region 143–5555 nm (1800–70,000 cm−1) with continuous, nickel–helium hollow cathode discharge sources. An extensive analysis of identified Ni ii lines resulted in the confirmation and revision of 283 previously reported energy levels, from the ground state up to the 3d8(ML)6s subconfigurations. Typical energy-level uncertainties are a few thousandths of a cm−1, representing at least an order-of-magnitude reduction in uncertainty with respect to previous measurements. Twenty-five new energy levels have now been established and are reported here for the first time. Eigenvector compositions of the energy levels have been calculated using the orthogonal operator method. In total, 159 even and 149 odd energy levels and 1424 classified line wavelengths of Ni ii are reported and will enable more accurate and reliable analyses of Ni ii in astrophysical spectra.

Journal article

Nave G, Clear C, 2021, Reference wavelengths of Si ii, C ii, Fe i, and Ni ii for quasar absorption spectroscopy, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 502, Pages: 5679-5685, ISSN: 0035-8711

Journal article

Pickering JC, Teresa Belmonte M, Clear CP, Liggins F, Concepcion-Mairey Fet 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.

Journal article

Belmonte MT, Pickering JC, Clear CP, Mairey FC, Liggins Fet 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.

Journal article

Rhodin AP, Belmonte MT, Engstrom L, Lundberg H, Nilsson H, Hartman H, Pickering JC, Clear C, Quinet P, Fivet V, Palmeri Pet 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.

Journal article

Nave G, Sansonetti CJ, Townley-Smith K, Pickering JC, Thorne AP, Liggins F, Clear Cet 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

Journal article

Belmonte MT, Pickering JC, Clear C, Liggings F, Thorne APet 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

Conference paper

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