Anne P. Thorne

A Fourier transform spectrometer for the vacuum ultraviolet: design and performance

A Fourier transform spectrometer designed to operate at high resolution at wavelengths down to 170 nm is described. The principal instrumental parameters are: mirror travel, 200 mm; resolving limit, 0.025 cm-1; collimator aperture ratio, f/25; overall dimensions of vacuum tank, 1.5 m*0.25 m*0.25 m. Test results show (i) a signal-to-noise ratio in the transformed spectrum at 200 nm better than 1000:1 for an iron-neon hollow cathode lamp at a resolving power of quarter of a million, (ii) fully resolved line profiles in the same source at a resolving limit of 0.03 cm-1 (resolving power 1.5*106), (iii) relative wavenumbers of Fe II emission lines reproducible to +or-0.0006 cm-1 (3.4 fm), and (iv) a significant luminosity gain over grating spectrometers operating in the same region.

Spectrum and term system of neutral nickel, Ni I

Spectra of nickel, emitted from hollow cathode discharges, have been recorded in the region 1700-55 000 A (58 800-1800 cm−1). Fourier transform spectrometers were used above 1800 A, yielding very high accuracy and resolution. The number of classified Ni I lines has increased from 1071 to 1996. 289 of the lines have been resolved in 2-4 isotope components. The term system derived from the observations comprises 286 energy levels. The term structure and the coupling conditions have been studied by means of ab initio and parametric theory.

High‐resolution photoabsorption cross‐section measurements of SO2 at 160 K between 199 and 220 nm

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) B-2 -(X) over tilde (1)A(1) system. Our results are compared with literature values obtained at lower resolutions and with 295 K cross sections recorded earlier with the same instrument.

Band oscillator strengths of the Herzberg I bands of O2

Photoabsorption cross section measurements of the Herzberg I bands (A3Σu+−X3Σg−) of O2 have been made by Fourier transform spectrometry with a resolution of 0.06 cm−1 in the wavelength region 240‐270 nm. Precise band oscillator strengths of the (4,0)–(11,0) bands are obtained by direct measurement, and for some of the strong bands, they are significantly higher than the previous experimental values. The rotational line strengths and the branching ratios are also presented for the same bands. The dissociation energy of O2 is discussed.

Ghosts and artifacts in Fourier-transform spectrometry

Ghosts in Fourier-transform spectrometry are important for three reasons: they can give rise to spurious coincidences of frequency differences in spectral analysis, distort the phase correction, and set a limit to the attainable signal-to-noise ratio. The various types of ghost, originating from amplitude modulation, phase modulation, and intermodulation, are described and discussed, together with some hardware and software artifacts. Recipes are given for identifying these features and, where possible, avoiding harmful effects from them.

Precision Fe i and Fe ii wavelengths in the ultraviolet spectrum of the iron–neon hollow-cathode lamp

The wave numbers of 167 Fe i lines between 26 000 and 34 100 cm−1 (385–293 nm), 146 Fe i lines between 33 700 and 51 400 cm−1 (297–195 nm), and 221 Fe ii lines between 35 900 and 54 600 cm−1 (279–183 nm) are measured with a relative precision of 30 parts in 109 by Fourier-transform spectrometry. Bridging techniques are used to place the measurements on the same Ar ii–based absolute wave-number scale as previously measured visible spectra. The uncertainty in the absolute wavelengths is limited by small source shifts and the accuracy of the available standards and is estimated to be 0.002 cm−1 (0.008 pm at 200 nm). This is an order of magnitude better than that of the majority of current UV standards.

High-resolution photoabsorption cross-section measurements of SO2 at 198 K from 213 to 325 nm

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 (C) over tilde B-1(2) - (X) over tilde (1)A(1) and (B) over tilde B-1(1) - (X) over tilde (1)A(1) 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 approximate to 450,000) using Fourier transform spectrometry and are compared to other high-resolution measurements in the literature.

Fourier transform spectroscopy and cross section measurements of the Herzberg III bands of O2 at 295 K

Fourier transform spectroscopic measurements of the absorption bands of the Herzberg III (A′ 3Δu–X 3Σg−) of O2 at 295 K have been made with a resolution of 0.06 cm−1 in the wavelength region 240 to 275 nm. Rotational line positions are determined with an accuracy of 0.005 cm−1, and rotational term values are presented for the vibrational levels, v′=4–11. Precise band oscillator strengths of the (4,0)–(11,0) bands are obtained for the first time by direct measurement by summing the cross sections of individual rotational lines of the bands. The rotational line strengths and the branching ratios are also presented for the same bands. The continuity relationship for the band oscillator strengths to photodissociation continuum cross sections has been applied to the three-band systems.

The application of a VUV Fourier transform spectrometer and synchrotron radiation source to measurements of: II. The δ(1,0) band of NO

Line-by-line photoabsorption cross-sections of the NO δ(1,0) band were measured with the VUV Fourier transform spectrometer from Imperial College, London, using synchrotron radiation at Photon Factory, KEK, Japan, as a continuum light source. The analysis of the NO δ(1,0) band provides accurate rotational line positions and term values as well as the photoabsorption cross-sections. The molecular constants of the C(1) 2Π level are found to be T0=54 690.155±0.03 cm−1, Bv=1.944 06±0.000 62 cm−1, Dv=(5.91±0.42)×10−5 cm−1, AD=−0.0187±0.0050 cm−1, p=−0.0189±0.0037 cm−1, and q=−0.015 21±0.000 20 cm−1. The sum of the line strengths for all rotational transitions of the NO δ(1,0) band is determined as 4.80×10−15 cm2 cm−1, corresponding to a band oscillator strength of 0.0054±0.0003.

Fourier transform spectrometry in the vacuum ultraviolet: applications and progress

The useful range of Fourier transform spectrometry (FTS) has been extended into the vacuum ultraviolet (VUV) region as far as about 1400 A. The advantages offered by FTS over other methods for the acquisition of the high quality laboratory data now required to exploit observations from satellite-borne instruments are discussed. The efficiency of FTS is compared with that of grating spectrometry to show how an effective short wavelength limit of the former is reached, and possibilities for extending this are mentioned. Some examples of VUV FTS data of astrophysical significance are given.