R. C. M. Learner

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.

New studies of the visible and near‐infrared absorption by water vapour and some problems with the HITRAN database

New laboratory measurements and theoretical calculations of integrated line intensities for water vapour bands in the near-infrared and visible (8500–15800 cm−1) are summarised. Band intensities derived from the new measured data show a systematic 6 to 26% increase compared to calculations using the HITRAN-96 database. The recent corrections to the HITRAN database [Giver et al., J. Quant. Spectrosc. Radiat. Transfer, 66, 101–105, 2000] do not remove these discrepancies and the differences change to 6 to 38%. The new data is expected to substantially increase the calculated absorption of solar energy due to water vapour in climate models based on the HITRAN database.

The contribution of unknown weak water vapor lines to the absorption of solar radiation

Thousands of unknown water vapour weak lines in the spectral region between 13200 and 22700 cm -1 are deduced from extrapolations of experimental results. These lines are then included in the HITRAN database and used in line-by-line calculations of atmospheric opacity with standard atmospheric profiles. The weak lines predicted by a theoretical model are also used in the line-by-line model to estimate their contribution to the absorption of solar radiation. The additional absorption of solar radiation is of order 1.5 to 2.5 W/m 2 at 45 mm precipitable water, about 8.5% to 14% of the absorption due to HITRAN lines in the spectral region. The effect is also compared with that of a continuum model.

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.

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.

Laboratory spectroscopy of molecular oxygen in the visible and near-infrared

The spectrum of molecular oxygen goes from the pure rotational system in the sub-millimetre region to the K edge at 2.3 nm.

High resolution FTS of atoms and molecules in the ultra-violet

The extension of high resolution FTS into the ultra-violet (λ≥175 nm) is described briefly and illustrated by two applications. In the hollow cathode spectrum of iron, wavelength measurements have been made with an accuracy from 1∶109 in the visible to 1∶108 in the UV. In atomic emission spectroscopy with ICP or glow discharges, full spectral information, with completely resolved line profiles, is obtained.