Molecular Spectroscopy

The group's work in the last few years on the spectra of molecules of importance in the atmosphere has been in several areas: sulphur dioxide (SO₂) in the ultraviolet; water in the visible and near infrared; oxygen (O₂) in the red and near infrared, and O ₂ and NO in the ultraviolet, . We are currently starting work on a study of diatomic sulphur .

Sulphur Dioxide

The UV molecular spectroscopy has included measurements of absorption cross-sections in the two UV band systems of sulphur dioxide, which is an important pollutant in the Earth's atmosphere. The longer wavelength system, around 300 nm, was investigated as part of an ESA project. However, SO₂ is also an important constituent of planetary atmospheres (Venus and Io, for example). Measurements of absorption cross-sections in the 220-198 nm band systems of SO₂ have been carried out in collaboration with the Harvard Smithsonian Center for Astrophysics (CfA) and Prof Glenn Stark (Wellesley College, USA) and have subsequently been extended to the low temperatures relevant to planetary atmospheres, for example atmospheres of Venus and Io. For this work we developed a new technique for establishing a reliable baseline by using the second output from the FT interferometer to monitor the background continuum source. Our results have been published in a series of papers, and are also available in the HITRAN database. The work was supported by PPARC and STFC of the UK, and NASA grants of our US collaborators.

Isotopolgues of Sulphur Dioxide

We have been studying the isotopologues of SO₂ at high resolution to measure accurate photoabsorption cross sections needed in studies of the early Earth atmosphere. This project is a collaboration between the Imperial College group and Dr J Lyons (UCLA) and Prof Glenn Stark (Wellesley College, USA). The timing of the oxygenation of the Earth's atmosphere is a central issue in understanding the Earth's paleoclimate. The discovery of mass-independent fractionation (MIF) of sulphur isotopes deposited within Archean and Paleoproterozoic rock samples (> 2.4 Gyrs) and the transition to mass-dependent fractionation found in younger samples, could provide a marker for the rise in oxygen concentrations in the Earth's atmosphere especially what is termed The Great Oxidation Event. Published laboratory experiments have suggest isotopic self shielding during gas phase photolysis of SO₂ present at wavelengths shorter than 220 nm as the dominant mechanism for MIF. The UV absorption of SO₂ is dominated by the C¹B₂ - X ¹A₁ electronic system which comprises strong vibrational bands extending from 170 - 230 nm. Within an atmosphere consisting of low O₂ and O₃ concentrations, such as that predicted for the early Earth, UV radiation would penetrate deep into the ancient Earth’s atmosphere in the 180 - 220 nm range driving the photolysis of SO₂. We have conducted the first ever high resolution measurements of the photo absorption cross sections of several isotopologues of SO₂, namely ³²SO₂, ³³SO₂, ³⁴SO₂ and ³⁶SO₂, using the Imperial College UV Fourier transform spectrometer which is ideal for high resolution, broad-band, VIS/UV measurements. These cross sections are being included by our collaborator J Lyons (UCLA) in photochemical models of the early Earth's atmosphere in order to reliably interpret the sulphur isotope ratios found in ancient rock samples. From the photochemical models it could be possible to link photochemical models of the Earth's early atmosphere with recorded sulphur isotope ratios contained within rock samples. Due to the small size of the variations in the sulphur isotope ratios uncertainties of the order of 1% are required for the measured cross sections. For this purpose the measurements were made using the dual output set-up of the IC UV FTS to enable continuous monitoring of the source continuum and eliminate the effect of source intensity drift. Further precautions and techniques have been used to meet the required error budget. This work has been supported at Imperial College by the Leverhulme Trust of the UK, and our US collaborators through the NASA Exobiology and Astrobiology Program.

Diatomic Sulphur

S₂ along with SO₂ have been identified as important parent molecules emitted from the active volcanoes of the Jovian moon Io: through photochemical reactions with these two molecules a variety of sulphur and oxygen based molecules are formed. Although short lived within Io's atmosphere, should S₂ be regularly emitted in volcanic plumes, it could drastically alter the predicted composition of Io's atmosphere. However despite its importance to the understanding of Io's atmosphere, no comprehensive measurement of the UV absorption cross sections has ever been performed. Through a collaboration with Wellesley College, Boston, USA, a sulphur furnace is being constructed for use with the IC UV FTS. This dual temperature furnace will generate the S₂ vapour from elemental sulphur, the dual temperature being required to drive the equilibrium reaction in favour of S₂ at the expense of the other possible sulphur allotropes. We plan to measure photoabsorption cross sections of S₂. This work is supported by STFC of the UK at Imperial College, and NASA Planetary Atmospheres Program at Wellesley College USA.

Water Vapour

The motivation for our work on water was the difference between observation and modelling of the absorption of solar radiation by the atmosphere. Even in the simplest case, a completely cloudless sky, theory and observation were seen to differ by some 25 Wm^-2. As calculations of the very important greenhouse effect are concerned with changes of only a tenth of this amount, sources of "missing opacity" are much needed. We undertook two such programmes. The first was to assess the opacity due to very weak water lines that had not hitherto been observed in the laboratory. Funded by NERC the group’s study of weak water vapour spectral lines in the near-infrared and visible spectral regions yielded parameters for over 3000 lines, almost 1000 newly identified lines, and showed a systematic 6-26% increase in band intensities compared to the HITRAN database. In atmospheric models this led to increases in downward solar fluxes, and changes in atmospheric heating rates of up to 4%.   A second programme on water involved re-measurements of strong water lines in the visible and near infrared already in the HITRAN database, where data quality was uncertain.  The experimental work was paralleled by computation of the spectrum at UCL. The experimental data for about 5000 lines was used to validate theory, which could then be used with confidence to treat a large number of weak lines. Initially it was thought that the strong line widths were not accurately known, but the new data showed that a much more important parameter - the line strengths in the existing database - were subject to large systematic errors. A paper was written to warn of problems with the HITRAN database, with two further papers giving detailed figures, supported by a CD ROM with data on some 36000 lines.

Oxygen

Work on the A-band of O₂ in the red part of the spectrum, included accurate values for line parameters, transition probabilities and pressure broadening coefficients for ¹⁶O₂ and also for its isotopomer ¹⁶O¹⁸O, for which there were significant differences. We then investigated, in collaboration with J.W.Brault (National Solar Observatory, U.S.A), the very weak Delta (0,0) band in the near infrared. A further project, also in collaboration with Brault at NSO, concerns the strengths and band profiles of the collision-induced absorption bands obtained at high pressures of O₂.

Further UV molecular spectroscopy projects: oxygen and NO

Much of our work on UV molecular spectroscopy has been concerned with the penetration of UV solar radiation into the Earth's atmosphere; this work has also been done in collaboration with the CfA, in particular with K.Yoshino. In the wavelength region above 205 nm (and below the ozone absorption bands), the atmospheric absorption is due to the continuum adjoining the three systems of Herzberg bands of oxygen, O₂. Measurements on these very weak, forbidden bands in the region 250-230 nm was carried out with a 3-metre multi-pass absorption cell in front of the FT spectrometer to achieve adequate absorption. Long integration times were required to obtain a satisfactory signal-to-noise ratio. There are three overlapping band systems, all of which have now been analysed at CfA. For the Herzberg I bands, line wavelengths and line intensities were published separately. The publications for the weaker Herzberg II and Herzberg III bands report both wavelengths and intensities. The band parameters are essential for reliable calculation of the regions of weak continuous absorption adjoining the discrete bands.

At shorter wavelengths, below 200 nm, the penetration of solar radiation is controlled by the discrete absorption lines in the Schumann-Runge bands of O₂ and the various band systems of NO. Accurate line positions and intensities of both species are required to model the process. Again in collaboration with the CfA, FT absorption spectra were obtained in 1995 by taking one of our instruments to Photon Factory at KEK, Japan, in order to use the synchrotron radiation as an intense source of background continuum. The three months there yielded extensive data on the Schumann-Runge bands of O₂ from 195 nm to 175 nm (where the continuous absorption begins) and on the many overlapping bands of NO from 195 nm to 160 nm. The high resolution of FTS is necessary to avoid errors in intensity arising from saturation of the absorption, which cannot be detected when the instrumental width exceeds the true line width, as well as to obtain accurate line positions. Analysis of all this data is complete (undertaken mainly in Japan and at CfA); to date the results for the NO b (9,0) band, the NO d (1,0) band, the NO e (1,0) band are published. (check publication record for updated list of NO publications).