Fourier Transform Spectroscopy
Spectroscopy Links
RAL - SRG
UCL - TAMPA
Harvard CfA AMP
Activity Report
The group has continued its work in the areas of atomic and molecular spectroscopy, and the development of Fourier transform spectrometers that operate in the ultra-violet and vacuum ultra-violet regions of the spectrum. These instruments, of uniquely high resolving power, efficiency and range, are being used to provide the data needed for the better understanding of the atmospheres both of stars and of the earth.
Laboratory Astrophysics
The spectra of stars are usually extremely complex: all the elements of the periodic table may contribute, each in more than one stage of ionisation and blends of several lines are the rule rather than the exception. Much of the spectral data in the literature (atomic energy levels, transition wavelengths, oscillator strengths and hyperfine structure) is too inaccurate and incomplete to resolve these blends, especially in the ultra-violet. We have recently published new analyses of the spectra of three cosmically abundant species, Fe, Ni, Ti and Co; work is in progress on Mn I, Cr I, V I and V II. A recently acquired Penning discharge source has allowed spectra of doubly ionised species to be recorded, of importance for hot stars. Studies in progress include Fe III, Mn III and Ni III. The spectral range of our instruments in well matched to that of the high resolution spectrograph on the Hubble Space Telescope, as a result of which our data on the spectra of elements such as Pt and Pb have been used to establish isotopic abundance anomalies in hot stars. New accurate atomic data is vital for reliable interpretation of high resolution astrophysical spectra, such as those recorded with the UHRF (Ultra High Resolution Facility) on the Anglo-Australian telescope, and in the future with the planned HROS (High Resolution Optical Spectrograph) on GEMINI. More details of atomic spectroscopy projects for astrophysics applications:
Group research report: Atomic Spectroscopy
Juliet C. Pickering: Research summary
Anne P. Thorne: Research summary
Richard Blackwell-Whitehead: Research Summary
Molecular Spectroscopy of Atmospheric Relevance
Our work on molecules relevant to the terrestrial atmosphere concentrates on spectral line parameter analysis giving essential data for precise analysis of observations made from downward looking satellites and is important for studies of the greenhouse effect and global warming. Measurements of the SO2 bands in the near and middle UV, required for planetary and terrestrial atmospheres and measurements of molecular oxygen in the visible have been completed. The spectral analysis of the visible O2 data included the two most abundant oxygen isotopomers and yielded results of unprecedented precision. Molecular constants and line parameters in the visible oxygen A band (Fig. 1) were measured, and this data is of direct application to atmospheric pollution measurements, and the accurate modelling of the penetration of solar radiation into the upper atmosphere. Furthermore, the oxygen data is of crucial importance for the remote sensing of cloud-top height and coverage for satellite based measurement of atmospheric ozone and other atmospheric trace gases, and so of application to European satellite systems such as GOME (Global Ozone Monitoring Experiment) and SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography). A study, collaborating with J.W.Brault, ( U.S.A), of the very weak O2 Delta bands in the near-infrared is also being carried out.
Projects funded by ESA and NERC (with University College London and the Molecular Spectroscopy Facility at Rutherford Appleton Laboratory, (RAL) investigate the water vapour spectra in the near-infrared and visible spectral regions aiming to provide precise spectral line parameters, as well as data on weak, previously unobserved, spectral absorptions. The results of these studies not only add a significant amount of weak lines to the water database, but also identified serious errors in the line intensities in the existing database. In collaboration with J.Haigh and W.Zhong (SPAT, IC), the new data has been included in atmospheric radiative transfer calculations and showed, that it will substantially increase the calculated absorption of solar energy in climate models by a moist cloud-free atmosphere. This data will significantly contribute to the solution of the so-called "missing absorber problem" and will have a large impact on the modelling of atmospheric radiative transfer and climate.
Current studies include work on the far UV bands of SO2 for investigations of terrestial and planetary atmospheres; for example knowledge of SO2 absorption cross-sections enables SO2 to be used to monitor volcanic activity on Io, a moon of Jupiter. Further work is planned on weak absorptions of water in the visible and near-infrared, which may have a significant contribution to the earth's solar radiation budget and may explain the existing serious discrepancies between the observed and modelled atmospheric radiation budgets, an effect which is much larger than the probable magnitude of greenhouse effects and thus has an impact on the credibility of this very serious topic. These programmes involve collaborative work with the CfA Atomic and Molecular Physics Laboratory at the Harvard-Smithsonian Center for Astrophysics, Wellesley College, U.S.A., the Theoretical Atomic and Molecular Physics and Astrophysics Group at the University College London and the Spectroscopy and Remote Sensing Group at the Rutherford Appleton Laboratory.
