Knowledge Bank

The Imperial College VUV-FT spectrometer has been equipped with optically contacted, beam splitters made from single crystals of $MgF2$ and the short wavelength performance has been demonstrated down to $\sim 139nm$. To make ultrahigh resolution VUV photoabsorption cross section measurements with the VUV-FTS require a pure continuum source below 190 nm and the best choice: is synchrotron radiation from a storage ring facility. Moreover a suitable zero-dispersion predisperser is available on beam line 12-B of the synchrotron radiation source at the Photon Factory. We therefore moved the IC VUV FT spectrometer from Imperial College. London to the Photon Factory, Japan to exploit the bandwidth-limited synchrotron radiation as a background source for FT absorption spectroscopy. With resolution of 0.06 or $0.12cm-1$, we extended the linewidth measurements of the Schumann-Range (S-R) bands with $\nu 12$ in the wavelength range 181 to $175nm$. The observed line widths consist of the instrumental, Doppler, and predissociation widths. The line profiles resulting from the convolution of the first two of these contributions are very closely Gaussian with widths of 0.12 and $0.15cm-1$ for spectra recorded with resolutions of 0.06 and $0.12cm-1$, respectively. The observed linewidths of the S-R bands with $\nu \prime \ge 12$ will be presented. This work is partly supported by NSF Division of Atmospheric Sciences grant ATM-94-22854 to Harvard College Observatory, and by NASA Upper Atmospheric Research Program under Giant No. NAG5-484 to the Smithsonian Astrophysical Observatory. We also acknowledge the support of the UK Science and Engineering Research Council and the Paul Instrument Fund of the Royal Society for the development of the UV-FT spectrometer.