Cavity ring-down spectroscopy of a van der Waals molecule
Cavity ring-down spectroscopy (CRDS), which measures the rate of absorption via the "ring-down" decay time of cavity light intensity rather than absorption directy, offers the advantages of direct absorption spectroscopy together with the high sensitivity required for recording spectra of optically thin samples. Meijer, et al. have introduced a novel variation to CRDS, which utilises the transverse mode density in an optical cavity to fill the spectral region between longitudinal cavity modes, thereby providing a more complete spectral transmission response. Cavity ring-down spectroscopy was introduced by O'Keefe, et al. and some additional theoretical aspects associated with the use of confocal cavities have been given by Zalicki and Zare. In this paper, the spectral characteristics of a CRD apparatus are discussed with a view to optimising the TEM mode density within the cavity and the requirements for meaningful signal recovery are evaluated. CRD measurements of the electronic absorption spectrum of the p-difluorobenzene-Ar van der Waals molecule formed in a supersonic jet expansion are presented.