Cavity ring down spectroscopy with a free-electron laser

A cavity ring down (CRD) absorption experiment is performed with a free-electron laser (FEL) operating in the 10-11 mu m region. A short infrared pulse of approximately 20 ns, sliced from the much longer FEL pulse, is used to measure CRD spectra of ethylene in two different ways. First, ''ordinary'' CRD spectra with a resolution determined by the bandwidth of the FEL (approximate to 5 cm(-1)) are recorded, Second, Fourier transform (FT) CRD spectra with a resolution that is in principle determined by the FT-spectrometer are obtained by analyzing the light exiting the ring down cavity with a FT-spectrometer while the FEL is operated in broadband mode. (C) 1997 Elsevier Science B.V.