CAVITY RING DOWN SPECTROSCOPY WITH A FREE-ELECTRON LASER

A cavity ring down (CRD) absorption experiment is performed with a fre e-electron laser (FEL) operating in the 10-11 mu m region. A short inf rared pulse of approximately 20 ns, sliced from the much longer FEL pu lse, is used to measure CRD spectra of ethylene in two different ways. First, ''ordinary'' CRD spectra with a resolution determined by the b andwidth of the FEL (approximate to 5 cm(-1)) are recorded, Second, Fo urier transform (FT) CRD spectra with a resolution that is in principl e determined by the FT-spectrometer are obtained by analyzing the ligh t exiting the ring down cavity with a FT-spectrometer while the FEL is operated in broadband mode. (C) 1997 Elsevier Science B.V.