Probing molecular species by cavity ringdown laser absorption spectroscopy, application to the spectroscopy and dynamics of jet-cooled NO2

The Cavity Ringdown Laser Absorption Spectroscopy (CRLAS or CRDS) technique has acquired a enviable audience in the spectroscopy community during the past decade. Based on a high-Q optical cavity, it largely bypasses the adva ntages of multipass absorption cells, offering ppm range sensitivities or b etter, and emulates rapid developments of the experimental configurations. The basic idea consists of measuring the intracavity electromagnetic field time behavior which reflects the cavity optical properties and medium losse s. This article is divided in three main parts. The first one is devoted to the description of the CRLAS technique, including: (i) a brief formalism a bout the principles of an empty high-Q cavity (Fabry-Perot) coupled to an i ncoming electromagnetic field and (ii) the absorption model allowing one to deal with absorbing species inserted inside the cavity. The second part su ccinctly reviews and compares some of the usual highly sensitive spectrosco py techniques and the main applications of the CRLAS technique are presente d. The last part of the paper reports the recent results obtained at the la boratory concerning the NO2 molecular species excited by a CW single mode l aser source and under slit jet expansion conditions. Two energy ranges are primarily investigated, firstly the region around 800 nm in which three kin ds of behaviors are identified Doppler-limited linear absorption. Doppler-f ree two-photon absorption and saturation absorption. Secondly. by using rad iation at 397 nm, the lowest photodissociation threshold of NO2 is interrog ated in order to address the unimolecular reaction processes. (C) 2001 Acad emie des sciences/Editions scientifiques et medicales Elsevier SAS.