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 advantages of multipass absorption cells, offering ppm range sensitivities or better, 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 losses. 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 about the principles of an empty high-Q cavity (Fabry–Perot) coupled to an incoming electromagnetic field and (ii) the absorption model allowing one to deal with absorbing species inserted inside the cavity. The second part succinctly reviews and compares some of the usual highly sensitive spectroscopy techniques and the main applications of the CRLAS technique are presented. The last part of the paper reports the recent results obtained at the laboratory concerning the NO₂ molecular species excited by a CW single mode laser source and under slit jet expansion conditions. Two energy ranges are primarily investigated, firstly the region around 800 nm in which three kinds of behaviors are identified: Doppler-limited linear absorption, Doppler-free two-photon absorption and saturation absorption. Secondly, by using radiation at 397 nm, the lowest photodissociation threshold of NO₂ is interrogated in order to address the unimolecular reaction processes.