Non-linear effects by continuous wave cavity ringdown spectroscopy in jet-cooled NO2

A new method of high resolution cavity ringdown spectroscopy (CRDS) was rec ently developed in our laboratory, where a narrow line, continuous wave (CW ) single-frequency laser is used instead of a pulsed laser. Here, we will f irst discuss the main differences between the 'traditional' pulsed CRDS and CW-CRDS. Then, we will describe our results exploiting the high intracavit y power that can be achieved with CW-CRDS. Using a single-mode Ti:Sa laser, we obtained CRDS spectra where the excitation power of a single cavity mod e is close to 20 W. In the virtually collisionless regime of a supersonic s lit jet, we observed saturation in some of the weak rovibronic transitions of NO2 around 796 nm, as evidenced by loss of absorption intensity and form ation of Doppler-free Lamb dips. In addition, in coincidence with absorptio n by these near infrared transitions, an appreciable fluorescence signal wa s detected in the visible range. According to our interpretation, this fluo rescence is from NO2 levels excited by two photons in a stepwise incoherent process, with a strongly allowed second step. Since the fluorescence spect rum has the same Lineshapes as the CRDS absorption spectrum, it seems that the first transition step is the one limiting the overall two-step process. In addition, we also observed very narrow fluorescence features, not coinc ident with any absorption feature. These must be coherent (non-stepwise), D oppler-free, two-photon transitions. Interesting new questions arise from t hese preliminary data, and we believe that more measurements of this kind w ill provide new information about the rovibronic states of NO2 in the disso ciation region. (C) 1999 Elsevier Science B.V. All rights reserved.