LIFETIME DISTRIBUTIONS, STATE DENSITIES, AND THE CHARACTERIZATION OF PREDISSOCIATING STATES OF HIGHLY EXCITED $NO_{2}$ FROM OPTICAL DOUBLE-RESONANCE SPECTROSCOPY IN A FREE JET

Abstract

The $NO_{2}$ molecule is an excellent model system for quantum state resolved investigations of unimolecular dissociation, IVR and collisional relaxation of small molecules and radicals at chemically significant internal energies. We have used a high resolution "fluorescence depletion pumping" double resonance technique (FDP) in a free jet to access and to assign predissociating rovibronic states of $NO_{2}$ above and stable molecular eigenstates below the dissociation threshold $E_{0}$. From the double resonance spectra linewidth distributions and densities of states $\rho (E)$ at around $25130 cm^{-1}$ have been determined. From sequential double resonance spectra just below $E_{0}$ the nature of the predissociating states above the threshold has been inferred. Controlling the conditions of the jet expansion we have been able to measure the linewidths of a large number of isolated transitions to predissociating states with defined energy E, symmetry, and total angular momentum J. The homogeneous linewidths have been converted to specific rate constants k(E, J). The rate constants and its fluctuations will be compared with recent results from "state resolved" SACM calculations, a statistical lifetime distribution model, and ps- time domain measurements.