Nonexponential unimolecular decay of jet-cooled NO2: Comparison of time-resolved measurements and quantum mechanical calculations

We present time-resolved measurements for the dissociative decay of NO2 in its ground electronic state using pump laser pulses with durations of ca. 6 50 fs, The temporal evolution of the coherent wave packet is probed by exci tation to a Rydberg state and detection of the subsequent fluorescence, The main experimental result is the observation of nonexponential decay, which is most pronounced at very low excess energies close to the dissociation t hreshold. This is compared with the results of quantum dynamics calculation s performed on a recently calculated global potential energy surface. The m easured decay curves are satisfactorily reproduced by summation of many exp onential decay terms e(-knt), with the state-specific dissociation rates k( n) being determined from the widths of resonance poles in the complex energ y plane. The key theoretical result is the observation that the calculated rates fluctuate by 1-2 orders of magnitude, These strong fluctuations are r esponsible for the distinct nonexponential behavior. The smallest calculate d rates just at threshold are of the same order of magnitude, ca. 2 x 10(10 ) s(-1), as the experimental rates previously extracted from energy-resolve d spectra.