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. 650 fs. The temporal evolution of the coherent wave packet is probed by excitation 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 threshold. This is compared with the results of quantum dynamics calculations performed on a recently calculated global potential energy surface. The measured decay curves are satisfactorily reproduced by summation of many exponential decay terms e^-k_nt, with the state-specific dissociation rates k_n being determined from the widths of resonance poles in the complex energy plane. The key theoretical result is the observation that the calculated rates fluctuate by 1−2 orders of magnitude. These strong fluctuations are responsible for the distinct nonexponential behavior. The smallest calculated rates just at threshold are of the same order of magnitude, ca. 2 × 10¹⁰ s^-1, as the experimental rates previously extracted from energy-resolved spectra.