Triatomic Photofragment Spectra. I. Energy Partitioning in NO2 Photodissociation

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Abstract
The photofragment spectrum of NO2 has been measured in the near ultraviolet at 28 810 cm−1. A molecular beam of NO2 is crossed with brief pulses of polarized laser light and measurements are made on the distributions of speed and direction of the recoiling O and NO fragments produced by photodissociation. The average translational energy of the fragments is about 60% of the available energy. There are at least two prominent peaks in the translational energy distribution. We conclude that the two peaks most likely correspond to nearly equal probability of recoil with the NO fragment in the v=0 and v=1 vibrational states. Such vibrationally excited NO fragments produced by photodissociation in polluted atmospheres could perhaps react with different rates than ground state fragments. The positions and widths of the peaks indicate that there is a significant rotational distribution. Statistical and direct models for photodissociation energy partitioning are briefly explored, and their predictions compared with the experimental measurements. A model is tentatively endorsed in which the nascent NO fragment is formed with vibrational excitation already present in the NO2excited state.Dissociation is then assumed to occur quickly before statistical equilibration is reached. This is the first of a series of four papers on the photofragment spectra of triatomic molecules, using NO2 and NOCl as examples.

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