Vibrationally mediated photodissociation

We describe a new experimental technique for studying the spectroscopy and photodissociation dynamics of highly vibrationally excited polyatomic molecules. The technique, vibrationally mediated photodissociation, involves one-photon excitation to a high-energy O-H stretching overtone vibration followed by excitation with a second photon (of the same or a different energy) to a dissociative electronic state that produces OH radical fragments. Laser-induced fluorescence of OH(A ²∑⁺ ← X ²Π) monitors the yield of dissociation products and the OH internal state distribution. We have applied the technique to molecules of the general formula HOX (where X = OH, NO₂, and Bu^tO) and, in this paper, present an overview of the experimental results. Because the OH fragment yield depends on the first photon being resonant with the vibrational overtone state, variation of its wavelength provides an excitation spectrum. Such a spectrum supplies information that is complementary to that obtained from laser photoacoustic spectroscopy. The spectroscopy of the fragment OH radicals reveals details of the photodissociation dynamics, which may be controlled both by the initial vibrational overtone excitation and by the nature of the excited electronic surface from which the parent molecule dissociates.