Time- and state-resolved measurements of nitric oxide dimer infrared photodissociation

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Abstract
Picosecond and nanosecond lasers and pulsed molecular beam techniques have been used to measure the infrared photodissociation spectra, the product state distributions, and the predissociation lifetimes of vibrationally excited nitric oxide dimer (NO)2 . Results for the ν1 (v=1) symmetric NO stretching mode and the ν4 (v=1) antisymmetric NO stretching mode are presented. Predissociation lifetimes are determined by time-resolved laser induced fluorescence probing of the NO monomer product appearance rate. A dramatic mode dependence of the predissociation lifetimes is observed with the higher energy ν1 mode decaying in approximately 1 ns, and the lower energy ν4 mode decaying in approximately 40 ps. The mode dependence is independent of which product state is probed. The product state distributions show that 75% to 80% of the available energy is channeled into relative translational energy of the fragments for both modes. Rotational state distributions are Boltzmann-like with temperatures ranging from 71 to 112 K depending on both the initially excited mode and on the NO product spin–orbit state. Predissociation from ν1 produces NO fragments in the 2Π1/2 and 2 Π3/2 states with equal probability. Predissociation from ν4 exhibits a propensity for producing the lower energy 2 Π1/2 spin–orbit state. The observations are discussed in terms of various vibrational predissociation mechanisms, including vibrational potential coupling and electronically nonadiabatic predissociation.