The dynamics of photodissociation of cluster ions. II. Photodissociation of the (NO)+3 cluster in the visible wavelength range

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Abstract
The photodissociation of the (NO)+3 cluster ion has been studied in the visible region of the spectrum (460–660 nm) using a crossed high energy ion beam/laser beam experiment. Total photodissociation cross sections, product branching ratios, product relative kinetic energy distributions, and information about the product angular distributions have been measured. The total photodissociation cross section peaks to the blue end of the visible spectrum with a maximum in excess of 5×10−17 cm2. The only ionic products observed were NO+ and (NO)+.2. The fraction of (NO)+⋅2 formed is 26±4% and is essentially independent of wavelength in the range studied. For (NO)+⋅2 the product kinetic energy distributions are narrow and characteristic of dissociation occurring on a repulsive surface. At 610 nm, 24% of the available energy appears as product relative kinetic energy, this fraction increases to 32% at 488 nm. The (NO)+⋅2 angular distributions are well described by an angular distribution of the form 1+βP2(θ) with β=1.5 indicating that dissociation occurs rapidly with respect to a rotational period. Product stability restrictions play a major role in the dynamics of stable (NO)+⋅2 formation and the mechanism of dissociation effectively channels energy into internal excitation of the neutral NO⋅ product. The NO+ kinetic energy distributions are bimodal for large photon energies suggesting that NO+ is generated by two mechanisms. The higher energy component was assigned to direct formation of NO+ on a time scale short compared to a rotational period and the lower energy component was assigned to unimolecular dissociation of excited (NO)+⋅2 photoproducts. The energy released in the unimolecular dissociation of (NO)+⋅2 forming the lower energy NO+ product is on the order of 0.1 eV which is consistent with statistical dissociation.