A line-tunable CO laser has been used to record infrared absorption profiles for (NO)n+ and (NO)n+.Ar clusters for n in the range 2–19 and at wavelengths between 1600 and 1900 cm−1. The presence of an argon atom is shown to have the effect of reducing the internal energy content of the ions, and for the case of (NO)2+. Ar is the only circumstance under which infrared absorption can be detected for the dimer ion. The trimer ion, (NO)3+, is found to have a very strong absorption feature at ∼1700 cm−1, which remains both static and comparatively narrow in subsequent cluster ions containing up to 19 nitric oxide molecules. It is concluded that (NO)3+ forms a stable core in larger clusters and that there is very little additional charge delocalization.

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An experimental study of the photofragmentation of Ar3+ at wavelengths >650 nm has been undertaken in order to investigate the reaction channel Ar3++hν→Ar2++Ar. As a function of photon energy, measurements have been made of the Ar+/Ar2+ branching ratio and the relative kinetic energy of the photofragments. A steady decline observed for the latter as a function of decreasing photon energy, suggests that the dimer ions emerges from the photodissociation processes with a very high level of internal excitation. Supporting semiclassical calculations of the decay dynamics confirm the trend seen for the Ar+/Ar2+ branching ratio, but do not reproduce the quantitative details. The calculations do, however, provide strong support for an experimental analysis of the energetics of fragmentation, and show that Ar2+ can appear either in the electronic ground state, but with a very high degree of vibrational excitation, or in the weakly bound Πg2 electronic excited state.

Using two separate IR lasers, we have attempted to characterise the increase in internal temperature that should accompany the partitioning of a single quantum of vibrational energy within small SF, -(NO),+ cluster ions. An initial temperature is established by measuring a kinetic-energy release associated with the unimolecular (metastable) decay of each ion. Using a CO, laser, ca. 950 cm-' of the vibrational energy is deposited into the SF, moiety and with a CO laser, the (NO),,+ moiety is excited with ca. 1700 cm-I of vibrational energy. In both cases, the ions are observed to photodissociate and the corresponding kinetic-energy releases are measured. Using Klots' model ( J . Chem. Phys., 1973, 58, 5364), an attempt is made to predict the photofragment kinetic energies on the assumption that the energy of each photon is partitioned statistically and contributes to an overall increase in temperature for each ion. The results show that at the higher photon energy, events are dominated by angular momentum conservation; a factor that is not an integral part of the model. In contrast, the photoexcitation of SF, at ca.