Photodissociation of Ar-H[sub 2]O at 121.6nm: high lying metastable rotational levels of Ar-OH.

The total kinetic energy release spectra of H atoms from the photolysis at 121.6 nm of water entrained in an argon jet show a number of peaks with widths of 100-150 cm[sup -1]. The relative intensity of these peaks varies with the physical conditions, which suggests that they should be attributed to molecular clusters. Calculation of the energy levels of Ar-OH with a known potential shows that there exists a wide range of states in which the OH moiety is excited to levels of the A[sup 2]Σ[sup +] state with high internal angular momentum, but which are remarkably stable to dissociation to Ar + OH. Indeed their stability increases with increasing rotation, even though their internal energy is many times the binding energy of the complex. The lowest frequency vibrational mode of the complex (ν < 10 cm[sup -1]) is a nutation of the OH rotational axis in the potential field of the argon atom. On this basis the broad peaks in the observed spectra have been simulated as unresolved bands involving these metastable levels. The calculated lifetimes for these levels are up to a nanosecond and more, so that in principle transitions involving them may give rise to resolvable spectra. It is also concluded from a comparison of spectra with and without clustering that photodissociation of Ar-H[sub 2]O leads exclusively to levels of Ar-OH(A). This is attributed to the blocking by the argon atom of the conical intersections which would be the pathways to Ar-OH(X), and which dominate in the dissociation of bare H[sub 2]O to give OH(X). [ABSTRACT FROM AUTHOR]