Following the photoexcitation of argon cluster ions, Ar+n for n in the range 4–25, kinetic energy release measurements have been undertaken on the fragments using two quite separate techniques. For Ar+4–Ar+6, fragment ion kinetic energy spectra were recorded at 532 nm in a crossed beam apparatus as a function of the angle of polarization of the laser radiation with respect to the incident ion beam. Only Ar+ from Ar+4 was observed to exhibit a polarization dependence together with a comparatively high kinetic energy release. The principal fragment ion Ar+2 was found both to emerge with a low kinetic energy release and to display no dependence on the angle of polarization of the radiation. In a second series of experiments, mass and kinetic energy resolved cluster ions were photodissociated in the entrance to a time-of-flight (TOF) device of variable length. The subsequent deflection of all ions allowed for time resolved measurements to be undertaken on the neutral photofragments. Following the absorption of a photon, all cluster ions up to Ar+25 were found to eject one/two neutral atoms with comparatively high kinetic energies. Any remaining internal energy appears to be dissipated through the loss of further neutral atoms with low kinetic energies. An analysis of the laser polarization dependence of these events, shows that those atoms identified as having high kinetic energies are ejected on a time scale which is short compared with the rotation period of a cluster (≂10 ps). These experimental observations are consistent with the results of recent molecular dynamics simulations of excited states in rare gas clusters by Landman, Jortner, and co-workers [J. Phys. Chem. 91, 4890 (1987); J. Chem. Phys. 88, 4273 (1988)]. Kinetic energy releases calculated from the TOF spectra exhibit marked fluctuations as a function of cluster size, with Ar+15 showing a minimum and Ar+19 a maximum. It is suggested that such behavior is part of a dynamic response to changes in structure as the cluster ions increase in size. A qualitative explanation is provided through the assumption that the cluster ions take the form of solvated Ar+2 structures. Considerations of the energy available from the photon and the relative contribution each TOF feature makes to the total signal, places an upper limit of two as the number of high kinetic energy atoms ejected by the larger cluster ions.

A supersonic nozzle has been used to generate Ar2+ in the ion source of a mass spectrometer via the electron impact ionization of neutral dimers and clusters. Following mass selection, the dimer ion has been photodissociated with infrared radiation from a C02 laser in a crossed-beam configuration. By changing the plane of polarization of the laser radiation, two electronic transitions have been identified. One transition corresponds to excitation to a repulsive state, a process that has been established from previous photodissociation studies using visible radiation. It is proposed that the second transition is associated with excitation to quasibound levels in a shallow ion-induced-dipole well, situated in the long-range region of an otherwise repulsive electronic state.

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Abstract. Reaction studies of carbon clusters C, in the range n = 8-37, produced by laser vaporisation in a supersonic nozzle, have been investigated using time-offlight mass spectrometry. Clear differences in reaction products formed on hydrogenation are detected which show that even and odd clusters behave quite differently and furthermore that at least two different types of even cluster appear to exist. The reactivity patterns for clusters C, with n = t6, 18 and 22 are in a different class from those with n = 20, 24, 26 .... a behaviour consistent with the existence of closed cage fullerene structures for even clusters with 20 or more carbon atoms (other than n = 22).