Development and utilization of laser-ion beam photodissociation

Abstract

Laser induced photodissociation of fast (3-8 keV) ion beams has been developed for use in both a coaxial and perpendicular beam arrangement. The laser-ion beam photodissociation technique has been used as a probe for elucidating and differentiating the structure(s) of small gas phase ions such as C3H3- and C4H4+ and for differentiating geometric isomers of C5H8- and C6H10- radical cations. In addition, it has been used to demonstrate that the 2.4-hexadiyne C6H6- radical cations do not isomerize to the more stable benzene radical cation prior to dissociation, but retain their structural integrity. Although the technique is useful for structural studies, it is also useful for studying dissociation dynamics and detecting dissociative excited states of the sampled ion and may be useful in investigating the mechanism of energy randomization in the dissociation process as well as the mechanism for the extensive fragmentation observed in multi-photon ionization experiments. The various types of experiments that can be performed provide different types of information. The laser beam polarization experiments are most useful for detecting isomerization following photon absorption. If the ions isomerize prior to photon absorption, the appearance of the photodissociation spectrum will reflect that isomerization and/or the presence of more than one isomer. Branching ratio measurements can be used to evaluate the effects of internal energy on the photodissociation process. Measuring the kinetic energy release as a function of photon energy can be used to detect isomerization reactions following photon absorption and to study the redistribution of excess internal energy into translational degress-of-freedom. This measurement, as well as the appearance of the photodissociation spectrum can detect dissociation from repulsive excited states.