Photodissociation Spectroscopy and Dynamics of Negative Ion Clusters. II. CO-3.(H2O)1,2,3

摘要

Energy resolved photodissociation studies of CO(3-)(H20)n, n = 1,2,3 are reported for photon energies ranging from 1.95 to 2.2 eV. The only dissociation channel observed is the loss of all attached water molecules to give unclustered CO(3-) as the sole photofragment ion. The cross section for this mechanism is substantially higher than that for the bare ion, and the sharp structure observed in the spectrum of the bare ion is nearly lost in the clusters. Analysis of the kinetic energy distributions for the photofragment ions places an upper limit of 20 microseconds on the lifetime of the excited clusters, and demonstrates that approximately 95% of the excess energy in the cluster remains in the CO(3-) containing fragment rather than being partitioned into relative translation of the photofragments or into internal motion of the water fragments. The dissociation mechanism begins with a bound-bound 2Bl to 2Al transition within the core CO(3-)ion. Interal conversion returns the core ion to the electronic ground state with substantical vibrational excitation; redistribution of this vibrational energy results in vibrational predissociation of the cluster. The relations of this mechanism to those that occur in the bare ion and to other vibrational predissociation experiments on clusters are discussed.