Gas-phase ion–molecule
complexes of silver cation with benzene
or toluene are produced via laser vaporization in a pulsed supersonic
expansion. These ions are mass-selected and photodissociated with
tunable UV–visible lasers. In both cases, photodissociation
produces the organic cation as the only fragment via a metal-to-ligand
charge-transfer process. The wavelength dependence of the photodissociation
produces electronic spectra of the charge-transfer process. Broad
structureless spectra result from excitation to the repulsive wall
of the charge-transfer excited states. Additional transitions are
detected correlating to the forbidden1S →1D silver cation-based atomic resonance and to the HOMO–LUMO
excitation on the benzene or toluene ligand. Transitions to these
states produce the same molecular cation photofragments produced in
the charge-transfer transitions, indicating an unanticipated excited-state
curve-crossing mechanism. Spectra measured for these ions are compared
to those for ions tagged with argon atoms. The presence of argon causes
a significant shift on the energetic positions of these electronic
transitions for both Ag+(benzene) and Ag+(toluene).