Charge-Transfer Spectroscopy of Ag+(Benzene) and Ag+(Toluene).

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 forbidden 1S → 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).

Communication: Electronic transition of the l-C6 + cation at 417 nm.

A new electronic transition is reported for the linear C6 + cation with an origin at 416.8 nm. This spectrum can be compared to the matrix isolation spectra at lower energies reported previously by Fulara et al. [J. Chem. Phys. 123, 044305 (2005)], which assigned linear and cyclic isomers, and to the gas phase spectrum reported previously by Campbell and Dunk [Rev. Sci. Instrum. 90, 103101 (2019)], which detected the same cyclic-isomer spectrum reported by Fulara. Comparisons to electronically excited states and vibrations predicted by various forms of theory allow assignment of the spectrum to a new electronic state of linear C6 +. The spectrum consists of a strong origin band, two vibronic progression members at higher energy and four hot bands at lower energies. The hot bands provide the first gas phase information on ground state vibrational frequencies. The vibrational and electronic structure of C6 + provide a severe challenge to computational chemistry.

The Role of Tunneling in the Spectra of H5+ and D5+ up to 7300 cm-1.

The spectra for H5+ and D5+ are extended to cover the region between 4830 and 7300 cm-1. These spectra are obtained using mass-selected photodissociation spectroscopy. To understand the nature of the states that are accessed by the transitions in this and prior studies, we develop a four-dimensional model Hamiltonian. This Hamiltonian is expressed in terms of the two outer H2 stretches, the displacement of the shared proton from the center of mass of these two H2 groups, and the distance between the H2 groups. This choice is motivated by the large oscillator strength associated with the shared proton stretch and the fact that the spectral regions that have been probed correspond to zero, one, and two quanta of excitation in the H2 stretches. This model is analyzed using an adiabatic separation of the H2 stretches from the other two vibrations and includes the non-adiabatic couplings between H2 stretch states with the same total number of quanta of excitation in the H2 stretches. Based on the analysis of the energies and wave functions obtained from this model, we find that when there are one or more quanta of excitation in the H2 stretches the states come in pairs that reflect tunneling doublets. The states accessed by the transitions in the spectrum with the largest intensity are assigned to the members of the doublets with requisite symmetry that are localized on the lowest-energy adiabat for a given level of H2 excitation.