Spectroscopic characterization of cations via their electronic transitions

Abstract

Our recent studies and developments undertaken aimed at spectral characterization of open-shell cations are outlined. The analyses of the rotational structure in the laser excitation spectra of the halocyanide and diacetylene cations have enabled their complete r_s geometries to be inferred. These should serve as a benchmark for calculations in open-shell cations containing either a heavy atom of many atoms. A new direction pursued aims at spectral characterization of fragment ions. The first success is the identification of the [Btilde] ⁴Σ_u ⁻—[Xtilde] ⁴Σ_g ⁻ transition of C₂ ⁺, which was observed initially in absorption in a 5 K neon matrix and then in the gas phase by laser excitation. The stimulated emission pumping approach was also used to probe higher vibrational levels of the [Xtilde] ⁴Σ_g ⁻ state. In the studies of new ions by laser methods, two obstacles are encountered. One concerns the uncertainty in the energy region of the transitions sought and the other is the identification of the carrier. We have therefore been developing methods to provide such information. In one, mass-selected ions are co-deposited with neon to form a matrix at 5 K. The electronic absorption spectrum of the trapped ions can then be measured. The first results illustrate the potential of this new technique. In a gas-phase approach, mass-selected ions are investigated by two-photon absorption within a triple-quadrupole instrument. One laser photon induces a transition between bound states of the ion and the second photon leads to fragment ion production. By monitoring the fragment ions as a function of either photon wavelength, both bound-bound and bound-pre-dissociative transitions can be studied. Such results have been obtained on CS₂ ⁺ and CO₂ ⁺.