Rotational and Vibronic Structure in the Electronic Spectra of Linear Open-Shell Cations

The vibrational and rotational structure of the electronic transitions of open-shell cations of linear polyatomics has been studied by three complementary techniques. In one, the emission spectra of ions of unstable species, XCP⁺ and XBS⁺, with X = H, D and F, are obtained by electron-impact excitation on effusive and supersonic free jets. The vibronic structure of their tilde{A}²Σ ⁺-> tilde{X}²Pi _i band systems could be analysed. The changes and distinct features of the rotational profiles at low rotational temperatures in the emission spectra of diacetylene cation have been used to establish the absolute numbering in the rotational analysis of the tilde{A}²Pi _u-tilde{X}²Pi _g transition. The second approach is based on the measurement of the laser excitation spectra of ions and has been used to confirm the vibrational assignment of complex emission spectra, such as the tilde{B}²Pi -> tilde{X}²Pi transition of ClCN⁺, and to resolve the rotational structure in the tilde{A}²Pi _3/2<- tilde{X}²Pi _3/2 transitions of the haloacetylene cations XCCH⁺, X = Cl, Br, I. The most recent development is the demonstration that the stimulated emission pumping approach can be used to probe the electronic structure of ions in a flow system.