Infrared Laser Spectroscopy of Cations [and Discussion]

The high-resolution infrared diode laser absorption spectra of several cations have been recorded and analysed. Most of the spectra were detected with an AC discharge and the velocity-modulation technique, with frequencies up to 60 kHz. In addition to ions, this method can also detect transient neutral species by recording at twice the modulation frequency (concentration modulation). As an example, spectra of an infrared electronic transition in H$_{2}$ are described briefly. The diatomic cationic species discussed are CO$^{+}$ (X$^{2}\Sigma ^{+}$) and SH$^{+}$ (X$^{3}\Sigma ^{-}$), which are compared with the isoelectronic neutral free radicals CN and PH respectively, also in their ground states. Polyatomic cations that have been studied include HCO$^{+}$ and H$_{3}$O$^{+}$. The bending mode of HCO$^{+}$ (2$_{0}^{1}$) shows a distinctive Q-branch with many resolved components at Doppler-limited resolution and also some hot-band lines. The HCO$^{+}$ spectrum can be combined with microwave measurements in the ground vibrational level to yield predictions for (2$_{1}$) rotational transitions for astrophysical searches. The most recent spectra of the $\nu _{2}$ `umbrella' mode of H$_{3}$O$^{+}$, the 2$^{+}\leftarrow $ 1$^{-}$ band, provide spectroscopic data on vibrational levels approximately 1000 cm$^{-1}$ above the inversion potential maximum.