Knowledge Bank

High resolution infrared spectra of the $1--O+(10\mu m)$ $1+-O-(20\mu m)$ and $1+-1+(27\mu m)\nu 2$ vibration-inversion bands of $H3O+$ have been observed in an ac glow discharge by using tunable diode lasers and velocity modulation technique. The three band origins were found to be at 954.4003 (25) $cm-$, 525.8237(13) $cm-1$ and 373.2304(47) $cm-1$, respectively. The rotational and centrifugal distortion constants of these three bands were also obtained from the least-squares fitting of observed transitions. From the molecular constants obtained above, we have determined the ground state inversion splitting of $H3O+$ molecular ion to be 55.3462 (55) $cm-1$ and predicted the ground state inversion spectrum in the far IR region and the inversion-rotation spectrum in the millimeter wave region. These predictions may be used to detect this fundamental molecular ion in interstellar space. A comparison has been made between our experimental molecular constants and the relative strength of these three bands with those ab-initio theoretical predictions for this long-standing problem. The discharge chemistry of $H3O+$ is discussed with reference to the observed line strength - gas composition relation and known ion-molecule reactions.