Knowledge Bank

The vibrational spectra of the hydrogen cluster ions $Hn+(n=5,7,9,11,13,and15)$ and the clusters $H3O+\cdot (H2)m(m=1,2,and3)$ have been observed from 3000 to $4200cm-1$, by detecting the laser-induced vibrational predissocation of the clusters trapped in a radio-frequency octopole ion $trap.1$ We have observed the $H2$ stretching modes near $4000cm-1$ of the clusters $Hn+$, with $n=5,7,9,11,13,and15$, as well as the ""symmetric"" $H3+$ stretch in $H5+$. With the exception of $H5+$, the frequencies are in good agreement with scaled values predicted by ab initio theory. The results are consistent with the theoretical structure of $H2$ molecules ""solvating"" a $H3+$ core, and support the prediction that $H9+$ is a particularly stable species, having a completed inner shell of three $H2$, one at each corner of the $H3+$. Despite an intensive search of the $H5+$ spectrum at a resolution of $0.1cm-1$, no rotational structure has yet been observed, suggesting that $H5+$ dissociates very rapidly. We have also obtained vibrational spectra of clusters containing one to three $H2$ solvating the hydronium ion $H3O+$. We have observed all (free and hydrogen-bonded) OH vibrations, as well as the $H2$ stretching modes of these clusters. The frequencies are in good agreement with unpublished ab initio calculations of Remington and Schaefer. At a laser resolution of $0.5cm-1$, we have observed partially resolved rotational structure in two of the bands of the cluster $H3O+(H2)$, the first evidence that rotational information can be obtained from cluster ion predissociation spectroscopy.