INFRARED SPECTRA OF THE HYDRATED HYDRONIUM ION CLUSTERS $H_{3}O^{+} \cdot (H_{2}O)_{m}, m=1.3$

$^{1}$ H. A. Schwarz, J. Chem. Phys. 67, 5525 (1977). $^{2}$ M. Okumura, L. I. Yeh, J. D. Myers, and Y. T. Lee, J. Chem. Phys. 85, 3705 (1986).Author Institution: Department of Chemistry, The University of Chicago; Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Chemistry Department, University of CaliforniaVibrational spectra from 2300 to $4000 cm^{-1}$ of the hydrated hydronium ion in the gas phase have been observed using two new variations of the ion trapping/photodissociation technique. Spectra are obtained by mass-selecting the cluster ion, trapping, exciting the ions with a tunable IR laser, and detecting dissociation products. The new methods have been developed because these clusters are too strongly bound to be dissociated by exciting a single vibrational quantum. In the first experiments, we have observed the vibrational predissociation spectrum of the clusters $H_{3}O^{+} \cdot (H_{2}O)_{m}(H_{2}).{^{2}}$ Excitation of an OH stretch leads to loss of the $H_{2}$. Because the $H_{2}$ binds weakly to the ion, the observed predissociation spectrum provides a low resolution $(1 cm^{-1})$ spectrum of the cluster $H_{3}O^{+} \pm (H_{2}O)_{m}$. Perturbations induced by the $H_{2}$ are observable, and provide additional information. In the second set of experiments, the spectra of the $H_{3}O^{+} \cdot (H_{2}O)$ clusters are observed by dissociating the clusters themselves. Clusters with OH vibrations excited by a tunable infrared laser are preferentially dissociated(relative to unexcited ions) by multiphoton dissociation(MPD) with a $CO_{2}$ laser. The spectra confirm earlier low resolution$(40 cm^{-1})$ gas phase spectra observed by $Schwarz,^{1}$ and agree with ab initio predictions of the vibrational frequencies. Both free and hydrogen-bonded OH stretches have been observed at low resolution$(1 cm^{-1})$ for $H_{3}O^{+} \cdot (H_{2}O)_{m}$, with $m = 1, 2$, and 3. Using the two laser MPD approach, rotational structure has also been observed in some bands of $H_{3}O^{+} \cdot (H_{2}O)$ and $H_{3}O^{+} \cdot (H_{2}O)_{3}$ with Doppler-limited linewidths of $0.02 cm^{-1}$