# VIBRATIONAL PREDISSOCIATION SPECTROSCOPY OF CLUSTER IONS: INFRARED SPECTRA OF THE CLUSTERS $H^{+}_{n}$ AND $H_{3}O^{+}\cdot (H_{2})_{m}$

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 Title: VIBRATIONAL PREDISSOCIATION SPECTROSCOPY OF CLUSTER IONS: INFRARED SPECTRA OF THE CLUSTERS $H^{+}_{n}$ AND $H_{3}O^{+}\cdot (H_{2})_{m}$ Creators: Okumura, Mitchio; Yeh, L. I.; Lee, Y. T. Issue Date: 1987 Publisher: Ohio State University Abstract: The vibrational spectra of the hydrogen cluster ions $H^{+}_{n} (n = 5, 7, 9, 11, 13, and 15)$ and the clusters $H_{3}O^{+} \cdot (H_{2})_{m} (m = 1, 2, and 3)$ have been observed from 3000 to $4200 cm^{-1}$, by detecting the laser-induced vibrational predissocation of the clusters trapped in a radio-frequency octopole ion $trap.^{1}$ We have observed the $H_{2}$ stretching modes near $4000 cm^{-1}$ of the clusters $H^{+}_{n}$, with $n = 5, 7, 9, 11, 13, and 15$, as well as the ""symmetric"" $H^{+}_{3}$ stretch in $H^{+}_{5}$. With the exception of $H^{+}_{5}$, the frequencies are in good agreement with scaled values predicted by ab initio theory. The results are consistent with the theoretical structure of $H_{2}$ molecules ""solvating"" a $H^{+}_{3}$ core, and support the prediction that $H^{+}_{9}$ is a particularly stable species, having a completed inner shell of three $H_{2}$, one at each corner of the $H^{+}_{3}$. Despite an intensive search of the $H^{+}_{5}$ spectrum at a resolution of $0.1 cm^{-1}$, no rotational structure has yet been observed, suggesting that $H^{+}_{5}$ dissociates very rapidly. We have also obtained vibrational spectra of clusters containing one to three $H_{2}$ solvating the hydronium ion $H_{3}O^{+}$. We have observed all (free and hydrogen-bonded) OH vibrations, as well as the $H_{2}$ 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 The frequencies are in good agreement with unpublished ab initio calculations of Remington and Schaefer.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 $0.5 cm^{-1}$, we have observed partially resolved rotational structure in two of the bands of the cluster $H_{3}O^{+} (H_{2})$, the first evidence that rotational information can be obtained from cluster ion predissociation spectroscopy. Description: $^{1}$ M. Okumura, L. I. Yeh, and Y. T. Lee, J. Chem. Phys. 83, 3705 (1985). Author Institution: Department of Chemistry, The University of Chicago; Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Chemistry Department, University of California URI: http://hdl.handle.net/1811/17304 Other Identifiers: 1987-TA-9