The first infrared spectra of protonated hydrogen clusters in the gas phase have been observed. Predissociation spectra were taken with a tandem mass spectrometer: mass selected hydrogen cluster ions were irradiated inside a rf ion trap by a tunable infrared laser, and the fragment ions created by photodissociation of the clusters were mass selected and detected. Spectra for each product channel were measured by counting fragment ions as a function of laser frequency. Low resolution spectra (Dgr;ngr;=10 cmminus;1) in the region from 3800 to 4200 cmminus;1were observed for the ions H+5, H+7, and H+9at 3910, 3980, and 4020 cmminus;1, respectively. A band was also observed for H+5at 3532 cmminus;1. No rotational structure was resolved. The frequencies of the band maxima agree well with the frequencies predicted by previousabinitiocalculations for the highest modes.
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