Infrared spectrum of the silicon hydride cation SiH₇⁺

We report the infrared spectrum of the silicon hydride cluster ion SiH₇⁺ obtained by vibrational predissociation spectroscopy. SiH₇⁺ ions were excited by a tunable infrared laser, and the resulting SiH₅⁺ photofragment intensity was measured as a function of the laser wavelength. We observed a vibrational band centered at 3866 cm^-1, which we assigned as a perturbed H₂ stretch. The absence of a second band between 3500 and 4200 cm^-1 suggests that the ion forms a symmetric complex with the structure H₂·SiH₃⁺·H₂, in contrast to the species CH₇⁺, which has the structure CH₅⁺·H₂.