Laser Photofragment Spectroscopy of Near-Threshold Resonances in SiH+

Near-threshold resonances in the photodissociation cross section of SiH⁺ have been recorded by the detection of photofragment Si⁺ ions. The photodissociation spectrum is found to be dominated by Feshbach resonances. The first evidence for multichannel resonances in the photodissociation spectrum of a diatomic molecule is presented. Laser photodissociation spectra between 15 600 cm^-1 and 18 750 cm^-1 were recorded at a resolution of 0.0012 cm^-1 by coaxial laser irradiation of a fast ion beam of SiH⁺. Over 70 transitions were observed, the majority of which involve excited-state levels (resonances) that lie between the Si⁺(²P_3/2) + H(²S) and Si⁺(²P_1/2) + H(²S) dissociation limits, which are separated by 287 cm^-1. The assignments were made by combining experimental information with predictions of the vibrational and rotational energy levels obtained by numerical solution of the radial Schrodinger equation and calculations of the rotational line intensities. The experimental data were obtained by measurement of the transition frequencies, line intensities, linewidths and hyperfine splittings and through examination of the effect of laser power on the linewidth. The kinetic energy released on dissociation and the photofragment angular distribution were also determined.