Multichannel resonances in near-threshold photodissociation spectra of CH+, SiH+ and GeH+

Evidence for multichannel character in resonances which appear in near-threshold photodissociation spectra of the isovalent molecules, CH+, SiH+ and GeH+ is presented. The spectra were obtained through laser-induced excitation from the X-1 Sigma(+) state to near-threshold levels of electronic states which correlate with the lowest dissociation limits, P-2(3/2,1/2)(C+, Si+, Ge+) + S-2(H), by monitoring the production of the atomic fragment ions. Complementary resonance-width calculations have been carried out using a close-coupling approach. In CH+ we report the discovery of a shape resonance with multichannel character; in SiH+, where excitations to Feshbach resonances dominate the photodissociation spectrum, we present the first results of multichannel calculations; in GeH+ we report observation of the first photodissociation spectrum involving near-threshold dissociated levels.