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-in duced excitation from the X-1 Sigma(+) state to near-threshold levels of electronic states which correlate with the lowest dissociation limi ts, P-2(3/2,1/2)(C+, Si+, Ge+) + S-2(H), by monitoring the production of the atomic fragment ions. Complementary resonance-width calculation s have been carried out using a close-coupling approach. In CH+ we rep ort the discovery of a shape resonance with multichannel character; in SiH+, where excitations to Feshbach resonances dominate the photodiss ociation spectrum, we present the first results of multichannel calcul ations; in GeH+ we report observation of the first photodissociation s pectrum involving near-threshold dissociated levels.