Near-threshold ion photofragment spectroscopy

We have recorded high-resolution laser photofragment spectra of the astrophysically important ions CH+ and SiH+. The experiments are designed to give information on the region of internuclear distance between the atomic and molecular limits for systems where dissociation occurs into atoms possessing electronic angular momentum, in this case C+(2P) or Si+(2P). A-keV beam of Ch+ (SiH+) is irradiated coaxially with cw single-mode tunable laser radiation which induces transitions from the X1∑+ ground state to predissociated levels of the A1∏ state and other electronic states which correlate with the C+(2P) + H(2S) or Si+(2S) limit. An electrostatic analyzer provides mass and energy selection of the photofragment C+ or Si+ ions which are detected with an electron multiplier. Information is obtained from transition frequencies, hyperfine splittings, predissociation linewidths, and also from the photofragment kinetic energies and angular distributions. Over thirty transitions to 1∏ levels have been detected in CH+ (Ref. 1); in SiH+ we have observed transitions to levels which almost certainly lie between the Si+(2P1/2,2P3/2) + H(2S) fine structure limits. We have also recorded numerous resonances in both ions which arise from transitions to triplet states which correlate to the same dissociation limit and are observable because of strong nonadiabatic couplings.