High-resolution laser photofragment spectroscopy of molecular ions

Abstract

High-resolution laser photofragment spectra of CH⁺ and SiH⁺ have been recorded by the detection of C⁺and Si⁺ ions using a fast-ion-beam apparatus. Information on near-threshold dissociation is obtained by the observation of transitions to predissociated (quasibound) levels, some of which exhibit predissociation lifetime-broadening and nuclear hyperfine splittings. Measurement of the centre-of-mass kinetic energy release and the photofragment angular distribution is also described.

The predissociation mechanisms for three rotationally quasibound levels of the A¹Π state of CH⁺ are investigated through analysis of lifetime-broadening of rotational lines of the A¹Π-X¹Σ⁺ electronic transition. Comparison is made with the results of a calculation of the tunnelling and radial contributions to the predissociation rates.

The first predissociation spectra of SiH⁺ are reported and transitions to two levels which lie only ca. 80 cm^–1 above the dissociation limit are discussed in detail. Evidence is found for non-adiabatic interactions near to dissociation.

It is proposed that ‘near-threshold’ photofragment spectra of CH⁺ observed in the 18 350–19 600 cm^–1 region arise from transitions between triplet levels near to the C⁺(²P)+ H(²S) dissociation limit and a shallow (bound) triplet state which correlates to the C(³P)+ H⁺ limit. Radiative decay to the continuum and near-threshold levels above the C⁺(²P)+ H(²S) limit yields low-energy C⁺ ions.