LASER SPECTROSCOPY OF MOLECULAR-IONS - THE (A)OVER-TILDE-(X)OVER-TILDE TRANSITION OF THE ACETYLENE RADICAL-CATION

Resonance-enhanced multiphoton-dissociation spectroscopy has been used to measure the first highly resolved UV spectrum of the acetylene rad ical cation. The bent structure of the ionic (A) over tilde state coul d be proved. In the (A) over tilde-(X) over tilde-spectrum progression s of all three bending modes of bent acetylene. Their frequencies, anh armonicities and estimated potential thresholds are given. Furthermore , for the origin and the 5a(0)(2) band (out-of-plane cis-bending mode) rotationally resolved spectra have been obtained and rotational const ants A, B, and D-K have been determined as well as a directly measured value of the (X) over tilde-state spin-orbit splitting. From rotation al linewidths or envelopes information about single vibrational lifeti mes has been deduced; a mode-specific behavior of these lifetimes has been found. Several nonradiative processes involving isomerization to vinylidene seem to be important in the energy region of our spectrum. (C) 1995 American Institute of Physics.