Laser spectroscopy of molecular ions: The A–X 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 radical cation. The bent structure of the ionic à state could be proved. In the ÖX̃-spectrum progressions of all three bending modes of bent acetylene appear. Their frequencies, anharmonicities and estimated potential thresholds are given. Furthermore, for the origin and the 5a20 band (out-of-plane cis-bending mode) rotationally resolved spectra have been obtained and rotational constants A, B, and DK have been determined as well as a directly measured value of the X̃-state spin–orbit splitting. From rotational linewidths or envelopes information about single vibrational lifetimes 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.