Spectroscopic Investigations of Intersystem Crossing and Triplet State Structure in Acetylene

A multispectral study employing both Ultraviolet Laser Induced Fluorescence (UV-LIF) and Surface Electron Ejection by Laser Excited Metastables (SEELEM) spectro-scopies has yielded a complete characterization of singlet-triplet interactions relevant to intersystem crossing in the region of the 3v3 vibrational level of the A 1A, excited electronic state of acetylene. Intersystem crossing proceeds through a "doorway" state which has been plausibly assigned as a specific low-lying vibrational level of the third triplet electronic state, T3. The predominantly T1,2 dark states observed in the SEELEM spectrum show the remarkable property of spectral regularity, in the sense that they are able to be arranged into smooth, well-behaved "vibrational levels " as judged from reduced rotational term value plots. This fact, together with the obser-vation of essentially all the expected triplet density of states, strongly suggest that these triplet states fall into the "strongly coupled, " highly mixed regime