FT-IR STUDIES OF OPTICALLY PUMPED C-3 IN RARE-GAS MATRICES - THE VIBRATIONAL AND ELECTRONIC ABSORPTION-SPECTRUM OF THE (A)OVER-TILDE(3)PI(U) STATE

Through a double-resonance technique, the FT-IR absorptions of the lowest-lying triplet state of C3 have been observed in argon and neon matrices. The long-lived a 3PI(u) state is populated by an intersystem crossing from the excited A 1PI(u) state. The difference spectra obtained with and without C3 A 1PI(u) laser excitation exhibit a large depletion of the electronic ground state and a simultaneous appearance of the vibrational and electronic a 3PI(u) absorption spectra. The b 3PI(g)<--a 3PI(u) spectrum extends from almost-equal-to 6500-8400 cm-1 and shows an extensive vibrational structure. The assignments of the two stretching vibrations of the a 3PI(u) state are verified through a harmonic force field fit to nine isotopic frequencies.