SPECTROSCOPY, STRUCTURE, ANOMALOUS FLUORESCENCE, AND MOLECULAR QUANTUM BEATS OF SILYLIDENE (H2C=SI), THE SIMPLEST UNSATURATED SILYLENE

The jet-cooled (B) over tilde B-1(2)-(X) over tilde (1)A(1) spectrum o f silylidene, the simplest unsaturated silylene, has been observed for the first time. H2C=Si and D2C=Si have been produced by an electric d ischarge through tetramethylsilane and tetramethylsilane-d(12) vapor d iluted in argon at the exit of a supersonic expansion. Rotational anal ysis of the 0(0)(0) bands yielded the following substitution structure s: r(s) ''(CSi) = 1.706(5) Angstrom, r(s) ''(CH) = 1.099(3) Angstrom, theta(s) ''(HCH)=114.4(2)degrees, r(s)'(CSi) = 1.815(5) Angstrom, r(s) '(CH) = 1.073(4) Angstrom, and theta(s)'(HCH)=133.7(1)degrees. The ele ctronic transition consists primarily of strong electronically allowed perpendicular bands, but a weaker system of vibronically induced para llel bands has also been assigned. Transitions involving Delta upsilon =2 changes in the upsilon(6) (b(2)) mode show up prominently in the sp ectrum, due to a very large change in the vibrational frequency on exc itation. Silylidene has very interesting excited state decay dynamics. Anomalous S-2-S-0 fluorescence is observed due to the very large S-2- S-1 energy gap. Rotational level specific intensity anomalies are foun d in the laser induced fluorescence spectra. Collision-free fluorescen ce decay curves exhibit superimposed quantum beats for almost all the accessible rotational levels in the 0(0)(0) bands of H2CSi and D2CSi. Density of states arguments lead to the conclusion that most of the br at patterns are due to coupling with high vibrational levels of the gr ound state, although two examples of hyperfine splittings associated w ith singlet-tripler interactions have also been found. (C) 1997 Americ an Institute of Physics.