The jet-cooled (B) over tilde B-1(2)-(X) over tilde (1)A(1) spectrum of silylidene, the simplest unsaturated silylene, has been observed for the first time. H2C=Si and D2C=Si have been produced by an electric discharge through tetramethylsilane and tetramethylsilane-d(12) vapor diluted in argon at the exit of a supersonic expansion. Rotational analysis of the 0(0)(0) bands yielded the following substitution structures: 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 electronic transition consists primarily of strong electronically allowed perpendicular bands, but a weaker system of vibronically induced parallel bands has also been assigned. Transitions involving Delta upsilon=2 changes in the upsilon(6) (b(2)) mode show up prominently in the spectrum, due to a very large change in the vibrational frequency on excitation. 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 found in the laser induced fluorescence spectra. Collision-free fluorescence 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 brat patterns are due to coupling with high vibrational levels of the ground state, although two examples of hyperfine splittings associated with singlet-tripler interactions have also been found. (C) 1997 American Institute of Physics.