The electronically forbidden (A) over tilde (1)A(2)-(X) over tilde (1)A(1) band system of jet-cooled silylidene (H2CSi and D2CSi) has been detected for the first time using laser-induced fluorescence (LIF) and stimulated emission pumping (SEP) techniques. The very weak, vibronically induced 4(0)(1) and 3(0)(1)4(0)(1) bands were detected by LIF along with the corresponding 6(0)(1) and 3(0)(1)6(0)(1) bands which gain intensity through excited state Coriolis coupling. SEP spectra, obtained by pumping the 0(0)(0) band of the S-2 state and stimulating transitions down to the S-1 state through the allowed S-2-S-1 transition, revealed many more bands, including the 0(0)(0) bands, which were studied at high resolution and rotationally analyzed. From the upper state rotational constants of H2CSi and D2CSi, the excited state structure was obtained as r(0)' (SiC)=1.873(2) Angstrom, r(0)(')(CH)=1.099(5) Angstrom, and theta(0)'(HCH)=113.9(3)degrees. The four lowest energy excited state vibrational frequencies of both isotopomers have also been determined. High level ab initio predictions of the ground and excited state properties of silylidene are also reported and found to be in good agreement with the experimentally determined values. (C) 2003 American Institute of Physics.