Discovery of the optically forbidden S1–S transition of silylidene (H2C=Si)

Abstract

The electronically forbidden à 1A2–X̃ 1A1 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 401 and 301401 bands were detected by LIF along with the corresponding 601 and 301601 bands which gain intensity through excited state Coriolis coupling. SEP spectra, obtained by pumping the 000 band of the S2 state and stimulating transitions down to the S1 state through the allowed S2–S1 transition, revealed many more bands, including the 000 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 r0′ (SiC)=1.873(2) Å, r0′(CH)=1.099(5) Å, and θ0′(HCH)=113.9(3)°. 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.