The electronic spectrum of silicon methylidyne (SiCH), a molecule with a silicon-carbon triple bond in the excited state

The (A) over tilde (2)Sigma(+)-X (2)Pi(i) transition of jet-cooled silicon methylidyne, SiCH, has been recorded by laser-induced fluorescence in the 850-600 nm region. The radical was produced in an electric discharge using tetramethylsilane as the precursor. Fifteen cold bands of SiCH and 16 bands of SiCD have been assigned vibrationally, giving the upper state frequencies as v(2)(’)=715/558 cm(-1) and v(3)(’)=1168/1127 cm(-1) for SiCH/SiCD. Rotational analysis of the 0(0)(0) and 3(0)(3) bands for each isotopomer has given the following molecular structures: r(0)(’)(Si-C)=1.692 52(8), r(0)(’)(C-H)=1.0677(4), r(0)(’)(Si-C)=1.6118(1), and r(0)(’)(C-H)=1.0625(5) Angstrom. The silicon-carbon bond length in the X (2)Pi ground state of SiCH (electron configuration ...sigma(2)pi(3)) is typical for a Si=C double bond; in the (A) over tilde (2)Sigma(+) excited state (...sigma(1)pi(4)) it corresponds to a triple bond. This work provides the first experimental measurement of the length of the carbon-silicon triple bond. (C) 2000 American Institute of Physics. [S0021-9606(00)00508-0].