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

Abstract

The à 2Σ+–X̃ 2Πi 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 v2′=715/558 cm−1 and v3′=1168/1127 cm−1 for SiCH/SiCD. Rotational analysis of the 000 and 303 bands for each isotopomer has given the following molecular structures: r0″(Si–C)=1.692 52(8), r0″(C–H)=1.0677(4), r0′(Si–C)=1.6118(1), and r0′(C–H)=1.0625(5) Å. The silicon–carbon bond length in the X̃ 2Π ground state of SiCH (electron configuration …σ2π3) is typical for a Si=C double bond; in the à 2Σ+ excited state (…σ1π4) it corresponds to a triple bond. This work provides the first experimental measurement of the length of the carbon–silicon triple bond.