Excited electronic states of the SiF2 radical studied by resonance enhanced multiphoton ionisation spectroscopy and by ab initio methods

SiF2 radicals have been produced by the gas-phase reaction of SiH4 with F atoms in a flow reactor and detected by mass-resolved resonance enhanced multiphoton ionisation (REMPI) spectroscopy. Two band systems are observed. One, a series of one-photon resonances in the wavelength range 210–230 nm, we associate with the previously documented Ã1B1 « X̃1A1 transition. The observed long progression in the excited state bending mode, ν2, reflects the increase in equilibrium bond angle that accompanies this electronic transition. We also identify three two-photon resonances in the wavelength range 315–325 nm. Two we can definitely associate with the B̃1B2 « X̃1A1 transition. Polarisation studies show the third resonance to involve an excited state of 1A1 symmetry. Guided by the results of companion ab initio calculations we present evidence for and against assignment of this feature as the (vibronically induced) B̃ « X̃301 band, or as a hitherto unidentified 1A1 ← X̃1A1 electronic origin. The calculations also enable prediction of the term values of a further eight, as yet unobserved, excited states of SiF2.