State-to-state collisional energy transfer in electronically excited SiCl radicals

The collisional quenching of selected vibrational levels of electronically excited SiCl B′²Δ radicals by the series of nonpolar molecules He, Ar, H₂, N₂, CO₂, CH₄, and CF₄ was investigated experimentally. The results for He and Ar agreed well with previous independent measurements. Rate constants (and cross sections) for total removal of SiCl B′²Δ v′ = 0 and 1 are relatively large for all collision partners and quite well correlated with the attractive nature of the intermolecular interaction. Rate constants and cross sections were measured for transfer to the near-degenerate B²Σ⁺ electronic state. A substantial, but variable, fraction of the total removal proceeds via this channel for all quenchers. Nascent vibrational distributions over the v′ = 0, 1, and 2 vibrational levels of the B²Σ⁺ state were determined by exploiting the relatively much shorter radiative lifetime of this electronic state. No obvious, single molecular property could be identified to explain these vibrational distributions. There is, however, a trend toward removal of more energy by polyatomic molecules. The cross sections for the single, near-resonant state-to-state process B′²Δ v′ = 0 to B²Σ⁺ v′ = 2 were found to be well-correlated with attractive intermolecular forces. Selected higher resolution measurements confirmed the lack of any significant vibrational relaxation within the B′²Δ state competitive with collisional transfer to the B²Σ⁺ state and revealed no evidence for preferred interstate transfer through specific "gateway" rotational levels.