Photophysics of C−2 (B 2Σ+u) in rare gas lattices: Vibrational relaxation through intermediate a 4Σ+u levels

Time and wavelength resolved fluorescence techniques are used to investigate C−2 (B 2Σ) radiationless transitions following pulsed, tunable laser excitation. The purely radiative lifetimes in each host are Ne, ?64 nsec; Ar, ?40 nsec; Kr, ≃38 nsec; and Xe, ≃34 nsec. The principal B 2Σ vibrational relaxation mechanism is a sequential process: crossing into a 4Σ, vibrational relaxation within 4Σ, and reverse crossing into B 2Σ. The observed intermediate lifetimes strongly suggest that v=0 4Σ lies between v=0 and 1 of B 2Σ; this fact allows assignment of gas phase B 2Σ↔a 4Σ rotational perturbations reported by Herzberg and Lagerqvist. The unperturbed a 4Σ constants are Te=19 448.4 cm−1, ωe=1074.04 cm−1, Be=1.1348 cm−1, ωexe=24.68 cm−1, and αe=0.0035 cm−1. Vibrational relaxation within a 4Σ, and intersystem crossing, show a strong multiphonon relaxation energy gap law. The variation in behavior among the 12–12, 12–13, and 13–13 isotopes indicates that the photophysics is dominated by accidental near degeneracies with A 2Π (and probably X 2Σ) levels in addition to 4Σ levels.