Photophysics of C-2 (B 2Σ+/subu/) in rare gas lattices: Vibrational relaxation through intermediate a 4Σ+/subu/ 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, approx. =64 nsec; Ar, approx. =40 nsec; Kr, approx. =38 nsec; and Xe, approx. =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 ν=0 4Σ lies between ν=0 and 1 of B 2Σ; this fact allows assignment of gas phase B 2Σbold-arrow-left-righta 4Σ rotational perturbations reported by Herzberg and Lagerqvist. The unperturbed a 4Σ constants are T/sube/=19 448.4 cm-1, ω/sube/=1074.04 cm-1, B/sube/=1.1348 cm-1, ω/sube/x/sube/=24.68 cm-1, and α/sube/=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 2Pi (and probably X 2Σ) levels in addition to 4Σ levels