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

Abstract

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 T_e=19 448.4 cm⁻¹, ω_e=1074.04 cm⁻¹, B_e=1.1348 cm⁻¹, ω_ex_e=24.68 cm⁻¹, and α_e=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.