Severely perturbed vibrational structure in the 266–310 nm electronic transition of C3

Abstract

The laser induced fluorescence (LIF) spectra of C3 are measured under jet-cooled conditions in the wide UV region of 266–310 nm, and 173 vibronic bands are identified. Among them, 77 and 68 vibronic bands were found to exhibit Σ–Σ type and Π–Σ type rotational structures, respectively. From the rotational analysis, the band-origin wave numbers and the rotational constants of the upper Σ and Π vibronic levels and K-type doubling parameters for the Π vibronic levels are determined. The rotational constants for the Σ and Π vibronic levels exhibit, respectively, similar distributions with almost the same mean values, B̄′=0.395(14) and 0.398(17) cm−1, indicating that the Σ and Π vibronic levels have the same electronic origin and that the average C–C bond distance of the upper electronic state in the observed energy range is about 1.331(25) Å, which is longer than the electronic ground X̃ 1Σg+ state by 0.054(25) Å. The convolution and Fourier-transform analyses are performed separately for the spectra composed only of Σ–Σ type and Π–Σ type vibronic bands to derive estimates of the vibrational fundamentals in the upper electronic state. The low ν1 fundamental of 940(60) cm−1 derived from these analyses and the seemingly regular ν1 progression in the convoluted spectra indicate that the bright character is carried primarily by the transitions to the vibrational levels in the Δu1 electronically excited state and is distributed into a large number of the originally dark transitions to the vibrational levels in the Πg1 electronically excited state, resulting in the exceedingly complex vibronic band system. © 2000 American Institute of Physics.