C2 and C2− Spectra Produced by the X Irradiation of Acetylene in Rare-Gas Matrices

Abstract

The absorption and emission spectra induced by the x irradiation of acetylene trapped in Ar, Kr, and Xe matrices have been studied in the visible region. Two transitions of diatomic carbon dominate the spectrum. The B 2Σu+ ↔ X 2Σg+ transition of C2−, first identified in a matrix by Milligan and Jacox, is present in all three solvents in both emission and absorption. The A 3Πg → X′ 3Πu Swan-band system of C2 is present in emission only, and then only in Ar and Kr matrices. This result is consistent with the gas-phase ordering of states, which places the X′ 3Πu state of neutral C2 610 cm−1 above the ground state. Spectroscopic constants have been obtained by making a least-squares fit of the data. Although the deviations from the gas-phase ωe's are 1%–2% in the ground state of C2− and in the upper state of the C2 Swan system, there can be no doubt of the assignment. The ωe's of the upper C2− state and the lower state of the Swan system differ from the gas-phase values by only 1–2 cm−1. Experiments with carbon-13 enriched acetylene confirmed the vibrational assignments. The emission spectra of both molecules are particularly interesting. Emission was observed from the υ′ = 4, υ′ = 2, and υ′ = 1 levels of C2 indicating that the vibrational lifetime is as long as or longer than the radiative lifetime of the A 3Πg state. The vibrational quantum numbers of the emitting levels were found to depend on the isotopic species involved, but the same vibronic levels emitted in both Ar and Kr matrices indicating that the relaxation process is strongly dependent on the spacing of the vibrational levels but is relatively independent of the phonon structure of the surrounding medium. Vibrational relaxation was also found to be slow in the B 2Σu+ state of C2−. Emission was observed from both the υ′ = 1 and υ′ = 0 levels of this molecule in all three rare gases. The paths of electronic and vibrational excitation and relaxation in both C2 and C2− are discussed qualitatively.