A Theoretical Study of the Low-Lying Electronic States of the AlCCH Radical and Its Ions

The AlCCH radical is a photolysis product of the aluminum-acetylene adducts and has been considered as a molecule with potential interest in astrophysics. In this study, the low-lying electronic states of the AlCCH radical, cation, and anion have been studied by using complete active space self-consistent field and multicontigurational second-order perturbation theory. The geometrical parameters, electron configurations, excitation energies, oscillator strengths, and harmonic vibrational frequencies are calculated in C-S symmetry. For the X-1 Sigma(+) state of AlCCH, the calculated C-C and C-Al stretching modes are in good agreement with experimental reports. Moreover, the vertical excitation energy (T-v) of 1(1)Pi is 3.68 eV, which is close to the experimental value of 3.57 eV. The electron transitions of AlCCH+, X-2 Sigma(+) -> 1(2)Pi, X-2 Sigma(+) -> 2(2)Sigma(+), and X-2 Sigma(+) -> 1(2)Sigma(-), are predicted at 2.57, 4.51, and 4.61 eV, respectively. For AlCCH-, the transition X-2 Pi -> 1(2)Sigma(-) occurs at 3.02 eV. The ionization potentials of AlCCH are computed in order to provide a theoretical guidance to the photoelectron spectrum of the AlCCH radical.