Vibronic Spectra of Aluminium Monochloride Relevant to Circumstellar Molecule

The A1Π → X1 Σ+ transition system of aluminium monochloride is determined by using ab initio quantum chemistry. Based on the multi-reference configuration interaction method in conjugate to the Davidson correction (MRCI + Q), the potential energy curves (PECs) of the three electronic states are obtained. Transition dipole moments (TDMs) and the vibrational energy levels are studied by employing the aug-cc-pwCV5Z-DK basis set with 4220-active space. The rovibrational constants are first determined from the analytic potential by solving the rovibrational Schrödinger equation, and then the spectroscopic constants are determined by fitting the vibrational levels, and these values are well consistent with the experimental data. The effect of spin–orbit coupling (SOC) on the spectra and vibrational properties are evaluated. The results show that the SOC effect has almost no influence on the spectroscopic constants of AlCl molecules. For the A1Π → X1 Σ+ transition, the highly diagonalized Frank–Condon factor (FCF) is f 00 = 0.9988. Additionally, Einstein coefficients and radiative lifetimes are studied, where the vibrational bands include ν″ = 0–19 → ν′ = 0–9. The ro-vibrational intensity is calculated at a temperature of 296 K, which can have certain astrophysical applications. At present, there is no report on the calculation of AlCl ro-vibrational intensity, so we hope that our results will be useful in analyzing the interstellar AlCl based on the absorption from A1Π → X1 Σ+.