A CASSCF study of the potential curves for the X 1Σ+, A 1Π, and C 1Σ+ states of the AlH molecule

The complete active space SCF (CASSCF) method was applied to compute the potential curves for the X 1Σ+, A 1Π, and C 1Σ+ states of the AlH molecule. Molecular constants were obtained from the potential curves by solving the radial Schrödinger equation. The results obtained for the ground state are in excellent agreement with experiment, and show that the intershell correlation effects only have a very small effect on the properties of AlH. Calculated values for ΔGv+1/2 differ from experiment within 4.4–5.6 cm−1for the five known band origins (v=0–4). The calculated dissociation energy (De) is 3.11 eV (experiment, 3.16 eV). The A state has a shallow minimum followed by a maximum. Two bound vibrational states have been found, the upper predissociative. The calculated value for De is 0.12 eV, which is 0.12 eV less than the experimental value. All deviations from experiment can be derived from this error. Rotational constants are in excellent agreement with experiment. Radiative lifetimes for the two bound states have been computed to be 62 (v=0) and 102 ns (v=1). The corresponding experimental values are 66±4 and 83±6 ns, respectively. The C state exhibits a double minimum in the potential curve, with the outer minimum lower in energy, in contrast to the corresponding state in BH.The dissociation energy (D0), 0.61 eV agrees well with the value 0.64 eV derived from experimental data. The second minimum is located at R=3.76 Å and has a depth of 1.16 eV. Transition moments from the six lowest levels of the ground state to any of the outer minimum levels are all very small and this minimum is not expected to be seen in absorption. The rotational levels for the second vibrational band in the inner minimum are heavily perturbed by interaction with outer minimum levels, and a normal analysis of the spectrum in terms of Bv and Dv is not possible.