Interaction of an aluminum atom with an alkaline earth atom: Spectroscopic and ab initio investigations of AlCa

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Abstract
A spectroscopic analysis of diatomic AlCa generated by laser vaporization of a 2:1 Al:Ca metal alloy followed by supersonic expansion has been completed using resonant two‐photon ionization spectroscopy. Four excited electronic states have been identified and investigated in the energy region from 13 500 to 17 900 cm−1. These are the [13.5] 2Πr, the [15.8] 2Σ, the [17.0] 2Δ3/2(?), and the [17.6] 2Δ3/2 states. From rotational analysis excited state bond lengths have been measured for three of the four excited states, and the ground state has been unambiguously determined as a 2Πr state with a weighted least squares value of the ground state bond length of r0‘ = 3.1479± 0.0010 Å. The ionization energy of the molecule has also been directly determined as 5.072±0.028 eV. Ab initio calculations for the potential energy curves of seven low‐lying states of AlCa [X 2Πr, 2Σ+, 4Σ−, 4Πr, 2Πr(2), 2Δ, and 2Σ−] and for the X 1Σ+ ground electronic state of AlCa+ have been carried out. In agreement with experiment, 2Πr is calculated to be the ground electronic state of the neutral molecule. The dissociation energies of AlCa (X 2Πr) into Al(3s23p1,2P0)+Ca(4s2,1S) and for AlCa+ (X 1Σ+) into Al+(3s2,1S)+Ca(4s2,1S) are calculated to be 0.47 and 1.50 eV, respectively. The excited 2Σ+, 4Σ−, 4Πr, 2Πr(2), 2Δ, and 2Σ− states are calculated to lie 0.2, 0.7, 0.7, 1.1, 1.1, and 1.1 eV above X 2Πr, respectively, and the vertical and adiabatic ionization energies of AlCa have been calculated to be 5.03 and 4.97 eV, respectively.