Journal ArticleA spectroscopic analysis of diatomic AlCa generated by laser vaporization of a 2:1 AI: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] ?II,, the [15.8] ??, the [17.0] ??3/2(?), and the [ 17.61 2?3/2 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 ?II, state with a weighted least squares value of the ground state bond length of r"0 = 3.1479 ? 0.00 10 A. The ionization energy of the molecule has also been directly determined as 5.072?0.028 eV. Ab inirio calculations for the potential energy curves of seven low-lying states of AlCa [X ?IIr, ??+, 4?-, 4IIr, ?II,(2), ??, and X l?+ and for the X 1?+ ground electronic state of AlCa+ have been carried out. In agreement with experiment, ?II, is calculated to be the ground electronic state of the neutral molecule. The dissociation energies of AlCa (X ?II,) into Al(3s?3p?,?Po) +Ca(4s?,?S) and for AlCa+ (X ??+) into A1+(3s?,?S) +Ca(4s?,?S) are calculated to be 0.47 and 1.50 eV, respectively. The excited ??+, 4?-, 4II,, 2II(2), ??, and ??+ states are calculated to lie 0.2, 0.7, 0.7, 1.1, 1.1, and 1.1 eV above X ?IIr,, respectively, and the vertical and adiabatic ionization energies of AlCa have been calculated to be 5.03 and 4.97 eV, respectively
