A study of the ground and excited states of Al3 and Al3(-). II. Computational analysis of the 488 nm anion photoelectron spectrum and a reconsideration of the Al3 bond dissociation energy.

Computational results are reported for the ground and low-lying excited electronic states of Al(3)(-) and Al(3) and compared with the available spectroscopic data. In agreement with previous assignments, the six photodetachment transitions observed in the vibrationally resolved 488 nm photoelectron spectrum of Al(3)(-) are assigned as arising from the ground X (1)A(1) (')((1)A(1)) and excited (3)B(2) states of Al(3)(-) and accessing the ground X (2)A(1)(')((2)A(1)) and excited (2)A(2)(")((2)B(1)), (4)A(2), and (2)B(2) states of Al(3) (with C(2v) labels for D(3h) states in parentheses). Geometries and vibrational frequencies obtained by PBE0 hybrid density functional calculations using the 6-311+G(3d2f) basis set and energies calculated using coupled cluster theory with single and double excitations and a quasiperturbative treatment of connected triple excitations (CCSD(T)) with the aug-cc-pVxZ {x=D, T, Q} basis sets with exponential extrapolation to the complete basis set limit are in good agreement with experiment. Franck-Condon spectra calculated in the harmonic approximation, using either the Sharp-Rosenstock-Chen method which includes Duschinsky rotation or the parallel-mode Hutchisson method, also agree well with the observed spectra. Possible assignments for the higher-energy bands observed in the previously reported UV photoelectron spectra are suggested. Descriptions of the photodetachment transition between the Al(3)(-) and Al(3) ground states in terms of natural bond order (NBO) analyses and total electron density difference distributions are discussed. A reinterpretation of the vibrational structure in the resonant two-photon ionization spectrum of Al(3) is proposed, which supports its original assignment as arising from the X (2)A(1)(') ground state, giving an Al(3) bond dissociation energy, D(0)(Al(2)-Al), of 2.403+/-0.001 eV. With this reduction by 0.3 eV from the currently recommended value, the present calculated dissociation energies of Al(3), Al(3)(-), and Al(3)(+) are consistent with the experimental data.