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 Al₃^- and Al₃ 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 Al₃^- are assigned as arising from the ground X ¹A₁^′(¹A₁) and excited ³B₂ states of Al₃^- and accessing the ground X ²A₁^′(²A₁) and excited ²A₂^″(²B₁), ⁴A₂, and ²B₂ states of Al₃ (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 Al₃^- and Al₃ 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 Al₃ is proposed, which supports its original assignment as arising from the X ²A₁^′ ground state, giving an Al₃ bond dissociation energy, D₀(Al₂–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 Al₃, Al₃^-, and Al₃⁺ are consistent with the experimental data. [ABSTRACT FROM AUTHOR]