A study of the ground and excited states of Al₃ and Al ₃ ^-. I. 488 nm anion photoelectron spectrum

The vibrationally resolved, 488 nm anion photoelectron spectrum of aluminum trimer displays transitions from two electronic states of Al₃- to four states of Al3. Franck-Condon analyses of the spectra in the independent harmonic oscillator, parallel mode approximation provide information concerning equilibrium bond length and bond angle differences among the observed states. The electron affinity of Al3 is measured to be 1.916±0.004 eV. In the X̃ A2 1′ Al₃ ground state, fundamental symmetric stretching (v₁) and bending (v₂) vibrational frequencies are 357±10 and 240±10 cm-1. In the X̃ A1 1′ Al 3- ground state, these values are 365±15 and 257±15 cm^-1, and the equilibrium bond lengths are the same as those of Al3 to within 0.02 Å. The transition between the Al 3- and Al3 ground states displays only weak activity in the bending mode, consistent with essentially D3h structures for both states. An excited B3 2 Al 3- state at 0.409±0.004 eV (T ₀) has vibrational frequencies of 330±20 (v₁) and 200±10 cm-1 (v₂). This C2v state has a 65±1° apex bond angle and its two equal bond lengths are within 0.01 Å of the ground state value. Liquid nitrogen cooling of the downstream portion of the ∼60 cm long, 0.4-0.7 Torr flow tube anion source increases the observed relative population of this excited triplet state among the sampled anions, evidently slowing its relaxation to the singlet ground state. A A2 ₂″ excited state of Al3 lies 0.192±0.004 eV above the ground state and has frequencies of 315±15 (v1) and 197±10 cm-1 (v2) and bonds 0.10±0.03 Å longer than in the ground state. A A4 2 Al3 excited state at 0.300±0.004 eV displays 315±15 (v1) and 140±10 cm-1 (v₂) vibrational frequencies. The Franck-Condon analysis of this state, which is accessed only from the 3B₂ anion, indicates a C _2v structure with a 69±2° apex bond angle and bonds 0.06±0.02 Å longer than in the ground state. A B2 2 Al3 excited state at 0.706±0.005 eV is also accessed from the 3B₂ anion. The lack of vibrational features observable over overlapping transitions indicates similar structures for the B2 2 and B3 2 states. Primary stretching force constants (mdyn/Å) are reported for the Al 3- (0.70±0.06) and Al3 (0.63±0.04) ground states and for three excited states. In the following paper, computational predictions for the ground and excited states of Al₃ ^- and Al₃ are reported and compared with these results.