A study of the ground and excited states of Al3 and Al3(-). I. 488 nm anion photoelectron spectrum.

The vibrationally resolved, 488 nm anion photoelectron spectrum of aluminum trimer displays transitions from two electronic states of Al(3)(-) to four states of Al(3). 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 Al(3) is measured to be 1.916+/-0.004 eV. In the X (2)A(1)(') Al(3) ground state, fundamental symmetric stretching (nu(1)) and bending (nu(2)) vibrational frequencies are 357+/-10 and 240+/-10 cm(-1). In the X (1)A(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 Al(3) to within 0.02 A. The transition between the Al(3)(-) and Al(3) ground states displays only weak activity in the bending mode, consistent with essentially D(3h) structures for both states. An excited (3)B(2) Al(3)(-) state at 0.409+/-0.004 eV (T(0)) has vibrational frequencies of 330+/-20 (nu(1)) and 200+/-10 cm(-1) (nu(2)). This C(2v) state has a 65+/-1 degree apex bond angle and its two equal bond lengths are within 0.01 A of the ground state value. Liquid nitrogen cooling of the downstream portion of the approximately 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 (2)A(2)(") excited state of Al(3) lies 0.192+/-0.004 eV above the ground state and has frequencies of 315+/-15 (nu(1)) and 197+/-10 cm(-1) (nu(2)) and bonds 0.10+/-0.03 A longer than in the ground state. A (4)A(2) Al(3) excited state at 0.300+/-0.004 eV displays 315+/-15 (nu(1)) and 140+/-10 cm(-1) (nu(2)) vibrational frequencies. The Franck-Condon analysis of this state, which is accessed only from the (3)B(2) anion, indicates a C(2v) structure with a 69+/-2 degree apex bond angle and bonds 0.06+/-0.02 A longer than in the ground state. A (2)B(2) Al(3) excited state at 0.706+/-0.005 eV is also accessed from the (3)B(2) anion. The lack of vibrational features observable over overlapping transitions indicates similar structures for the (2)B(2) and (3)B(2) states. Primary stretching force constants (mdyn/A) are reported for the Al(3)(-) (0.70+/-0.06) and Al(3) (0.63+/-0.04) ground states and for three excited states. In the following paper, computational predictions for the ground and excited states of Al(3)(-) and Al(3) are reported and compared with these results.