AB-INITIO STUDY FOR THE LOW-LYING ELECTRONIC STATES OF AL-3 AND AL-3(-) - THE PHOTOELECTRON-SPECTROSCOPY OF AL-3(-)

The low-lying electronic states of Al-3 2(1),(4)A(2),B-4(1),B-2(2),(2) A(1),B-4(2),(6)A(2)) and Al-3(-) ((1)A(1),B-3(2),(3)A(1),(3)A(2),B-3(1 ),(5)A(2)) are studied by coupled-cluster methods with a [6s5p2d1f] ba sis set. The geometries and harmonic frequencies are calculated by the coupled-cluster single double triple (CCSD(T)) correlation method wit h frozen core and virtual molecular orbitals. The energetic splittings at CCSD(T) geometries are calculated also by the CCSDT method. The ca lculated vibrational frequencies of the observed states of Al-3 ((2)A( 1), B-2(1), and (4)A(2)) and Al-3(-) ((1)A(1) and B-3(2)) are in excel lent agreement with experimental results. Other frequencies of this wo rk are expected to be correct within +/-20 cm(-1). It is shown that (4 )A(2)-B-4(1)(E-4 '') and B-2(2)-(2)A(1)(E-2') Of Al-3 as well as B-3(2 )-(3)A(1)(E-3') and (3)A(2)-B-3(1)(E-3 '') Of Al-3(-) are pairs of min ima and transition states on a potential energy surface of a pseudorot ation of the corresponding degenerate states. The vertical excitation energies of additional states of Al-3(2E',E-4',(2)A(1)') and Al-3(-)(E -1 '',E-1') are calculated by the electron-excitation equation-of-moti on CC method and the electron-attachment equation-of-motion CC method. The possible processes of ionizations and vibronic transitions are an alyzed based on the calculated results. All features of the recent pho toelectron spectroscopic study of Al-3(-) are explained consistently. It is also shown that the photoelectron signals of electron binding en ergies of 2.65 and 4.4 eV in earlier experiments correspond to the ion ization of the ground state of Al-3(-) to higher-lying excited states of Al-3. The two states of the resonant two-photon ionization experime nt are assigned to the lowest quartet state and the third quartet stat e, E-4 ''-->E-4', Without ambiguity. The anticipated features of five more electronic excitations with transition energies of 0.22, 0.69, 0. 77, 0.98, and 1.06 eV are discussed. (C) 1998 American Institute of Ph ysics.