Coupled-cluster electronic spectra for the Ca+-acetylene pi complex and comparisons to its alkaline earth analogs

Equation of motion coupled-cluster (EOM-CCSD) predictions of structures and electronic excitation energies for the recently detected Ca+-acetylene pi- complex confirm three experimental state assignments, but suggest reinterpr etation of the signals associated with the (2) B-2(1) and (2) B-2(2) states that correlate to the P-2 <--S-2 Ca+ atomic transition. The originally ass igned 0(0)(0) band for the (2) B-2(1) state corresponds to the computed exc itation energy to the (2) B-2(2) state and simple reassignment is proposed. The true (2) B-2(1) state was not assigned in the original spectrum. Howev er, the computed oscillator strength is large and its optimized geometry is similar to that of the ground state. Furthermore, the experimental band te ntatively attributed to the onset of the symmetric C-H stretching progressi on of the assigned state has a relative energy conspicuously close to the c omputed electronic energy for the unassigned (2) B-2(1) state. Based on the computed energy separations of the optimized EOM-CCSD structures, reassign ment of this vibronic band to the 0(0)(0) line of the (2) B-2(1) state is p roposed. The newly assigned bands are also compared to the analogous transi tions in the beryllium-and magnesium-acetylene pi complexes. (C) 2000 Ameri can Institute of Physics. [S0021-9606(00)30326-9].