Theoretical and experimental study of the A(2)Pi(u)-X-2 Pi(g) band system of C-7(-)

The A(2)Piu-X(2)Pig electrdnic system of gaseous C-7(-) is examined experim entally in the light-of theoretical predictions. Ab initio calculations at the RHF, RCCSD(T) and MRCI levels using the aug-cc-pVQZ basis set indicate that the transition is accompanied by a small elongation in the molecule an d a significant reduction in the spin-orbit coupling constant. On the basis of these predictions the band profiles of the 0(0)(0), 1(0)(1), 2(0)(1) an d (1)(0) transitions were recorded using photodetachment spectroscopy. Thes e spectra revealed the spin-orbit component bands for each transition as we ll as providing band contourswhich show partially resolved rotational struc ture. The experimental spectra are compared to simulations based upon the c alculated spectroscopic constants and the possible causes of the main featu res in the band contours are accessed by least-squares fitting of the profi les for the 0(0)(0) and 1(0)(1) transitions. The implications for the recen t observation of coincidences between the A(2)Pi (u)-X(2)Pig vibronic bands of C-7(-) and the diffuse interstellar bands are discussed. (C) 2000 Ameri can Institute of Physics. [S0021-9606(00)01945-0].