Theoretical and experimental study of the A2Πu-X2Πg band system of C7-

The A(2)Piu-X(2)Pig electrdnic system of gaseous C-7(-) is examined experimentally 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 and 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) and (1)(0) transitions were recorded using photodetachment spectroscopy. These spectra revealed the spin-orbit component bands for each transition as well as providing band contourswhich show partially resolved rotational structure. The experimental spectra are compared to simulations based upon the calculated spectroscopic constants and the possible causes of the main features in the band contours are accessed by least-squares fitting of the profiles for the 0(0)(0) and 1(0)(1) transitions. The implications for the recent 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 American Institute of Physics. [S0021-9606(00)01945-0].