ABSTRACT
In the case of electron vibrational transitions in large molecules, clusters, and condensed media, lines responsible for individual electron vibrational transitions overlap each other so extensively that the entire spectrum looks as their envelop, sometimes with a discernible vibronic structure. Pekar in his seminal review presented several particular cases when the infinite series of FC factors for an optical transition band could be approximately summed within the Condon approximation. The GF formalism, introduced initially by Lax only as a formal technique facilitating the calculation of the FC profile of a vibronic band, turned out to be an exceptionally useful concept because it allows the problem of calculating the FC profile of a vibronic band to be connected with the dynamics of the nuclear subsystem after an electronic transition. A qualitative illustration of the possible behavior of the magnitude of the AF of a wave packet and the possible behavior of the corresponding vibronic spectrum in given.
