Physical Review A
Vibronic coherence between the sequential transitions in two-photon processes induced by phase-incoherent light beams
Yoshihiro Nomura, Yuichi Fujimura
Abstract
Mechanisms of two-photon transitions of a molecular ensemble induced by stationary-phase-incoherent light beams are theoretically studied. The time evolution of the vibronic coherence created by the phase-incoherent light beams is analyzed by using the Liouville-space Feynman diagram properly taking into account vibronic dephasings. It is shown that effects of coherence between sequential transitions make a dominant contribution to the time profile of the pump-probe two-photon transition induced by the phase-incoherent light beams. The time profiles are calculated within a simple model to demonstrate differences in the time evolution of the vibronic coherence between the phase-incoherent and pulsed-coherent light spectroscopy. A theoretical consideration is also given to nonresonant two-photon fluorescence of coumarin 440 in solution excited by incoherent light beams, which shows subfemtosecond oscillation with the incident photon-field frequencies.