Nonlinear optical response functions for a chromophore with linear and quadratic electron–vibration coupling

Abstract

Two models for the third-order response function of a two-electronic level chromophore are investigated. The first assumes an excited state vibrational Hamiltonian whose phonon modes exhibit both linear and diagonal quadratic electron–phonon coupling. Impulsive stimulated photon echoes are calculated for this model. The second assumes linear electron–phonon coupling including vibrational relaxation and pure electronic dephasing. Impulsive two-pulse photon echo signals and their dependence on temperature and electron–phonon coupling strength are calculated for this model. The initial fast nonexponential (free-induction) decay due to all multiphonon transitions, quantum beats and the slow decay component due to the zero-phonon line (pure electronic dephasing) are identified and correlated with features of the single-site absorption spectrum whose relationship to the hole burned spectrum is well understood. Pure electronic dephasing associated with the zero-phonon line contributes to the decay of the quantum beats. This contribution may be non-negligible at high temperatures in certain systems. An application is made to the special pair absorption band of the bacterial reaction center.