Changes in Dipole Moments and Polarization of a Two-Level System with Intramolecular Coupling #

Abstract

Motivation. Polyatomic molecules can be described by a two–level system represented by two crossed harmonic potential curves (including vibrational levels) in order to study their nonlinear signal response. The variation of the coupling parameter has an important influence on the behavior of the dipole moments and, consequently, on the polarization on the new coupled–state basis. Method. The calculation method employed is based on the Liouville formalism for the density matrix, where the conventional optical Bloch equations are solved in Fourier space to obtain the density matrix elements associated to the polarization of the system. Changes in the coupling parameters V0 (energetic difference between the minima of the potential curves), S (height to which the vibronic coupling occurs) and v (coupling parameter) produce modifications in the behavior of the dipole moments and Polarization on the coupled system. In this communication, two studies are presented: (a) variation of the parameters S and v, while keeping V0 constant and, (b) variation of S and V0, while keeping v constant. In order to study the effects of these changes, the rotating–wave approximation is neglected, which permits us to analyze the processes that occur out of the resonance region. Moreover, the permanent dipole moments of the states in the uncoupled basis are included. Results. First case (variation of S and v): an important increment of dipole moments and polarization magnitude values were observed for low values of v and high values of S. Second case (variation of S and V0): an abruptly increment in the polarization magnitude were found at high values of S and low values of V0. Conclusions. The observed changes in the nonlinear response are associated to modifications in the overlap integral and consequently to the dipole moments of the states in the new coupled basis.