Femtosecond time-resolved nonlinear spectroscopy

The calculation of four-wave mixing signals with emphasis on ultrafast dynamics of molecular systems constitutes the main thrust of our research. The goal is to give a description and an interpretation of experimental signals obtained in short time-resolved nonlinear spectroscopy experiments but also to analyze the influence on such signals of parameters which are characteristic of the dynamics of the studied systems. For example, dephasing processes are essential to characterize the optical properties of molecular systems and it is important to understand how ultrafast nonlinear spectroscopy can be used to describe the dephasing processes. On the other hand, we stress the time-dependent approaches which can follow the time evolution of the system. A theoretical approach of such experiments requires a model for the studied system and an analysis of the nonlinear optical processes involved. This last point has been considered for many years and it is generally based on a perturbative description of the field-matter interaction. It is important to note that this perturbative approach is instrumental in characterizing and classifying nonlinear optical processes. The description of the physical system under investigation requires the development of methods to describe quantum dynamics in dissipative systems. They have often been represented by a few discrete levels with Markovian rate constants. Such descriptions are inadequate to take into account the short time behaviors of chromophores in condensed phase for instance. Therefore, one tries to construct the reduced dynamics of a system coupled to a dissipative environment. To this end, the stochastic model was extensively used in electronically resonant spectroscopy. But this kind of approach is inappropriate to describe vibrational transitions. While the decay time of population in electronically excited state is long compared to the dephasing of optical transitions, the decay time of vibrational excited stated states is sometimes comparable to the time scale of vibrational dephasing. Therefore, we generally use a microscopic description of the bath and of the system-bath coupling to take into account energy and phase relaxation. Thus we are not only able to describe four-wave mixing experiments involving electronic states, but also to analyse vibrational dynamics or nonadiabatic coupling in electron-transfer systems, and even to investigate photon-echo experiments or multidimensional spectroscopies in the infrared domain. Selected publications : * “Perturbative simulation of ultrafast four-wave mixing in electron-transfer systems” J.P. Lavoine, A.J. Boeglin and C. Meier Chem. Phys. Lett. 416(2005) 192 * “Nonadiabatic coupling effects on the short time signal in four-wave mixing experiments” J.P. Lavoine and A.J Boeglin J. Chem. Phys. 118(2003) 7882 * ” Influence of nonadiabatic coupling on oscillatory dephasing” J.P. Lavoine and A.J Boeglin Chem. Phys. Lett. 360(2002) 320