D1. Relaxation mechanisms in molecular systems
Chemical reactions or photophysical transformations of molecules have their origin in the primary events taking place at the nanometer scale and in the femtosecond temporal scale. These events strongly depend on the molecular dynamics that can be observed using femtosecond optical spectroscopy techniques. In figure 1 several physical mechanisms are represented in a potential in a simplified configuration space. A molecule having three electronic singlet states S0, S1, Sn, is excited with an ultrashort laser pulse with a duration of a few femtoseconds. The initial optical transition from the ground state to S1 (mechanism 1 in dark blue) contributes to excite a coherent superposition of states, i.e. a wave packet that is out of equilibrium and moves forth and back on the excited potential (mechanism 2 in red). A wave packet is also excited in the ground state (not represented). The energy and periodicity of the vibronic wave packets naturally depend on the specific molecular vibrations that are excited (stretching, bending, rotational degrees of freedom of the molecule bonds) as well as on the specific shape of the potential surfaces in the configuration space (Frank-Condon connected potentials, anharmonicity…).