Investigation of intramolecular vibrational energy flow in polyatomic molecules by the femtosecond pump-probe technique

The present studies are devoted to the investigation of the intramolecular vibrational energy redistribution (IVR), the processes of transfer of energy from some initially excited vibrational state to other nearly isoenergetic states. IVR with a great variety of redistribution scenarios and the corresponding timescales is one of the most important primary processes for chemical kinetics. In the common quasi-equilibrium theories of reaction dynamics, such as transition state theory, RRKM theory (Rice-Ramsperger-Kassel-Marcus) and SACM (statistical adiabatic channel model), it is assumed that IVR is much faster than chemical reaction and thus IVR leads to quasi-equilibrium prior to reaction. However, understanding the principles of vibrational energy flow may open a path for new developments in reaction kinetics such as mode-selective chemistry. Indeed, IVR might be used as a powerful instrument for controlling chemical reactions. Different approaches for the description of the IVR processes have been used. One theoretical approach would be classical molecular dynamics on ab initio or empirical potential energy hypersurfaces. A more exact numerical treatment consists in the numerical integration of the Schrödinger equation with the molecular hamiltonian drawn from ab initio calculations. Such an approach is limited to small molecules of perhaps four to six atoms at most and the accuracy is severely limited by the limited accuracy of ab initio potentials. A more realistic time evolution in IVR can be obtained experimentally with the molecular hamiltonian derived from the analysis of high-resolution infrared spectra. Finally, statistical models are sometimes used for cases of relatively large systems when the molecular hamiltonian can not be obtained in explicit form. The relaxation dynamics can then be obtained experimentally from spectroscopic line shapes or kinetic measurements. Theoretically it can be calculated in this case from the