Vibrational energy relaxation, or vibrational population relaxation, is a process in which the population distribution of molecules in quantum states of high energy level caused by an external perturbation returns to the Maxwell–Boltzmann distribution.
In solution, the process proceeds with intra- and intermolecular energy transfer.
Vibrational energy relaxation has been studied with time-resolved spectroscopy.
By the excitation of the pump pulse, the population distribution of the vibrationally excited state is made by infrared absorption or a Raman process when the molecule is in the electronic ground state.
In addition, by the electronic transition, the molecule often moves to the vibrationally excited state of the electronic excited state.