If a selected vibrational state of a polyatomic molecule is populated by a short laser pulse this population may be transfered from the initially populated state to other isoenergetic states even in the absence of collisions or any other interaction of the molecule with its surrounding. Further on, these secondary populated states may realax to other isoenergetic states an so on. This redistribution process of vibrational energy is called IVR. To investigate IVR different methods have been proposed and applied.
The impact of IVR in all of chemistry can hardly be overstated and is at the center of any first principles description of chemical reaction rates. In thermal reaction theory the redistribution of vibrational energy, initially deposed in one or a certain number of vibrational modes, has to be significantly faster than the thermal rate constants k(T), calculated by statistical methods. And in IR-multiphoton excitation experiments the possibility of a mode selective chemistry, based on a simple excitation scheme, is essentially limited by the fast redistribution of the vibrational energy even at intermediate excitation levels