Intramolecular vibrational redistribution spectroscopy

In order to define how the nuclei move as a reaction progresses from reactants to transition structure to products, one must choose a definition of how a reaction occurs. There are two such definitions in common use. One definition is the minimum energy path (MEP), which defines a reaction coordinate in which the absolute minimum amount of energy is necessary to reach each point on the coordinate. A second definition is a dynamical description of how molecules undergo intramolecular vibrational redistribution until the vibrational motion occurs in a direction that leads to a reaction. The MEP definition is an intuitive description of the reaction steps. The dynamical description more closely describes the true behavior molecules as seen with femtosecond spectroscopy. 

Intramolecular vibrational redistribution (IVR) is an important nonradiative process in an isolated large molecule, and it is being extensively studied experimentally and theoretically. Especially, IVR in electronically excited state has been studied by various experimental means such as fluorescence excitation, dispersed fluorescence spectroscopies and the measurement of fluorescence life-time. As a result, our information on IVR for electronically excited states is now considerably accumulated. On the other hand, the study on IVR in electronically ground-state is very few. This is due to lack of suitable experimental means. The study by infrared radiation is an orthodox way. However, because of poor time response of the infrared detection and of severe selection rule for infrared absorption, the IVR study of a ground state molecule by infrared light is greatly restricted. Instead of direct vibrational excitation by... 

Callegari A, Rebstein J, Muenter J S, Jost R and Rizzo T R 1999 The spectroscopy and intramolecular vibrational energy redistribution dynamics of HOCI in the u(OH) = 6 region, probed by infrared-visible double resonance overtone excitation J. Chem. Phys. 111 123-33... 

Boyarkin O V and Rizzo T R 1996 Secondary time scales of intramolecular vibrational energy redistribution in CFgH studied by vibrational overtone spectroscopy J. Chem. Phys. 105 6285-92... 

An alternative spectroscopic approach would be to determine the rotational temperature from the intensities in a rotationally resolved vibrational or electronic spectrum. In this case, even if one cannot resolve individual rovihrational transitions, one can still estimate the temperature by simulating the rotational contour of an individual vibronic band [120]. For this, one needs to know the rotational constants of the molecule and the direction of the transition moment however, even rough estimates of these quantities can lead to a reasonable temperature estimate. In measuring either Doppler widths or rotational band contours, the linewidth one obtains may contain a contribution from the finite lifetime of the molecule, determined by its intramolecular vibrational energy redistribution and/or dissociation rate if some type of photofragment spectroscopy is used, and this can make the temperature appear to be higher than it really is. 

Gatti F, lung C (2009) Intramolecular vibrational energy redistribution and infrared spectroscopy. In Meyer H-D, Gatti F, Worth GA (eds) Multidimensional quantum dynamics MCTDH theory and applications. WUey, Weinheim, pp 275—291... 

---