The intramolecular energy distribution of infrared multiphoton excited molecules


Polyatomic molecules in the electronic ground state can absorb a large number of infrared photons from a resonant high power infrared laser. For sufficiently high laser power, most molecules will even reach the dissociation limit. When this phenomenon was discovered in 1973 it was hoped that infrared multiphoton excitation would lead to the realization of 'bond-selective' laser- controlled photochemistry. Despite the selectivity of infrared excitation at low energy, however, at high excitation vibrational energy is no longer confined to the pump mode because of the interaction between vibrational modes. This thesis explores the intramolecular dynamics of infrared multiphoton excited molecules. Time-resolved spontaneous and coherent anti-Stokes Raman spectroscopy was employed to measure the energy distribution among vibrational modes immediately following infrared excitation. Both cases of intramolecular equilibrium below the dissociation threshold, as well as cases of nonequilibrium close to dissociation were found. The results are consistent with nonlinear dynamics theory.