Slow infrared laser dissociation of molecules in the rapid energy exchange limit

An improved model for the slow infrared laser photodissociation of large biomolecules is proposed. As has been previously shown by master equation and Monte Carlo calculations, large, laser-heated molecules reach steady-state internal energy distributions that very closely approximate Boltzmann internal energy distributions. By approximating the internal energy distribution of laser-heated molecules as a Boltzmann distribution, one can derive a very simple relationship between the relative laser power experienced by the ions and the ion temperature. This relationship can be substituted into the Arrhenius equation, yielding a new Arrhenius-like relationship for slow laser dissociation. The model presented here is a modified version of the laser dissociation model proposed by Dunbar in 1991 (J. Phys. Chem. A 2000,104, 3188-3196). Dunbar's model, while making the correct qualitative predictions, significantly underestimates the activation energy of large molecules measured by laser dissociation. The present laser dissociation model differs from the Dunbar model in that stimulated and spontaneous emission at all frequencies is included in the analysis. A significant result of this model is that there is a parametrized relationship between the laser power and the dissociation rate constant and that the parametrization for different classes of polymers can be determined computationally. This new model gives activation energies via CO2 laser dissociation that are in good agreement with activation energies measured by blackbody infrared radiative dissociation.