Theory and Control of Photo-excited Polyatomic Reactions

Abstract

An important process in nature is the photo-excitation of a polyatomic molecule, which initially is at the local surface temperature of the earth. Before excitation, the internal energy distribution of the molecule is the equilibrium Boltzmann distribution. The fate of this distribution after photo-excitation of the molecule from a ground electronic state (S₀) to an excited electronic state (S₁) has been rather ignored. The nature of the nascent vibrational distribution after photo-excitation is the topic of this paper. Our work in recent years has shown that the photo-excitation process can lead to a significant change in the vibrational population of the molecule in the excited state1,2. Within the Condon approximation, if the excitation wavelength is to the blue of the transition frequency from the ground vibrational state of the ground electronic state, to the ground vibrational state in the excited electronic state (ω₀₀), then the molecule is usually heated. Interestingly, at the ω₀₀ transition frequency or somewhat to the red of it one may expect under rather general conditions² that the nascent distribution will be cooled. The cooling effect is predicted to be generic for polyatomic molecules² and is caused by the lowering of vibrational frequencies in the excited electronic surface (in general this lowering reflects the weakening of the chemical bonds due to the electronic excitation).