Hydrocarbon Substructures of Organic Molecules as Antennas and Amplifiers for External mid-IR Radiation

Abstract

We consider organic polyatomic molecules, which contain hydrocarbon antennasi.e.substructures of identical periodically located C–H dipoles. In these antennas collective vibrational excitations (excimols) are coherently produced by resonant absorption of photons in the mid-IR energy range. The energy of an excimol is lower than the energy of the first vibrational excited state of an isolated antenna dipole and the excimol lifetime does not exceed 10-11s. The excimols are excited successively and independently in the C–H dipoles resulting in energy accumulation in the antenna. The accumulated vibrational energy can be transmitted to a trap bond, which is also forming a dipole inside the molecule, however, not acting as an antenna. The trap bond is cleaved, if the absorbed excimol energy is equal to its dissociation energy. Under certain conditions the accumulated energy can be transformed to electronic excitation of a molecular constituent resulting in molecular luminescence. The emitted luminescence photons have energies much higher than the energy of each of the IR photons, which induce an excimol. Thus the hydrocarbon antennas in organic molecules are able to work as amplifiers of external IR radiation. These intramolecular energy transmission processes complete within a time period less or equal to the excimol lifetime and thus can be referred to as super-fast compared to typical time scales calculated by statistical models. The model presented here predicts that the probabilities of the considered processes resonantly depend on the frequency and intensity of IR radiation. The presented theoretical model is supported by the results of our experimental studies of the considered type of molecular fragmentation.