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Collisional effects after selective laser excitation of polyatomic molecules

Application of multistep schemes for excitation of polyatomic molecules makes it possible to improve bond-selected excitation with light. Fast collisional energy transfer is one of the most important processes that hinder bond-selected reactions. Collisional effects after multistep laser excitation of molecules is an active area of research at present time because of both the little studied characteristics of relaxation processes for polyatomic molecules in vibrational quasi-continuum and possible practical applications. In this report, the intensities and decay rates of the time-resolved delayed fluorescence (DF) activated by several ways of multistep laser excitation of complex organic molecules (acetophenone, benzophenone, anthraquinone, fluorenone) were used to study collisional processes after nonequilbirium vibrational excitation of triplet molecules mixed with bath gases N2, CO2, NH3, H2O, C2H2, CCl4, C6H6, C5H12, many of which participate in important chemical and photochemical organic molecules transformations that occur in nature. The quantitative characteristics of collisional processes in vibrational quasicontinuum were obtained. Analysis was made of rate constant dependences for near-resonant vibration-vibration (V-V) and vibration-translation (V-T) energy transfer processes on such factors as: properties of excited molecules and bath gases; vibrational energy of excited molecules; temperature, etc. Conclusions were made that collisional efficiencies of V-V process in mixture with polyatomic bath gases were governed by long-range attractive interactions. Upper levels, initially populated following laser excitation relaxed to vibrational distribution after several collisions. Majority of the collision took place only in V-T transfer of relatively small energies. The regularities of this process reflected the dominant role of short-range repulsive forces.