6. Energy Relaxation and Photothermal Process in Solution

　　 In the field of chemistry, we are very much interested in dynamics of molecules with hoping that we can ultimately understand chemical reactions completely; the 'dynamics' include dynamical processes of molecules in space as well as energy transfer processes among various intramolecular states or intermolecular systems. The energy flow process from solute to solvent is of major importance not only in chemistry but also in other fields of science such as laser ablation in industrial technology or biology, and has been a subject of extensive researches during the last two decades. The excess energy due to the nonradiative transition between electronic states (internal conversion and/or intersystem crossing) will be first transferred to several energy-accepting vibrational modes, so that the total energy is conserved before and after the transition. The energy ultimately goes to the translational freedom (heating). This phenomenon is known as the photothermal effect and is one of the most commonly observed phenomena after the photoexcitation in condensed phase. Revealing the elementary step of this energy relaxation process has been one of the central topics in physical chemistry. The energy transfer process from the photoexcited molecule to the matrix has been studied so far by monitoring the population decay from highly excited vibrational states by the time-resolved Raman scattering, transient IR spectroscopy the hot band detection of the solvent or solute molecules, or the stimulated emission detection. A number of these results so far show that most of the vibrational relaxations of polyatomic molecules in solution are in a range of 100 ps - 10 ps. The temperature of the matrix rises because of the released energy from the highly excited vibrational states. Hence, it is reasonable to consider that the translational temperature rise also in a range of 100 ps-10 ps even if the equilibrium process in the translational freedom is extremely fast. However, by using the direct temperature detection, we showed that this consideration is not correct.
　　Compared with these rather extensive investigations of the vibrational cooling processes, studies of the heating process of the matrix, i.e., increase of the thermal energy, which is the ultimate energy-accepting mode in the condensed phase, are very rare. So far, several photothermal detection methods have been developed for measuring the temperature increase. However, although improvement on the sensitivity has been pursued extensively for a long time, it is not so much with respect to the time resolution. This inherent time response comes from the fact that most of the photothermal detection methods are using a matrix density change by the temperature increase as the source of the signal. In an effort to understand the thermalization from a point of view of the translational energy of the solvents, we developed four new methods: temperature lens, temperature grating, an acoustic peak shift method and molecular heater-molecular thermometer system. Our results showed that the temperature rise after the decay of the electronic state is quite fast in water. When the solvent motions are coupled strongly with the large energy fluctuation, that fluctuation may efficiently transfer the internal energy to the kinetic energy. Hence it is expected that the energy transfer depends on the nature of the solute-solvent interaction as well as that of the solvent-solvent interaction. The mechanism and the rate of the temperature rise were further explored in various systems by changing the matrices using the acoustic peak shift method. A new molecular integrated system: molecular heater-molecular thermometer, is described as a new trial for the study of the temporal and spatial propagation of the thermal energy from the "hot" molecules.




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