Intramolecular relaxation processes

With the development of shorter and shorter laser pulses, the inter- and intramolecular relaxation processes in the nano- and picosecond windows have been progressively resolved in real time down to the vibrational and reactive dynamics in the femtosecond window [1]. These observations have... 

The internal viscosity of the macromolecule is a consequence of the intramolecular relaxation processes occurring on the deformation of the macromolecule at a finite rate. The very introduction of the internal viscosity is possible only insofar as the deformation times are large, compared with the relaxation times of the intramolecular processes. If the deformation frequencies are of the same order of magnitude as the reciprocal of the relaxation time, these relaxation processes must be taken explicitly into account and the internal viscosity force have to be written, instead of (2.26) as... 

The most important point to note is that the electron-transfer process can be purely electronic. There are no nuclear motions required to sample the continuum states such that there is no fundamental limit (apart from defining the initial state) to the electron-transfer dynamics. This finding is in contrast to electron transfer to localised molecular states (Section 2.3.5). In this latter case, the solvent or intramolecular relaxation process dictates the rate of passage into the product channel. 

Of fundamental importance for a proper understanding of unimolecular reactions is the difference between small molecule and large molecule behaviour [81.S3] in the sense that these terms are used in the discussion of intramolecular relaxation processes this classiHcation into small and large is closely parallel to our division into weak and strong... 

In some nematics, e.g., PCB (4-cyano-4 -pentylbi-phenyl), the natural absorption is quite small at the second harmonics of the Nd YAG laser. To enhance the absorption constant and thus the nonlinearity, it is possible to dissolve some dyes that absorb at around 0.53 /xm, e.g., rhodamine 6G. Experiments have been conducted [13], [17] in nematic films with traces of dissolved dyes which showed that the required laser energies for several nonlinear processes based on the thermal effect were drastically reduced. A detailed study of the rise portion of the He-Ne diffraction also shows that the intramolecular relaxation processes that transfer the excitation from the dye to the liquid crystal take place probably at much shorter time scales than the laser pulse length, as the diffraction from a dyed sample has exactly the same rise time as the undyed sample. 

The laser-induced fluorescence (LIF) of molecules can be used as a powerful probe of the photodynamics of the optically prepared molecule participating in intramolecular relaxation processes and in intermolecular processes. Fluorescence depolarization (and its ground state analogue, transient dichroism) offers a versatile probe of the orientational relaxation of the excited molecules and has found much application in studies of slip and stick boundary conditions and in tests of hydrodynamic descriptions (17,23-26). 

The DD relaxation mechanism is combined of both intramolecular and intermolecular processes. A detailed account of the derivation of Ti from the various intermolecular and intramolecular dipole-dipole interactions has been described by Bloembergen et al. (30). The intramolecular relaxation process is governed by the angular reorientation of the vector connecting the spin one-half (5) nuclei—in this case either H or in benzene or perfluorobenzene. The relaxation rate (1/Ti) is... 

The situation with molecules is much more involved for several reasons. First, for polyatomic molecules, the intramolecular relaxation processes that occur on a subpicosecond timescale are essential. It was for exactly this reason that the first successful experiments were conducted on the noncoherent laser control of polyatomic molecules with intermolecular selectivity. Second, the phase relaxation time in a condensed medium is also on a subpicosecond scale because of the interaction between the quantum system and its surroundings. Therefore, it was only the creation of relatively simple and inexpensive femtosecond lasers that made it possible to set about realizing the ideas of the coherent laser control of unimolecular processes (Tannor and Rice 1985 Brumer and Shapiro 1986 Judson and Rabitz 1992), particularly the... 

Data illustrating the intramolecular relaxation processes of the Sj states of thiones (Scheme 1.1) in perfluoroaUcane solvents are given in Table 1.2. The purely photophysical channel for Sj relaxation is vibronically induced Sj-Sj internal conversion. Direct Sj - Sq internal conversion can be neglected due to its much larger electronic energy gap (AE(S2 - Sq) = 20,000 to 25,000 cm ) compared with that for Sj - Sj (ca. 7,000 to 11,000 cm ) [50,58,60]. The photochemical channel contains two reversible H-abstraction processes, one of which leads to the Sj state via an Sj transient and a second one that leads directly to the ground state via a second... 

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