Relaxation polyatomic

For a monatomic gas, where the heat capacity involves only translational energy, V is independent of sound oscillation frequency (except at ultra-high frequencies, where a classical visco-thermal dispersion sets in). For a relaxing polyatomic gas this is no longer so. At sound frequencies, where the period of the oscillation becomes comparable with the relaxation time for one of the forms of internal energy, the internal temperature lags behind the translational temperature throughout the compression-rarefaction cycle, and the effective values of CT and V in equation (3) become frequency dependent. This phenomenon occurs at medium ultrasonic frequencies, and is known as ultrasonic dispersion. It is accompanied by... 

For polyatomics, ordinarily only the last two tenns of equation (C3.5.6), the cubic and quartic anlrannonic tenns, need be considered [34]. In a cubic anlrannonic process, excited vibration D relaxes by interacting with two other states, say airother vibration cr aird one phonon (or alternatively two phonons). In the quartic process, D relaxes by interacting with tlrree other states, say two vibrations aird one phonon. The total rate constairt for energy loss from Q for cubic... 

Understanding VER in condensed phases has proven difficult. The experiments are hard. The stmcturally simple systems (diatomic molecules) involve complicated relaxation mechanisms. The stmctures of polyatomic molecules are obviously more complex, but polyatomic systems are tractable because the VER mechanisms are somewhat simpler. 

Fig. 11. (a) Diagram of energy levels for a polyatomic molecule. Optical transition occurs from the ground state Ag to the excited electronic state Ai. Aj, are the vibrational sublevels of the optically forbidden electronic state A2. Arrows indicate vibrational relaxation (VR) in the states Ai and Aj, and radiationless transition (RLT). (b) Crossing of the terms Ai and Aj. Reorganization energy E, is indicated. 

For polyatomic gases in porous media, however, the relaxation rate commonly decreases as the pore size decreases [18-19]. Given that the relaxation mechanism is entirely different, this result is not surprising. If collision frequency determines the Ti, then in pores whose dimensions are in the order of the typical mean free path of a gas, the additional gas-wall collisions should drastically alter the T,. For typical laboratory conditions, an increase in pressure (or collision frequency) causes a proportional lengthening of T1 so the change in T, from additional wall collisions should be a good measure of pore size. 

R. L. Armstrong 1987, (Magnetic resonance relaxation effects in polyatomic gases), Magn. Reson. Rev. 12, 91-135. 

The HF results generated for representative polyatomic molecules have used the /V-derivatives estimated by finite differences, while the -derivatives have been calculated analytically, by standard methods of quantum chemistry. We have examined the effects of the electronic and nuclear relaxations on specific charge sensitivities used in the theory of chemical reactivity, e.g., the hardness, softness, and Fukui function descriptors. New concepts of the GFFs and related softnesses, which include the effects of molecular electronic and/or nuclear relaxations, have also been introduced. 

Maier JP, Seihneier A, Laubereau A, Kaiser W (1977) Ultrashort vibrational population lifetime of large polyatomic-molecules in vapor-phase. Chem Phys Lett 46 527 Shank CV, Ippen EP, Teschke O (1977) Sub-picosecond relaxation of large organic-molecules in solution. Chem Phys Lett 45 291... 

Sarkar, N., Takeuchi, S., and Tahara, T. 1999. Vibronic relaxation of polyatomic molecule in nonpolar solvent. J. Phys. Chem. A 103 4808. 

However, the chromophoies used in SD experiments imdergo small changes in the solute intramolecular potential. Fmthermore, since they are large polyatomics with many intramolecular vibrational modes, vibrational energy relaxation is expected to be very rapid. Thus, AE = AE. In all theories and in most simulations of SD, with a few exceptions, the intramolecular contribution to AE is neglected. 

Fluorescence always occurs from the lowest singlet state even if the initial excitation is to higher energy state (Kasha s rule). Azulene and some of its derivatives are exceptions to this rule. Because of vibrational relaxation of initially excited vibronic state, the fluorescence spectrum may appear as a minor image of the absorption spectrum for large polyatomic molecules. The shape of the emission spectrum is independent of the exciting wavelength. 

These higher-order correlation functions play a large role in determining many physical properties of polyatomic systems. For example, the vibrational relaxation can, in some cases, be expressed in terms of the rotational kinetic energy autocorrelation function.27... 

It was pointed out in Section 4.2 that most polyatomic molecules show only a single relaxation process, owing to rapid intramolecular vibration-vibration transfer between modes. This corresponds to a state of affairs where Vibrational energy enters the molecule via process (a), which is rate-determining,... 

For a few polyatomic molecules, where there is a large difference between Vj and v2, the rate of the complex process (c) is much slower, and the condition P2 > P12 > Pi applies. This gives rise to a double relaxation phenomenon. Process (b) is again too slow to play any role, but process (a) is now faster than process (c). The vibrational energy of mode 2 (and any upper modes) relaxes... 

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