Relaxation rotational

Another, purely experimental possibility to obtain a better estimate of the friction coefficient for rotational motion in chemical reactions consists of measuring rotational relaxation times of reactants and calculating it according to equation (A3,6,35) as y. =... 

This is no longer the case when (iii) motion along the reaction patir occurs on a time scale comparable to other relaxation times of the solute or the solvent, i.e. the system is partially non-relaxed. In this situation dynamic effects have to be taken into account explicitly, such as solvent-assisted intramolecular vibrational energy redistribution (IVR) in the solute, solvent-induced electronic surface hopping, dephasing, solute-solvent energy transfer, dynamic caging, rotational relaxation, or solvent dielectric and momentum relaxation. 

The relation between the microscopic friction acting on a molecule during its motion in a solvent enviromnent and macroscopic bulk solvent viscosity is a key problem affecting the rates of many reactions in condensed phase. The sequence of steps leading from friction to diflfiision coefficient to viscosity is based on the general validity of the Stokes-Einstein relation and the concept of describing friction by hydrodynamic as opposed to microscopic models involving local solvent structure. In the hydrodynamic limit the effect of solvent friction on, for example, rotational relaxation times of a solute molecule is [ ]... 

Figure A3.6.6. Viscosity dependence of rotational relaxation times of (ran.s -stilbene hr ethane (open circles) andn-octane (fiill circles) [89].

Dote J L, Kivelson D and Schwartz R N 1981 A molecular quasi-hydrodynamic free-space model for molecular rotational relaxation J. Phys. Chem. 85 2169-80... 

Spectroscopic detemiination of the HE rotational distribution is another story. In both the chemical laser and infrared chemiluminescence experiments, rotational relaxation due to collisions is faster or at least comparable to the time scale of the measurements, so that accurate detemiination of the nascent rotational distribution was not feasible. However, Nesbitt [40, 41] has recently carried out direct infrared absorption experiments on the HE product under single-collision conditions, thereby obtaining a fiill vibration-rotation distribution for the nascent products. 

FigureBl.5.16 Rotational relaxation of Coumarin 314 molecules at the air/water interface. The change in the SFI signal is recorded as a fimction of the time delay between the pump and probe pulses. Anisotropy in the orientational distribution is created by linearly polarized pump radiation in two orthogonal directions in the surface. (After [90].)...

Castro A, Sitzmann E V, Zhang D and Eisenthal K B 1991 Rotational relaxation at the air-water interface by time-resolved second-harmonic generation J. Phys. Chem. 95 6752-3... 

McClung RED 1996 Spin-rotation relaxation theory Encyclopedia of Nuclear Magnetic Resonance ed D M Grant and R K Harris (Chichester Wiley) pp 4530-5... 

Weitz E and Flynn G W 1974 Laser studies of vibrational and rotational relaxation in small moleoules Annu. Rev. Rhys. Chem. 25 275-315... 

Depending on the method of pumping, the population of may be achieved by — Sq or S2 — Sq absorption processes, labelled 1 and 2 in Figure 9.18, or both. Following either process collisional relaxation to the lower vibrational levels of is rapid by process 3 or 4 for example the vibrational-rotational relaxation of process 3 takes of the order of 10 ps. Following relaxation the distribution among the levels of is that corresponding to thermal equilibrium, that is, there is a Boltzmann population (Equation 2.11). 

Quantum well interface roughness Carrier or doping density Electron temperature Rotational relaxation times Viscosity Relative quantity Molecular weight Polymer conformation Radiative efficiency Surface damage Excited state lifetime Impurity or defect concentration... 

The perturbation theory presented in Chapter 2 implies that orientational relaxation is slower than rotational relaxation and considers the angular displacement during a free rotation to be a small parameter. Considering J(t) as a random time-dependent perturbation, it describes the orientational relaxation as a molecular response to it. Frequent and small chaotic turns constitute the rotational diffusion which is shown to be an equivalent representation of the process. The turns may proceed via free paths or via sudden jumps from one orientation to another. The phenomenological picture of rotational diffusion is compatible with both... 

Storer model used in this theory enables us to describe classically the spectral collapse of the Q-branch for any strength of collisions. The theory generates the canonical relation between the width of the Raman spectrum and the rate of rotational relaxation measured by NMR or acoustic methods. At medium pressures the impact theory overlaps with the non-model perturbation theory which extends the relation to the region where the binary approximation is invalid. The employment of this relation has become a routine procedure which puts in order numerous experimental data from different methods. At low densities it permits us to estimate, roughly, the strength of collisions. 

For strong collisions (y = 0), is still equal to //to, but qj does not exist and the diffusion mechanism of rotational relaxation is replaced by a hopping mechanism. 

It is clear that J-diffusion is a good approximation for rotational relaxation as a whole, if the centre of equilibrium distribution over J is within the limits of non-adiabatic theory. In the opposite case m-diffusion is preferable. Consequently, the J-diffusion model is applicable, if the following inequality holds ... 

Here p is the radius of the effective cross-section, (v) is the average velocity of colliding particles, and p is their reduced mass. When rotational relaxation of heavy molecules in a solution of light particles is considered, the above criterion is well satisfied. In the opposite case the situation is quite different. Even if the relaxation is induced by collisions of similar particles (as in a one-component system), the fraction of molecules which remain adiabatically isolated from the heat reservoir is fairly large. For such molecules energy relaxation is much slower than that of angular momentum, i.e. xe/xj > 1. 

Markovian perturbation theory as well as impact theory describe solely the exponential asymptotic behaviour of rotational relaxation. However, it makes no difference to this theory whether the interaction with a medium is a sequence of pair collisions or a weak collective perturbation. Being binary, the impact theory holds when collisions are well separated (tc < to) while the perturbation theory is broader. If it is valid, a new collision may start before the preceding one has been completed when To < Tc TJ = t0/(1 - y). 

Inequality (1.88) defines the domain where rotational relaxation is quasi-exponential either due to the impact nature of the perturbation or because of its weakness. Beyond the limits of this domain, relaxation is quasi-periodic, and t loses its meaning as the parameter for exponential asymptotic behaviour. The point is that, for k > 1/4, Eq. (1.78) and Eq. (1.80) reduce to the following ... 

The qualitative difference between low-density and high-density rotational relaxation is clearly reflected in the Fourier transform of the normalized angular momentum correlation function ... 

It is clear that the high-density spectra of rotational relaxation g(co) have two maxima and these occur when... 

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