Electron-molecule collisions correlation

Molecules and Atoms, Electronic Correlation in (Nesbet) Molecules and Atoms, Electronically Excited, Collisions of (Muschlitz). ... 

Electrons switch between levels characterized by Ms values. Let us examine now an ensemble of n molecules, each with an unpaired electron, in a magnetic field at a given temperature. The bulk system is at constant energy but at the molecular level electrons move, molecules rotate, there are concerted atomic motions (vibrations) within the molecules and, in solution, molecular collisions. Is it possible to have information on these dynamics on a system which is at equilibrium The answer is yes, through the correlation function. The correlation function is a product of the value of any time-dependent property at time zero with the value at time t, summed up to a large number n of particles. It is a function of time. In this case the property can be the Ms value of an unpaired electron and the particles are the molecules. The correlation function has its maximum value at t = 0 since each molecule has one unpaired electron, the product of the... 

Coincidence experiments explicitly require knowledge of the time correlation between two events. Consider the example of electron impact ionization of an atom, figure Bl.10.7. A single incident electron strikes a target atom or molecule and ejects an electron from it. The incident electron is deflected by the collision and is identified as the scattered electron. Since the scattered and ejected electrons arise from the same event, there is a time correlation... 

If the rotational motion of the molecules is assumed to be entirely unhindered (e.g., by any environment or by collisions with other molecules), it is appropriate to express the time dependence of each of the dipole time correlation functions listed above in terms of a "free rotation" model. For example, when dealing with diatomic molecules, the electronic-vibrational-rotational C(t) appropriate to a specific electronic-vibrational transition becomes ... 

In symmetric complexes where the excited electronic levels are high in energy, for S > 1/2, the most efficient electron relaxation mechanism seems to be due to the modulation of transient ZFS with a correlation time independent of xr. As already seen, this time is ascribed to the correlation time for the collisions of the solvent molecules, responsible for the deformation of the coordination polyhedron causing transient ZFS. In complexes where a static ZFS is also present, modulation of this ZFS with a correlation time related to xr is another possible electron relaxation mechanism. 

Quantum Systems in Chemistry and Physics is a broad area of science in which scientists of different extractions and aims jointly place special emphasis on quantum theory. Several topics were presented in the sessions of the symposia, namely 1 Density matrices and density functionals 2 Electron correlation effects (many-body methods and configuration interactions) 3 Relativistic formulations 4 Valence theory (chemical bonds and bond breaking) 5 Nuclear motion (vibronic effects and flexible molecules) 6 Response theory (properties and spectra atoms and molecules in strong electric and magnetic fields) 7 Condensed matter (crystals, clusters, surfaces and interfaces) 8 Reactive collisions and chemical reactions, and 9 Computational chemistry and physics. 

The Time Dependent Processes Section uses time-dependent perturbation theory, combined with the classical electric and magnetic fields that arise due to the interaction of photons with the nuclei and electrons of a molecule, to derive expressions for the rates of transitions among atomic or molecular electronic, vibrational, and rotational states induced by photon absorption or emission. Sources of line broadening and time correlation function treatments of absorption lineshapes are briefly introduced. Finally, transitions induced by collisions rather than by electromagnetic fields are briefly treated to provide an introduction to the subject of theoretical chemical dynamics. 

The photofragmentation that occurs as a consequence of absorption of a photon is frequently viewed as a "half-collision" process (16)- The photon absorption prepares the molecule in assorted rovibrational states of an excited electronic pes and is followed by the half-collision event in which translational, vibrational, and rotational energy transfer may occur. It is the prediction of the corresponding product energy distributions and their correlation to features of the excited pes that is a major goal of theoretical efforts. In this section we summarize some of the quantum dynamical approaches that have been developed for polyatomic photodissociation. For ease of presentation we limit consideration to triatomic molecules and, further, follow in part the presentation of Heather and Light (17). 

The proton NMRD profile of an ethyleneglycol solution containing GdCb is reported in Fig. 5.53 at two different temperatures. The correlation time for electron relaxation, xv, is longer than in water. This could indicate that collisions of solvent molecules with the ion are slowed down in viscous solvents. r5o, which is related to the magnitude of the instantaneous ZFS induced by collisions, instead, does not change much. Therefore, the decrease of thermal motion as well as the... 

When more than one state correlates with the electronic states of the separated species, collisions populate the various molecular states at statistically controlled relative rates. If more than one such state is bound, then it may be stabilized in a third-order process. For complex species, the rate of predissociation of the energy-rich complex, that is, k i[Rt], depends on its dissociation energy, so the ground-state complex will survive longest and have the highest chance of being collisionally stabilized. For diatomic molecules, for example, N2, 02, and NO, dipole transitions from these excited states to the ground state are not fully allowed and the excited species are almost certainly quenched in collisions. 

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