Electronic relaxation, collisional effects

Considerable experimental effort has been aimed at elucidating the collision-free unimolecular dynamics of excited molecules. Processes of interest include the dynamics of highly excited vibrational states, which have been reached by multiphoton absorption, and the various electronic relaxation processes that can occur in electronically excited states of moderate to large molecules, etc. The idealized collision-free limit is approached either by extrapolating data to the limit of zero pressure or by performing experiments in molecular beams. Alternatively, estimates of expected collisional effects are made by using collision cross-sections that are computed from hard-sphere collision rates. These estimates are then utilized to determine whether the experiments are performed in the collision-free domain. 

A previous review provides a description of the theory of electronic relaxation in polyatomic molecules with particular emphasis on the vibronic state dependence of radiationless transition rates. A sequal review considers the general question of collisional effects on electronic relaxation, while the present one covers only the special phenomenon of collision-induced intersystem crossing. It departs from the other collisional effects review in presenting only a qualitative description of the theory the full theoretical details can be obtained from the previous review and the original papers.As a review of the basic concepts of radiationless transitions theory is necessary as a prelude to a discussion of collision-induced intersystem crossing, considerable overlap exists between this section and Section II of the previous collision effects review. However, since many concepts from radiationless transition theory, such as the nature and criteria for irreversible decay, the role of the preparation of the initial state, the occurrence of intramolecular vibrational relaxation, etc. pervade the other papers on laser chemistry in these volumes, it is useful to recall the primary results of the theory of electronic relaxation in isolated molecules and its relevance to the material in the present volume as well as to this review. 

By definition irreversible electronic relaxation processes cannot occur in isolated small and too-many level small (intermediate) case molecules because of the insufficient density of final levels. For long times the molecule senses the presence of a finite number of possible final levels instead of the effective continuum that is required to drive irreversible electron relaxation. When collisional processes are appended, it is clear that the continuous density of states of the colliding pair can provide the necessary driving force for irreversible relaxation. The observed magnitudes of electronic relaxation rates as well as dependencies on the initial state, perturbing molecules, temperature, and so on, are the aspects of the processes that are of central interest. 

In the past few years some attention has been given to collisional effects on electronic relaxation of gas-phase molecules. By now, a substantial amount of experimental results and some theoretical studies are available. 

In Section II, we describe briefly the primary collisional effects, vibrational and rotational relaxation and dephasing processes, and discuss their influence on the time evolution of an electronically excited molecular system. 

If this interpretation of the weak-field effect is correct (its confirmation in the case of other molecules would be necessary), application of weak magnetic fields in the study of collisional electronic relaxation opens new, interesting possibilities without affecting the structure of (j) levels, the weak magnetic field modifies the character of the / manifold. 

A. Tramer, A. Nitzan, Collisional effects in electronic relaxation. Adv. Chem. Phys. 47(2), 337 (1981)... 

Collisional effects on electronic relaxation processes. Adv. Chem. Phys. 42, 207. 

Observable effects in the quenching of fluorescence are usually the result of competition between radiation and bimolecular collisional deactivation of electronic energy, since vibrational relaxation is normally so rapid, especially in condensed phases, that emission derives almost entirely from the ground vibrational level of the upper electronic state. The simplest excitation-deactivation scheme, which does not allow for intramolecular radiationless... 

From inspecting the atomic database of the EIRENE code [31], which is used in many applications to a large number of different tokamaks, including for the ITER design, in particular its collisional-radiative models for molecules, it was clear that matters can be more complicated. The relaxation time for establishing a vibrational distribution of the ground state molecule is comparable to the transport time of the molecule, hence the applicability of local collisional-radiative approximations is questionable. Furthermore, one of the two atoms created in dissociative recombination is electronically excited, and, hence, can be ionized very effectively even at low divertor plasma temperatures (instead of radiative decay). In this case, the whole chain of reactions would be just an enhanced ( molecular activated ) dissociation (MAD, i.e., dissociative excitation of those H]]", which have been produced... 

E.E.Nikitin, Non-adiabatic effects in collisional vibrational relaxation of diatomic molecules, in The Physics of Electronic and Atomic Collisions, ed. J.S.Risley, R.Geball, University of Washington Press, Seattle and London, 1975... 

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