Collisional electronic relaxation processes

Collisional Electronic Relaxation Processes, a singlet excited state (S ) of an aldehyde or ketone can be electronically quenched by a collision partner molecule,... 

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. 

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. 

There exists, in the past few years an increasing interest in the influence of external (magnetic and electric) fields on the dynamics of excited molecular states. This interest is not surprising if we are reminded of the role played by this kind of studies in the development of the atomic physics. We will limit our discussion to the phenomena related to the collisional electronic relaxation application of magnetic fields in the studies of predissociation and of dephasing processes in isolated molecules will not be treated here. 

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

The Orbach-type process as well as the collisional process (inducing either ZFS, g anisotropy or hyperfine coupling modulation) are mechanisms that can provide electron relaxation independently on reorientation. Electron relaxation is certainly not modulated by reorientational motions... 

Vibrational Relaxation. Stochastic processes, including vibrational relaxation in condensed media, have been considered from a theoretical standpoint in an extensive review,502 and a further review has considered measurement of such processes also.503 Models have been presented for vibrational relaxation in diatomic liquids 504 and in condensed media,505 using a master-equation approach. An extensive development of quantum ergodic theory for relaxation processes has been published,506 and quantum resonance effects in electronic to vibrational energy transfer have been considered.507 A paper has also considered the coupling between vibrational relaxation and molecular electronic transitions.508 A theory has also been outlined for the time-resolved electronic absorption spectrum of a molecule undergoing collisional vibrational relaxation.509... 

The percent contributions of the KrF relaxation channels are shown in Fig. 10. Sixty to eighty percent of the KrF excimers can contribute to the stimulated emission as an intracavity laser flux, depending on the excitation rate. Other relaxation processes are by a slow electron, F2, Kr, and Ar. In these collisional relaxation reactions the reaction KrF + Kr forms the Kj 2F trimer, and the reaction KrF + Ar forms the ArKrF trimer. 

Henry, B.R. and Siebrand, W. (1972) Collisional relaxation processes for the n = 3 states of helium. 1. Excitation transfer by normal atoms and by electrons. /. Chem. Phys.. 56, 1072. 

A study of collisional relaxation of the electronically excited OH (A 2S+) radical has been reported by Welge, Filseth, and Davenport [229]. The excited species were formed by monochromatic photodissociation of H20 and were created in vibrationally and rotationally excited levels v, K. By monitoring the OH A - Xemission spectrum from the UV-irradiated H20 and mixtures of H20 with N2 and argon, these investigators were able to study the population of individual levels in v = 0 and 1 and to observe shifts in the population due to collisions with foreign gases. The process... 

In contrast to the relatively limited number of experimental approaches utilized to determine electron collisional information for C02 laser species, many different types of experiments have been employed in the determination of heavy particle rates as a function of temperature, for temperatures slightly below room temperature up to several thousand degrees. At room temperature, measurements have been obtained using sound absorption and/or dispersion as well as impact-tube and spectrophone techniques. High temperature rate data have been obtained primarily from shock tube experiments in which electron beam, infrared emission, schlieren, and interferometric diagnostic techniques are employed. For example, as many as 36 separate experiments have been conducted to determine the relaxation rate of the C02 bending mode in pure C02 [59]. The reader is referred to the review by Taylor and Bitterman [59] of heavy-particle processes of importance to laser applications for a detailed description and interpretation of available experimental and theoretical data. 

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. 

The efficient quenching of the atomic and molecular fluorescence by collisions has been observed in early studies of the luminescence of gaseous compounds (for a review of early work see Ref. 2). In a large number of cases these processes have been explained by the electronic-to-vibrational energy transfer, charge transfer or excited-complex (excimer or exciplex) formation. There remains, however, an important class of collisional processes corresponding to the essentially intramolecular relaxation induced (or assisted) by collisions with chemically inert partners. In such... 

The mechanism by which a vibralionally excilcd species relaxes to the nearest electronic stale involves a transfer of its excess energy to other atoms in the system through a series of collisions. As noted, this process lakes place at an enormous speed. Relaxation from one electronic stale to another can also occur by collisional ttaiisfer of energy, but the rate of this process is slow enough that relaxation by photon release is favorctl. 

The kinetics of plasma-chemical reactions of vibrationally excited molecnles is determined not only by their concentration but mostly by the fraction of highly excited molecnles able to dissociate or participate in endothermic chemical reactions. The formation of highly vibrationally excited molecules at elevated pressures is due not to direct electron impact but to collisional energy exchange called W relaxation. Most conventional resonant W processes usually imply vibrational energy exchange between molecules of the same kind, for example, N2(w = 1) + N2(w = 0) N2(w = 0) - - = 1), and are char-... 

As it can be seen from the presented plots the splash of electric current in the nanotube occurs before the saturation regime is set up. Moreover, the greater phonon relaxation time the greater difference between the peak value of the electric current and its saturation value is observed. The results of our calculations have revealed (see, for example, Fig. 2) that at Fo 1 MV/m and T 100 fs the pronounced damping oscillation of the current dependence on time is even observed. Such a behavior is mainly related to the processes responsible for the energy redistribution between the electron and phonon gases (owing to Eq. (2)). The collisional operator I does not depend on time at t = 0, and I depends on time dramatically at r — +oo). 

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