Collision effect electronic transition

Dagdigian (1997) reviewed state-resolved studies of collision-induced electronic transitions carried out up to 1997, with a particular emphasis on understanding the role of spectroscopic perturbations in enabling this process. Three different types of collision experiments have been carried out (1) those in which the initial vibronic level is selectively populated, (2) those in which the final vi-bronic level is selectively detected, and (3) those in which both the initial and final levels are resolved. The third type of experiment is the most informative in revealing the importance of spectroscopic doorways, but is the most demanding. Such experiments usually employ the OODR technique, and appropriate optical transitions must be available to prepare (populate or label) and detect the initial and final levels. The role of spectroscopic perturbations can be revealed in experiments which define only the initial or final level if there is a strong doorway effect in the system. 

Intermolecular forces involving sulfur hexafluoride molecules have been discussed in several papers (91, 121, 122, 194, 350, 296). Other studies include (a) molecular volume (254), (b) stopping of alpha particles (16,117), (c) transfer of energy by collision (205), (d) mutual diffusion of H2 and SF6 (291), (e) mutual solubilities of gases, including SF , in water (197), (f) salting out of dissolved gases (219), (g) compressibility (193) (h) Faraday effect (161), (i) adsorption on dry lyophilized proteins (14), (j) effect of pressure on electronic transitions (231), (k) thermal relaxation of vibrational states (232), (1) ultraviolet spectrum (295), (m) solubility in a liquid fluorocarbon (280). 

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. 

Radiative Processes.—A large number of papers concerned with the various aspects of the electronic absorption and emission processes in atomic species have appeared. Of general interest is a paper which presents an expression suitable for the evaluation of the infinite sum describing absorption due to a hydrogenic series of Lorenzian lines.444 The Doppler lineshape in atomic transitions,448 collision effects on lineshapes of atomic transitions,44 the effect of metastable dimers on the radiative transition in pairs of atoms 447 and in donor-acceptor pairs,448 and other aspects of excitation transfer in two-atom systems,449- 480... 

The target molecule can be considered in terms of the isolated electron involved in the transition, charge q, and a charge, q", determined by the dipole moment of the molecule, Pq, and its orientation with respect to the electron projectile. The effective charge in the collision is given by = q + where. 

If, however, one replaces the H2 molecules by deuterium hydride (HD), new effects appear. While the electronic structure of HD still does not differ much from those of H2 or D2, a small, permanent dipole moment exists in the case of HD which gives rise to the allowed transitions with J — J + 1, the so-called Rq(J) lines. In Fig. 3.19, three such lines with J = 0... 2 are clearly discernible. These are superimposed with a collision-induced background, the So(0) line of HD, that peaks around 280 cm-1. Although this is not obvious from the figure, a detailed analysis shows an interference between allowed and induced lines that will concern us below. 

Suppose that absorption promotes a molecule from the ground electronic state, S0, to a vibra-tionally and rotationally excited level of the excited electronic state S, (Figure 18-13). Usually, the first process after absorption is vibrational relaxation to the lowest vibrational level of Sj. In this radiationless transition, labeled R, in Figure 18-13, vibrational energy is transferred to other molecules (solvent, for example) through collisions, not by emission of a photon. The net effect is to convert part of the energy of the absorbed photon into heat spread through the entire medium. 

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