Excited molecules chemical dissociation

It is also possible for an excited molecule to be formed in a photochemical primary step. It could then also undergo subsequent physical transformations. In Figure 3, the excited molecule A dissociates to give two species, one a stable product, B, and the other an excited molecule, G, in the photochemical primary step 35. then may follow any of the paths b, c, and d, of which b and d are physical and c is chemical. These three steps are secondary steps, but not of the usual kind. Although examples of this behavior are rare, they may perhaps not remain so. It is, however, difficult to distinguish this situation from the case that G is first formed and then excited by energy transfer. 

How are those radicals formed In a liquid that is exposed to ionizing radiation the formation of radicals is preceded by a number of rather complex steps. Basically, in the primary steps that follow the absorption of ionizing radiation the molecules of the absorbent become ionized or electronically excited. In nonpolar liquids such as alkanes, the charge neutralization processes are fast and therefore the lifetime of the charged species, molecular cations (RH ") and electrons, is very short. The directly formed excited molecules (RH ) or those that are created in the neutralization processes (RH ) can lose their energy in processes such as radiative and nonradiative conversion to the ground state, collisional deactivation, etc. These processes do not result in a chemical change in the system. Alternatively, these excited molecules can dissociate into molecular products and free radicals. This latter chain of events, that leads to the formation of radicals, is summarized by reactions 2-11. 

In chemiluminescence, some of the chemical reaction products developed remain in an excited state and radiate light when the excitation is discharged. This is particularly so at low pressures, when the collision frequency is low the excitation is discharged as light radiation. The extra energy bound to the excited molecule can discharge through impact or molecular dissociation. 

In very pure hydrogen, there can be hardly any permanent chemical change produced by irradiation. However, the ion-molecule reaction (5.1) does occur in the mass spectrometer, and it is believed to be important in radiolysis. The H2 molecule can exist in the ortho (nuclear spin parallel) or para (antiparallel) states. At ordinary temperatures, equilibrium should favor the ortho state by 3 1. However, the rate of equilibration is slow in the absence of catalysts but can be affected by irradiation. Initially, an H atom is produced either by the reaction (5.1) or by the dissociation of an excited molecule. This is followed by the chain reaction (H. Eyring et al, 1936)... 

It should be stressed that the reversible chemical reactions give us better chance to observe many-particle effects since there is no need here to monitor vanishing particle concentrations over many orders of magnitude. Indeed, the fluctuation-controlled law of the approach to the reaction equilibrium similar to (2.1.61) was observed recently experimentally [85] for the pseudo-first-order reaction A + B AB of laser-excited ROH dye molecules which dissociate in the excited state to create a geminate proton-excited anion pair. The solvated proton is attracted to the anion and recombines with it reversibly. After several dissociation-association cycles it finally diffuses to long distances and further recombination becomes unobservable. 

Abstract Photochemistry is concerned with the interaction between light and matter. The present chapter outlines the basic concepts of photochemistry in order to provide a foundation for the various aspects of environmental photochemistry explored later in the book. Electronically excited states are produced by the absorption of radiation in the visible and ultraviolet regions of the spectrum. The excited states that can be produced depend on the electronic structure of the absorbing species. Excited molecules can suffer a variety of fates together, these fates make up the various aspects of photochemistry. They include dissociation, ionization and isomerization emission of luminescent radiation as fluorescence or phosphorescence and transfer of energy by intramolecular processes to generate electronic states different from those first excited, or by intermo-lecular processes to produce electronically excited states of molecules chemically different from those in which the absorption first occurred. Each of these processes is described in the chapter, and the ideas of quantum yields and photonic efficiencies are introduced to provide a quantitative expression of their relative contributions. 

Polanyi and his co-workers have observed infrared chemiluminescence from vibrationally excited molecules formed in simple chemical reactions. In some cases [101-103], excitation was thought to occur in recombination reactions. The highest vibrational level observed in these experiments was always considerably below the dissociation energy of the excited molecule, but no firm conclusion can be drawn from this fact because there is little doubt that the observed distribution was considerably relaxed from the first stabilizing collisions. 

In some physical chemistry texts, the primary photochemical process is incorrectly considered to be no more than the absorption of radiation. Such a definition is not acceptable because absorption is not a chemical transformation and, more important, because it does not correspond to current usage in photochemistry. These texts then label such diverse processes as fluorescence, dissociation of an excited molecule, and chain reactions, all as different types of secondary photochemical processes. This is also unacceptable because "secondary" has come to have a specific meaning (as is discussed in Section III.A.3) which does not apply to all of these transformations, and also because some of them are not chemical. Photochemists instead use primary and secondary in the original sense of Bodenstein. 

In another physical chemistry book, all of the elementary steps that involve the excited molecule are called "primary photochemical processes." Vibrational relaxation, fluorescence, energy transfer, isomerization, and dissociation are all considered to be different types of primary photochemical processes. But the first three listed do not involve any chemical change and cannot... 

The dissociation of weakly bound van der Waals complexes is a special case of unimolecular dissociation [20]. Because of the exceedingly weak coupling between the dissociation coordinate and the mode (or modes) initially excited, and the very low density of states of the energized complex, narrow resonances are the dominant features of van der Waals spectra. There are, of course, many similarities between the dynamics of physically bound and chemically bound molecules. The dissociation dynamics of these special molecules (or clusters) has been studied in great detail, both experimentally and theoretically. Exhaustive review articles are available [85-89] and therefore van der Waals molecules will not be particularly considered in this chapter. However, one must keep in mind that, as the density of states of van der Waals molecules increases, their dynamics becomes more and more comparable with the dynamics of strongly bound molecules [90,91]. 

Another possible process is internal conversion, radiationless transition to an electronic state of the same spin multiplicity, for example, S - 82, involving excited singlet states. This can be followed by delayed fluorescence. Chemical bonds in electronically excited molecules can also dissociate or rearrange themselves, thereby taking part in photochemical reactions. 

Chemical lasers are pumped by reactive processes, whereas in photodissociation lasers the selective excitation of certain states and the population inversion are directly related to the decomposition of an electronically excited molecule. Photolysis has been the only source of energy input employed in dissociation lasers, although it appears quite feasible to use other energy sources, e.g. electrons, to generate excited states. Table 4 lists the chemical systems where photolysis produces laser action. It is appropriate to begin the discussion of Table 4 with the alkali-metal lasers since Schawlow and Townes in 1958 35> chose the 5 f> 3 d transitions of potassium for a first numerical illustration of the feasibility of optical amplification. These historical predictions were confirmed in 1971 by the experimental demonstration of laser action in atomic potassium, rubidium and cesium (Fig. 14). 

Fig. 9. Potential energy diagram for breaking chemical bonds in an energetic molecule. The specific coordinate R shown here is identified as the reaction coordinate. In ascending energy these levels are the electronic ground state, a bound excited state and a dissociative excited state. Thermal cleavage of a bond in the electronic ground state requires a minimum energy Dq. In bound electronic states the bond dissociation energy Do is usually smaller than Do, so thermochemistry often has a lower barrier electronic excited states. Chemical bonds can also be broken by electronic excitation to predissociative or dissociative electronic states.

The fate of the excitation energy depends on the nature of the molecule and on the amount of energy is receives. The excited molecule may give off the energy as radiation (fluorescence), dissipate it by collisions (quenching), utilize the energy for chemical transformations (isomerization, dissociation, ionization, etc.), transfer all or part of the energy to other molecules that then react further (sensitization), or enter into chemical reactions directly. Several of these processes are written in Table 2-4 in the form of chemical reactions. They are considered primary processes in the sense that they all involve the excited molecule formed initially by photon absorption. 

The ability of both pulsed and cw infrared lasers to induce chemical reactions is well known. CO2 lasers are now common equipment in many laboratories. The infrared laser-induced process studied most extensively is multiplephoton excitation of molecules (using megawatt COj laser radiation) to high vibrational states from which reaction, usually dissociation, may occur. This field is the subject of intense effort by many research groups, and a number of excellent review articles have been written about multiplephoton excitation. At lower laser intensities it is possible to prepare molecules in specific initial vibrational states below the dissociation threshold and to study their subsequent bimolecular and unimolecular (isomerization) reactions. In this chapter we shall restrict ourselves to considering only the results of low-level vibrational excitation on chemical reactions. 

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-... 

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