Absorption-ionization-dissociation

The ionization of ammonia clusters (i.e. multiphoton ionization,33,35,43,70,71 single photon ionization,72-74 electron impact ionization,75 etc.) mainly leads to formation of protonated clusters. For some years there has been a debate about the mechanism of formation of protonated clusters under resonance-enhanced multiphoton ionization conditions, especially regarding the possible alternative sequences of absorption, dissociation, and ionization. Two alternative mechanisms63,64,76,77 have been proposed absorption-ionization-dissociation (AID) and absorption-dissociation-ionization (ADI) mechanisms see Figure 5. 

Two mechanisms have been proposed to account for the formation of protonated ammonia clusters under multiphoton resonant ionization conditions. They are absorption-ionization-dissociation (AID) (Echt et al. 1984, 1985 Shinohara and Nishi 1987 Tomoda 1986) and absorption-dissociation-ionization (ADI) (Cao... 

The alternative absorption-dissociation-ionization mechanism is expressed as ... 

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. 

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. 

Furthermore, there might seem to be a possible inconsistency between the observation of Ag made here and the energies reported in [419]. If the energy of the dissociation Agg -> AgJ -h Ag is about 2.9 eV and the ionization potential of Aga is approximately 5.7 eV, as [419] gives, then the dissociative ionization could only occur from vibrationally excited molecules. Of course the neutrals produced by photodetachment are expected to be quite vibrationally excited, since they should be nearly linear, but the energy deficit of 2.66 eV seems to be too large to make this explanation plausible. Simultaneous three-photon absorption has an energy deficit of over 2eV and no... 

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. 

Figure 1 Illustration of (A) sequential and (B) simultaneous two-photon excitation from state A to state B. Also shown in (B) are three possible fates of the excited state B fluorescence, dissociation and further photon absorption that ionizes the molecule. This latter process it termed 2+1 resonance-enhanced multiphoton ionization (REMPI).

Figure 13 shows the images of masses m/e = 42, corresponding to C3D3, at various delay times. The image of m/e = 42 was disk-like and its width did not change with the delay time. Therefore, it was totally from the dissociation of hot benzene after the ionization. Photolysis laser fluence dependence study showed that it was from one-photon absorption. 

The molecular time scale may be taken to start at 10 14 s following energy absorption (see Sect. 2.2.3). At this time, H atoms begin to vibrate and most OH in water radiolysis is formed through the ion-molecule reaction H20+ + H20 H30+ + OH. Dissociation of excited and superexcited states, including delayed ionization, also should occur in this time scale. The subexcitation electron has not yet thermalized, but it should have established a quasi-stationary spectrum its mean energy is expected to be around a few tenths of an eV. 

Excited states may be formed by (1) light absorption (photolysis) (2) direct excitation by the impact of charged particles (3) ion neutralization (4) dissociation from ionized or superexcited states and (5) energy transfer. Some of these have been alluded to in Sect. 3.2. Other mechanisms include thermal processes (flames) and chemical reaction (chemiluminescence). It is instructive to consider some of the processes generating excited states and their inverses. Figure 4.3 illustrates this following Brocklehurst (1970) luminescence (l— 2)... 

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