Absorption-dissociation-ionization mechanism

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

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... 

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)... 

As was previously explained for metals, during hydrogen absorption, the molecule is first dissociated on the surface of the oxide. Subsequently, the adsorbed hydrogen atoms are ionized, and are incorporated directly into the material as protons and electrons, e, through interaction with the oxide ions, and, as explained later by another mechanism, interstitially located in tetrahedral and octahedral sites. Besides, since the proton will interact with the neighboring electron density, it, consequently takes, in a certain way, the form of an hydrogen atom [33], Therefore, it is possible to consider that in this case, a neutral dissociation of hydrogen occurs as follows... 

The continuous spectrum is also present, both in physical processes and in the quantum mechanical formalism, when an atomic (molecular) state is made to interact with an external electromagnetic field of appropriate frequency and strength. In conjunction with energy shifts, the normal processes involve ionization, or electron detachment, or molecular dissociation by absorption of one or more photons, or electron tunneling. Treated as stationary systems with time-independent atom - - field Hamiltonians, these problems are equivalent to the CESE scheme of a decaying state with a complex eigenvalue. For the treatment of the related MEPs, the implementation of the CESE approach has led to the state-specific, nonperturbative many-electron, many-photon (MEMP) theory [179-190] which was presented in Section 11. Its various applications include the ab initio calculation of properties from the interaction with electric and magnetic fields, of multiphoton above threshold ionization and detachment, of analysis of path interference in the ionization by di- and tri-chromatic ac-fields, of cross-sections for double electron photoionization and photodetachment, etc. 

The primary action of light on the absorbing molecule is very essential from the standpoint of the photochemical reaction mechanism. Depending on the light frequency and the structural features of absorbing molecules, the photochemical activation can result in excitation, ionization or dissociation of the molecule. In many cases, the nature of the primary photochemical step can be evaluated from data on the absorption spectrum structure. 

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