Photodissociation and photoionization

In the photochemistry of larger molecules the same physical principles apply and the sequence of events is essentially the same, although descriptions are more complex and less precise. Opposite to diatomic, the polyatomic molecules can yield a multitude of different sets of products. To establish the photodissociation mechanism, the nature of the elementary chemical process undergone by an electronically excited molecular entity (primary photoreaction) yielding primary photoproducts should be known, eg in the case of alkanes both the radical fission 

Photodissociation in the gas phase is of considerable importance in various aspects of photochemistry, especially many of the radicals, ions, and electrons present in the Earth s atmosphere which are formed as a result of these pathways [21, 22]  

In the condensed phase the close proximity of neighbouring molecules can have a considerable effect on the subsequent fate of the excited species. Solvation can reduce molecular energies, affect the selection rules, and greatly increase the number of collisions undergone by each molecule [23]. The last effect can increase the photophysical decay and decrease the photoreaction quantum yield. Moreover, the 

Photoionization can also proceed in solutions of the coordination compounds containing low-valence, readily oxidizable centres, eg V2+, Cr2, Fe2+. The process proceeds at the expense of moderate excitation energy and relies on solvated electron generation, with concomitant oxidation of the relevant metal species [24, 25] (see below). 

It was assumed in the preceding chapters that optical transitions between bonded states of molecules take place with no change in their chemical composition. Photodissociation and photoionization form a general class of photofragmentation processes in collisions between a molecule and a photon which lead to simple chemical reactions of disintegration into atomic or molecular fragments, or into ions and electrons. 

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Moseley J T 1985 Ion photofragment spectroscopy Photodissociation and Photoionization ed K P Lawley (New York Wiley)... 

Figure 12. Slice of the potential energy surfaces of vinyl chloride, showing the excitation paths for photodissociation and photoionization. (Reproduced with permission from Ref. 62, Copyright 2006 American Physical Society.)...

Balint-Kurti, G.G. and Shapiro, M. (1985). Quantum theory of molecular photodissociation, in Photodissociation and Photoionization, ed. K.P. Lawley (Wiley, New York). 

FIGURE 6 Experimental coherent phase control of the photodissociation and photoionization of HI. The three panels show the signals for the ionization of H2S (top), which is used as phase reference, the ionization of HI (middle), and the dissociation of HI (bottom). [Reproduced with permission from Fiss, J. A., Zhu, L., Gordon, R. J., and Seideman, T. (1999). Phys. Rev. Lett. 82, 65.]... 

Thus, the polarization aspects of such a wide class of photoprocesses, as discussed in the present section, namely photodissociation and photoionization, make it possible to obtain information both on the stereodynamics of the process and on the properties (for instance, symmetry types) of the states through which the transition takes place. It ought to be mentioned that photodissociation can be considered not only as a reaction of a photon with a molecule, but as a halfcollision , in which only the second stage of a collision is present, namely the departure of the products without their previous approach. In the following section we will dwell on the polarization of molecules in full collision, both reactive and non-reactive. 

Figure 6.14 Phase lag spectrum (top) for the photodissociation and photoionization of HI fftircles) and for photoionization of a mixture of HI and H2S (triangles). Bottom two panels are vtfte one- and three-photon ionization speclTa of HI. (From Fig. 7, Ref. [221].)...

TelHnghuisen, The Franck-Condon principle in bound free transitions, in K.P. Lawley (Ed.), Photodissociation and Photoionization, Willey, New York, 1985. 

Ultraviolet radiation with the very highest energy is absorbed during photodissociation and photoionization in the upper atmosphere. Because most of this harmful radiation does not reach Earth s surface, life can exist. 

Photodissociation and photoionization are important processes that absorb high-energy ultraviolet radiation in the upper atmosphere. 

Interpreting Data Why are photodissociation and photoionization reactions more common in the upper atmosphere than in the lower atmosphere ... 

Since the decomposition process of Cgo is well known and simple, it provides fundamental information on the formation of polyynes from carbon materials. We found that C2 H2 polyynes are produced under irradiation of pulse Nd YAG laser into Cgo particles suspended in hexane or methanol solution at room temperature. Laser photodissociation and photoionization of Cgo in the gas phase leading to neutral and ionic carbon clusters has been studied by O Brien et al. [21] and Lykke and Wurz [22,23]. According to their results C2-I0SS processes are major dissociation... 

Photodissociation and photoionization are two aspects of a more general process photofragmentation (Greene and Zare (1982)). Photodissociation of a diatomic molecule... 

To obtain information concerning energy disposals, product kinetic and internal energy distributions, and identification of primary product structures resulting from the UV photolysis of organosulfur pollutants, we have performed UV laser photodissociation and photoionization mass spec-trometric studies of a series of sulfur-containing species in recent years [40-45,49,50,54-61]. Ab initio calculations [49,50,55-59,65,66,69] have also been made to compare with the energetic measurements and to interpret the experimental observations. This review mostly summarizes and evaluates the results of these experimental and theoretical studies. Related results obtained by other laboratories are also discussed. 

Figure 22. Cross section view of the TOF mass spectrometer (I) photodissociation-photoionization chamber (2) photodissociation and photoionization region (3) beam-source chamber (4) pulsed valve (5) skimmer (6) TOF tube and (7) microchannel plate detector. Taken from ref. 58.

Describe the processes of photodissociation and photoionization and their role in the upper atmosphere. (Section 18.1)... 

It is convenient to divide photodissociation and photoionization processes into two broad categories, i.e. direct processes and indirect processes. In this context, we note that unimolecular processes are essentially aU indirect. Direct processes are characterized by the instantaneous and smooth departure of the fragments on an unbound potential energy surface (PES), without any re-collisions. Direct processes are... 

In most photodissociation and photoionization studies attention has been focused on characterizing the potential surfaces involved. However, when large molecules are considered, such as those of biological interest, the complexity of their spectra presents a... 

Once a well-characterized beam of van der Waals molecules is produced, experiments such as photodepletion, photodissociation and photoionization can be undertaken, yielding information about the electronic structure and photofiragmentation dynamics of these molecules. Essentially, four methods have been... 

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