Photochemical process

A number of TRMS studies focus on photochemical reactions. While most of them can be regarded as fundamental studies, they have implications for atmospheric chemistry. 

Over the past three decades the combination of pulse (e.g., femtosecond) lasers with MS has attracted considerable attention (for an overview, see [20]). TRMS has found applications in the studies of desorption processes induced by energetic light beams. As an illustration. Van Breemen et al. [21] investigated desorption of non-volatile organic salts following a laser pulse. The desorbed species were further analyzed by time-of-flight mass spectrometry (TOF-MS). von der Linde and Danielzik [22] demonstrated MS detection in 

Phenomenologically, we can distinguish the following photochemical reactions photoisomerization, photocyclization (photoaddition), photocleavage, hydrogen abstraction, photo-concerted reaction, etc. For a photochemical reaction to occur, efficient absorption of ultraviolet or visible light is necessary, thus the photoreactive molecules should contain in their structure one of the bond types listed in Table 1.13. The characteristics of the typical photochemical reactions of C = C and C = O groups are summarized as follows. 

n ) transition homolysis (free-radical type) photocyclization (addition), etc. 

The photochemical processes of excited benzophenone are listed in Table 1.8 to illustrate typical photochemical reactions of the carbonyl group. 

A detailed explanation of each reaction path is not given here, but the reader should understand that the type of reactions can be estimated to some extent by considering the localized site of the excited electron. We have shown in Section 1.4.2 that the transition probability of an electronic system can be determined from theoretical considerations. Therefore, by controlling the radiation field it becomes possible to select photochemical reactions and to steer their outcome. 

Numerous examples of primary photochemical processes are known, such as the following. 

These reactions are likely to play an important role in the photo-oxidation of polymers. 

This type of reaction could be responsible for the initiation of the photodegradation of polyvinylchloride  

Such a reaction is involved in the photodegradation of many partly oxidized polymers. 

The first two processes occur with quantum yields very close to unity, probably through a dissociative excited state. The photolysis of ketones is more complex and it is suggested that the Norrish type I split partly occurs through excitation above the dissociation limit of an upper state. 

The ESR method is useful in two aspects for studies of photochemical processes involving paramagnetic species the qualitative mechanistic aspect and the quantitative rate measurement. For the mechanistic approach we have seen from Section IV how transient radicals can be observed during photolysis. The major disadvantages are (a) It is not practical to determine accurately the quantum efficiency of radical production. Thus the radical process evidenced by ESR can well have been a very minor photochemical process, due to the high sensitivity of the modern ESR spectrometer. (b) In static 

Molecule % o2 to ( ) converted °2 (lA) Triplet-State 3 Energy Level (ev) Pho sphor es cence 3 Lifetime (sec) 77°K 

As stated earlier, chemical reactions in the atmosphere are initiated mainly by photochemical processes rather than by thermal activation. It becomes necessary, therefore, to supplement the preceding survey of thermal reactions with a brief account of the principles of photochemistry. 

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. 

Any subsequent thermal chemical reactions or physical energy dissipation processes are of a secondary nature. Specific examples for both primary and secondary photochemical reactions will be given in later chapters. 

The quantitative assessment of photochemical activity is facilitated by introducing the quantum yield. In the practice of photochemistry a variety of quantum yield definitions are in use depending on the type of application. There are quantum yields for fluorescence, primary processes, and final products, among others. In atmospheric reaction models, the primary and secondary reactions usually are written down separately, so that the primary quantum yields become the most important parameters, and only these will be considered here. Referring to Table 2-4, it is evident that an individual quantum yield must be assigned to each of the primary reactions shown. The quantum yield for the formation of the product Pf in the ith primary process is the rate at which this process occurs in a given volume element divided by the rate of photon absorption within the same volume element. 

The quantum yield thus represents the probability for the occurrence of a selected process compared to the total probability for all the primary processes taken together. The sum of the individual primary quantum yields is unity. 

Collisional deactivation and energy transfer play important roles in tropospheric chemistry. For example, electronically excited S02 in the 3B, state can be deactivated by 02 (as well as by N2 and H20) to the ground ( A,) state, with part of this process occurring via triplet-triplet energy transfer to generate singlet electronically excited states of 02  

In contrast to the photo physical processes just described, photochemical processes produce new chemical species. Such processes can be characterized by the type of chemistry induced by light absorption photodissociation, intramolecular rearrangements, photoisomerization, photodimerization, hydrogen atom abstraction, and photosensitized reactions. 

Of these, photodissociation is by far the most pervasive and important in atmospheric chemistry. For example, the photodissociation of N02 into ground-state oxygen atoms, 

The reader will encounter numerous other examples of photodissociation throughout this text, so it will not be treated further here. However, as will become obvious in examining the chemistry of both the troposphere and stratosphere in later chapters, it is photochemistry that indeed drives the chemistry of the atmosphere. 

The wavelength (A) of the electromagnetic spectrum varies over 17 orders of magnitude (Table 6.1) and the frequency (v) in cycles s is expressed as a function of wavelength and velocity, c (3.0 x 10V-s ). 

Radiation also acts as a particle, a photon, whose energy, E, is expressed as a function of frequency  

Chemical Concepts in Pollutant Behavior, Second Edition, by Ian Tinsley ISBN 0-471-09525-7 2004 John Wiley Sons, Inc. 

TABLE 6.1 Approximate Wavelength Ranges for Different Forms of Electromagnetic Radiation 

The ability of a compound to absorb radiation is readily determined using a spectrophotometer that compares the light absorbed by a solution with that of the solvent, and the absorbance. A, is expressed as a function of the wavelength of the incident light. Absorbance is defined by 

---

Modem photochemistry (IR, UV or VIS) is induced by coherent or incoherent radiative excitation processes [4, 5, 6 and 7]. The first step within a photochemical process is of course a preparation step within our conceptual framework, in which time-dependent states are generated that possibly show IVR. In an ideal scenario, energy from a laser would be deposited in a spatially localized, large amplitude vibrational motion of the reacting molecular system, which would then possibly lead to the cleavage of selected chemical bonds. This is basically the central idea behind the concepts for a mode selective chemistry , introduced in the late 1970s [127], and has continuously received much attention [10, 117. 122. 128. 129. 130. 131. 132. 133. 134... 

Rice S A 1975 Some comments on the dynamics of primary photochemical processes Excited States ed E C Urn (New York Academic) pp 111-320... 

The conmron flash-lamp photolysis and often also laser-flash photolysis are based on photochemical processes that are initiated by the absorption of a photon, hv. The intensity of laser pulses can reach GW cm or even TW cm, where multiphoton processes become important. Figure B2.5.13 simnnarizes the different mechanisms of multiphoton excitation [75, 76, 112], The direct multiphoton absorption of mechanism (i) requires an odd number of photons to reach an excited atomic or molecular level in the case of strict electric dipole and parity selection rules [117],... 

The absorjDtion of a photon initiating photophysical and photochemical processes can itself be an extremely rapid... 

Knowledge of the underlying nuclear dynamics is essential for the classification and description of photochemical processes. For the study of complicated systems, molecular dynamics (MD) simulations are an essential tool, providing information on the channels open for decay or relaxation, the relative populations of these channels, and the timescales of system evolution. Simulations are particularly important in cases where the Bom-Oppenheimer (BO) approximation breaks down, and a system is able to evolve non-adiabatically, that is, in more than one electronic state. 

A number of electronic and photochemical processes occur following band gap excitation of a semiconductor. Figure 5 illustrates a sequence of photochemical and photophysical events and the possible redox reactions which might occur at the surface of the SC particle in contact with a solution. Absorption of light energy greater than or equal to the band gap of the semiconductor results in a shift of electrons from the valence band (VB) to... 

Fig. 5. Photophysical and photochemical processes in a semiconductor cluster whereand represent chemical species, adsorbed on the surface of the semiconductor particle, which are capable of undergoing reduction and oxidation at rates and respectively. The subscript...

Photochemistry. The most important photochemical processes that proceed from the excited state are geometrical isomerization and photochromic reactions. 

Pesticides can be transported away from the site of appHcation either in the atmosphere or in water. The process of volatili2ation that transfers the pesticide from the site of appHcation to the atmosphere has been discussed in detail (46). The off-site transport and deposition can be at scales ranging from local to global. Once the pesticide is in the atmosphere, it is subject to chemical and photochemical processes, wet deposition in rain or fog, and dry deposition. 

Chlorine free radicals used for the substitutioa reactioa are obtaiaed by either thermal, photochemical, or chemical means. The thermal method requites temperatures of at least 250°C to iaitiate decomposition of the diatomic chlorine molecules iato chlorine radicals. The large reaction exotherm demands close temperature control by cooling or dilution, although adiabatic reactors with an appropriate diluent are commonly used ia iadustrial processes. Thermal chlorination is iaexpeasive and less sensitive to inhibition than the photochemical process. Mercury arc lamps are the usual source of ultraviolet light for photochemical processes furnishing wavelengths from 300—500 nm. 

Dye Stability. The dyes used in photographic systems can degrade over time, both by thermal reactions and, if the image is displayed for extended periods of time, by photochemical processes. The relative importance of these two mechanistic classes, known as dark fade and light fade. 

Anotlrer consideration in the production of thin fllms by photochemical processes is that the fraction of the beam which is not used in photodecomposition will heat any substrate on which it is desired to form the fllm. The power of tire light source which can be used for photodecomposition in the gaseous phase only is therefore limited by the transmission of energy. Clearly this transmitted beam represents a constant source of energy which... 

Fig. 13.1. Energy level diagram and summary of photochemical processes.

Because photochemical processes are very fast, special techniques are required to obtain rate measurements. One method is flash photolysis. The excitation is effected by a diort pulse of light in an apparatus designed to monitor very fast spectroscopic changes. The rate characteristics of the reactions following radiation can be determined from these spectroscopic changes. 

Ring closure of more highly substituted cyclohexadienes also follows the Woodward-Hoffmarm rules and, indeed, provided the initial examples of the dichotomy between thermal and photochemical processes that led to development of the concepts underlying the Woodward-Hofimann rules. ... 

Notice that the orbital array is of the Mobius topology with a phase change depicted between the C-1 and C-2 positions. This corresponds to an allowed photochemical process since there are six electrons involved in bonding changes. 

---