Excited molecules chemical processes

The record m the number of absorbed photons (about 500 photons of a CO2 laser) was reached with the CgQ molecule [77]. This case proved an exception in that the primary reaction was ionization. The IR multiphoton excitation is the starting pomt for a new gas-phase photochemistry, IR laser chemistry, which encompasses numerous chemical processes. 

Molecular Interaction. The examples of gas lasers described above involve the formation of chemical compounds in their excited states, produced by reaction between positive and negative ions. However, molecules can also interact in a formally nonbonding sense to give complexes of very short lifetimes, as when atoms or molecules collide with each other. If these sticky collisions take place with one of the molecules in an electronically excited state and the other in its ground state, then an excited-state complex (an exciplex) is formed, in which energy can be transferred from the excited-state molecule to the ground-state molecule. The process is illustrated in Figure 18.12. 

Photolytic methods are used to generate atoms, radicals, or other highly reactive molecules and ions for the purpose of studying their chemical reactivity. Along with pulse radiolysis, described in the next section, laser flash photolysis is capable of generating electronically excited molecules in an instant, although there are of course a few chemical reactions that do so at ordinary rates. To illustrate but a fraction of the capabilities, consider the following photochemical processes ... 

In studies of molecular dynamics, lasers of very short pulse lengths allow investigation by laser-induced fluorescence of chemical processes that occur in a picosecond time frame. This time period is much less than the lifetimes of any transient species that could last long enough to yield a measurable vibrational spectrum. Such measurements go beyond simple detection and characterization of transient species. They yield details never before available of the time behavior of species in fast reactions, such as temporal and spatial redistribution of initially localized energy in excited molecules. Laser-induced fluorescence characterizes the molecular species that have formed, their internal energy distributions, and their lifetimes. 

The majority of heterogeneous chemical and physical-chemical processes lead to formation of the intermediate particles - free atoms and radicals as well as electron- and oscillation-excited molecules. These particles are formed on the surface of solids. Their lifetime in the adsorbed state Ta is determined by the properties of the environment, adsorbed layer, and temperature. In many cases Ta of different particles essentially affects the rate and selectivity of heterogeneous and heterogeneous-homogeneous physical and chemical processes. Therefore, it is highly informative to detect active particles deposited on surface, determine their properties and their concentration on the surface of different catalysts and adsorbents. 

While for thermal reactions one usually does not correlate the energy input with the amount of product formed, electrochemists and photochemists are certainly more energy-minded . The first ones use the current yield to define the amount of product formed per electrons consumed. The latter ones use the so called quantum yield which is defined as the ratio of number of molecules undergoing a particular process from an excited state over moles of photons absorbed by the system, or in other words, the ratio of the rate constant for the process defined over the sum of all rate constants for all possible processes from this excited state (1.4). Thus, if for every photon absorbed, a molecule undergoes only one chemical process, the quantum yield for this process is unity if other processes compete it will be less than unity. 

The primary process consists of raising of the electronic quantum level of molecule by absorption of energy from photon. The excited molecule may then behave in different ways. The energy of the photon is transformed into heat and temperature of absorbing system is raised but the excited molecule may behave in other ways resulting in a chemical change. 

The much larger energy difference between Si and S0 than between any successive excited states means that, generally speaking, internal conversion between Si and S0 occurs more slowly than that between excited states. Therefore, irrespective of which upper excited state is initially produced by photon absorption, rapid internal conversion and vibrational relaxation processes mean that the excited-state molecule quickly relaxes to the Si(v0) state from which fluorescence and intersystem crossing compete effectively with internal conversion from Si. This is the basis of Kasha s rule, which states that because of the very rapid rate of deactivation to the lowest vibrational level of Si (or Td, luminescence emission and chemical reaction by excited molecules will always originate from the lowest vibrational level of Si or T ... 

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