Molecules, excited states

If the temperature were raised, more molecules would attain the excited state, but even at 50,000°C there would be only one excited-state atom for every two ground-state atoms, and stimulated emission would not produce a large cascade effect. To reach the excess of stimulated emissions needed to build a large cascade (lasing), the population of excited-state molecules must exceed that of the ground state, preferably at normal ambient temperatures. This situation of an excess of excited-state over ground-state molecules is called a population inversion in order to contrast it with normal ground-state conditions. 

Interaction of an excited-state atom (A ) with a photon stimulates the emission of another photon so that two coherent photons leave the interaction site. Each of these two photons interacts with two other excited-state molecules and stimulates emission of two more photons, giving four photons in ail. A cascade builds, amplifying the first event. Within a few nanoseconds, a laser beam develops. Note that the cascade is unusual in that all of the photons travel coherently in the same direction consequently, very small divergence from parallelism is found in laser beams. 

The reaction path shows how Xe and Clj react with electrons initially to form Xe cations. These react with Clj or Cl- to give electronically excited-state molecules XeCl, which emit light to return to ground-state XeCI. The latter are not stable and immediately dissociate to give xenon and chlorine. In such gas lasers, translational motion of the excited-state XeCl gives rise to some Doppler shifting in the laser light, so the emission line is not as sharp as it is in solid-state lasers. 

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. 

All of the atomic species which may be produced by photon decomposition are present in plasma as well as the ionized states. The number of possible reactions is therefore also increased. As an example, die plasma decomposition of silane, SiH4, leads to the formation of the species, SiH3, SiHa, H, SiH, SiH3+ and H2+. Recombination reactions may occur between the ionized states and electrons to produce dissociated molecules either direcdy, or tlrrough the intermediate formation of excited state molecules. 

Many other measures of solvent polarity have been developed. One of the most useful is based on shifts in the absorption spectrum of a reference dye. The positions of absorption bands are, in general, sensitive to solvent polarity because the electronic distribution, and therefore the polarity, of the excited state is different from that of the ground state. The shift in the absorption maximum reflects the effect of solvent on the energy gap between the ground-state and excited-state molecules. An empirical solvent polarity measure called y(30) is based on this concept. Some values of this measure for common solvents are given in Table 4.12 along with the dielectric constants for the solvents. It can be seen that there is a rather different order of polarity given by these two quantities. 

Note that prior applications of the proton inventory technique were to reactants in their ground states. The particular example cited, however, refers to an excited state molecule (and indeed was the first of its kind). An implicit assumption made in the... 

The excited-state molecules may either undergo radiationless decay to the ground state leading to the formal generation of heat under conditions of high radiation flux or radiative decay (i.e., phosphorescence), thereby emitting light. 

The theoretical significance of SO MO s in the excited-state molecules was discussed in detail 10M ). One of these SO s, or both, play important parts in excited-state reactions. 

If a reaction can yield products in the ground state or in an electronically excited state, the activation energy for the formation of the latter will therefore be less than that required for the formation of the products in the ground state — provided that there is no significant change in the configuration of the excited-state molecules as compared with the reactant molecules. 

Electrogenerated chemiluminescence (ECL) is the process whereby a chemiluminescence emission is produced directly, or indirectly, as a result of electrochemical reactions. It is also commonly known as electrochemiluminescence and electroluminescence. In general, electrically generated reactants diffuse from one or more electrodes, and undergo high-energy electron transfer reactions either with one another or with chemicals in the bulk solution. This process yields excited-state molecules, which produce a chemiluminescent emission in the vicinity of the electrode surface. 

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 ... 

The chemical association of the exciplex results from an attraction between the excited-state molecule and the ground-state molecule, brought about by a transfer of electronic charge between the molecules. Thus exciplexes are polar species, whereas excimers are nonpolar. Evidence for the charge-transfer nature of exciplexes in nonpolar solvents is provided by the strong linear correlation between the energy of the photons involved in exciplex emission and the redox potentials of the components. 

We can approximate simple potential energy surfaces for the reacting ground-state and excited-state molecules in the manner shown in Figure 7.5. 

Photochemical reactions are the reactions of excited-state molecules initiated by photon absorption whereas thermal reactions are the reactions of ground-state molecules usually initiated by heat. The energy of photoexcitation of molecules can be provided by photon absorption even at very low temperatures and is of the same order as the activation energies for ground-state molecules. Provided the process of photoexcitation can be utilised in order to... 

Since the molecular geometry is related to electron distribution within the molecule, the ground-state and (n,it ) excited-state molecules have different geometries (Figure 7.7). The C-O bond length of the Si and Ti excited states is longer than that of S0, due to the excited states having less double-bonded character than S0. 

Excited-state molecules are more reactive than the corresponding ground-state molecules because ... 

The long lifetime also has important consequences for the effect of specific quenching between the chromophore and surrounding quenching species. The probability of bimolecular collisions is related to the duration of the excited state. The triplet excited state molecule is more susceptible than the singlet excited molecule to quenching simply because it has more time to interact with the surroundings. 

M = ground state molecule M = excited state molecule P = Photochemical product X= second molecule... 

A number of methods have been proposed for calculations of the geometries of molecules in excited states. These include CIS (Configuration Interaction Singles) and variations on CIS to account for the effect of double substitutions, as well as so-called time dependent density functional models. Except for CIS (the simplest of the methods) there is very little practical experience. There is also very little solid experimental data on the geometries of excited-state molecules. 

In some cases, simultaneously with the quenching of the normal fluorescence a new structureless emission band appeals at about 6000 cm-1 to the red side of the monomer fluorescence spectrum (Figure 6.4). This phenomenon was first observed in pyrene solution by Forster and was explained as due to transitory complex formation between the ground and the excited state molecules since the absorption spectrum was not modified by increase in concentration. Furthermore, cryoscopic experiments gave negative results for the presence of ground state dimers. These shortlived excited state dimers are called pxcimers to differentiate them from... 

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