Excited Molecules — Physical Processes

By absorption of light a molecule is promoted to a higher electronic state. The monomolecular physical processes for the dissipation of the excess energy are outlined in Fig. 5 in a so called Jablonski diagramm. In principle one has to differentiate between radiative and non-radiative deactivation on the one side and on the other side one has to consider if the multiplicity of the system is conserved or not. Radiative deactivation, i.e. deactivation accompanied by emission of light, is termed fluorescence if the transition occurs with spin conservation and phosphorescence, if spin inversion occurs. 

The radiationless transition between two states of same spin is called internal conversion, the one occuring with inversion of spin being termed intersystem crossing. In both processes the excess energy is liberated as heat. All these transitions between different electronic states are customarily preceded by vibrational relaxation, i.e. the deactivation from a higher vibronie level to the v0-level of the same electronic state (Fig. 5). 

As shown in Fig. 6, there is a correlation between absorption spectrum and emission spectrum. Taking into consideration the Franck-Condon principle, which states that there is no motion of the atoms during an electronic transition, one has to differentiate between the two following possibilities in the one the geometry of the excited state is similar to the one of the ground state (same interatomic distances), 

In the first case the so called 0—0 transition will be the most probable one, both for absorption and emission, and therefore the most intense 

Besides the excited molecule can interact physically with a second molecule, i.e. undergo bimolecular processes. These are either energy transfer (1.7) or exciplex formation (1.8) depending on the relative excitation energies of the molecule to be studied and its partner. 

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Excited states can also be quenched. Quenching is the same physical process as sensitization, but the word quenched is used when a photoexcited state of the reactant is deactivated by transferring its energy to another molecule in solution. This substance is called a quencher. 

TABLE 7.4 Physical Processes Undergone by Excited Molecules"... 

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. 

In this section we first (Section IV A) derive a formal expression for the channel phase, applicable to a general, isolated molecule experiment. Of particular interest are bound-free experiments where the continuum can be accessed via both a direct and a resonance-mediated process, since these scenarios give rise to rich structure of 8 ( ), and since they have been the topic of most experiments on the phase problem. In Section IVB we focus specifically on the case considered in Section III, where the two excitation pathways are one- and three-photon fields of equal total photon energy. We note the form of 8 (E) = 813(E) in this case and reformulate it in terms of physical parameters. Section IVC considers several limiting cases of 813 that allow useful insight into the physical processes that determine its energy dependence. In the concluding subsection of Section V we note briefly the modifications of the theory that are introduced in the presence of a dissipative environment. 

The physical significance of Eq. (53) is clear. At an isolated resonance the excitation and dissociation processes decouple, all memory of the two excitation pathways is lost by the time the molecule falls apart, and the associated phase vanishes. The structure described by Eq. (53) was observed in the channel phase for the dissociation of HI in the vicinity of the (isolated) 5sg resonance. The simplest model depicting this class of problems is shown schematically in Fig. 5d, corresponding to an isolated predissociation resonance. Figures 5e and 5f extend the sketches of Figs. 5c and 5d, respectively, to account qualitatively for overlapping resonances. 

TABLE 7.4 Physical processes undergone by excited molecules... 

As we have seen, an excited organic molecule may undergo several physical and chemical processes. The relative importance of the various processes depends, of course, on the structure of the compound and on its environment (e.g., type of solvent, presence of solutes). For each individual process j, we may, for a given environment, define a quantum yield ,y(A) which denotes the fraction of the excited molecules of a given compound i that react by that particular physical or chemical pathway ... 

The properties of the F band are, unfortunately, not known, making assignment of the time constants to a specific physical process difficult. In general, the growth time X, about 1 ps, appears to be independent of cluster size since the (S02) clusters produce similar values. The F band decay, x2, is altered substantially in the (S02)n clusters where the decay is slowed with increasing cluster size from about 13 ps to 65 ps for the n = 1 to 5 size range. The decay is attributed to a relaxation process that is slowed by interactions between the excited-state species and the surrounding cluster molecules. However, the exact nature of this relaxation process has not yet been determined. 

In summary, spectrally resolved 3-pulse 2-colour photon echoes provide a potential tool to study the molecular structure dynamics on a femtosecond time scale and will be used to study chemical and physical processes involving nonequilibrium relaxation in both ground and excited states of molecules. 

Figure 18-13 Physical processes that can occur after a molecule absorbs an ultraviolet or visible photon. S0 is the ground electronic state. S, and T, are the lowest excited singlet and triplet electronic states. Straight arrows represent processes involving photons, and wavy arrows are radiationless transitions. R denotes vibrational relaxation. Absorption could terminate in any of the vibrational levels of S,. not just the one shown. Fluorescence and phosphorescence can terminate in any of the vibrational levels of Sq.

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