Electronic Excitation in Molecules

The frequency absorbed or emitted by a molecule and the energy of radiation are related by AE = hv. The actual amount of energy required depends on the difference in energy between the ground state and the excited state E of the electrons. The relationship is described by 

Fj is the energy of the excited state Eq is the energy of the ground state 

---

In considering absorption of light by molecules, we have been principally concerned with transitions between electronic states. However, it is not possible to explain fully the effects of electronic excitation in molecules unless we also take into account the motions of the nuclei. 

Optical spectroscopy involves electronic excitations in molecules, making it well suited to extending the atomic structural information... 

Electronic excitation in molecules is accompanied by changes in vibrational or rotational energy levels. The energy levels for these excitations are considerably smaller than the energy differences between electronic excitations. These transitions are superposed on the electronic excitations, which results in a large number of absorption peaks so closely spaced that the spectrophotometer cannot resolve them. For this reason, UV-visible absorption peaks usually are much broader than IR absorption peaks. 

Classic examples are the spontaneous emission of light or spontaneous radioactive decay. In chemistry, an important class of monomolecular reactions is the predissociation of metastable (excited) species. An example is the fonnation of oxygen atoms in the upper atmosphere by predissociation of electronically excited O2 molecules [12, 13 and 14] ... 

Sensitivity levels more typical of kinetic studies are of the order of lO molecules cm . A schematic diagram of an apparatus for kinetic LIF measurements is shown in figure C3.I.8. A limitation of this approach is that only relative concentrations are easily measured, in contrast to absorjDtion measurements, which yield absolute concentrations. Another important limitation is that not all molecules have measurable fluorescence, as radiationless transitions can be the dominant decay route for electronic excitation in polyatomic molecules. However, the latter situation can also be an advantage in complex molecules, such as proteins, where a lack of background fluorescence allow s the selective introduction of fluorescent chromophores as probes for kinetic studies. (Tryptophan is the only strongly fluorescent amino acid naturally present in proteins, for instance.)... 

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. 

Electronic transitions in molecules in supersonic jets may be investigated by intersecting the jet with a tunable dye laser in the region of molecular flow and observing the total fluorescence intensity. As the laser is tuned across the absorption band system a fluorescence excitation spectrum results which strongly resembles the absorption spectrum. The spectrum... 

Shorter-wavelength radiation promotes transitions between electronic orbitals in atoms and molecules. Valence electrons are excited in the near-uv or visible. At higher energies, in the vacuum uv (vuv), inner-shell transitions begin to occur. Both regions are important to laboratory spectroscopy, but strong absorption by make the vuv unsuitable for atmospheric monitoring. Electronic transitions in molecules are accompanied by stmcture... 

Semiconductor materials are rather unique and exceptional substances (see Semiconductors). The entire semiconductor crystal is one giant covalent molecule. In benzene molecules, the electron wave functions that describe probabiUty density ate spread over the six ting-carbon atoms in a large dye molecule, an electron might be delocalized over a series of rings, but in semiconductors, the electron wave-functions are delocalized, in principle, over an entire macroscopic crystal. Because of the size of these wave functions, no single atom can have much effect on the electron energies, ie, the electronic excitations in semiconductors are delocalized. 

It can be assumed that in cycloadditions only one reactant is electronically excited, in view of the short lifetimes of excited species in solution and the consequently low probability of a collision between two excited molecules. Also, the cycloadditions are conducted with light of wavelengths above 2800 A... 

Energy is transferred from molecules electronically excited in a chemical reaction to other molecules which emit the accepted excitation energy in the form of light alternatively the accepting molecules can undergo photochemical transformations. First examples of this photochemistry without light were described by E. H. White and coworkers 182>. Thus the trans-stilbene hydrazide 127, on oxidation, yielded small amounts of the cis- 128 beside the trans-stilbene dicarboxylate in a luminol-type reaction. 

In an effort to understand the mechanisms involved in formation of complex orientational structures of adsorbed molecules and to describe orientational, vibrational, and electronic excitations in systems of this kind, a new approach to solid surface theory has been developed which treats the properties of two-dimensional dipole systems.61,109,121 In adsorbed layers, dipole forces are the main contributors to lateral interactions both of dynamic dipole moments of vibrational or electronic molecular excitations and of static dipole moments (for polar molecules). In the previous chapter, we demonstrated that all the information on lateral interactions within a system is carried by the Fourier components of the dipole-dipole interaction tensors. In this chapter, we consider basic spectral parameters for two-dimensional lattice systems in which the unit cells contain several inequivalent molecules. As seen from Sec. 2.1, such structures are intrinsic in many systems of adsorbed molecules. For the Fourier components in question, the lattice-sublattice relations will be derived which enable, in particular, various parameters of orientational structures on a complex lattice to be expressed in terms of known characteristics of its Bravais sublattices. In the framework of such a treatment, the ground state of the system concerned as well as the infrared-active spectral frequencies of valence dipole vibrations will be elucidated. 

The vibrational and electronic excitation of molecules has received less attention over the years, but the understanding of excitation processes is important for a number of reasons. Thus, in this paper, we will review the various mechanisms leading to vibrational and/or electronic excitation of molecules in strong laser fields. As we do this, it will become clear that exploring these mechanisms (1) reveals new features of the strong field interaction ... 

Lemus, R., and Frank, A. (1991b), An Algebraic Model for Molecular Electronic Excitations in Diatomic Molecules, Ann. Phys. 206, 122. 

At the end of the physical stage, which is within about 10 sec of the passage of the ionizing particle through the liquid, the track made by the particle contains H20", subexcitation electrons e , and electronically excited water molecules H2O in small clusters called spurs. From about 10 to 10 sec, the following processes are thought to occur and comprise the physicochemical stage [9,10] ... 

The electron excitation of molecules in nearly all classes of organic compounds can be performed by visible or ultraviolet light (lex > 190 nm, < 6.5 eV). There are a few ex-... 

---