Physical Properties of Excited States

Electronically excited organic molecules are chemical entities, just as are groimd state molecules. Even though they have short lifetimes, they have characteristic bond angles, dipole moments, bond strengths, and vibrational 

Shetlar, M. D. Mol. Photochem. 1974,6,191 reported a general form of the Stem-Volmer equation for a system with multiple excited states. Green, N. J. B. Pimblott, S. M. Tachiya, M. /. Phys. Chem. 1993,97,196 presented generalizations for cases in which the quenching requires description with a time-dependent rate constant. 

Potential energy surfaces for dissociation of a proton from a ground state or photoexcited compound. 

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Excited State Wavefunctions 937 Physical Properties of Excited States 944 The Sensitivity of Fluorescence—Good News and Bad News 946... 

Physical Properties of Excited States Jablonski Diagram... 

Photophysics and photochemistry are relatively young sciences, a real understanding of light-induced processes going back some 50 or 60 years. The development of quantum mechanics was an essential step, as classical physics cannot account for the properties of excited states of atoms and molecules. In the past 30 years the advent of new experimental techniques has given a major impetus to research in new areas of photochemistry, and these are the subject of this final chapter. It must of course be realized that these developments advance all the time, and that we talk here of a moving frontier, as it is in 1992. 

Very importantly, an excited state must be considered as a new chemical species in comparison with the molecule in its ground state and can present different chemical and physical properties. The excited states and the ground one differ, in fact, for the distribution of the external electrons that are the ones interacting with the environment, and, therefore, the ones determining the chemical reactivity of a substance. An example is reported in Fig. 3.1 that compares some characteristics of formaldehyde in the ground state and in its lower excited state. 

At low temperature or energy, most degrees of freedom of quark matter are irrelevant due to Pauli blocking. Only quasi-quarks near the Fermi surface are excited. Therefore, relevant modes for quark matter are quasi-quarks near the Fermi surface and the physical properties of quark matter like the symmetry of the ground state are determined by those modes. High density effective theory (HDET) [7, 8] of QCD is an effective theory for such modes to describe the low-energy dynamics of quark matter. 

There are several important differences between ground state and excited state molecules with respect to the physical properties of shape, bond lengths and charge distribution. These properties are related also to the chemical properties of these molecules, so they have a direct bearing on photochemistry. 

The situation for reactions in solids is much more complex and is treated in a separate section (4.7.4, p. 153). Physical diffusion of molecules can be neglected within the lifetimes of excited states, but exciton interactions can become important. These have no counterpart in dark reactions and can lead to unusual photochemical properties in crystals and polymers. 

The Hamiltonian Eq. (7) provides the basis for the quantum dynamical treatment to be detailed in the following sections, typically involving a parametrization for 20-30 phonon modes. Eq. (7) is formally equivalent to a class of linear vibronic coupling (LVC) Hamiltonians which have been used for the description of excited-state dynamics in molecular systems [66] as well as the Jahn-Teller effect in solid-state physics. In the following, we will elaborate on the general properties of the Hamiltonian Eq. (7) and on quantum dynamical calculations based on this Hamiltonian. 

In this chapter, we have developed the information content of different excited state spectroscopic methods in terms of ligand field theory and the covalency of L—M bonds. Combined with the ground-state methods presented in the following chapters, spectroscopy and magnetism experimentally define the electronic structure of transition metal sites. Calculations supported by these data can provide fundamental insight into the physical properties of inorganic materials and their reactivities in catalysis and electron transfer. The contribution of electronic structure to function has been developed in Ref. 61. 

It must be pointed out that the classification into MC, LC and CT transitions (or excited states) is somewhat arbitrary and loses its meaning whenever the states involved cannot be described with localized MO configurations. The chemical and physical properties of these orbitally different excited states have been examined in detail by several authors2 3>6> 8-10) and will not be further discussed here. 

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