Information from Electronic Spectra

The reactive excited state in a photochemical reaction is usually either the Si state or the Ti state of the reactant molecule. These states can be characterised by reference to the absorption and emission spectra of the reactant. 

Generally speaking, luminescence spectra (fluorescence and phosphorescence) provide more information about excited states than do absorption spectra. This is because luminescence measurements are much more sensitive than absorption measurements, and the two types of emission can be studied separately due to their widely differing lifetimes. 

Excited-state lifetimes can be measured directly by monitoring the decay of luminescence, but impurities present affect both the lifetime and the luminescence spectrum. Also, because low temperatures are necessary for phosphorescence studies, the excited-state properties determined may differ from those at room temperature. 

The energy of the first excited singlet state can be determined from the wavelength at which the first vibrational band in the absorption spectrum coincides with the vibrational band in the fluorescence spectrum  

There is no vibrational information in the phosphorescence spectrum. It is assumed that 0-0 is close to the maximum at 384nm  

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Analysis of the rotational fine structure of electronic bands is also possible, and the rotation constant for a molecule can in principle be determined for each of many vibrational states of each electronic state involved in the transition. In practice, smdies have largely been limited to diatomic molecules, partly because of the high resolution needed to observe rotational fine structure of an electronic band, and partly because the excited state can have a very short lifetime, with a consequent uncertainty in the energy levels because of the Uncertainty Principle, leading to broadening of the lines. More information about obtaining rotational information from electronic spectra is available in the on-line supplement for Chapter 7. 

The first factor is associated with the electronic dipole transition probability between the electronic states the second factor is associated between vibrational levels of the lower state v" and the excited state V, and is commonly known as the Franck-Condon factor, the third factor stems from the rotational levels involved in the transition, J" and /, the rotational line-strength factor (often termed the Honl-London factor). In particular, the Franck-Condon information from the spectrum allows one to gain access to the relative equilibrium positions of the molecular energy potentials. Then, with a full set of the spectroscopic constants that are used to approximate the energy-level structure (see Equations (2.1) and (2.2)) and which can be extracted from the spectra, full potential energy curves can be constructed. 

Structural Information from EELS. Besides yielding chemical composition, EELS is also capable of providing structural information on an atomic scale. It has been known (54) for some time that the fine-structure in the energy-loss spectrum close to an ionization edge reflects the energy dependence of the density of electronic states above the Fermi level. 

Until recently, experimental studies of AI were limited to the identification of the process and, in some cases, to the determinations of cross sections or rate constants. It was not possible to draw definite conclusions from this experimental information regarding the involved mechanisms. In recent studies of the AI systems R -H, with R = Ar(3P20), Kr(3P20), Xe(3P2 o), it was verified by electron spectroscopy that the mechanism of Fig. 34b is dominant for these systems.99-101 In all three cases the observed electron spectra extended to the rather high energies of e —1.45 eV (Ar),= 1.0 eV (Kr), and 1.2 eV (Xe) and showed structure resulting from population of different vibrational rotational states as expected for the mechanism of Fig. 34b. As an example, the AI electron spectrum for Ar(3F2.0)-H is shown in Fig. 35. 

Additional information on electronic structure comes from a NMR study of a closely related series of compounds of the type (Cp )2Hf(X)(H), where X is H, Me, OH, NH2, NHMe, or NMe2. Bercaw and co-workers observed that in the H-NMR spectrum the metal-bonded H atom becomes more shielded as the tt donor ability of X increases (51). The hydride chemical shifts corresponding to X are 8 15.6, 13.1, 10.2, 9.3, 9.1, and 11.5, respectively. Clearly, the tt interaction of the main group ligand with the metal is important in compounds of this type. 

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