Excitation, electronic

4 Electronic Excitation Although each atom has a specific electronic structure, the disnibution of electrons can be modified through the deposition of energy through photon, electron, or ion impact. The atom/ion then responds through either  

The production of core holes (energies are greater than 20 eV with lifetimes 10 i s) 

Which occurs depends on the energy deposited upon the atom/ion and the electronic structure of the atoms that make up Ae solid. These are discussed in greater detail in the following sections. 

The distribution of filled states above the Fermi edge is most commonly defined through the Boltzmann distribution as  

The temperature parameter (T) is present because such excitations are within the range of energies over which temperature variations are observed. 

Although the valence ji-ji excitation spectra of benzene derivatives have been extensively studied over the past 65 years both experimentally and theoretically, much less is known about that of phenol, apart from its lowest excited state. In general, absorption and fluorescence spectroscopy of a benzene ring can be used to detect its presence in a larger compound and to probe its environment. While the relative constancy of the valence jt-n excitation spectrum allows a qualitative identification of spectral bands by a correspondence with those in free benzene, detailed quantitative differences could indicate the nature of substituents, ligands or medium. Key information on substituted benzene includes the excitation energies, transition moments and their direction, and electrostatic 

knowledge of the transition moment direction of a phenol band could help in interpreting the fluorescence spectrum of a tyrosine chromophore in a protein in terms of orientation and dynamics. The absorption spectrnm of the first excited state of phenol was observed around 275 nm with a fluorescence peak aronnd 298 nm in water. The tyrosine absorption was reported at 277 nm and the finorescence near 303 nm. Fluorescent efficiency is about 0.21 for both molecules. The fluorescent shift of phenol between protic and aprotic solvents is small, compared to indole, a model for tryptophan-based protein, due to the larger gap between its first and second excited states, which resnlts in negligible coupling .  

The Rydberg states have not yet been detected experimentally, bnt CASPT2 calculations indicated the existence of at least six n —tt Rydberg states that range from 6.3 to 7.6 eV and arise from the promotion of 3jr and 4 7r electrons to 3p and 3d orbitals. There are also no less than twelve a jr Rydberg states ranging from 5.8 to 7.8 eV. 

Solvent effects were found to have minimal influence on the excitation energies of phenol in aqueous solution using a quantum Monte Carlo simulation , which is in line with experimental observations on its absorption spectra . Reaction field calculations of the excitation energy also showed a small shift in a solution continuum, in qualitative agreement with fluorescent studies of clusters of phenol with increasing number of water molecules . The largest fluorescent shift of 2100 cm was observed in cyclohexane. 

In substituted phenols, the excited 5 i states are again dominated by the LUMO HOMO and LUMO -I-1 HOMO — 1 transitions and the corresponding excitation energies apparently differ from that of phenol by, at most, 0.6 eV. Results obtained using time-dependent density functional theory computations in conjunction with a systematic empirical correction are recorded in Table 32. CASSCF(8,7) calculations on both Sq and 

The potential curves of different excited states of the C2-molecule illustrate the effect of electronic excitation on bond energies (Fig. 4.2). We would like to point out that the dissociation energies of the different molecular states shown range from 350 to 119 kJ/mol (84 to 28 kcal/mol). These dissociation energies are generally smaller than the energy differences caused by the electronic excitation. The excited states of the (many-electron) C-atoms are denoted in the usual manner as S, P, D. 

Since two excited atoms may combine in a number of ways there are more than one molecular state corresponding to any two atomic states. 

The complete removal of an electron from a molecule, i.e. the ionization, requires more energy than electron excitation and has an even stronger effect on the dissociation energy. In Table 4.6 the dissociation and ionization potentials of the first few hydrocarbons have been collected from various sources. It is assumed that ionization is first accomplished through removal of the least bound electron from the highest occupied orbital and also that this orbital embraces the whole molecule. 

F rom Landolt Bornstein 1/3, 6. Auflage, Springer Verlag, Berlin 1951, p. 363. 

From F. A, Elder, C. Giese, B. Steiner, M. Inghram, J. Chem. Phys. 36, 3292 (1962). 

Decomposition reactions, A -+ B+C, provide two extreme cases. The uni-molecular decomposition that involves rupture of a single bond, A-B - A+B, usually has an activation energy almost equal to the bond dissociation energy. Excitation is absent, although either A or B may be unstable relative to other products and may isomerize in elementary steps with little or no activation energy. Decompositions which involve considerable bond rearrangement (bond shortening is the simplest example) may produce excited molecules. 

Electronic excitation is found in the thermal decomposition of diazomethane 

The methylene is produced in the singlet state, the one characterized by insertion reactions112. It is this state, not the ground triplet state, that correlates with the reactant. 

Singlet carbenes are also favored as intermediates in the decomposition of 

Vibrational excitation of triplet methylene appears to be possible. Photolysis of diazomethane in the presence of alkenes shows product distributions as a function of pressure that could indicate a change in the methylene singlet/triplet proportions in the mixture97. Internal conversion from singlet to triplet has been suggested115. These triplet methylene molecules should be excited. 

Everything in the future is a wave, everything in the past is a particle. 

The interaction of light with a molecular system is generally an interaction between one molecule and one photon, which can be written  

The short lifetime is a consequence of the high energy of the molecule in excited state, which exceeds that of the ground state, mostly by 150-600 kJ/mol. Besides energy, the excited-state molecule differs from that in the ground state in 

Bioinorganic Photochemistry Grazyna Stochel, Malgorzata Brindell, Wojciech Macyk, Zofia Stasicka, Konrad Szacilowski 2009 Grazyna Stochel, Malgorzata Brindell, Wojciech Macyk, Zofia Stasicka, Konrad Szacilowski. ISBN 978-1 -05-16172-5 

The molecule in its excited state cannot only be generated by light absorption, but also the excitation energy can be obtained in some chemical, biochemical, electrochemical processes, or by conversion of yet another kind of energy, eg ultrasonic or mechanical. The excited-state behaviour is, however, independent of its origin. 

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Heterogeneous photochemical reactions fall in the general category of photochemistry—often specific adsorbate excited states are involved (see, e.g.. Ref. 318.) Photodissociation processes may lead to reactive radical or other species electronic excited states may be produced that have their own chemistry so that there is specificity of reaction. The term photocatalysis has been used but can be stigmatized as an oxymoron light cannot be a catalyst—it is not recovered unchanged. 

A DIET process involves tliree steps (1) an initial electronic excitation, (2) an electronic rearrangement to fonn a repulsive state and (3) emission of a particle from the surface. The first step can be a direct excitation to an antibondmg state, but more frequently it is simply the removal of a bound electron. In the second step, the surface electronic structure rearranges itself to fonn a repulsive state. This rearrangement could be, for example, the decay of a valence band electron to fill a hole created in step (1). The repulsive state must have a sufficiently long lifetime that the products can desorb from the surface before the state decays. Finally, during the emission step, the particle can interact with the surface in ways that perturb its trajectory. 

These results do not agree with experimental results. At room temperature, while the translational motion of diatomic molecules may be treated classically, the rotation and vibration have quantum attributes. In addition, quantum mechanically one should also consider the electronic degrees of freedom. However, typical electronic excitation energies are very large compared to k T (they are of the order of a few electronvolts, and 1 eV corresponds to 10 000 K). Such internal degrees of freedom are considered frozen, and an electronic cloud in a diatomic molecule is assumed to be in its ground state f with degeneracy g. The two nuclei A and... 

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] ... 

Lee M, Flaseltine J N, Smith A B III and Flochstrasser R M 1989 Isomerization processes of electronically excited stiibene and diphenylbutadiene in liquids Are they one-dimensional J. Am. Chem. See. Ill 5044-51... 

Detailed analyses of the above experiments suggest that the apparent steps in k E) may not arise from quantized transition state energy levels [110.111]. Transition state models used to interpret the ketene and acetaldehyde dissociation experiments are not consistent with the results of high-level ab initio calculations [110.111]. The steps observed for NO2 dissociation may originate from the opening of electronically excited dissociation chaimels [107.108]. It is also of interest that RRKM-like steps in k E) are not found from detailed quantum dynamical calculations of unimolecular dissociation [91.101.102.112]. More studies are needed of unimolecular reactions near tln-eshold to detennine whether tiiere are actual quantized transition states and steps in k E) and, if not, what is the origin of the apparent steps in the above measurements of k E). 

Such electronic excitation processes can be made very fast with sufficiently intense laser fields. For example, if one considers monochromatic excitation with a wavenumber in the UV region (60 000 cm ) and a coupling strength / he 4000 (e.g. 1 Debye in equation (A3.13.59), / 50 TW cm ),... 

Another example of a teclmique for detecting absorption of laser radiation in gaseous samples is to use multiphoton ionization with mtense pulses of light. Once a molecule has been electronically excited, the excited state may absorb one or more additional photons until it is ionized. The electrons can be measured as a current generated across the cell, or can be counted individually by an electron multiplier this can be a very sensitive technique for detecting a small number of molecules excited. 

Liebsch A 1997 Electronic Excitations at Metal Surfaces (New York Plenum)... 

So far we have exclusively discussed time-resolved absorption spectroscopy with visible femtosecond pulses. It has become recently feasible to perfomi time-resolved spectroscopy with femtosecond IR pulses. Flochstrasser and co-workers [M, 150. 151. 152. 153. 154. 155. 156 and 157] have worked out methods to employ IR pulses to monitor chemical reactions following electronic excitation by visible pump pulses these methods were applied in work on the light-initiated charge-transfer reactions that occur in the photosynthetic reaction centre [156. 157] and on the excited-state isomerization of tlie retinal pigment in bacteriorhodopsin [155]. Walker and co-workers [158] have recently used femtosecond IR spectroscopy to study vibrational dynamics associated with intramolecular charge transfer these studies are complementary to those perfomied by Barbara and co-workers [159. 160], in which ground-state RISRS wavepackets were monitored using a dynamic-absorption technique with visible pulses. 

In the ideal case for REMPI, the efficiency of ion production is proportional to the line strength factors for 2-photon excitation [M], since the ionization step can be taken to have a wavelength- and state-mdependent efficiency. In actual practice, fragment ions can be produced upon absorption of a fouitli photon, or the ionization efficiency can be reduced tinough predissociation of the electronically excited state. It is advisable to employ experimentally measured ionization efficiency line strengdi factors to calibrate the detection sensitivity. With sufficient knowledge of the excited molecular electronic states, it is possible to understand the state dependence of these intensity factors [65]. 

For two Bom-Oppenlieimer surfaces (the ground state and a single electronic excited state), the total photodissociation cross section for the system to absorb a photon of energy ai, given that it is initially at a state x) with energy can be shown, by simple application of second-order perturbation theory, to be [89]... 

In a defect-free, undoped, semiconductor, tliere are no energy states witliin tire gap. At 7"= 0 K, all of tire VB states are occupied by electrons and all of the CB states are empty, resulting in zero conductivity. The tliennal excitation of electrons across tire gap becomes possible at T > 0 and a net electron concentration in tire CB is established. The electrons excited into tire CB leave empty states in tire VB. These holes behave like positively charged electrons. Botli tire electrons in the CB and holes in tire VB participate in tire electrical conductivity. 

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.)... 

Ireland J F and Wyatt PAH 1976 Acid-base properties of electronically excited states of organic molecules Adi/. Rhys. Org. Chem. 12 131-221... 

Agranovioh V M and Galanin M D 1982 Electronic Excitation Energy Transfer in Condensed Maffer (Amsterdam Elsevier/North-Flolland)... 

Barzykin A V, Barzykina N S and Fox M A 1992 Electronic excitation transport and trapping in micellar systems— Monte-Carlo simulations and density expansion approximation Chem. Rhys. 163 1-12... 

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