Localized molecular-ion states

V i O.leV for motion along the polymeric backbone of pendant group polymers, and otherwise V << O.leV (1, 8). Therefore inequality (3) is clearly satisfied for these materials, so injected charges form intrinsic (pendant-group polymers) or extrinsic (molecular-ly-doped polymers) local molecular-ion states (j, 2, 7> 8) A similar situation seems to prevail for molecular glasses, although the parameter values are not yet firmly established in this case. For bulk molecular crystals in the absence of defects... 

Relaxation Energy Shifts and Localized Molecular-Ion States in Aromatic Pendant Group Polymers. The electronic structure of polystyrene (PS) and poly vinyl pyridine (PVP) have been studied using a variety of electronic spectroscopies and model calculations (W). Here, we review the results of the UPS and ultra violet absorption spectroscopy (UAS) portion of that study, and discuss the results in a phenomenological manner. The aim of this... 

In the usual space-charge limited theory, electrons are injected into the insulator conduction band, and some of these electrons are immobilized in localized defect states. We have considered an alternate mechanism more appropriate to the polymer structure. Contact charge transfer studies in Polyethylene Terephthalate (PET) and other polymers (15-16) suggest that the electronic states accessible from metal contacts are localized molecular-ion states located deep in the forbidden energy gap. Charge transport is by hopping between localized states. 

Therefore inequality (5) is clearly satisfied for these materials, so injected charges form intrinsic (pendant-group polymers) or ex r a c j olecularly-doped polymers) local molecular-ion states. A similar situation seems to prevail for... 

The purpose of this paper is the presentation of a brief overview of recent literature in which new models of electronic states in polymers and molecular solids have been proposed (, 2, 5-16). Since localized (e.g., molecular-ion) states seem prevalent in these materials, I indicate in Sec. II the physical phenomena which lead to localization. Sec. Ill is devoted to the description of a model which permits the quantitative analysis of the localized-extended character of electronic states and to the indication of the results of spectroscopic determinations of the parameters in this model for various classes of polymeric and molecular materials. I conclude with the mention in Sec. IV of an important practical application of these concepts and models The contact charge exchange properties of insulating polymers ( 7, 17, 18, 19). 

Intermolecular relaxation effects are a central issue in the interpretation of the ultraviolet photoelectron spectroscopy (UPS) of molecular solids. These relaxation effects result in several significant characteristics of UPS valence spectra, intermolecular relaxation phenomena lead to localized electron molecular-ion states, which are responsible for rigid gas-to-solid molecular spectral energy shifts, spectral line broadening, and dynamic electronic localization effects in aromatic pendant group polymers. 

The local or extended nature of molecular-ion states in molecular solids is determined by a competition between fluctuations in the local site energies of these states (which tend to localize them) and the hopping integrals for inter-site excitation transfer (which tend to delocalize them). In order to define this fluctuation-induced localization concept more precisely, consider the model defined by the one-electron Hamiltonian... 

This paper is devoted to the presentation of a brief overview of a recently-developed "relaxation-localization" model of localized molecular-ion and exciton states in polymers and molecular glasses. This model was proposed initially to interpret photoemission measurements from two pgn ant-group polymers polystyrene and p ly(2-vinyl pyridine.) It ext was utilized in the prediction and subsequent observation of surface states of molecular solids as well as of the temperature dependence of photoe iss on and UV absorption linewidths of molecular films. Having proven successful in describing the spectroscopic properties of typical pendant-group polymers and molecular glasses, the model most recently has been extended to provide a description of electron-transfer processes in both these materials and molecularly-doped polymers. Therefore it affords a unified and experimentally-verified microscopic description of electron ionization, excitation and transfer processes in a variety of molecular and polymeric materials. 

Dealing with a molecular ion it is necessary to identify its ground state, that is to remove an electron from the highest occupied molecular orbital (HOMO). The most favorable sites for the charge and unpaired electron localization may be established by taking away an electron with minimal ionization energy. The energy requirements in this case are similar to these known in UV-spectroscopy for the electron transitions a < tt< n. 

This effect can be illustrated by Fig. 14.2. The effective range of local modification of the sample states is determined by the effective lateral dimension 4ff of the tip wavefunction, which also determines the lateral resolution. In analogy with the analytic result for the hydrogen molecular ion problem, the local modification makes the amplitude of the sample wavefunction increase by a factor exp( — Vi) 1.213, which is equivalent to inducing a localized state of radius r 4tf/2 superimposed on the unperturbed state of the solid surface. The local density of that state is about (4/e — 1) 0.47 times the local electron density of the original stale in the middle of the gap. This superimposed local state cannot be formed by Bloch states with the same energy eigenvalue. Because of dispersion (that is, the finite value of dEldk and... 

The factor 0.47 is the relative intensity of the tip-induced local states, estimated from an analogy to the hydrogen molecular ion problem. For example, for (E/r- ,) = 4 eV and kf = 1/4 , when atomic resolution is achieved, that is, r < 2 A, the energy resolution is about 0.2 eV. Even for the reconstructed surfaces, such as Si(lll) 7X7, when the adatoms are barely resolved, that is, t aA, the energy resolution is 0.12 eV. 

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