John J. Ritsko

Electron energy loss spectroscopy and the optical properties of polymethylmethacrylate from 1 to 300 eV

Energy loss spectra of 80 keV electrons transmitted through thin films polymethylmethacrylate (PMMA) were measured with a resolution of 0.1 eV for energy losses from 1 to 300 eV. From the loss spectra, the dielectric response function of PMMA was obtained from 1 to 100 eV and compared with recent synchrotron radiation results. The spectrum of valence excitations from 5 to 13 eV is shown to be characteristic of the pendant group and is compared to experimental gas phase spectra and molecular orbital (CNDO/S) calculations of model molecules. The spectrum of core electron excitations above 285 eV provides a measure of the distribution of empty molecular orbitals and, when the carbon 1s binding energies are taken into account, a qualitatively accurate description of the observed spectrum is made from the CNDO ground state calculation. The energy loss spectra of 20, 40, and 100 eV electrons reflected from the surface of PMMA exhibit a triplet excitation at 4.2 eV and indicate the extreme sensitivity of this ma...

Core excitons and the dielectric response of polystyrene and poly(2‐vinylpyridine) from 1 to 400 eV

The complete spectra of valence and core electronic excitations of atactic polystyrene, PS, and poly(2‐vinylpyridine), PVP, from 1 to 400 eV were measured by electron energy loss spectroscopy. Energy losses of 80 keV electrons transmitted through 1000 A thick films were measured with a resolution of 0.1 eV. The spectrum of PS contains several previously unreported broad peaks between 9 and 20 eV which coincide with structure observed in gas phase benzene spectra and which have been attributed to Rydberg states, but which are described here as valence molecular orbital excitations. The dielectric response function of PS and PVP from 1 to 100 eV is computed from energy loss data. The calculated reflectivity and refractive index are in excellent agreement with recent optical measurements. The spectrum of carbon core electron excitations above 285 eV consists of a series of sharp peaks due to transitions to empty molecular states (core excitons) followed by transitions to a free‐electron‐like continuum of sta...

Electronic states in polypyrrole

Abstract Electron energy loss spectra of valence excitations in polypyrrole doped with boron tetrafluoride, undoped, and subsequently redoped with arsenic pentafluoride were measured. Energy loss peaks associated with intraband and interband excitations which show characteristically different momentum dependence were observed. Intraband peaks exhibit weak negative dispersion while interband excitations show strong positive dispersion. This suggests that interband excitations occur between wide energy bands (∼5 eV) in long polymetric molecules while intraband excitations may be due to intrinsic or extrinsic intermolecular charge transfer transitions in a narrow band (∼0.5 eV).

Electron energy loss spectroscopy of pristine and radiation damaged polyethylene

The spectra of valence and core electronic excitations of polyethylene from 1 to 390 eV were measured with a resolution of 0.11 eV by transmission electron energy loss spectroscopy. Structure at 9.5, 13.2, 14.7, 16.6, 19.6, 21.6 eV and a continuum of core excitations beginning at 287.5 eV was measured and compared with theoretical calculations of molecular excitons and interband transitions. The momentum dependence of the fundamental absorption threshold at 7.2 eV indicates that the absorption edge is due to highly localized excitons. From the energy loss spectra the complex dielectric response function and optical properties were calculated and compared with existing optical data. Radiation induced changes in the electronic structure were observed by their characteristic excitation spectra which were shown to be due to formation of polyenyl chromophores in the polymer backbone.

Photoemission from ferrocene, decamethylferrocene, and decamethylferrocene‐bis(7,7,8,8‐tetracyano‐p‐quinodimethane)

We present the first vacuum ultraviolet photoemission measurements of condensed ferrocene, decamethylferrocene, and the highly conductive charge transfer complex decamethylferrocene‐7,7,8,8‐tetracyano‐p‐quinodimethane, at photon energies from 7.7 to 21.2 eV. We compare the electronic spectra of these compounds and discuss substituent effects. The first ionization potentials are 6.1, 5.1, and 5.3 eV, respectively. The charge transfer complex is a semiconductor with its highest occupied states 0.8 eV below EF. The electron scattering lengths in ferrocene and decamethylferrocene are ?60 A for electrons 0.5 eV above the vacuum level and decrease to ?2.5 A as the energy is increased to 16 eV. From this result we estimate the electron affinity of solid ferrocene and decamethylferrocene to be about 1 eV.

Energy of metal states in intercalated graphite

Abstract X-ray photoemission and electron energy loss spectroscopy of carbon and potassium core levels are used to measure the energy distribution of potassium 4s states in KC8. Contrary to many earlier theoretical calculations these states form a completely empty band with a minimum energy 2.2 eV above the Fermi level in good agreement with the most recent ab initio KKR calculations. Evidence for hybridized states near the Fermi level is presented.

Inelastic electron scattering spectroscopy of graphite-acceptor compounds

Abstract Electron energy loss spectroscopy has been used to measure the valence and core electronic excitations up to 300 eV in low stage AsF 5 -graphite residual intercalation compounds. The results are compared with similar measurements on FeCl 3 -graphite intercalation compounds and graphite which has been doped substitutionally with a small amount of boron. The AsF 5 -graphite samples showed well defined intraband plasmons near 1.2 eV which exhibited dispersion similar to that observed in stage-2 FeCl 3 -graphite. The excitation spectra of carbon 1s electrons in a stage-2 compound were used as a direct measurement of the Fermi level position which is shown to be 0.9 eV below that of pristine graphite. Using an empirical model for the density of states the charge transfer per carbon atom is estimated as 0.025 electrons in stage-2 AsF 5 -graphite.

Dielectric response and intraband plasmon dispersion in stage-1 FeCl3 intercalated graphite

Abstract A theory of the frequency and wavevector dependent longitudinal dielectric response functions in stage-1 FeCl3-intercalated-graphite is developed from a two-band model for the graphite π electrons. The theory predicts the observed wavevector dependence of the energy and intensity of the intraband plasmon induced by intercalation, and comparison with experiment provides a measurement of the degree of charge transfer in C6.6FeCl3.

Reactive diffusion and electronic structure at the nickel/selenium interface

Ultraviolet photoemission spectroscopy has been used to study reactive diffusion at the interface between clean nickel and evaporated selenium films in ultrahigh vacuum. Photoelectron‐energy‐distribution curves were measured at 10.2 eV for sputter‐cleaned nickel, evaporated selenium and nickel, and the reaction product nickel selenide, which is metallic. By depositing thick selenium films and monitoring the photocurrent as a function of time and temperature, the growth of nickel selenide in the interface region was measured. In agreement with the theory of reactive diffusion, the nickel selenide layer increased in thickness as the square root of the product of the growth constant k and time, where k=1×108 exp(−1.35eV/kBT) cm2 sec−1. This rate is independent of impurity concentrations in both the nickel and selenium up to several atomic percent. A thin (∼7 A) nickel oxide layer prevented nickel selenide formation. The electronic structures of selenides prepared with various compositions were determined, an...

Electron energy-loss spectroscopy of tetracyanoquinodimethane, TCNQ, tetrathiafulvalene, TTF, and the salt TTF-TCNQ

Abstract Energy loss spectra of low energy electrons (ELS) have been obtained for thin solid films of tetracyanoquinodimethane (TCNQ), tetrathiafulvalene (TTF), and the charge transfer salt TTF-TCNQ as a function of incident energy (20 to 100 eV). The incident energy dependence of energy loss features allows the identification of triplet and symmetry forbidden excitations in TCNQ and TTF. The ELS spectrum of TTF-TCNQ is compared with high energy electron loss data to show that the surface of evaporated films of TTF-TCNQ contains 20–50% neutral TCNQ molecules.