Paul Nielsen

Photoemission studies of TCNQ compounds

Abstract The densities of occupied states of a series of TNCQ compounds have been determined by u.v. photoemission. A clear difference between TCNQ weak charge transfer complexes and anion radical salts including TTF-TCNQ is found. Due to strong electron-molecule coupling narrow delocalized electron bands are not observable.

Inelastic electron scattering lengths and charge transfer in tetrathiofulvalene-tetracyanoquinodimethane (TTF-TCNQ) films

Abstract Inelastic scattering lenghts of hot electrons in TTF and TCNQ have been determined. The electronic structure of a 1 : 1 mixture of TTF and TCNQ, obtained by ultraviolet photoemission spectroscopy, shows that at least 40% of the surface molecules are charge transferred. Implications for crystalline TTF-TCNQ are discussed.

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.

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...

Substrate dependent ionization and polarization energies of molecules: Dibenztetrathiafulvalene

Abstract The ionization energy of the highest occupied orbital of dibenztetrathiafulvalene condensed at 25° C is 5.8 eV for polycrystalline gold substrates and 5.1 eV for oxidized aluminum ones. Corresponding polarization energies are 1.0 and 1.7 eV. This dependence of ionization and polarization energies on substrate is a consequence of the different crystallite orientation on the two substrates, and demonstrates for the first time that in the case of organic materials these energies depend on the surface through which the photoelectrons are removed.

Simple Feedback Electrostatic Voltmeter with a Transparent Probe

A simple feedback system has been developed for application to either a field‐interrupting electrostatic modulator or to a vibrating electrode modulator; this system substantially increases the stability, sensitivity, and accuracy of the electrostatic voltmeter in which it is employed. An accuracy of 0.2% and a response rate of 2000 V/sec have been achieved in this instrument, which incorporates a transparent, vibrating NESA glass electrode to facilitate sample illumination.