Showing papers on "Photoexcitation published in 1980"

The GaAs spin polarized electron source

Abstract: The design, construction, operation, and performance of a spin polarized electron source utilizing photoemission from negative electron affinity (NEA) GaAs are presented in detail. A polarization of 43±2% is produced using NEA GaAs (100). The polarization can be easily modulated without affecting other characteristics of the electron beam. The electron beam intensity depends on the intensity of the exciting radiation at 1.6 eV; beam currents of 20 μA/mW are obtained. The source is electron optically bright; the emittance phase space (energy‐area‐solid angle product) is 0.043 eV mm2 sr. The light optics, electron optics, and cathode preparation including the GaAs cleaning and activation to NEA are discussed in depth. The origin of the spin polarization in the photoexcitation process is reviewed and new equations describing the depolarization of photoelectrons in the emission process are derived. Quantum yield and polarization measurements for both NEA and positive electron affinity surfaces are reported. T...

High-resolution one and two photon excitation spectra of trans, trans-1,3,5,7-octatetraene

Abstract: One and two photon excitation spectra for the lowest energy singlet transition (2 1Ag←1 1Ag) of trans, trans 1,3,5,7‐octatetraene in n‐octane at 4.2 K(0–0 at 28 56l cm−1) have been measured and analyzed. The one photon spectrum is found to be vibronically induced while the two photon spectrum is electronically allowed. The contribution of the higher lying 1 1Bu←11Ag transition (0–0 at approximately 32 100 cm−1) to the observed two photon excitation spectrum is found to be negligible allowing vibrational overtones of the 2 1Ag←1 1Ag system to be observed. On the bases of frequency, intensity, and one or two photon allowedness, 128 measured lines are assigned as combinations of 4 antisymmetric and 11 symmetric normal modes.

Observation and Analysis of Surface States on TiO2 Electrodes in Aqueous Electrolytes

Abstract: When a electrode in an aqueous electrolyte is biased enough positive of its flatband potential and then photoexcited with light of wavelength shorter than 400 nm, photocurrent is observed. If the electrode is subsequently swept to more negative voltages in the dark a negative current is observed which is in excess of the normal dark current observed on a subsequent sweep with no photoexcitation between sweeps. The excess current is observed to have a maximum at a voltage positive of the flatband potential. In this paper, the growth and decay of this reduction peak are investigated experimentally. The peak is analyzed as a reduction by conduction band electrons of surface states that were oxidized by valence band holes during the photoexcitation. The observations favor the interpretation that these states are intermediates of the reaction leading to evolution. This interpretation, however, is not unequivocally established. It is clear that the conduction‐band electron reduction of a state that was previously oxidized by a valence‐band hole is in effect an electron‐hole recombination. This recombination controls the onset of photocurrent with voltage. There are 1013–1014 of these states per cm2 and the cross section for electron interaction with this surface state is estimated to be 10−16–10−17 cm2 based on the analysis used to describe the peak. The usefulness of this analysis in investigating these states and surface states due to surface coatings is discussed.

Photoexcitation and ionization in molecular oxygen - Theoretical studies of electronic transitions in the discrete and continuous spectral intervals

Abstract: Theoretical studies of valence-electron (1πg, 1πu, 3σg) photoexcitation and ionization cross sections in molecular oxygen are reported employing separated-channel static-exchange calculations and the Stieltjes–Tchebycheff moment-theory technique. As in previously reported investigations of photoexcitation and ionization in small molecules following this approach, canonical Hartree–Fock orbitals, large Gaussian basis sets, and many-electron eigenstates of correct symmetry are used in defining appropriate noncentral static-exchange potentials and in computations of the appropriate discrete and continuum transition strengths. It is particularly important in molecular oxygen to incorporate the appropriate ionic parentages of the various photoionization multiplet states in order to obtain the correct partial-channel cross sections. The calculated discrete series associated with 1πg excitation are found to be in good agreement with available experimental assignments and previously reported theoretical studies, and the predicted states associated with 1πu and 3σg excitations are in general accord with assignments for the higher series based on spectral and quantum-defect analysis. Although the observed photoelectron spectra and photoionization cross sections are relatively complex, the calculated total vertical electronic photoabsorption cross section and the partial-channel photoionization cross sections for production of X 2πg, a 4πu, A 2πu, 2 2πu, 3 3IIu, b 4∑g-, and B 2∑g-, ionic states are found to be in good accord with recent synchrotron radiation, line-source, electron-impact, and (e,2e) dipole oscillator-strength measurements when proper account is taken of the parentages of the various multiplet states. The partial-channel cross sections exhibit resonancelike structures that can be attributed to contributions from diabatic valencelike virtual states that appear in the appropriate photoionization continua, rather that in the corresponding discrete spectral intervals. These features in the dipole spectrum of molecular oxygen are discussed and are contrasted and compared with the results of previously reported related studies in molecular nitrogen and carbon monoxide.

Photoexcitation and ionization in molecular fluorine: Stieltjes-Tchebycheff calculations in the static-exchange approximation

Abstract: Theoretical investigation of outer (1pig, 1piu, 3sigmag) and inner (2sigmau, 2sigmag) valence-shell electronic photoexcitation and ionization cross sections in molecular fluorine are reported employing separated-channel static-exchange calculations and Stieltjes–Tchebycheff (S–T) moment-theory techniques. The discrete vertical electronic 1pig excitation series are found to be in good agreement with recent spectral assignments and previously reported theoretical studies, and those for 1piu, 3sigmag, 2sigmau and 2sigmag excitations are in general accord with position and intensity estimates based on quantum-defect analysis. Certain of the partial-channel photoionization cross sections in F2 are seen to exhibit resonancelike features similar to those reported recently in related S–T studies of photoionization in N2, CO, and O2. The resonances can be attributed to valencelike and pre-Rydberg diabatic states that cross the outer limbs of appropriate Rydberg series and corresponding ionic-state potential curves as functions of internuclear coordinate, giving rise to large continuum transition intensities at the ground-state equilibrium internuclear separation. In contrast to the situation in N2, CO, and O2, however, there is no evidence of a resonance like sigma-->sigma* feature in the 3sigmag-->ksigmau photoionization channel in F2. Rather, this resonance in F2 appears as a strong N-->Vg transition below the 3sigmag ionization threshold, and the corresponding partial-channel photoionization cross section is seen to be structureless. Although experimental studies of partial-channel photoionization cross sections are apparently unavailable for comparison, the calculations reported here should provide reliable approximations to the dipole excitation/ionization spectra in F2, and are helpful in understanding and clarifying the dependences of photoionization spectra in light diatomic molecules on shell occupancy and equilibrium internuclear separation when compared with the results of previous studies of photoionization in N2, CO, and O2.

Internal photoemission mechanisms at interfaces between germanium and thin metal films

Abstract: The quantum efficiency associated with the internal photoemission of electrons over the Schottky barrier (of height φ B ) at the metal-Ge interface has been studied experimentally for several metals (Au, Cu, Ag, Pb, and Ni). A theoretical description of this mechanism has been developed in which we take into account the front and back optical absorptance, hot electron scattering, and multiple reflections of excited electrons from the surfaces of the thin electrode film. We have found it necessary to impose a modification of the Fowler theory of photoemission when applied to internal photoemission from thin metal films over a Schottky barrier. This modification relates to an enhanced photoexcitation within the metal films which is attributed in the present theory to a density of states which exhibits a peaked distribution in energy rather than the simple parabolic bands assumed by Fowler. It is clear from the present study that the majority of photoelectron excitation occurs from a small region of energy of the order of a fraction of an electron volt near the Fermi energy. The theoretical model presented here defines two important parameters: a hot-electron mean free path (L e ) and an energy (E ef ) given by the difference between the Fermi level and the effective conduction hand minimum associated with the region of energy in the metal near the Fermi level where the electron distribution is strongly peaked. Values of L e for Au is 550 A, Ag is 570 A, Cu is 450 A, and Pb is 55 A. E ef for Au is 0.1 eV, Ag is 0.152 eV, Cu is 0.11 eV, Pb is 0.1 eV. The validity of this model is confirmed by the experimental finding that the parameters L e and E ef are independent of metal thickness.

Spin polarized optical density of states of cubic uranium compounds

Abstract: The conduction and valence band states of uranium monochalcogenides and monopnictides have been investigated by photoemission techniques. The energy distribution curves and the spin polarization of the photoelectrons show that the 5f states are located at the Fermi energy. The absence of final state multiplet-splittings and the participation of the 5f electrons in chemical bonding point to extended states, whereas the energy dependence of their photoexcitation cross section is typical for atom-like orbitals. For the ferromagnetic compounds the spin polarization of the photoemitted 6d electrons is opposite to the direction of the magnetization. It is proposed that a virtual bound state model accounts for these features.

Self-absorption of heliumlike satellite lines in high-density fusion plasmas

Abstract: The high densities obtained in recent laser fusion experiments have created a need for additional plasma density diagnostics. The ratio of two heliumlike satellite lines, the 1s2p /sup 3/P..-->..2p/sup 2/ /sup 3/P and 1s2s /sup 3/S..-->..2s2p /sup 3/P transitions, has shown promise as a spectral diagnostic when laser-imploded microballoons are seeded with medium-atomic-weight gases. In this study results are presented from a collisional-radiative ionization-dynamics model with photoexcitation processes included that indicate that the emission from these satellite lines is strongly affected by opacity in density and temperature regimes common to plasmas to which this diagnostic could be applied effectively. The radiation emission attenuated by photon reabsorption is presented for neon, aluminum, and argon plasmas and compared with results predicted when the calculation is undertaken in an optically-thin-plasma approximation. The opacity effects are seen to cause multivaluedness in the line ratio at several temperatures, and an overall loss of sensitivity of the ratio with density is predicted.

Photoionisation of helium to He+(n=2)

Abstract: A theoretical study of the simultaneous photoionisation and photoexcitation of a helium atom leaving its ion in an n=2 state is carried out by including various important electronic configurations in the initial and final state of the transition. The result is compared and found to be in good agreement with the most recent experimental measurements.

Photoinitiation of one electron reactions in dipeptides and proteins containing tryptophan and tyrosine

Abstract: — A radical transformation of the type Trṗ-TyrOH TrpH-TyrO, previously observed by pulse radiolysis, has been detected following 265 nm laser excitation of oxygen saturated neutral aqueous solutions of TrpH-TyrOH, TyrOH-TrpH and cyclo-TrpH-TyrOH. A similar process was observed after photoexcitation of the proteins α-lactalbumin and α-chymotrypsin.

Soft X-ray population inversion in laser plasmas by resonant photoexcitation and photon-assisted processes

Abstract: In this paper, the results of studies on soft X-ray population inversion by the resonant photoexcitation process are presented. The experimental studies in highly ionized multicomponent plasmas produced by high power glass laser indicate large population inversion densities at wavelengths \lsim 130 A. Intensity inversions of resonance lines from the n = 3 and n = 4 levels of Mg11+ions at electron densities as high as 1020cm-3have been observed, indicating that gains of \sim 5-10 cm-1have been achieved. These results and other photon-assisted soft X-ray population inversion schemes in inhomogeneous plasmas are discussed.

Enhanced photoexcitation from the surface of aluminum

Abstract: The enhancement of photoionization cross sections due to the spacially varying photon field at the surface of a metal has been observed by measuring the cross section for the surface state and Fermi level on Al(001) for photon energies between 9 and 23 eV. The data are in excellent agreement with the results of a self‐consistent jellium calculation which takes into account the induced short wavelength longitudinal fields at the surface of the metal. These fields are much more effective than the transverse fields in providing the momentum necessary for photoexcitation and this mechanism dominates all others, including direct vertical transitions, in the energy range considered. The implications of this effect on photoemission studies of surface and bulk properties will be discussed.

Semiconductor-catalysed [2 + 2]photocycloreversion of a strained cage molecule

Abstract: The ZnO- or CdS-catalysed photocycloreversion of the strained cage molecule (1) afforded the diene (2), which was also produced by a cerium ammonium nitrate-catalysed dark reaction and by photoexcitation of the charge-transfer complex of (1) with tetracyano-ethylene.

Excitation and emission bands of hydrogen atoms in a solid neon matrix

Abstract: H atoms have been stabilized in solid Ne matrices by photolysis of H 2 molecules. Photoexcitation results in the emission of L α and L β . The excitation spectrum of the L α line shows maxima at 10.69 eV, 13.51 eV and 14.17 eV. The two lower energetic bands are attributed to the n = 1 and n = 2 members of a series of exciton states of the H impurity. The third maximum is tentatively assigned to predissociation of the H 2 molecules leading to an excited H atom in the 2p state and subsequent decay by L α emission.

Ion-implanted PLZT for photoferroelectric image storage and display devices

Abstract: A substantial increase in photoferroelectric image storage sensitivity has been achieved in ferroelectric-phase PLZT ceramics by implanting protons (H+ ions) into the image storage surfaces. The 10- to 100-fold increase in sensitivity is attributed to increased densities of carrier photoexcitation and trapping centers and to an increase in photon absorption in the near-UV. Preliminary measurements indicate that these effects are primarily caused by implantation-produced disorder.

Photoelectron photoion molecular beam spectroscopy

Abstract: The use of supersonic molecular beams in photoionization mass spectroscopy and photoelectron spectroscopy to assist in the understanding of photoexcitation in the vacuum ultraviolet is described. Rotational relaxation and condensation due to supersonic expansion were shown to offer new possibilities for molecular photoionization studies. Molecular beam photoionization mass spectroscopy has been extended above 21 eV photon energy by the use of Stanford Synchrotron Radiation Laboratory (SSRL) facilities. Design considerations are discussed that have advanced the state-of-the-art in high resolution vuv photoelectron spectroscopy. To extend gas-phase studies to 160 eV photon energy, a windowless vuv-xuv beam line design is proposed.

Investigations of Cr in GaAs Using Phonons

Abstract: The paper reviews published spectroscopic work on Cr in GaAs using a variety of phonon techniques from 0.1 to 1000 GHz. All this work was done on semi-insulating material. It then describes recent Nottingham work on n-type, p-type and semi-insulating samples before and after photoexcitation using acoustic paramagnetic resonance, relaxation measurements, thermal conductivity and magnetothermal conductivity (frequency crossing). The spectra observed are thought to be largely due to Cr2+ but cannot be explained by the model with a static Jahn-Teller effect, which describes the EPR data at two very different frequencies. Possible explanations are advanced.

The Effect of the Strong Configuration Interaction of the Atomic Transition in a Many-Electron System

Abstract: As a first step in an attempt to carry out a systematic study of the effect of the strong configuration interaction to the atomic transition in a many-electron system, we have developed a new theoretical approach to study the simultaneous photoionization and photoexcitation process of the He atom. Detailed discussion will be given to the physical interpretation of a few new aspects which have emerged from our study.