Photoemission Source Research Articles

A robust and powerful green light photoemission source: The ferroelectric ceramics

The photoemission characteristics of ceramic disks of lead zirconate titanate lanthanum doped (PLZT), have been investigated. We observe 1 nC of extracted charge under an accelerating field of 20 kV/cm in poor vacuum conditions. The emission is clearly limited by space charge effects. The extrapolated quantum efficiency results in ≈10−6. The yield of a PLZT ceramic in the ferroelectric state and its slope versus light intensity have turned out higher than those of antiferroelectric ceramic. Samples in different experimental configurations have shown different nonlinear yields.

Photoemissive sources and quantum stochastic calculus

Just at the beginning of quantum stochastic calculus (QSC), Hudson and Parthasarathy proposed a quantum stochastic Schrodinger equation linked to dilations of quantum dynamical semigroups [7, 8, 11]. Such an equation has found applications in physics, mainly in quantum optics, but not in its full generality [6, 1, 2]. It has been used to give, at least approximately, the dynamics of photoemissive sources such as an atom absorbing and emitting light or matter in an optical cavity, which exchanges light with the surrounding free space. But in these cases the possibility of introducing the gauge (or number) process in the dynamical equation has not been considered. In this paper we want to show, in the case of the simplest photoemissive source, namely a two–level atom stimulated by a laser, how the full Hudson– Parthasarathy equation allows to describe in a consistent way not only absorption and emission, but also the scattering of the light by the atom. Let us recall the Hudson–Parthasarathy equation; this is just to fix our notations, while for the proper mathematical definitions and the rules of QSC we refer to the book by Parthasarathy [11]. We denote by F := F(X ) the Boson Fock space over the Hilbert space X := Z ⊗ L(R+) ≃ L(R+;Z), where Z is another separable complex Hilbert space. Let {ei, i ≥ 1} be a c.o.n.s. in Z and let us denote by Ai(t), A†i (t), Λij(t) the annihilation, creation and gauge processes associated with such a c.o.n.s. We denote by E(h), h ∈ X , the exponential vectors in F with normalization ‖E(h)‖2 = exp{‖h‖2}; E(0) is the Fock vacuum. We shall also use the Boson Fock spaces Ft := F ( L([0, t];Z) ) and F t := F ( L2((t,∞);Z) ) , for which we have F = Ft ⊗F , and the Weyl operators

Structure investigations of Xe-adsorbate layers by spin-polarized low-energy electron diffraction I. ( [formula omitted]) R30°- Xe/ Pt (111)

The ( 3 3 × 3 3 )R30° Xe layer on Pt(111) is studied by spin-polarized low-energy electron diffraction (SPLEED) . In the experiment spin-polarized electrons from a GaAs photoemission source are scattered at the target, and the spin-dependent intensities are measured in different diffracted beams. Corresponding calculations are performed using a relativistic LEED program. Measurements and calculations show strong contributions from multiple scattering between the substrate and the adlayer. The scattering contributions from the substrate are found to dominate the general structure of asymmetry and intensity profiles of integer order beams. In adsorbate-induced beams the contributions from the substrate are significantly weaker but nevertheless clearly present. The integer-order beams as well as the adsorbate-induced beams are suitable for structure investigations. Comparing measurements and calculations by means of r-factor analysis, we determine the local adsorption geometry. The Xe atoms are adsorbed in domains of fcc and hcp hollow sites on the Pt(111) surface with identical domain size probability and random distribution of the domains. The distance between the adsorbate layer and the topmost substrate layer is found to be 4.2 ±0.1 Å. Measurements and calculations agree very well in general.

Quantum trajectory theory for cascaded open systems.

The quantum trajectory theory of an open quantum system driven by a photoemissive source is formulated. The formalism is illustrated by applying it to photon scattering from an atom driven by strongly focused coherent light.

Relaxations in electron beams and adiabatic acceleration.

In order to produce electron beams with the minimum possible energy spread we have investigated the possibility of replacing the usual thermocathodes with a photoemissive source such as GaAs. A comparison carried out between these two sources indicated the latter as the best device to obtain a very-low-energy spread. Beam relaxation after emission also leads to an increase in the energy spread. Therefore an experimental study on the nature of relaxations occurring in electron beams, yielded both by a thermocathode and by a photocathode, has been performed. More specifically, we investigated the possibility of reducing the transverse-longitudinal and the longitudinal-longitudinal relaxations. With this aim, the features of adiabatic acceleration, which damp the pure longitudinal relaxation, have been examined. The experience gained during the measurement cycle demonstrated that an adiabatic structure, accelerating electrons emitted by a GaAs photocathode, leads to the best performances.

Mott and helium polarimetry of a spin polarized electron source

An absolute determination of the electron spin polarization of a GaAs photoemission source was made using the method of helium polarimetry. Measurements of the source polarization were also made using a Mott spin polarization analyzer. The authors report an electron beam polarization of (25.5+or-1)% and an effective Sherman function of 0.26+or-0.01 for the Mott detector at 31.8+or-0.6 keV incident electron energy. The Sherman function is also presented for a number of incident energies between 10 keV and 37 keV. A complete set of Stoke's parameters is reported for the 33P-23S (3889 AA) helium line. The characteristics of the source, Mott detector and helium polarimeter are discussed in detail.

Photoemissive ionisation source for ion mobility detectors

A combination of a photoemissive ionisation source coupled to an ion mobility detector has been developed which utilises a pulsed light source thereby eliminating the need for an ion injection gate. Factors influencing the performance of this negative ion detector have been investigated. With air as the sample and drift gas streams, the performance of the detector is comparable to conventional designs employing radioactive ionisers.

Xe-adlayers on Pt(111) and Pd(111): structure investigation by spin-polarized LEED

(√3 × √3)R30 ° Xe-layers on Pt(111) and Pd(111) are studied by spin-polarized LEED. In the experiment spin-polarized electrons from a GaAs photoemission source were scattered and the spin-dependent intensities are measured in different diffracted beams. Corresponding calculations were performed using a new relativistic LEED code. Measurements and calculations agree reasonably. They show strong contributions from multiple scattering between substrate and adlayer, and therefore spin-polarized LEED reveals strong sensitivity to structural parameters.

Inhomogeneous Electric Potential Distributions Induced by in Clusters Grown on P-WSE2 (0001) Surfaces

In deposited on UHV cleaved p-WSe 2 (0001) is investigated by photoelectron spectroscopy (XPS, UPS), low energy electron diffraction (LEED), scanning electron microscopy (SEM), and surface photovoltage (SPV) experiments. In forms an atomically abrupt, non-reactive metallic overlayer on the substrate. It grows in threedimensional clusters (Volmer-Weber growth mode). The experimentally observed band bending induced by In at RT is smaller than expected by the Schottky limit. Photoemission source induced SPV values at 100 K are saturated and different for the substrate and In overlayer emission lines. The results are explained by laterally inhomogeneous electric potential distributions due to the In clustering on the semiconductor surface.

Surface intervalley scattering on GaAs(110) studied with picosecond laser photoemission

Laser-based photoemission sources provide the unique opportunity to study dynamic electronic processes at surfaces and interfaces. Using angle-resolved, laser photoemission with < 1 ps time resolution, we have directly observed a new surface band at the X̄ point in the GaAs(110) surface Brillouin zone. The appearance of electron population in this valley occurs only as a result of scattering from the directly photoexcited valley at Г. The momentum resolution of our experiment has permitted us to isolate the dynamic electron population changes at both Г and X̄ and to deduce the scattering time between the two valleys.

A proposed superconducting photoemission source of high brightness

We describe the design of an electron injector for the production of a bunched cw electron beam of high brightness, low energy spread, and potentially high beam power. Electron beams of this kind are required for high efficiency and short wavelength free electron lasers. The injector consists of a superconducting reentrant cavity housing a photoemission cathode evaporated on a superconducting substrate. The cathode is illuminated with short light pulses of a mode-locked frequency doubled Nd: YAG laser. The experimental layout of the “superconducting photoemission source” is described together with its components: the photocathode preparation chamber, the cavity, the cryogenic setup, and the beam analysis system. The conceptual beam parameters are discussed and first results of calculations on beam dynamics using a particle-in-cell computer code are given.

High-brightness injectors for RF-driven free-electron lasers

Free-electron laser (FEL) oscillators require pulse trains of high peak brightness and, in some applications, high average power. Recent developments in the technology of photoemissive and thermionic electron sources in RF cavities for electron linac-injector applications offer promising advances over conventional electron injectors. Reduced emittance growth in high peak current electron injectors may he achieved by using high field strengths and by linearizing the radial component of the cavity electric field at the expense of lower shunt impedance. Harmonically excited cavities have been proposed for enlarging the phase acceptance of linac cavities and thereby reducing the energy spread produced in the acceleration process. Operation of injector linacs at a subharmonic of the main linac frequency is also proposed for enlarging the phase acceptance.

Electron-capture detector with a photoemissive electron source

Photoemission of electrons from thin metallic films exposed to ultraviolet light is used as a source of thermal electrons for a non-radioactive electron-capture detector. No positive ions or energised radicals are produced in the device, since the initial energy of the photoemitted electrons is less than 1 eV. Response is determined essentially by the transport properties of the electrons and negative ions, which inherently produces a gain mechanism. A sensitivity of at least one femtogram has been demonstrated for this new type of electron-capture detector, and it therefore has the potential for greater overall sensitivity than a conventional 63Ni detector.

Electron-capture detector with a photoemissive electron source

Photoemission of electrons from thin metallic films exposed to ultraviolet light is used as a source of thermal electrons for a non-radioactive electron-capture detector. No positive ions or energised radicals are produced in the device, since the initial energy of the photoemitted electrons is less than 1 eV. Response is determined essentially by the transport properties of the electrons and negative ions, which inherently produces a gain mechanism. A sensitivity of at least one femtogram has been demonstrated for this new type of electron-capture detector, and it therefore has the potential for greater overall sensitivity than a conventional 63Ni detector.

Calibration of a VUV spectrometer–detector system using synchrotron radiation

A new technique and apparatus have been developed for the measurement of absolute electron impact photoemission cross sections in the 30–150-nm wavelength range. Synchrotron light is used as the primary intensity standard for the calibration of the detection efficiency of a vacuum ultraviolet (VUV) spectrometer–detector system. A multiadjustable manipulator was used to position precisely a Seya-Namioka-type spectrometer–detector system with respect to a narrow ray of synchrotron radiation. By scanning the beam of synchrotron radiation across the surface of the grating in the spectrometer, precise simulation of the geometry of the light source encountered in the electron impact photoemission measurement was realized. Analysis of the results underscores the fact that for spectrometer calibrations in the VUV, the calibration procedure depends on the geometry of the experimental source. The simultaneous determination of the absolute apparatus response function of the spectrometer–detector system and the geometrical factors pertaining to the electron impact photoemission source allows photoemission cross sections in the VUV to be determined with unparalleled precision.

Electron Emission of over 200 A/CM2 from a Pulsed-Laser Irradiaied Photocaihode

High-current-density, bunched electron beams with low emittance are required for efficient operation of rf-linac-driven free-electron lasers (FELs). Laser-irradiated, photoemissive electron sources are suitable for this application. Currents of over 200 A have been generated in an ultrahigh vacuum chamber from a 1-cm2 Cs3Sb photocathode irradiated by a frequency-doubled, Q-switched pulse from a Nd: glass laser. These currents are over two times larger than previously reported from any photocathode. The duration of the electron pulse was 50 ns (FWHM), corresponding to the width of the 532-nm laser pulse.

Practical laser-activated photoemissive electron source

The design and performance of a laser-excited cesium antimonide (Cs3Sb) photoemissive electron source operating in an electron-gun configuration is described. Such an electron source has advantages of higher brightness, beam shapeability, low-temperature operation, external modulability, and ease of service compared with conventional thermal and field emission sources. With an uncooled photoemissive source operating at 2.9 A/cm2, cathode life exceeded 50 h without degradation. Photoemissive electron sources should be especially useful in applications requiring high-resolution imaging or high-frequency pulsed operation.

High current density photoemissive electron source

A high current, high current density cesium antimonide photoemissive electron source was constructed and tested over an extended period of time with small degradation. A photocurrent density of more than 3 A/cm2 was obtained with a performance drop of about 15% over a 70-h period. The slow decay was caused by cesium desorption from the cathode. Cesium evaporated onto the surface at the end of the test rejuvenated the photocathode to its original response. Such a photoemissive source has particular applications to electron beam lithography machines and high resolution, nondestructive electron microscopes.

Change of Energy-Level Distribution of Photoemission Sources in Al and Zn Caused by Scratching

Photoemission from Al (99.99% purity) specimens was measured for the photostimulation wavelength ranging from 190 to 700 nm. The specimens were scratched with a steel needle at temperatures from 77 to 290 K and at pressures from 10-5 to 10-7 Torr. Whereas prior to scratching the emission peaks from Al specimens appeared in a range from 250 to 290 nm, they shifted to a 290–350 nm range after scratching. Although this result is nothing but a manifestation of the so-called exoelectron emission, it has not been well recognized that scratching causes a change in the number of photoemission sources at various energy levels. The wavelength at the peak emission was influenced neither by the specimen temperature nor by the degree of vacuum. Similar results have been obtained for Zn specimens, though the yield observed was two orders of magnitude lower than that for Al.