Photoelectric Threshold Research Articles

Two approaches for the passive charge management of contactless test masses

Free floating test masses (TM) of inertial reference instruments accumulate charge mainly through the triboelectric effect during separation from their housings and, if in the space environment, from cosmic radiation. These charges will degrade the accuracy of high sensitivity accelerometers and drag-free sensors. We demonstrate in ground testing two passive bipolar charge management systems using photoelectrons emitted from gold coated surfaces under illumination by ultraviolet light emitting diodes (UV-LEDs) with 255 nm, 275 nm, and 295 nm central wavelengths. The first method uses fast photoelectrons, generated by two 255 nm UV-LEDs with adjustable-intensity (through fine-tuning of their excitation currents) and illuminating the TM and its housing respectively. A second technique uses slow photoelectrons generated by one UV-LED, of either 275 nm or 295 nm, directed at the TM. Fast and slow electrons are defined as having kinetic energies after photoemission above and below , where is the maximum allowable potential required for normal operation of the TM. In its optimized configurations and following an exposure of <30 s to UV, the fast-photoelectron control system converges to zero TM potential from ±1 V with a drift of ≈1.5 mV/day. The slow-photoelectron system requires about 5 min to converge to zero TM potential from 100 mV, with a similar drift of ≈2.0 mV/day. For reference, for the LISA and LISA pathfinder sensors. Insight into the slow photoelectrons charge control method is provided by our simple Gaussian model of the work function for gold films (that approximates their quasi-triangular measured shape): 4.6 eV center, 0.2 eV standard deviation, and 4.30 eV and 4.90 eV cutoffs. The longest wavelength (lowest energy) photoemission is obtained with the 295 nm LED (11 nm FWHM) at about 285 nm (4.35 eV), where only about 3.5% of the photons have an energy above the photoelectric threshold. While the two-source UV fast photoelectron method is a modification of the LISA Pathfinder approach, the slow photoelectron method is a new passive charge management technique. These two passive techniques were developed for instruments without sensing and activation systems. For instruments with electric fields surrounding the TM and more complex geometries additional adaptations are required.

Impact of the photoelectric threshold sensitivity on the work function determination—Revealing ultra-low work functions of caesiated surfaces

The exploitation of the photoelectric effect is a prominent method for the in situ measurement of the absolute work function of a surface. In the case of metallic surfaces, the Fowler theory is routinely applied for fitting or extrapolating the measured photoelectric yield data to determine the work function value. However, for the reliable application of the Fowler method, attention must be paid to the experimental sensitivity to the photoelectric behavior close to the threshold, which is mainly determined by the signal-to-noise ratio for photocurrent detection and the available photon energies used for irradiation. This is illustrated by means of applying a photoelectric work function measurement system during a Cs coating process of a metal surface, where insufficiently low photon energies or an unfavorable noise level can lead to a severe overestimation of the work function of the order of 1 eV. By a sufficient enhancement of the photoelectric sensitivity, it is now unveiled that ultra-low surface work functions of 1.25 ± 0.10 eV can be generated via caesiation of metallic surfaces (here molybdenum and stainless steel) under vacuum conditions of 10−6–10−5 mbar, which is most probably the result of the formation of an oxidized Cs adlayer.

An investigation of the changes in mass attenuation and energy absorption coefficients at K, L and M photoelectric absorption thresholds

An analysis of photoelectric attenuation and absorption data at the threshold energies for the K, L and M shell absorption discontinuities.

Photoelectron Emission Yield of Au Film: Theoretical Calculation and Measurement

Spacecraft in orbit are exposed to intense solar radiation in the space environment, causing the material surface to be charged and damaged. The incident electrons, ions, and photons on the surface of spacecraft materials produce secondary electrons, backscattered electrons, photoelectrons, and so on. The damage to the material caused by the emitted photocurrent is more serious when the incident energy is higher than the photoelectric threshold. In view of the significant threat of particle radiation to the charge protection and operational life of spacecraft materials, in this article, a theoretical model of the photoelectron emission yield (PEEY) is studied, and a test system of metal materials under ultraviolet light is developed. First, we derive a three-step model of the photoemission of metal materials and calculate the PEEY spectrum of Au. Second, the optical system is built by using a deuterium lamp, a monochromator, and an aperture. The installation structure of the photodiode is designed, and the photon number is obtained by measuring the incident luminous flux. Third, a central through-hole microchannel plate is used and a voltage division circuit module is designed to realize effective collection of photoelectrons emitted from the sample surface. This approach realizes a detectable range for an Au film from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−7</sup> el./ph in the spectral range of 130–258 nm. The reliability of the test system is proved by comparing the experimental results with the theoretical results.

Ab Initio Molecular Dynamics Simulations of Solvated Electrons in Ammonia Clusters.

We investigated excess electron solvation dynamics in (NH3)n– ammonia clusters in the n = 8–32 size range by performing finite temperature molecular dynamics simulations. In particular, we focused on three possible scenarios. The first case is designed to model electron attachment to small neutral ammonia clusters (n ≤ ∼10) that form hydrogen-bonded chains. The excess electron is bound to the clusters via dipole bound states, and persists with a VDE of ∼160 meV at 100 K for the n = 8 cluster. The coupled nuclear and electronic relaxation is fast (within ∼100 fs) and takes place predominantly by intermolecular librational motions and the intramolecular umbrella mode. The second scenario illustrates the mechanism of excess electron attachment to cold compact neutral clusters of medium size (8 ≤ n ≤ 32). The neutral clusters show increasing tendency with size to bind the excess electron on the surface of the clusters in weakly bound, diffuse, and highly delocalized states. Anionic relaxation trajectories launched from these initial states provide no indication for excess electron stabilization for sizes n < 24. Excess electrons are likely to autodetach from these clusters. The two largest investigated clusters (n = 24 and 32) can accommodate the excess electron in electronic states that are mainly localized on the surface, but they may be partly embedded in the cluster. In the last 500 fs of the simulated trajectories, the VDE of the solvated electron fluctuates around ∼200 meV for n = 24 and ∼500 meV for n = 32, consistent with the values extrapolated from the experimentally observed linear VDE–n–1/3 trend. In the third case, we prepared neutral ammonia cluster configurations, including an n = 48 cluster, that contain possible electron localization sites within the interior of the cluster. Excess electrons added to these clusters localize in cavities with high VDEs up to 1.9 eV for n = 48. The computed VDE values for larger clusters are considerably higher than the experimentally observed photoelectric threshold energy for the solvated electron in bulk ammonia (∼1.4 eV). Molecular dynamics simulations launched from these initial cavity states strongly indicate, however, that these cavity structures exist only for ∼200 fs. During the relaxation the electron leaves the cavity and becomes delocalized, while the cluster loses its initial compactness.

Effective decrease of photoelectric emission threshold from gold plated surfaces.

Many applications require charge neutralization of isolated test bodies, and this has been successfully done using photoelectric emission from surfaces which are electrically benign (gold) or superconducting (niobium). Gold surfaces nominally have a high work function (∼5.1 eV) which should require deep UV photons for photoemission. In practice, it has been found that it can be achieved with somewhat lower energy photons with indicative work functions of (4.1-4.3 eV). A detailed working understanding of the process is lacking, and this work reports on a study of the photoelectric emission properties of 4.6 × 4.6 cm2 gold plated surfaces, representative of those used in typical satellite applications with a film thickness of 800 nm, and measured surface roughnesses between 7 and 340 nm. Various UV sources with photon energies from 4.8 to 6.2 eV and power outputs from 1 nW to 1000 nW illuminated ∼0.3 cm2 of the central surface region at angles of incidence from 0° to 60°. Final extrinsic quantum yields in the range 10 ppm-44 ppm were reliably obtained during 8 campaigns, covering a period of ∼3 years but with intermediate long-term variations lasting several weeks and, in some cases, bake-out procedures at up to 200 °C. Experimental results were obtained in a vacuum system with a baseline pressure of ∼10-7 mbar at room temperature. A working model, designed to allow accurate simulation of any experimental configuration, is proposed.

Experimental Investigation of Total Photoemission Yield from New Satellite Surface Materials

Electron photoemission influences spacecraft surface potentials and the surrounding plasma, and many modern spacecraft use new uncharacterized materials. The angle-dependent photoemission properties were measured for niobium C103 alloy, molybdenum TZM alloy, tantalum tungsten alloy, Elgiloy, graphite lubricant epoxy (DAG213) and titanium nitride at the Bending for Emission Absorption and Reflectivity beam line. The properties of tungsten were also studied to verify the method with past data. The materials were prepared similarly to flight materials. The properties were measured from 0 (normal) to 80 deg (grazing) incidence preannealed and postannealed conditioning of the samples. The yield was measured in all cases for both p and s polarization at each angle of incidence. Results are presented for the photoelectric threshold and photoelectron yield for photon energies up to 30 eV. The work function was also found for each material tested. An analytical equation was then used to fit the normal photoelectron yield for each material to help obtain photocurrents, which were calculated assuming solar illumination at 1 astronomical unit (AU) at normal incidence.

Enhanced transient photovoltaic characteristics of core–shell ZnSe/ZnS/L-Cys quantum-dot-sensitized TiO2 thin-film**Project supported by the Natural Science Foundation of Hebei Province, China (Grant Nos. E2013203296 and E2017203029).

Photoanodic properties greatly determine the overall performance of quantum-dot-sensitized solar cells (QDSCs). In the present report, the microdynamic behaviors of carriers in the nanocomposite thin-film, a ZnSe QD-sensitized mesoporous La-doped nano-TiO2 thin-film, as a potential candidate for photoanode, are probed via nanosecond transient photovoltaic (TPV) spectroscopy. The results confirm that the L-Cys ligand has a dual function serving as a stabilizer and molecular linker. Large quantities of interface states are located at the energy level with a photoelectric threshold of 1.58 eV and a quantum well (QW) depth of 0.67 eV. This QW depth is approximately 0.14 eV deeper than the depth of QW buried in the ZnSe QDs, and a deeper QW results in a higher quantum confinement energy. A strong quantum confinement effect of the interface state may be responsible for the excellent TPV characteristics of the photoanode. For example, the peak intensity of the TPV response of the QD-sensitized thin-film lasts a long time, from 9.40 × 10−7 s to 2.96 × 10−4 s, and the end time of the PTV response of the QD-sensitized thin-film is extended by approximately an order of magnitude compared with those of the TiO2 substrate and the QDs. The TPV characteristics of the QD-sensitized thin-film change from p-type to n-type for the QDs before and after sensitizing. These properties strongly depend on the extended diffusion length of the photogenerated carries and the reduced recombination rate of photogenerated electron-hole pairs, resulting in prolonged carrier lifetime and an increased level of electron injection into the TiO2 thin-film substrate.

Sub-terahertz microsecond optically controlled switch with GaAs active element beyond the photoelectric threshold

We study an unusual working regime of a recently developed sub-terahertz microwave cavity-based switch. The resonator cavity includes a semiconductor plate which is illuminated by laser emission beyond the photoelectric threshold. Despite a significant change to the conventional process of photoelectric effect we have found that the switch works. Typical switching performance rate is about 1 μs for the regime. A process of carrier density relaxation beyond the photoelectric threshold is discussed. An idea of diagnostic method for the semiconductor's quality is proposed.

Photoacoustic and surface photovoltaic characteristics of L-Cysteine-capped ZnSe quantum dots with a core-shell structure

The study on photoelectronic characteristics of ZnSe quantum dots (QDs) is of significance for investigating its microelectronic structure and expanding its potential applications because ZnSe QD has low biologic toxicity. In the present paper, the surface photovoltaic and photoacoustic technologies, and laser Raman, X-ray diffraction, transmission electron microscopy and Foureier transform infrared spectroscopy spectrum are jointly used to probe the microstructures, the photoacoustic and surface photovoltaic characteristics of L-Cysteine-capped ZnSe QDs prepared by water-phase synthesis at different reflux temperatures. The results indicate that the ZnSe QDs with a mean grain size of about 3 nm has a core-shell ZnSe/ZnS/L-Cys structure, in which the sulfhydryl groups in ligand prefer reacting with Zn atom at the (220) face to form the ZnS shell layer between the core-ZnSe and ligand L-Cys. The results show that the QDs with n-type photovoltaic property display a wide range of surface photovoltaic response and weak photoacoustic signal upon the illumination of near ultraviolet to visible light as compared with others QDs with similar core-shell structures in II-VI group. Especially, the strong SPV response and the weak PA signal in a wavelength region of 350-550 nm imply that the photon energies in the range are almost all used to produce the surface photovoltaic (SPV) phenomenon instead of the thermal lattice vibration caused by non-radiative de-excitation process. This reveals the energy complementary relationship between the photoacoustic and the surface photovoltaic phenomena of the QDs. The PA signals appearing in a short wavelength range of 300-350 nm and the Raman peaks located in a high frequency ranges of 1120 cm-1, 1340 cm-1 and 1455 cm-1 are identified as relating closely to the multi-phonon vibration modes of ligand L-Cys. At low reflux temperature, the photoelectric threshold of the SPV response that relates to the core-ZnSe displays a red shift to a certain extent as compared with the bulk ZnSe. The narrowed bandgap may be attributed to quantum confinement effect of the QDs. In addition, the intensity of the SPV response that relates to the core-ZnSe gradually increases with the decrease of the reflux temperature. The results show that the above improved surface photovoltaic characteristics of the QDs may benefit from the reduced average grain size of the ZnSe QDs, thus causing its surface and small-size effects.

Ambient pressure photoemission spectroscopy of metal surfaces

We describe a novel photoemission technique utilizing a traditional Kelvin probe as a detector of electrons/atmospheric ions ejected from metallic surfaces (Au, Ag, Cu, Fe, Ni, Ti, Zn, Al) illuminated by a deep ultra-violet (DUV) source under ambient pressure. To surmount the limitation of electron scattering in air the incident photon energy is rastered rather than applying a variable retarding electric field as is used with UPS. This arrangement can be applied in several operational modes: using the DUV source to determine the photoemission threshold (Φ) with 30–50meV resolution and also the Kelvin probe, under dark conditions, to measure contact potential difference (CPD) between the Kelvin probe tip and the metallic sample with an accuracy of 1–3meV. We have studied the relationship between the photoelectric threshold and CPD of metal surfaces cleaned in ambient conditions. Inclusion of a second spectroscopic visible source was used to confirm a semiconducting oxide, possibly Cu2O, via surface photovoltage measurements with the KP. This dual detection system can be easily extended to controlled gas conditions, relative humidity control and sample heating/cooling.

Dual Mode Kelvin Probe: Featuring Ambient Pressure Photoemission Spectroscopy and Contact Potential Difference

We describe a novel dual-mode Kelvin probe featuring ambient pressure Photoemission Spectroscopy (PES), which yields information on the absolute work function (Φ) of a metal and the Ionisation Potential (IP) of a semiconductor, coupled with a high resolution Contact Potential Difference capability which can be extended to Surface Photovoltage measurements. The relative energy resolution are 50 meV for PES and 1-3 meV for CPD. To surmount the limitation of electron scattering in air the incident photon energy is rastered rather than applying a variable retarding electric field as is used UPS. We propose a mechanism of atmospheric ion generation and show that for the metal photoresponse obeys Fowler Theory. The relationship between CPD and photoelectric threshold is a useful tool in characterizing the electrical behavior of materials. We illustrate this with native oxide covered Cu and n-type Si. Further we show that the photoresponse can be used to generate the near Fermi-level Density of States (DOS) in Iron and Nickel-Phthalocyanine.

Two-photon ionization of ground state hydrogen from the photoelectric threshold up to 40 keV

Our study of the ionization of the 1s state of the hydrogen atom through the absorption of two identical photons covers a region of higher energies than those considered up to now; it aims to establish the region of validity of nonrelativistic dipole approximation for the total generalized cross section.

Triboelectron emission from metal surfaces in sliding contact with polytetrafluoroethylene: Relevance to work function and surface potential

The mechanism of the triboelectron emission (TriboEE) occurring in sliding contact between metals and polytetrafluoroethylene (PTFE) has been investigated. The mapping of the TriboEE intensity for 16 kinds of metals onto the periodic table revealed that the order of the TriboEE intensity between two adjoining metals in the same period was well correlated with that of the work function and photoelectric threshold of metal samples and with that of the surface potential of metal samples and mated PTFE riders after the TriboEE measurement. The relationship of TriboEE intensity to the electronegativity, the ionization energy, and the electron affinity of the elements and the electrical conductivity of metals was also examined.

Photoelectron imaging of tetrahydrofuran cluster anions (THF)n− (1≤n≤100)

Anionic tetrahydrofuran clusters (THF)(n) (-) (1≤n≤100) are studied with photoelectron imaging as gas-phase precursors for electrons solvated in THF. Photoelectron spectra of clusters up to n=5 show two peaks, one of which is attributed to a solvated open chain radical anion and the other to the closed THF ring. At n=6, the spectra change shape abruptly, which become more characteristic of (THF)(n) (-) clusters containing solvated electrons. From n=6-100, the vertical detachment energies (VDEs) of these solvated electron clusters increase from 1.96 to 2.71 eV, scaling linearly with n(-1/3). For fully deuterated (THF-d8)(n) (-) clusters, the apparent transition to a solvated electron cluster is delayed to n=11. Extrapolation of the VDEs to infinite cluster size yields a value of 3.10 eV for the bulk photoelectric threshold. The relatively large VDEs at onset and small stabilization with increasing cluster size compared to other solvated electron clusters may reflect the tendency of the bulk solvent to form preexisting voids that can readily solvate a free electron.

Photoelectric threshold of silicon wafer surfaces implanted with H, Si and Ar ions

AbstractThe effect of ion implantation on the photoemission spectral yield for native oxide‐covered Si(100) wafer surfaces has been studied. Three types of H, Si and Ar atoms were employed for the ion implantation. For one sample the yield curve was measured successively first at 25 °C, next after heating to 340 °C, and subsequently after cooling to 40 °C. All implanted samples showed the photoemission in the spectral yield with a much enhanced intensity compared to a nonimplanted sample, moreover, more strongly at 340 °C than at 25 and 40 °C. The analysis of the yield curve showed two distinct photothresholds of ϕ1 and ϕ2, which can be attributed to the interface states at the SiO2/Si interface and the silicon bulk, respectively, and the creation of a localized emitting center ascribed to E′ center (≡ Si ·). Both thresholds for the implanted samples were reduced about 0.3–0.5 eV compared to those for the nonimplanted sample. The localized center appeared at the photon energy of 5.66 eV (25 and 40 °C) and 5.76 eV (340 °C), and the amount of the centers created at 340 °C was much greater than that at 25 and 40 °C. Interestingly, the amount of the E′ centers at 25 °C for the H implanted sample tended to decrease with an increase in the concentration of implanted H atoms. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Experimental verification of light induced field emission

This letter reports the experimental verification of the recently predicted phenomenon that the electric field emission current from a negatively charged surface gets enhanced by incidence of light (even of frequency below the photoelectric threshold) on the cathode.

Relationships between strain and band structure in Si(001) and Si(110) nanomembranes

The flexibility of single-crystal Si nanomembranes allows strain to be applied elastically without introducing dislocations in the fabrication process, resulting in uniform strain. It is also relatively easier to apply different types and orientations of strain to Si using elastic-strain sharing than by the traditional graded-strained-layer approach. We use X-ray absorption spectroscopy to measure the effect of uniform biaxial strain on several features of the conduction band structure of Si with (001) and (110) orientations. By also measuring the $\text{Si}\text{ }2p$ photoelectric threshold, we are able to determine the absolute positions of features of the Si conduction band and their change with strain.

Influence of ferroelectricity on the photoelectric effect of LiNbO3

A comparison between domain dependent photochemical and photoelectric cation reduction in LiNbO3 is presented. The reduction in photoelectric threshold for LiNbO3 due to the depolarization field allows UV irradiation to produce free electrons that can participate in photochemical reduction in silver nitrate. This is in addition to domain directed photophysics, where influences on the space charge layer due to the internal dipole of a ferroelectric determine the carrier at the surface. We show that the interaction of photoelectric and domain dependent influences is observed in LiNbO3 due to the low electron affinity (∼1–1.5 eV) and band bending (0.3–0.8 eV).

Characteristics of thermo‐ and photo‐stimulated electron emission from silicon wafers

AbstractThe temperature dependence of photoelectric emission and its spectral yield has been studied for Si (100) wafer surfaces with a native oxide film. The emission measurement was performed using a gas‐flow Geiger counter developed for examining real surfaces. The saturation emission observed as a function of temperatures between 26 and 350 °C under illumination with far ultraviolet rays had the average activation energy of 0.27 eV in the Arrhenius plot. The analysis of the spectral yield observed after the preillumination gave two almost temperature independent photoelectric thresholds of 4.84 ± 0.11 and 5.57 ± 0.15 eV. The difference between the former value and the indirect optical excitation threshold for a clean, cleaved silicon surface reported by Gobeli and Allen approximately agrees with the activation energy. This energy is interpreted as being due to a thermal process for electron transport from the silicon substrate to the SiO2/Si interface states. A new emitting center with optical activation energy of about 5.7 eV was also found for the preilluminated surfaces. Copyright © 2008 John Wiley &amp; Sons, Ltd.