Photon-stimulated Desorption Research Articles

Lunar soil record of atmosphere loss over eons.

The Moon has a tenuous atmosphere produced by space weathering. The short-lived nature of the atoms surrounding the Moon necessitates continuous replenishment from lunar regolith through mechanisms such as micrometeorite impacts, ion sputtering, and photon-stimulated desorption. Despite advances, previous remote sensing and space mission data have not conclusively disentangled the contributions of these processes. Using high-precision potassium (K) and rubidium (Rb) isotopic analyses of lunar soils from the Apollo missions, our study sheds light on the lunar surface-atmosphere evolution over billions of years. The observed correlation between K and Rb isotopic ratios (δ 87Rb = 0.17 δ 41K) indicates that, over long timescales, micrometeorite impact vaporization is the primary source of atoms in the lunar atmosphere.

Understanding the Dust Environment at Mercury: From Surface to Exosphere

We provide an overview of our understanding of the dust environment at Mercury and the role that dust plays in shaping the planet's surface and exosphere. Our understanding of the role that dust impacts play in the generation of Mercury's atmosphere has evolved considerably with continued analysis of results from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. Recent models have provided evidence for the probable release of refractory species into Mercury's exosphere via impacts. However, there remain significant questions regarding the relative contributions of atoms released via impacts versus other mechanisms (e.g., photon-stimulated desorption) to the overall exospheric budget. We also discuss the state of observational and modeling efforts to constrain the dust environment at Mercury, including sources from the zodiacal cloud, cometary trails, and interstellar dust. We describe the advancements that will be made in our characterization of dust at Mercury with BepiColombo, providing observational constraints on the dust clouds themselves and the role that impacts play in exospheric generation. On Mercury's surface, there remain outstanding questions regarding the role that dust impacts play in the regolith cycling and development. We review how improved modeling efforts to understand grain lifetimes as a function of impactor flux will further our understanding of Mercury's regolith. Finally, there are few constraints on the role of dust impacts on the space weathering of Mercury's surface, particularly the expected chemical, physical, and spectral alterations to the regolith. Here we discuss the importance of laboratory experiments to simulate these processes for the interpretation of data from MESSENGER and BepiColombo.

Measurement of the Photoelectron Yield from the Synchrotron Radiation for the NEG-coated Tubes

NEG-coated chambers have been adopted as the beam ducts for large particle accelerators and synchrotron light sources for the sake of the lower yields of the photon stimulated desorption (PSD) and the photoelectrons (PE) from the NEG films in addition to their pumping performance. Measurement of the photoelectron yield (PEY) was performed at the BL19B (PSD) beamline of the 1.5 GeV Taiwan Light Source (TLS) which simultaneously measures the PSD-yield. An aluminium cathode was inserted in the tubes and a positive bias of voltage for extraction of the photoelectrons applied. The PEY was obtained by dividing the photoelectron current by the photon flux of the synchrotron radiation. Measurements of the PEY include various types of NEG-coated stainless steel tubes and the bare tubes of titanium and aluminium alloys for the comparison. The experimental system and the results will be described in this presentation.

Study on the vacuum properties of laser-etched oxygen-free copper

The performance of operating particle accelerators has been seriously affected by the electron cloud (e-cloud) effect. The secondary electron emission (SEE) and the e-cloud can be effectively suppressed through laser-etching the inner surface of the vacuum chamber. Oxygen-free copper (OFC) has become the first choice for the vacuum chambers of modern accelerators due to its high electric and thermal conductivity and effective radiation shielding property. It is necessary to study the vacuum properties of the laser-etched OFC for the application in the particle accelerators. In this paper, the photon stimulated desorption (PSD) yield and the outgassing rate of the laser-etched OFC were measured. The results show that the laser-etched OFC presents lower PSD yield compared to the untreated OFC, while the outgassing rates of the laser-etched and unetched samples are similar.

Electronic energy transfer ionization in naphthalene-CO2 clusters reveals excited states of dry ice.

Electronic energy relaxation and transfer shapes the photochemistry in molecules and materials that are exposed to UV radiation in areas ranging from astrochemistry to biology. The interaction between CO2 and polycyclic aromatic hydrocarbons (PAHs) specifically, is of paramount interest in astrochemically relevant ices, the transition to renewable energy and the development of green chemistry. We investigate the vacuum UV excitation of the naphthalene-CO2 complex and observe excited states of CO2 through a newly identified molecular electronic energy transfer ionization mechanism. We evaluate the spectral development upon cluster growth with time-dependent density functional theory and show that the photoionization spectrum of naphthalene-CO2 closely resembles the photon-stimulated desorption spectrum of CO2 ice. The molecular electronic energy transfer ionization mechanism may affect the energy redistribution and charge balance in the interstellar medium significantly and therefore we discuss its implications for astrochemical models.

Modeling of intrinsic photon-stimulated desorption yields under readsorption effect

In synchrotron radiation and electron storage rings, photon-stimulated gas desorption is a major contributor to gas load, leading to vacuum degradation and compromising the stability and quality of the beamline. Therefore, it is imperative to investigate the underlying mechanisms of photon-stimulated gas desorption to enhance the reliability of the experiments. In this paper, a theoretical model for analyzing the process of photon-stimulated desorption is presented, and the equations are solved numerically by considering the evacuation, bulk diffusion, thermal outgassing, adsorption, and desorption of surface gas molecules. The modified model can predict the yield of H2 gas desorption induced by photon stimulation when synchrotron radiation is directed vertically onto various sample surfaces, utilizing the coupled multi-physics solution. The calculated results are consistent with the experimental measurements obtained from the Shanghai Synchrotron Radiation Facility. The results show that the photodesorption yield is related to the amount of surface-adsorbed molecules, which explains the observed decrease in the photodesorption yield with increasing accumulated photon dose. Furthermore, the study provides a theoretical approach to predicting the pressure evolution of photon-stimulated desorption.

Development of highly durable Pd coatings with ultra-low photon-stimulated desorption and low resistivity values

Denser Pd coatings were prepared on the inner wall of the Cu tubes and the photon-stimulated desorption (PSD) of gases from the tubes in response to synchrotron radiation was assessed. In comparison with low PSD materials such as TiZrV coatings, the Pd coatings exhibited ultra-low PSD yields. Especially, the initial H2 PSD yield was lower one order. These ultra-low PSD yields could be maintained after several cycles of air exposure and followed heating. In addition, the denser Pd films showed significantly lower resistivities compared to the TiZrV films.

The Influence of Surface Binding Energy on Sputtering in Models of the Sodium Exosphere of Mercury

We have simulated the sodium (Na) exosphere of Mercury to show how the exosphere is affected by the assumed surface binding energy (SBE) of Na in the sputtered component. We constrained ion precipitation onto the surface using distributions for the cusp regions that are consistent with measurements by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging Fast Imaging Plasma Spectrometer instrument. We have simulated sputtering with SBEs of 0.27, 2.6, 4.4, and 7.9 eV, with the lowest value commonly used in exosphere models and the highest from recent molecular dynamics calculations for the Na-bearing feldspar end-member, albite. A gradual change in the exosphere is seen as the yield decreases and the ejecta energy increases with increasing SBE. We describe the corresponding exosphere source functions for ion sputtering (IS), as well as for the previously studied processes of micrometeoroid impact vaporization and photon-stimulated desorption (PSD), along with their release energy distributions and spatial distributions. We have summed the contributions of the various source processes to explain how and when the different sources can be distinguished by observations. The modeled exosphere scale heights range from 72 km for PSD to over 1000 km for IS using a SBE of 7.9 eV. We find that the processes responsible for generating Mercury's Na exosphere are separable by measuring line-of-sight column densities tangent to the planet at various altitudes and positions around the planet. Our initial results are consistent with the Na being sputtered from a high-SBE material such as feldspar, which has been predicted to be abundant on the Mercury's surface.

Plasma-induced surface cooling

Plasmas are an indispensable materials engineering tool due to their unique ability to deliver a flux of species and energy to a surface. This energy flux serves to heat the surface out of thermal equilibrium with bulk material, thus enabling local physicochemical processes that can be harnessed for material manipulation. However, to-date, there have been no reports on the direct measurement of the localized, transient thermal response of a material surface exposed to a plasma. Here, we use time-resolved optical thermometry in-situ to show that the energy flux from a pulsed plasma serves to both heat and transiently cool the material surface. To identify potential mechanisms for this ‘plasma cooling,’ we employ time-resolved plasma diagnostics to correlate the photon and charged particle flux with the thermal response of the material. The results indicate photon-stimulated desorption of adsorbates from the surface is the most likely mechanism responsible for this plasma cooling.

Synchrotron radiation-stimulated desorption from Pd or Pd/TiZrV coated copper tubes

Gas desorption from copper tubes coated with Pd or Pd/TiZrV in response to synchrotron radiation was investigated. Following activation (250 °C for 4 h), Pd/TiZrV film exhibited H2 desorption yield that was over one order of magnitude lower than that from TiZrV film. The Pd and Pd/TiZrV films also desorbed smaller amount of CO and CO2 during exposure. Compared to the TiZrV film, the Pd film exhibited a higher H2 pumping rate and was more likely to release H2 and CO during activation. These characteristics are considered to have reduced the photon-stimulated desorption yield of the Pd layer.

Photon‐Stimulated Desorption of MgS as a Potential Source of Sulfur in Mercury's Exosphere

AbstractMercury has a relatively high sulfur content on its surface, and a signal consistent with ionized atomic sulfur (S+) was observed by the fast ion plasma spectrometer (FIPS) instrument on the MESSENGER spacecraft. To help confirm this assignment and to better constrain the sources of exospheric sulfur at Mercury, 193 nm photon‐stimulated desorption (PSD) of neutral sulfur atoms (S0) from MgS substrates was studied using resonance‐enhanced multiphoton ionization (REMPI) spectroscopy and time‐of‐flight (TOF) mass spectrometry. Though the PSD process is inherently nonthermal, the measured velocities of ejected S0 were fit using flux‐weighted Maxwellian distributions with translation energies <E> expressed as translational “temperatures” Ttrans = <E>/μkB. A bimodal distribution consisting of both thermal (Ttrans = 300 K) and suprathermal (Ttrans > 1,000 K) components in roughly a 2:1 ratio was found to best fit the data. The PSD cross‐section was measured to be approximately 4 × 10−22 cm2 and, together with the velocity distributions, was used to calculate the PSD source rate of S0 into the exosphere of Mercury. Exosphere simulations using the calculated rates demonstrate that PSD is likely the primary source of S0 in Mercury's exosphere at low (<1,000 km) altitudes.

Photon-stimulated Desorption Processes of Polymethylmethacrylate by Vacuum Ultraviolet Irradiations

Photon-stimulated Desorption Processes of Polymethylmethacrylate by Vacuum Ultraviolet Irradiations

Experimental research on photon-stimulated desorption from CuCrZr

In this study, photon-stimulated gas desorption yields from CuCrZr, which are used for manufacturing high-heat-load elements, were measured using a synchrotron light with a critical energy of 10.4 keV at the Shanghai Synchrotron Radiation Facility. CuCrZr samples were exposed to synchrotron radiation (SR) under both normal and grazing incidences. For comparison, a Glidcop sample was also exposed to SR under normal incidence. The experimental apparatus and measurement principles for photon desorption yield are described in this paper and the results of measurements are discussed.

Commissioning of vacuum system for SuperKEKB positron damping ring

The first stage commissioning of the new positron damping ring, which was constructed in the upgraded injector system of the SuperKEKB, was carried out from February to July 2018. Vacuum scrubbing in the arc sections, which occupy most of the ring, progressed smoothly, and a beam lifetime of 1000 s was obtained with a stored beam current of 10 mA and a beam dose of 0.7 A h. However, the beam lifetime did not increase with subsequent vacuum scrubbing because the beam lifetime was primarily determined by the Touschek effect. The residual gas composition during beam operation was typical of what would be expected for a vacuum system pumped by nonevaporable getter pumps. Rough Molflow+-based estimates of the photon-stimulated desorption yields of the beam pipe at the arc sections revealed that they decreased to 1.4 × 10−4 molec. photon−1 for H2 and to 2.3 × 10−5 molec. photon−1 for CO at a beam dose of 2 A h, which corresponds to a photon dose of 1 × 1023 photons m−1.

Diffusion and Photon-Stimulated Desorption of CO on TiO2(110)

Thermal diffusion of CO adsorbed on rutile TiO2(110) was studied in the 20–110 K range using photon-stimulated desorption (PSD), temperature-programmed desorption (TPD), and scanning tunneling microscopy. During UV irradiation, CO desorbs from certain photoactive sites (e.g., oxygen vacancies). This phenomenon was exploited to study CO thermal diffusion in three steps: first, empty these sites during a first irradiation cycle, then replenish them with CO during annealing, and finally probe the active site occupancy in the second PSD cycle. The PSD and TPD experiments show that the CO diffusion rate correlates with the CO adsorption energy—stronger binding corresponds to slower diffusion. Increasing the CO coverage from 0.06 to 0.44 monolayer (ML) or hydroxylation of the surface decreases the CO binding and increases the CO diffusion rate. Relative to the reduced surface, the CO adsorption energy increases and the diffusion decreases on the oxidized surface. The CO diffusion kinetics can be modeled satisfa...

Velocity distribution function of Na released by photons from planetary surfaces

In most surface-bound exospheres Na has been observed at altitudes above what is possible by thermal release. Photon stimulated desorption of adsorbed Na on solid surfaces has been commonly used to explain observations at high altitudes. We investigate three model velocity distribution functions (VDF) that have been previously used in several studies to describe the desorption of atoms from a solid surface either by electron or by photon bombardment, namely: the Maxwell-Boltzmann (M-B) distribution, the empirical distribution proposed by [1] for PSD, and the Weibull distribution. We use all available measurements reported by [2, 3] to test these distributions and determine which one fits best (statistically) and we discuss their physical validity. Our results show that the measured VDF of released Na atoms are too narrow compared to Maxwell-Boltzmann fits with supra-temperatures as suggested by [3]. We found that a good fit with M-B is only achieved with a speed offset of the whole distribution to higher speeds and a lower temperature, with the offset and the fit temperature not showing any correlation with the surface temperature. From the three distributions we studied, we find that the Weibull distribution provides the best fits using the temperature of the surface, though an offset towards higher speeds is required. This work confirms that Electron-Stimulated Desorption (ESD) and Photon-Stimulated Desorption (PSD) should produce non-thermal velocity (or energy) distributions of the atoms released via these processes, which is expected from surface physics. We recommend to use the Weibull distribution with the shape parameter κ = 1.7, the speed offset v0 = 575 m/s, and the surface temperature to model PSD distributions at planetary bodies.

Adsorption and Photodesorption of CO from Charged Point Defects on TiO2(110).

The adsorption and photochemistry of CO on rutile TiO2(110) are studied with scanning tunneling microscopy (STM), temperature-programmed desorption, and angle-resolved photon-stimulated desorption (PSD) at low temperatures. Site occupancies, when weighted by the concentration of each kind of adsorption site on the reduced surface, show that the adsorption probability is the highest for the bridging oxygen vacancies (VO). The probability distribution for the different adsorption sites corresponds to very small differences in CO adsorption energies (<0.02 eV). UV irradiation stimulates diffusion and desorption of CO at low temperature. CO photodesorbs primarily from the vacancies with a bimodal angular distribution, indicating some scattering from the surface, which also leads to photostimulated diffusion. Hydroxylation of VO's does not significantly change the CO PSD yield or the angular distribution, which suggests that photodesorption can be initiated by recombination of photogenerated holes with excess electrons localized near the charged point defect (either VO or bridging hydroxyl).

Beam scrubbing of beam pipes during the first commissioning of SuperKEKB

The first (Phase-1) commissioning of SuperKEKB—an electron-positron collider with asymmetric energies located at KEK, in Tsukuba, Japan—started in February 2016, after more than five years of upgrading work on KEKB, and successfully ended in June of the same year. This paper describes one major task of Phase-1 commissioning: beam scrubbing the surface of the beam pipes, to prepare them for a sufficiently long beam lifetime and low background noise in the next commissioning, when a new particle detector will be installed. The pressure rises per unit beam current (dP/dI [Pa A−1]) were continuously monitored, and the coefficient of photon-stimulated desorption (PSD), η [molecules photon−1], was evaluated in the arc sections. The value of η decreased steadily with the beam dose, as expected. For arc sections in the positron ring, where most of the beam pipes were newly fabricated, the decrease in η against the photon dose (D) was similar to that previously reported; that is: η∝D−0.5∼0.8. At high storage beam currents, the evolution of η was affected by gas desorption resulting from the multipacting of electrons—that is, the electron cloud effect (ECE), which is a phenomenon particular to high-intensity positron rings. For the arc sections in the electron ring, η also decreased smoothly with the photon dose D, approximately as ∝ D−0.8. Given that most of these beam pipes were reused from KEKB, the value of η was much lower than that of the positron ring, and also lower than that of the electron ring of KEKB from the early stages of D. This implies that the surface of the reused beam pipes remembered the conditions in the KEKB, which is a known memory effect. The results obtained for η are compared with those obtained in various other accelerators.

Replacement of the TPS bending chamber with an on-site cutting method

To minimize the impact on the machine downtime due to replacement of a TPS bending chamber, an on-site cutting method had been developed. This method can shorten the replacement time of a bending chamber from 50 to 4 days and avoid breaking the vacuum in the frond ends and adjacent insertion devices. We also developed a plug to prevent micro dust, generated from saw-cutting of the vacuum chamber, contaminating the beam duct. Three different preconditioning methods, including baking, prebaking and no-bake treatments, were applied to the newly installed arc-cell vacuum systems to evaluate their efficacy on reducing the photon stimulated desorption (PSD) in the vacuum chambers. The PSD data show no obvious discrepancy between these preconditioning methods.

Doping-Dependent Adsorption and Photon-Stimulated Desorption of CO on GaN(0001)

CO is used as a chemical probe molecule to elucidate the properties of differently n-type doped GaN(0001) surfaces under ultrahigh vacuum conditions. Doping-dependent sticking of CO is observed by temperature-programmed desorption, which is not influenced by the surface composition of the semiconductor substrates. By the excitation of the semiconductor with UV photons a low-temperature desorption of CO is stimulated. The absolute photon-stimulated desorption intensity strongly depends on the surface composition. However, the desorption kinetics do not significantly depend on the surface composition, semiconductor doping, or UV excitation wavelength. A model is introduced, which is based on the electronic characteristics of the GaN substrates and describes the doping-dependent adsorption as well as the photochemical behavior.