Photostimulated Desorption Research Articles

Direct Determination of the Activation Energy for Diffusion of OH Radicals on Water Ice

Using a combination of photostimulated desorption and resonance-enhanced multiphoton ionization methods, the behaviors of OH radicals on the surface of an interstellar ice analog were monitored at temperatures between 54 and 80 K. The OH number density on the surface of ultraviolet-irradiated compact amorphous solid water gradually decreased at temperatures above 60 K. Analyzing the temperature dependence of OH intensities with the Arrhenius equation, the decrease can be explained by the recombination of two OH radicals, which is rate-limited by thermal diffusion of OH. The activation energy for surface diffusion was experimentally determined for the first time to be 0.14 ± 0.01 eV, which is larger than or equivalent to those assumed in theoretical models. This value implies that the diffusive reaction of OH radicals starts to be activated at approximately 36 K on interstellar ice.

Photo-stimulated hydrogen desorption from magnesium nanoparticles

Hydrogen remains an attractive energy carrier because it is abundant, environmentally friendly and has the highest gravimetric energy density of any known substance. Despite this high gravimetric energy density, hydrogen suffers from a low volumetric energy density as a room-temperature gas. To maximize volumetric energy density, storing hydrogen as a magnesium hydride is an efficient and economically viable route, owing to the low weight and high earth abundance of magnesium. A long-lasting obstacle for using magnesium is the high temperature required to release hydrogen once absorbed by magnesium. Although nanoscale magnesium is known to have a favorable effect on the hydrogen desorption temperature, it is not sufficient. In this work, hydrogen absorption and release was investigated by measuring optical changes, which correspond to certain hydrogen concentrations in magnesium nanoparticles. Remarkably, hydrogen desorption from the magnesium nanoparticle assembled thin films at room temperature could be achieved by illumination. This photo-stimulated hydrogen desorption introduces an effective and simple method to enable reversible hydrogen storage in magnesium. The sensitivity of the optical method here used is demonstrated by the fact that even hydrogen absorption from ambient air at 1 ppm has been measured. This work demonstrates that hydrogen can be efficiently stored and released from magnesium nanoparticles using only photons.

Photostimulated desorption performance of the future circular hadron collider beam screen

Synchrotron radiation (SR) originated at superconducting bending magnets is known to be at the origin of several beam detrimental effects related to vacuum instabilities. One of the major challenges in the design of the vacuum beam pipes of high-energy hadron colliders is the SR coping strategy. In the case of the future circular hadron collider (FCC-hh), a Cu-coated beam screen (BS) operating in the range of 40--60 K has been designed with the aim of protecting the superconducting magnet cold bores from direct synchrotron irradiation. In order to experimentally study the FCC-hh BS vacuum and cryogenic performance, two sample prototypes were manufactured and installed in the beam screen test-bench experiment (BESTEX) at the Karlsruhe Research Accelerator (KARA) at the Karlsruhe Institute of Technology (KIT). The emitted SR has a critical energy of 6.2 keV, very similar to the 4.6 keV of FCC-hh. Irradiation at both room (RT) and cryogenic (77 K) temperatures showed a significant reduction of the molecular photostimulated desorption yields ($\ensuremath{\eta}$) of the FCC-hh beam screen compared to those of Cu samples. A first approximation of $\ensuremath{\eta}$ and its evolution with the photon dose accumulated on the FCC-hh BS prototype at 77 K allows to estimate that a machine conditioning period of $\ensuremath{\sim}1.2$ months would be needed to reduce the photostimulated molecular density at the necessary levels to ensure a 100 h beam lifetime at nominal FCC-hh operation.

Visible-Light-Activated Response Originating from Carrier-Mobility Modulation of NO2 Gas Sensors Based on MoS2 Monolayers.

Some gas sensors exhibit significant increases in their sensitivity and response/recovery rates under light illumination. This photoactivation of the gas response is considered a promising alternative to conventional thermal activation, which requires high power consumption. Thin layers of molybdenum disulfide (MoS2) are known to exhibit an effective photoactivated gas response under visible light. However, the mechanism of the photoactivated response has not yet been studied in detail. In this study, we fabricated field-effect-transistor (FET) gas sensors based on MoS2 monolayers and investigated their photoactivated gas responses to NO2 gas under illumination at various irradiances of visible light. A photocurrent was generated mainly due to the photovoltaic effect, which decreased upon exposure to NO2. The conductance-based sensor response showed a dependence on NO2 concentration according to the Langmuir adsorption isotherm, thereby suggesting that the response is proportional to the surface coverage of NO2 molecules on the MoS2 layer. The response and recovery rates showed a linear increase with increasing irradiance. Analysis based on the Langmuir adsorption model revealed that both photostimulated adsorption and desorption are involved in the photoactivated response. In contrast, despite the strong dependence of the photocurrent on the irradiance, the magnitude of the sensor response was independent of the irradiance. Based on this result and the change in transfer characteristics of the FET during NO2 exposure, we concluded that the fast response/recovery of the photoactivated response is due to the carrier mobility modulation of MoS2, which is caused by the dipole scattering of adsorbed NO2 molecules.

Photostimulated desorption of OH radicals from amorphous solid water: Evidence for the interaction of visible light with an OH-ice complex

Using a combination of photostimulated desorption and resonance-enhanced multiphoton ionization techniques, OH radicals photodesorbed from amorphous solid water at low temperatures are detected. It was found that the OH photodesorption can be caused by a one-photon process on the surface even at 532 nm, where both isolated OH and ${\mathrm{H}}_{2}\mathrm{O}$ are transparent. Quantum chemical calculations reveal that the electronic excitation of OH dramatically changes depending on the adsorption sites and the number of surrounding ${\mathrm{H}}_{2}\mathrm{O}$ molecules, and that OH having three hydrogen bonds with neighboring ${\mathrm{H}}_{2}\mathrm{O}$ molecules can absorb photons of around 532 nm, which supports the experimental findings.

Direct measurement of fast ortho-para conversion of molecularly chemisorbed H2 on Pd(210)

Ortho-para conversion of molecularly chemisorbed ${\mathrm{H}}_{2}$ on a Pd(210) surface at a surface temperature of 50 K is reported. A combination of a pulsed molecular beam, photostimulated desorption, and resonance-enhanced multiphoton ionization techniques was used for probing the change in the rotational states of molecularly chemisorbed ${\mathrm{H}}_{2}$ on the surface. Our result shows that fast ortho-para conversion of chemisorbed ${\mathrm{H}}_{2}$ occurs. The conversion time was experimentally determined to be about 2 s, which is in good agreement with a previous theoretical calculation. This agreement supports that the ortho-para conversion mechanism of the molecularly chemisorbed ${\mathrm{H}}_{2}$ on Pd(210) is a two-step process based on the hyperfine-Coulomb excitation.

Experimental Approach to Physicochemical Hydrogen Processes on Cosmic Ice Dust

Hydrogen is the most abundant element in the universe, and thus atomic and molecular hydrogen play important roles in chemical evolution in space. In particular, the physicochemical processes of hydrogen on cosmic dust, such as the diffusion of H atoms and nuclear spin conversion of H2 molecules at low temperatures, are recognized to significantly influence the subsequent chemical evolution. However, it is not easy to track the hydrogen on the surface by conventional experiments. We have recently succeeded in applying a combination of photostimulated desorption and resonance-enhanced multiphoton ionization methods to detect hydrogen on cosmic dust analogues. In this paper, we present a brief review of our recent experiments for elucidating the behavior of hydrogen on water ice, pure solid carbon monoxide, and diamond-like carbon as cosmic dust analogue surfaces.

Photo-stimulated desorption of trimethyl acetic acid on cross-linked (1 × 2) TiO2(1 1 0) probed by scanning tunneling microscopy

Using in situ scanning tunneling microscopy (STM), we report site-specific photo-stimulated desorption of trimethyl acetic acid (TMAA) on cross-linked, (1 × 2) reconstructed TiO2(1 1 0). The comparison of the same area reveals that a significant fraction of the adsorbates still remains on the reduced surface even after prolonged UV irradiation in UHV. However, the photo-desorption of TMAA in troughs can be substantially enhanced with the combined exposure to O2. The enhancement of photo-desorption is attributed to co-adsorbed oxygen capturing excess electrons, which can partially screen photo-hole driven reactions of TMAA on the reduced surface. Such impeded photo-reactivity of TMAA is illustrated also with isolated strands on (1 × 1) TiO2(1 1 0), which exhibits much slower photo-desorption, compared to the rest of (1 × 1) surface. Moreover, the restoration of slow photo-reactivity with oxygen treatment suggests that surface can be tuned to control photoreaction of TMAA through reduction and re-oxidation cycles.

Interactions of Atomic and Molecular Hydrogen with a Diamond-like Carbon Surface: H2 Formation and Desorption

Abstract The interactions of atomic and molecular hydrogen with bare interstellar dust grain surfaces are important for understanding H2 formation at relatively high temperatures (>20 K). We investigate the diffusion of physisorbed H atoms and the desorption energetics of H2 molecules on an amorphous diamond-like carbon (DLC) surface. From temperature-programmed desorption experiments with a resonance-enhanced multiphoton ionization (REMPI) method for H2 detection, the H2 coverage-dependent activation energies for H2 desorption are determined. The activation energies decrease with increasing H2 coverage and are centered at 30 meV with a narrow distribution. Using a combination of photostimulated desorption and REMPI methods, the time variations of the surface number density of H2 following atomic and molecular hydrogen depositions are studied. From these measurements, we show that H2 formation on a DLC surface is quite efficient, even at 20 K. A significant kinetic isotope effect for H2 and D2 recombination reactions suggests that H-atom diffusion on a DLC surface is mediated by quantum mechanical tunneling. In astrophysically relevant conditions, H2 recombination due to physisorbed H-atoms is unlikely to occur at 20 K, suggesting that chemisorbed H atoms might play a role in H2 formation at relatively high temperatures.

Behaviour of radicals on interstellar dust analogues

AbstractSurface reactions of radicals play important roles in the formation of complex molecules on interstellar dust grains. Under interstellar conditions, because the coverage of adsorbates on dust is significantly low, surface reactions are often limited by precedent processes, namely, the adsorption and diffusion of reactants. Therefore, to appropriately incorporate dust surface reactions into chemical models, information on the adsorption and diffusion of radicals is crucial. However, it is not easy to follow the behaviour of radicals on surfaces by conventional experimental methods. To monitor radicals on interstellar dust analogues, we have recently succeeded in applying a combination of photostimulated desorption and resonance-enhanced multiphoton ionization methods. In this paper, we briefly review our recent experiments for clarifying radical behaviour on water ice, pure solid CO and diamond-like carbon.

Measurements of the Activation Energies for Atomic Hydrogen Diffusion on Pure Solid CO

Abstract The diffusion of hydrogen atoms on dust grains is a key process in the formation of interstellar H2 and some hydrogenated molecules such as formaldehyde and methanol. We investigate the adsorption and diffusion of H atoms on pure solid CO as an analog of dust surfaces observed toward some cold interstellar regions. Using a combination of photostimulated desorption and resonance-enhanced multiphoton ionization methods to detect H atoms directly, the relative adsorption probabilities and diffusion coefficients of the H atoms are measured on pure solid CO at 8, 12, and 15 K. There is little difference between the diffusion coefficients of the hydrogen and deuterium atoms, indicating that the diffusion is limited by thermal hopping. The activation energies controlling the H-atom diffusion depend on the surface temperature, and values of 22, 30, and ∼37 meV were obtained for 8, 12, and 15 K, respectively.

Space Weathering of Super-Earths: Model Simulations of Exospheric Sodium Escape from 61 Virgo b

Abstract Rocky exoplanets are expected to be eroded by space weather in a similar way as in the solar system. In particular, Mercury is one of the dramatically eroded planets whose material continuously escapes into its exosphere and further into space. This escape is well traced by sodium atoms scattering sunlight. Due to solar wind impact, micrometeorite impacts, photo-stimulated desorption and thermal desorption, sodium atoms are released from surface regolith. Some of these released sodium atoms are escaping from Mercury’s gravitational-sphere. They are dragged anti-Sun-ward and form a tail structure. We expect similar phenomena on exoplanets. The hot super-Earth 61 Vir b orbiting a G3V star at only 0.05 au may show a similar structure. Because of its small separation from the star, the sodium release mechanisms may be working more efficiently on hot super-Earths than on Mercury, although the strong gravitational force of Earth-sized or even more massive planets may be keeping sodium atoms from escaping from the planet. Here, we performed model simulations for Mercury (to verify our model) and 61 Vir b as a representative super-Earth. We have found that sodium atoms can escape from this exoplanet due to stellar wind sputtering and micrometeorite impacts, to form a sodium tail. However, in contrast to Mercury, the tail on this hot super-Earth is strongly aligned with the anti-starward direction because of higher light pressure. Our model suggests that 61 Vir b seems to have an exo-base atmosphere like that of Mercury.

Surface Temperature Dependence of Hydrogen Ortho-Para Conversion on Amorphous Solid Water.

The surface temperature dependence of the ortho-to-para conversion of H_{2} on amorphous solid water is first reported. A combination of photostimulated desorption and resonance-enhanced multiphoton ionization techniques allowed us to sensitively probe the conversion on the surface of amorphous solid water at temperatures of 9.2-16K. Within a narrow temperature window of 8K, the conversion time steeply varied from ∼4.1×10^{3} to ∼6.4×10^{2} s. The observed temperature dependence is discussed in the context of previously suggested models and the energy dissipation process. The two-phonon process most likely dominates the conversion rate at low temperatures.

Application of nondiffracting Bessel beams for shaping of surface metal microstructures

A novel method of laser-controlled shaping of metal microstructures based on the processes of metal atoms adsorption on the surface of crystalline substrate and simultaneous control of photostimulated desorption of atoms by spatially modulated nondiffracting laser beam illumination is presented. The experiments were performed for sodium atoms deposition to the sapphire substrate, which was illuminated by Bessel beam at 532 nm wavelength and 2 W/cm2 intensity. Experiments showed that the optical pattern was well reproduced in the sodium deposits thus creating the annularly microstructured metal film with few tens nanometre thickness.

Surface analysis by photo-stimulated desorption using tunable VUV radiation

We have demonstrated the photon-stimulated surface spectroscopy using broadband vacuum ultraviolet (VUV) emissions from a laser-produced plasma, in which adsorbed atoms or molecules on material surfaces were to be desorbed and dissociated as a result of absorption of the wavelength-selected VUV photons. Desorbed and dissociated atoms or molecules were then detected by a mass spectrometer. Mass spectra of polyethylene and polyvinyl chloride samples were obtained as a function of the irradiation wavelength. We have found sufficient characteristic differences of the mass spectra in the irradiation wavelength shorter than 200 nm in each sample. The desorption and dissociation of atoms or molecules from material surfaces depended on bond energy or molecular structure.

Micrometeoroids flux on the Moon

Context. The Moon has a tenuous exosphere consisting of atoms that are ejected from the surface by energetic processes, including hypervelocity micrometeoritic impacts, photon-stimulated desorption by UV radiation, and ion sputtering. Aims. We calculate the vapor and neutral Na production rates on the Moon caused by impacts of meteoroids in the radius range of 5−100 μm. We considered a previously published dynamical model to compute the flux of meteoroids at the heliocentric distance of the Moon. Methods. The orbital evolution of dust particles of different sizes is computed with an N-body numerical code. It includes the effects of Poynting-Robertson drag, solar wind drag, and planetary perturbations. The vapor production rate and the number of neutral atoms released in the exosphere of the Moon are computed with a well-established formulation. Results. The result shows that the neutral Na production rate computed following our model is higher than previous estimates. This difference can be due to the dynamical evolution model that we used to compute the flux and also to the mean velocity, which is 15.3 km s −1 instead of 12.75 km s −1 as reported in literature. Conclusions. Until now, the micrometeoritic impacts have been considered a negligible source for the release of neutral sodium atoms into the exosphere compared to other mechanisms, but according to our calculations, the contribution may be 8% of the photostimulated desorption at the subsolar point, becoming similar in the dawn and dusk regions and dominant on the night side.

THE MECHANISM OF SURFACE DIFFUSION OF H AND D ATOMS ON AMORPHOUS SOLID WATER: EXISTENCE OF VARIOUS POTENTIAL SITES

To understand elementary processes leading to H{sub 2} formation, and the hydrogenation and deuteration reactions of adsorbed species on dust grains in dense clouds, we experimentally investigated the diffusion of atomic hydrogen and deuterium on amorphous solid water (ASW) at temperatures of 8-15 K. The present study extended our previous study for selective detections of H and D atoms, and of H{sub 2} (J = 0 and 1) and D{sub 2} (J = 0 and 1) molecules adsorbed on ASW using both photo-stimulated desorption and resonance-enhanced multiphoton ionization, to investigate potential sites on ASW for diffusion, recombination dynamics, and the diffusion mechanism of H and D atoms. Our results demonstrate that the ASW surface contains various potential sites that can be categorized into at least three groups: very shallow, middle-, and deep-potential sites, with diffusion activation energies of { =}30 meV, respectively. The present study pictured the outline of H{sub 2} formation on cosmic ice dust at low temperatures: H atoms landing on the dust will diffuse rapidly at the abundant shallow and middle sites on ASW, and finally become trapped at deep sites. The H atoms that arrive next recombine with suchmore » trapped H atoms to yield H{sub 2} molecules. The small isotopic difference between the diffusion of H and D atoms on ASW indicates that the diffusion mechanism can be explained by thermal hopping, at least at middle-potential sites.« less

Photo-stimulated desorption from water and methane clusters on the surface of solid neon

Photo-stimulated desorption of ions from methane and water heteroclusters on the surface of solid neon was studied. The desorption yields of various photo-desorbed species showed strong dependence on the composition and size of the mother cluster. It was found that presence of a water molecule in the cluster significantly enhanced, or was almost essential for, desorption of all species observed. Systematic investigation of the correlation between the cluster size and desorption yield of each ion revealed the corresponding mother cluster for each species of the desorbed ions.

Creation of single oxygen vacancy on titanium dioxide surface

Abstract

Electric Charge of Single Au Atoms Adsorbed on TiO2(110) and Associated Band Bending

The influence of charge transfer from single Au atoms to TiO2(110) on the photostimulated desorption (PSD) of 18O2 has been studied using a measurement of the rate of hole transport which mediates PSD. Band bending effects observed experimentally and theoretically by density functional theory indicate that Auδ+ atoms are present. The presence of atomically dispersed Au on the TiO2(110) surface with a constant coverage of preadsorbed 18O2 depresses the photoinduced hole transport rate from the bulk to the TiO2 surface and decreases the 18O2 PSD yield. This indicates that single Au atoms donate a fraction of an electron to the surface, causing downward band bending. DFT calculations show that ∼0.2 electron transfers from single Au atoms to the O2/TiO2(110) surface and the valence and conduction bands bend ∼0.6 eV downward. With increasing Au coverage, the positive charge per Au atom decreases due to the formation of small Au clusters.