Desorption Cross Section Research Articles

Interaction of H2S with H atoms on grain surfaces under molecular cloud conditions

Context. Hydrogen sulfide (H2S) is thought to be efficiently formed on grain surfaces through the successive hydrogenation of sulfur atoms. Its non-detection so far in astronomical observations of icy dust mantles thus indicates that effective destruction pathways must play a significant role in its interstellar abundance. While chemical desorption has been shown to remove H2S very efficiently from the solid phase, in line with H2S gas-phase detections, possible ice chemistry triggered by the related HS radical have been largely disregarded so far, despite it being an essential intermediate in the H2S + H reaction scheme. Aims. We aim to thoroughly investigate the fate of H2S upon H-atom impact under molecular cloud conditions, providing a comprehensive analysis combined with detailed quantification of both the chemical desorption and ice chemistry that ensues. Methods. We performed experiments in an ultrahigh vacuum chamber at temperatures between 10 and 16 K in order to investigate the reactions between H2S molecules and H atoms on interstellar ice analogs. The changes in the solid phase during H-atom bombardment were monitored in situ by means of reflection absorption infrared spectroscopy (RAIRS), and desorbed species were complementarily measured with a quadrupole mass spectrometer (QMS). Results. We confirmed the formation of H2S2 via reactions involving H2S + H and quantified its formation cross section under the employed experimental conditions. Additionally, we directly assessed the chemical desorption of H2S by measuring the gas-phase desorption signals with the QMS, providing unambiguous desorption cross sections. Chemical desorption of H2S2 was not observed. The relative decrease of H2S ices by chemical desorption changed from ~85% to ~74% between temperatures of 10 and 16 K, while the decrease as the result of H2S2 formation was enhanced from ~15% to ~26%, suggesting an increasingly relevant sulfur chemistry induced by HS radicals at warmer environments. The astronomical implications are further discussed.

Direct Observation and Quantitative Measurement of OH Radical Desorption During the Ultraviolet Photolysis of Liquid Nonanoic Acid.

Ultraviolet (UV) photolysis of fatty acid surfactants─which cover the surfaces of atmospheric liquid aerosols and are found in the oceans─such as nonanoic acid (NA) has recently been suggested as a source of hydroxyl (OH) radicals in the troposphere. We used laser-induced fluorescence to directly observe OH radicals desorbed from the surface of neat liquid NA as a primary photoproduct following 213 nm irradiation. The upper limit of photoreaction cross section for the OH radical desorption was estimated to be 9.0(4.1) × 10-22 cm2, which is only 1.2 ± 0.8% of the photoreaction cross section established for the photolysis of gas-phase acetic acid monomers. Vibrational sum-frequency generation spectroscopy for liquid NA revealed the hydrogen-bonded, cyclic, dimer structure of the NA molecules at the liquid surface. This dimerization can inhibit the formation of OH radicals and lead the present low photochemical reactivity of liquid NA.

An Experimental Study of Chemical Desorption for Phosphine in Interstellar Ice

Abstract Phosphine (PH3), an important molecule for the chemistry of phosphorus (P)-bearing species in the interstellar medium (ISM) is considered to form primarily on interstellar grains. However, no report exists on the processes of PH3 formation on grains. Here, we experimentally studied the reactions of hydrogen (H) atoms and PH3 molecules on compact amorphous solid water, with a particular focus on the chemical desorption of PH3 at 10–30 K. After exposure to H atoms for 120 minutes, up to 50% of solid PH3 was lost from the icy surface. On the basis of experiments using deuterium atoms, it was concluded that the loss of PH3 resulted from chemical desorption through the reactions PH3 + H → PH2 + H2 and/or PH2 + H → PH3. The effective desorption cross-section was ∼5 × 10−17 cm2, which is three times larger than that of hydrogen sulfide measured under similar experimental conditions. The present results suggest that the formation of PH3, and possibly PH2 and PH, followed by their desorption from icy grains, may contribute to the formation of PN and PO in the gas phase, and thus may play a role in the P chemistry of the ISM.

Growth of cobalt films at room temperature using sequential exposures of cobalt tricarbonyl nitrosyl and low energy electrons

Cobalt thin films were grown at room temperature using sequential exposures of cobalt tricarbonyl nitrosyl (CTN, Co(CO)3NO) and low energy (75–175 eV) electrons. During this cyclic growth process, the CTN molecules were first adsorbed on the substrate. The electrons then induced the desorption of the carbonyl and nitrosyl ligands from the adsorbed CTN. The removal of CO and NO ligands produced new adsorption sites. Subsequent CTN exposures allowed CTN to react with these new adsorption sites on the substrate. In situ ellipsometry was utilized to monitor the film thickness during the electron enhanced growth. Co growth rates as high as 1.3 Å/cycle were observed by in situ ellipsometry depending on the reaction conditions. The in situ ellipsometry also observed the CTN adsorption and the removal of the carbonyl and nitrosyl ligands. Quadrupole mass spectrometer measurements confirmed the desorption of CO and NO during electron exposures. X-ray photoelectron spectroscopy (XPS) measured N XPS signals from the Co films deposited using electron exposures at 200 eV. The N/Co XPS signal ratio was consistent with the dissociation of 13% of the nitrosyl ligands on the CTN precursors that lead to Co deposition. In contrast, the negligible C XPS signals from the Co films indicated that the CO ligands were desorbed completely from CTN by the electron exposures at 200 eV. Under identical reaction conditions at lower incident electron currents, the maximum growth rate was obtained at an electron energy of 125 eV. Because the Co growth depends on the electron flux, the Co films were deposited only on the surface area irradiated by the electron beam. The spatial profile of the Co film deposited using long electron exposure times was mapped by ex situ spectroscopic ellipsometry. This spatial profile displayed a pronounced flat top that was consistent with the electron flux desorbing nearly all the CO and NO surface coverage in the central area of the electron beam during each reaction cycle. The spatial profile was used to calculate an electron induced desorption cross section of σ = 2 × 10−17 cm2 at 200 eV. This cross section was in approximate agreement with the cross sections for the electron impact dissociation of CTN in the gas phase.

Physico-chemical Behavior of Hydrogen Sulfide Induced by Reactions with H and D Atoms on Different Types of Ice Surfaces at Low Temperature

Abstract To elucidate the physico-chemical behavior of hydrogen sulfide (H2S) on icy grains in dense molecular clouds, we investigated the surface reactions of solid H2S with H and D atoms in low-temperature laboratory experiments. We confirmed that H2S was lost from the surface by reaction with H atoms via chemical desorption. We found no strong association between the effective desorption cross section and the ice structure (porous amorphous, non-porous amorphous, or crystalline) or temperature (10–30 K). At 10 K, the reaction rate constant for the H–D substitution of solid H2S with D atoms almost matched that for the D–H substitution of solid D2S with H atoms. The present experimental results clearly suggest that the observed abundances of H2S and its deuterated isotopologues (HDS and D2S) in the interstellar medium are controlled, at least partly, by surface reactions on interstellar icy grains.

Thermal and radiation chemistry of butyltin oxo hydroxo: A model inorganic photoresist

We have investigated the thermal and radiation chemistry of an organotin-based model photoresist to elucidate patterning mechanisms related to extreme ultraviolet (EUV) lithography. Butyltin oxide hydroxide (BuSnOOH) was dissolved in organic solvents and spin-coated to form uniform thin films. The thermal stability and radiation sensitivity of the thin films were characterized by temperature programmed desorption (TPD), electron stimulated desorption (ESD), and X-ray photoelectron spectroscopy (XPS). From the TPD studies, we determined that decomposition of BuSnOOH occurs at ~653 K through cleavage of the butyl-tin bond. Low kinetic energy electron (Ekin = 80 eV) and X-ray (1486.6 eV) exposure also resulted in cleavage of the butyl-tin bond with the resulting desorption of butyl ligands from the film. From the ESD data, we estimated that the butyl ligand desorption cross section was ~4.3 × 10−14 cm2 for electrons with 80 eV kinetic energy. From the XPS results, we found a significant reduction in C 1s intensity for extended X-ray exposures, which corresponds to butyl ligand loss. There were also changes in the O 1s spectra indicating loss of hydroxyl groups and tin oxide network formation during radiation exposure. These results indicate the preferential dissociation and desorption of butyl ligands that can occur through either thermal or radiation induced processes, and that butyl ligand loss leads to the solubility contrast.

An infrared measurement of chemical desorption from interstellar ice analogues

In molecular clouds at temperatures as low as 10 K, all species except hydrogen and helium should be locked in the heterogeneous ice on dust grain surfaces. Nevertheless, astronomical observations have detected over 150 different species in the gas phase in these clouds. The mechanism by which molecules are released from the dust surface below thermal desorption temperatures to be detectable in the gas phase is crucial for understanding the chemical evolution in such cold clouds. Chemical desorption, caused by the excess energy of an exothermic reaction, was first proposed as a key molecular release mechanism almost 50 years ago. Chemical desorption can, in principle, take place at any temperature, even below the thermal desorption temperature. Therefore, astrochemical net- work models commonly include this process. Although there have been a few previous experimental efforts, no infrared measurement of the surface (which has a strong advantage to quantify chemical desorption) has been performed. Here, we report the first infrared in situ measurement of chemical desorption during the reactions H + H2S -> HS + H2 (reaction 1) and HS + H -> H2S (reaction 2), which are key to interstellar sulphur chemistry. The present study clearly demonstrates that chemical desorption is a more efficient process for releasing H2S into the gas phase than was previously believed. The obtained effective cross-section for chemical desorption indicates that the chemical desorption rate exceeds the photodesorption rate in typical interstellar environments.

Excitation and desorption of physisorbed H2 via theΣu2 electron scattering resonance.

Our high-resolution electron energy-loss measurements concern physisorbed H2 and comprise differential cross sections for the excitation of the internal H2 modes and the H2-surface bonding mode and their combinations and extend over the electron impact energy range of the classical low-energy H2 Σu2 resonance. Comparison with corresponding data for the excitation of the internal modes of gas phase H2 reveals that strong elastic electron reflectivity from the Cu(100) substrate profoundly distorts the inelastic scattering pattern for physisorbed H2. We find that this influence can be corrected for and that the resulting peak cross sections agree with the H2 gas phase data, in accordance with theoretical predictions for the excitation of the internal H2 vibration. We have used corrected cross sections for the rotational mode spectra of physisorbed H2, HD, and D2 in a model concerning electron induced desorption via rotation-translation energy conversion. These spectra include transitions from the ground state as well as excited levels of the physisorption potential well. H2 and HD can desorb from all levels while D2, for energetic reason, can only desorb from the excited levels. This model gives a satisfactory account of the observed desorption cross sections and predicts characteristic velocity distributions of the desorbing molecules. The cross section data for H2 and HD reveals that direct bound-free transitions also contribute to the electron induced desorption.

Optical-power-dependent photodesorption kinetics of graphene studied by conductance response.

The photodesorption kinetics of graphene with various UV laser power is studied by conductance response. Analytical expressions of the power-dependent photodesorption kinetics of graphene in ambience are derived. The photodesorption time constant τd, steady current, and magnitude of modulation current, can be expressed as functions of the adsorption time constant τa, desorption cross section σ, and photon flux density. Under illumination the steady occupation ratio of adsorbed O2 on graphene is equal to τd/τa. It is suggested that the photodesorption of O2 on graphene is attributed the injection of photogenerated hot electrons and is restricted by the density of antibonding states of O2.

Hydrogen–deuterium exchange on plasma-exposed W and SS surface

The desorption cross section for hydrogen isotopes adsorbed on stainless steel (SS) and tungsten (W) has been evaluated experimentally to provide basic information on tritium exchange. One side of a sample sheet was alternately exposed to H and D plasma, and deuterium density on the surface was repeatedly observed using nuclear reaction analysis (NRA) under continuous plasma exposure. From the time dependent change in the deuterium density, the desorption cross sections for SS and W were estimated to be 6.9±2.3×10−23m2 and 4.6±1.0×10−23m2, respectively. No significant differences in the cross section between H and D plasma were observed. Recombinative desorption was found to dominate the desorption process owing to the low incident energy of hydrogen atoms.

Focused helium-ion-beam-induced deposition

The recent introduction of the helium ion microscope (HIM) offers new possibilities for materials modification and fabrication with spatial resolution below 10 nm. In particular, the specific interaction of He+ ions in the tens of keV energy range with materials—i.e., minimal deflection and mainly energy loss via electronic excitations—renders the HIM a special tool for ion-beam-induced deposition. In this work, an overview is given of all studies of helium-ion-beam-induced deposition (He-IBID) that appeared in the literature before summer 2014. Continuum models that describe the deposition processes are presented in detail, with emphasis on precursor depletion and replenishment. In addition, a Monte Carlo model is discussed. Basic experimental He-IBID studies are critically examined. They show deposition rates of up to 0.1 nm3/ion. Analysis by means of a continuum model yields the precursor diffusion constant and the cross sections for beam-induced precursor decomposition and beam-induced desorption. Moreover, it is shown that deposition takes place only in a small zone around the beam impact point. Furthermore, the characterization of deposited materials is discussed in terms of microstructure and resistivity. It is shown that He-IBID material resembles more electron-beam-induced-deposition (EBID) material than Ga-ion-beam-induced-deposition (Ga-IBID) material. Nevertheless, the spatial resolution for He-IBID is in general better than for EBID and Ga-IBID; in particular, proximity effects are minimal.

Mechanisms and cross sections for water desorption from a lunar impact melt breccia

AbstractDesorption of H2O (ν = 0) following 157‐nm irradiation of amorphous solid water on a lunar impact melt breccia was measured with resonance‐enhanced multiphoton ionization. Photofragments of vibrationally excited water were detected with nonresonant ionization. The average cross section for H2O (ν = 0) removal and destruction at 0.1 Langmuir (1 L = 10−6 Torr∙s) H2O exposure was measured to be (7.1 ± 1.9) × 10−19 cm2. Cross sections were also measured at 0.3, 1, 5, and 10 L exposures. Because these cross sections increase drastically with decreasing water coverage, water is not expected to remain intact as H2O on the sunlit lunar surface. Instead, photons are likely to cause H2O to desorb or dissociate. The OH+ fragment of H2O (ν*) increased in intensity with increasing irradiation as hydroxyl groups built up on the surface and then recombined. The OH+ signal eventually began to decrease after a dose of 5 × 1018 photons cm−2. Under these conditions, the cross section for H2O (ν*) photodesorption was measured to be 6.4 × 10−20 cm2 for an initial exposure of 5 L H2O.

Laboratory studies of electron and ion irradiation of solid acetonitrile (CH 3 CN)

The structure and bonding of solid acetonitrile (CH₃CN) films on amorphous silica are studied, and chemical and physical processes under irradiation with 200 keV protons and 250-400 eV electrons are quantified using transmission infrared spectroscopy, reflection-absorption infrared spectroscopy and temperature-programmed desorption, with the assistance of basic computational chemistry and nuclear materials calculations. The thermal desorption profiles are found to depend strongly on the balance between CH₃CN-surface and CH₃CN-CH₃CN interactions, passing from a sub-monolayer regime (binding energy: 35-50 kJ mol⁻¹) to a multilayer regime (binding energy: 38.2±1.0 kJ mol⁻¹) via a fractional order desorption regime characteristic of islanding as the coverage increases. Calculations using the SRIM code reveal that the effects of the ion irradiation are dominated by electronic stopping of incident protons, and the subsequent generation of secondary electrons. Therefore, ion irradiation and electron irradiation experiments can be quantitatively compared. During ion irradiation of thicker CH₃CN films, a cross section for secondary electron-promoted chemical destruction of CH3CN of 4 (±1) × 10⁻¹⁸ cm² was measured, while electron-promoted desorption was not detected. A significantly higher cross section for electron-promoted desorption of 0.82-3.2 × 10⁻¹⁵ cm² was measured during electron irradiation of thinner CH₃CN films, while no chemical products were detected. The differences between the experimental results can be rationalized by recognizing that chemical reaction is a bulk effect in the CH₃CN film, whereas desorption is a surface sensitive process. In thicker films, electron-promoted desorption is expected to occur a rate that is independent of the film thickness; i.e. show zeroth-order kinetics with respect to the surface concentration.

Dynamics Research on Electron-Stimulated Desorption of Fluorine from Fluorinated Graphene Surface

The experiment of stimulating fluorine to desorb from fluorinated graphene was carried out in order to make clear the dynamical mechanism of the electron-stimulated desorption (ESD) of fluorine. X-ray photoelectron spectroscopy (XPS) with an electron beam gun accessory was used not only to stimulate fluorine to desorb, but also to characterize the concentration of fluorine after ESD. The concentration of fluorine dropped quickly following electron beam irritation. The desorption cross section of fluorine is about 1.5×10-17 cm2. It was also found that electron beam with low current density did not damage the structure of fluorinated graphene. These results confirm that it is feasible to fabricate all graphene electronics by ESD technology.

Ultraviolet photodesorption as a driver of water migration on the lunar surface

We have studied the photodesorption of monolayer coverage of water ice coating amorphous carbon by 193nm photons. We found that the column density of water decreases exponentially with fluence during irradiation at 120K and obtain a desorption cross-section of (7.4±0.5)×10–19cm2. We estimate the lifetime for photodesorption to be ∼14h for average solar activity at the sub-solar point on the Moon. The photodesorbed molecules, based on other experiments, are expected to be slower than the escape velocity and to be re-adsorbed on the surface, contributing to water migration. For comparison, we determine the effect of 4keV He+ on water-coated Apollo 16 soil (65901) under similar experimental conditions. We find a sputtering cross-section of (1.2 ±0.4)×10–16cm2. We calculate that photodesorption by solar UV is four orders of magnitude more effective than sputtering by the solar wind in causing migration of adsorbed water on the lunar surface.

Laboratory investigations of irradiated acetonitrile-containing ices on an interstellar dust analog

Reflection-absorption infrared spectroscopy is used to study the impact of low-energy electron irradiation of acetonitrile-containing ices, under conditions close to those in the dense star-forming regions in the interstellar medium. Both the incident electron energy and the surface coverage were varied. The experiments reveal that solid acetonitrile is desorbed from its ultrathin solid films with a cross section of the order of 10−17 cm2. Evidence is presented for a significantly larger desorption cross section for acetonitrile molecules at the water–ice interface, similar to that previously observed for the benzene–water system.

Characterisation of the growth of a carbonaceous film on silicon

In this paper we present the first characterisation of growth of a carbonaceous film on a silicon substrate exposed to a metastable atom beam using an in situ rotating polariser ellipsometer. The initial deposition of oil due to a background partial pressure in vacuum is investigated. Subsequent exposure of the deposited oil to a high flux metastable neon (Ne*) beam results in cross-linking of the oil film, creating a polymerised carbonaceous layer. Values for the mean residence time, polymerisation cross-section, and desorption cross-section are calculated and compared to similar studies performed for ion bombardment. Simple estimates can provide reasonable values for application of the theory to other systems.

XUV Free-Electron Laser Desorption of NO from Graphite (0001)

We report results of femtosecond laser induced desorption of NO from highly oriented pyrolytic graphite using XUV photon energies of hν = 38 eV and hν = 57 eV. Femtosecond pulses with a pulse energy of up to 40 μJ and about 30 fs duration generated at FLASH are applied. The desorbed molecules are detected with rovibrational state selectivity by (1 + 1) REMPI in the A(2)Σ(+) ← X(2)Π γ-bands around λ = 226 nm. A nonlinear desorption yield of neutral NO is observed with an exponent of m = 1.4 ± 0.2. At a fluence of about 4 mJ/cm(2) a desorption cross section of σ(1) = (1.1 ± 0.4) × 10(-17) cm(2) is observed, accompanied with a lower one of σ(2) = (2.6 ± 0.3) × 10(-19) cm(2) observable at higher total fluence. A nonthermal rovibrational population distribution is observed with an average rotational energy of <E(rot)> = 38.6 meV (311 cm(-1)), a vibrational energy of <E(vib)> = 136 meV (1097 cm(-1)) and an electronic energy of <E(el)> = 3.9 meV (31 cm(-1)).

Photon- and electron-stimulated desorption from laboratory models of interstellar ice grains

The nonthermal desorption of water from ice films induced by photon and low energy electron irradiation has been studied under conditions mimicking those found in dense interstellar clouds. Water desorption following photon irradiation at 250 nm relies on the presence of an absorbing species within the H2O ice, in this case benzene. Desorption cross sections are obtained and used to derive first order rate coefficients for the desorption processes. Kinetic modeling has been used to compare the efficiencies of these desorption mechanisms with others known to be in operation in dense clouds.

Surface Patterning of Silver using an Electron‐ or Photon‐Assisted Oxidation Reaction

We have investigated a recently developed method of patterning Ag surfaces. The method uses an electron beam to irradiate Ag surfaces during NO(2) dosing at 300 K and leads to sharp oxide patterns on otherwise metallic surfaces. Investigations were performed on an Ag(111) single crystal and on an Ag foil with LEEM (low-energy electron microscopy), LEED (low-energy electron diffraction), MEM (mirror electron microscopy), and XPEEM (X-ray photo-emission electron microscopy). The oxidation reaction, which is based on the electron-induced desorption of NO molecules, proceeds in steps, from a layer of O atoms adsorbed on the metallic Ag via an intermediate phase to an amorphous Ag(2)O film. Our measurements evidence a high cross section for electron-induced NO desorption with 30-40 eV electrons, indicating that only a few electrons per adsorbing NO(2) molecule are required to initiate the process. The intermediate phase, which forms a partially ordered quadratic structure, contains oxygen species in an oxide-like environment, coexisting with an adsorbate covered metallic Ag(111) surface. While the intermediate phase dissolves within hours under UHV conditions, fully developed oxide patches, consisting of several layers of thick, amorphous Ag(2)O, are kinetically stable. The oxidation method also works with 40 eV (and 700 eV) photons instead of electrons. In preliminary experiments local patterns could also be created with photons, suggesting that mask techniques can be applied for the process.