Total Desorption Cross Sections Research Articles

Electron stimulated desorption of NO from step sites on Pt(112): The role of chemisorption site geometry on the cross section

The electronic excitation of adsorbed NO on Pt(112) has been observed to cause mainly desorption. Species-specific total desorption cross sections of terminally bound and bridged-bound NO present exclusively on the step sites of Pt(112) have been measured using Fourier transform infrared reflection absorption spectroscopy (FT-IRAS) as a sensitive detector of the surface coverage of various chemisorbed NO species. Other electron stimulated processes which might influence the measurement of these cross sections, such as electron stimulated fragmentation, site-exchange, and occupied site density effects have been found to be unimportant. It was found for 275 eV electrons that terminally bound NO desorbs with a larger cross section (2.3×10−18 cm2) than bridged-bound NO, which has a desorption cross section of 8.7×10−19 cm2. This is the expected relationship between the total desorption cross sections for these two species, with the smaller cross section being observed for the bridged NO.

Electron stimulated desorption of positive ions from an adsorbate-covered Si(100) surface

Electron stimulated desorption of positive ions (H +, C +, O +, CO + and CO 2 +) from the (100) surface of a silicon single crystal exposed to NO and CO at room temperature has been observed for electron energies in the range of 0–180 eV. The dependence of the ion yields on the incident electron beam energy shows thresholds in the 18.5–22.5 eV range and features at Si core level excitation energies. The total desorption cross sections for the ionic species are similar and at a value of approximately 10 −18 cm 2.

Argon ion impact desorption cross sections of chlorine and carbon from molybdenum

Total desorption cross sections have been measured for Cl (σ Cl) and C(σ C) on molybdenum by argon ion bombardment for an incidence angle of 60° from the surface normal. For the bombardment an ion gun with low current density ( i 0 ~ 1 × 10 −7 A cm −2) at low system pressure (~10 −9 Torr) was used. The detection was performed by AES and the data were sensitivity factor corrected. The AES analysis of the surface after adsorption showed that Mo, C and Cl contributed to more than 94% of the atomic composition. With known i 0, it is possible to obtain σ from the adsorbate signal vs ion bombardment time curve. For ion energies between 0.2 keV to 1.0 keV the measured value for σ Cl and σ C are 0.5−3 × 10 −15 cm 2 and 0.2−4 × 10 −15 cm 2, respectively. The possible effects of the surface roughness due to prebombardment are discussed.

Desorption of carbon from tungsten by argon ion bombardment

Since the desorption of adsorbed layers from the first walls of fusion reactors due to energetic particle bombardment produces plasma contamination, studies of the ion stimulated desorption mechanisms and finding of appropriate materials are of major concern. In the present work Auger Electron Spectroscopy (AES) has been used to measure the total desorption cross sections (TDCS) of C ( σ c) on tungsten by argon ion bombardment. From the decay of the carbon adsorbate signal versus the ion bombardment time, and with known current density of the ion beam causing the desorption, it is possible to evaluate σ c. With an ion impact angle of 60° from the surface normal and ion energies from 0.2 keV to 1 keV, the measured values are 0.2 to 1.6 × 10 −14 cm 2 for σ c. The possible influence of the surface roughness is discussed, since a prebombarded surface is used in the experiments. Preliminary data of the desorption of CO adsorbed on well annealed tungsten surfaces are reported. Using the binary collision model the TDCS for C on Ni, Mo, Ta and W under argon ion bombardment are discussed.

Argon ion impact desorption cross sections of chlorine and carbon from nickel

Total desorption cross sections have been measured for Cl (σCl) and C (σC) on Ni(111) by Argon ion bombardment for an incidence angle of 60 ° from the surface normal. For the bombardment an ion gun with low current density (i0∠1×10−7 A cm−2) at low system pressure (∠10−9 Torr) was used. The detection was performed by AES and the data was sensitivity factor corrected. The AES analysis of the surface after adsorption showed that Ni, C, and Cl contributed to more than 95% of the atomic composition. With known i0, it is possible to obtain σ from the adsorbate signal vs ion bombardment time curve. For ion energies between 0.2 to 1.0 keV the measured values for σCl and σC are 0.35–3.5×10−15 cm2 and 0.9–75×10−17 cm2, respectively.

Adsorption of oxygen at tungsten single crystal: the (110) face (Electron stimulated desorption)

The interaction of oxygen gas with the (110) face of tungsten single crystal has been investigated by the techniques of electron stimulated desorption combined with surface mass spectrometry. At 300K two states with desorption efficiencies of 6*10-7 and 5*10-6 ion/electron for production of O+ ions are observed in the adsorption scheme. The former state forms rapidly on introducing gas to the clean surface and is essentially complete at an exposure of 0.3*10-6L. The two states are identified with different temperature regimes of around 350 and 750K for the removal of ion current and with total desorption cross sections of respectively 2*10-17 and 5*10-19 cm2. Measurements of the energy distribution of desorbed ions reveal a difference of around 1.2 eV between the values of most probable ion energy from the two states.

Electron impact desorption of hydrogen on tungsten: I. Pure hydrogen lasers

The electron impact induced desorption of H + ions from hydrogen layers on thermally annealed polycrystalline tungsten ribbons has been investigated mass spectrometrically. In agreement with Benninghoven et al. (1972) and Madey (1973) the more strongly bound β 2-state has been found to have a higher EID cross section than the more weakly bound β 1-state. Ionic and total desorption cross sections have been measured at 100 eV; the obtained values agree with the lower values published to data, which suggests that the much higher values reported by some authors do not apply to pure hydrogen layers. The contributions of the different states have also been investigated using partial coverage with deuterium. The implications of the findings for the understanding of the adsorption states of hydrogen on tungsten and for the mechanism of EID are discussed.

Electron-Impact Desorption of Ions from Polycrystalline Tungsten

A cylindrical magnetic spectrometer has been constructed to study the electron-impact desorption of ions and neutrals from solid surfaces. The instrument has high-energy resolution and sensitivity and allows the determination of the charge-to-mass ratio of the emitted ions. The apparatus has been applied to the study of polycrystalline tungsten with ${\mathrm{O}}_{2}$, CO, C${\mathrm{O}}_{2}$, ${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, and ${\mathrm{H}}_{2}$O adsorbed. The ions which have been observed are ${\mathrm{O}}^{+}$ from ${\mathrm{O}}_{2}$/W, CO/W, and C${\mathrm{O}}_{2}$/W; C${\mathrm{O}}^{+}$ from CO/W; and ${\mathrm{H}}^{+}$ from ${\mathrm{H}}_{2}$/W and ${\mathrm{H}}_{2}$O/W. The ion energy distributions, ionic and total desorption cross sections, threshold energies, and other experimental results are presented and discussed.

Kinetics of Electron Impact Desorption of Ions and Neutrals from Polycrystalline Tungsten

A cylindrical magnetic spectrometer has been constructed to study electron impact desorption of ions and neutrals from solid surfaces. The apparatus has been applied to the study of polycrystalline tungsten with O2, CO, CO2, N2, H2, and H2O adsorbed. This paper reports the results of the measurements of the kinetics of the adsorption and desorption processes, the states of adsorption which are observed, the total desorption cross sections for these states, and electronic and thermal conversion between states.