eV Impact Energy Research Articles

Valence–shell electron energy-loss spectra of formic acid and acetic acid

Abstract Gas phase optical absorption spectra of formic acid and acetic acid in the energy ranges 5.0–11.3 and 4.4–11.3 eV, respectively, have been previously recorded. It is generally agreed that of the four successive bands observed in the spectra of these compounds, the first corresponds to n → π * transition. However, various assignments have been proposed for the following three bands. In the present study, electron energy-loss spectra of formic acid and acetic acid vapours at 70 eV impact energy and 0° scattering angle have been measured between 5–15 eV. The transition energies that have been observed coincide with those measured by optical methods in the corresponding range. To identify the valence shell transitions among the spectral features observed, the semi-empirical HAM/3 calculations have been performed. With the help of earlier studies on these and related compounds first and third bands are tentatively assigned as valence, whereas second and fourth bands are assigned as Rydberg.

Elastic and rotationally inelastic cross sections for low-energy electron scattering by SO2 molecules

We show rotationally summed and rotationally inelastic differential, integral and momentum transfer cross sections for electron scattering by SO2 in the 3-30 eV impact energy range in the static exchange approximation. Our results were obtained with the Schwinger multichannel method with pseudopotentials, including a first Born approximation to describe the influence of the molecular permanent dipole moment on the scattering cross sections. The rotational excitation cross sections were obtained through the adiabatic-nuclei-rotation approximation. Our results show good agreement with available experimental and theoretical data. The rotationally inelastic cross sections were found to be very large.

Separation of dipole and impact scatterings in high resolution electron energy loss spectroscopy: Experiment from model organic material

The interpretation of High Resolution Electron Energy Loss Spectroscopy (HREELS) spectra recorded from polymers remains a difficult task because of (1) the relatively poor resolution achieved on these non-ordered systems, (2) the large number of vibrational features induced by the numerous chemical groups in the monomer unit, (3) the superposition of the dipole, impact and resonant scattering mechanisms. In this report, the study of the physical mechanisms responsible for the electron–molecular vibration coupling is investigated, using a simple model system consisting of a well-ordered film of eicosanoic acid adsorbed on a GeS substrate and an hypothesis supposing that dipole scattering is extinguished in non specular geometry. The ordered character of the molecular layer allowed to separate the dipole from the impact contributions in the signal recorded in specular geometry. The results support the idea that any vibrational mode of an organic compound induced by electron excitation contains both contributions with different relative extent, depending on the chemical nature of the molecular group involved in the vibration and their orientation. Cross section measurements for both interaction mechanisms are in agreement with theoretical prediction, confirming the validity of this new and original method suggested for the separation of the interaction mechanisms. Furthermore, we measured a resonance near 6 eV impact energy as the intensity of the ν(C–D) band is enhanced for this impact energy. This observation again is in agreement with our hypothesis that the impact scattering is the dominant interaction mechanism involved in the vibrational excitation of the C–D group. This study allowed for a model of the geometry of adsorption of the molecule on the substrate. It is confirmed that the eicosanoic acid adsorbs the COO group on the substrate, with the long hydrocarbon chain standing up and the CD3 terminating group pointing out of the surface.

Elastic electron scattering by laser-excited 138Ba( ... 6s6p 1P1) atoms

The results of a joint experimental and theoretical study concerning elastic electron scattering by laser-excited 138Ba( ... 6s6p 1P1) atoms are described. These studies demonstrate several important aspects of elastic electron collisions with coherently excited atoms, and are the first such studies. From the measurements, collision and coherence parameters, as well as cross sections associated with an atomic ensemble prepared with an arbitrary in-plane laser geometry and linear polarization (with respect to the collision frame), or equivalently with any magnetic sublevel superposition, have been obtained at 20 eV impact energy and at 10°, 15° and 20° scattering angles. The convergent close-coupling (CCC) method was used within the non-relativistic LS-coupling framework to calculate the magnetic sublevel scattering amplitudes. From these amplitudes all the parameters and cross sections at 20 eV impact energy were extracted in the full angular range in 1° steps. The experimental and theoretical results were found to be in good agreement, indicating that the CCC method can be reliably applied to elastic scattering by 138Ba( ... 6s6p 1P1) atoms, and possibly to other heavy elements when spin-orbit coupling effects are negligible. Small but significant asymmetry was observed in the cross sections for scattering to the left and to the right. It was also found that elastic electron scattering by the initially isotropic atomic ensemble resulted in the creation of significant alignment. As a byproduct of the present studies, elastic scattering cross sections for metastable 138Ba atoms were also obtained.

Absolute total and partial electron impact ionization cross sections of hexamethyldisiloxane

We studied the electron impact ionization of hexamethyldisiloxane (HMDSO), Si 2O(CH 3) 6, which is widely used in plasma-enhanced polymerization applications. Appearance energies and absolute partial cross sections for the formation of fragment ions with relative intensities >1% of the most abundant ion, the Si 2O(CH 3) 5 + fragment ion, were measured in a high resolution double focusing sector field mass spectrometer with a modified ion extraction stage for electron energies from threshold to 100 eV. Dissociative ionization was found to be the dominant process. The main ionization channel removes a complete methyl group to produce the fragment ion Si 2O(CH 3) 5 + with a cross section of 1.7 × 10 −15 cm 2 at 70 eV. The stoichiometric and isotope composition of the various fragment ions was determined by using the high resolution ( m/Δ m = 40,000) of the mass spectrometer used in these studies. The methyl ion is formed with considerable excess kinetic energy, whereas all other fragment ions are formed with essentially no excess energy. The experimental total single ionization cross section of HMDSO (2.2 × 10 −15 cm 2 at 70 eV impact energy) is in good agreement with the result of a semiempirical calculation (2.1 × 10 −15 cm 2 at the same energy).

Electron impact excitation of the , , and states of CO at 10.0, 12.5 and 15.0 eV impact energies

We have measured the differential and integral cross sections for electron impact excitation of the , , and states of CO in the 10-15 eV impact energy region. These measurements fill the gap which existed between the low-energy (threshold to about 12 eV) data of Zobel et al and the intermediate energy (20-50 eV) results of Middleton et al. Recent electron time-of-flight measurements are utilized to place the relative cross sections (deduced from energy-loss spectra) on the absolute scale. The cross sections are in excellent agreement with the results obtained by Zobel et al at the overlapping impact energies and always smoothly extrapolate to their low-energy results. Extrapolation to the intermediate energy domain and comparison with the results of Middleton et al is not so clear. Comparison with theoretical differential and integral cross sections of Sun et al and the integral cross sections of Morgan and Tennyson reveals angular and energy dependences which resemble those found for the experimental results but there are serious discrepances concerning the absolute magnitudes.

Generation of “stabilized” surface species by low-energy interactions of ethylene ions with O- or N-precovered Cu(100) at room temperature

The dissociative adsorption and surface reactions induced by low-energy (≤200 eV) irradiation of unmass-selected ethylene ions on clean, O-precovered and N-precovered Cu(100) surfaces at room temperature (RT) were investigated by high-resolution electron energy loss spectroscopy (EELS) and temperature-programmed desorption mass spectrometry. Ion irradiation of a clean Cu(100) surface in ethylene at 200 eV impact energy was found to produce hydrocarbon fragments that adsorbed readily on the surface at RT and decomposed completely after annealing to above 600 K. For an O-precovered Cu(100) surface at RT, the hydrocarbon species so produced appeared to react with the pre-deposited O atoms to form “stabilized” CO. In addition to the red-shifted CO stretch observed at a low ethylene ion dose, a blue-shifted CO stretch was also found at a higher ethylene ion dosage. The observed red and blue shifts were attributed to adsorption of CO stabilized by a proposed direct interaction mechanism involving neighboring surface O atoms and C-containing species, respectively. In the case of low-energy ion irradiation of a N-precovered Cu(100) surface in ethylene, EELS features attributable to CN and N–H stretching vibrations were observed, giving support to the formation of CN and NH radicals as a result of surface reactions between the hydrocarbon species and the pre-deposited N atoms. The present work provides further evidence for in-situ formation and stabilized adsorption of unique surface species generated by the low-energy ion irradiation process.

Article

The vacuum ultraviolet (VUV) emissions of the hydrogen Lyman series following the dissociative excitation of the Si-organic compounds tetraethoxysilane (TEOS), tetramethylsilane (TMS), and hexamethyldisiloxane (HMDSO) by controlled electron impact were analyzed. Absolute photoemission cross sections from threshold to 200 eV and appearance potentials were determined for the Lyman-α and Lyman-β emissions for each of the three targets. The measured apparent emission cross sections at 100 eV impact energy for the Lyman-α lines are 6.0 x 10-19 cm2 for TMS, 6.5 x 10-19 cm2 for HMDSO and 1.1 x 10-18 cm2 for TEOS. The apparent Lyman-α cross sections for all three targets contained significant cascade contributions from the H Balmer series. Detailed studies of the near threshold regions indicated that several break-up mechanisms of the parent molecules contribute to the emissions. The lowest onsets for HMDSO, and TEOS were attributed to the removal of a single excited atomic hydrogen atom in the n=2 state. Additional onsets at higher energies could not be uniquely correlated in all cases with a particular break-up channel and (or) cascading, since the other fragments in these processes were not identified. The Lyman-β apparent emission cross sections were also measured and we found values of 1.3 x 10-19 cm2 for TMS, 1.6 x 10-19 cm2 for HMDSO, and 2.0 x 10-19 cm2 for TEOS at an impact energy of 100 eV. PACS Nos. 52.20Fs and 34.80Gs

Production of vacuum ultraviolet radiation by electron impact on silicon-organic molecules

The vacuum ultraviolet (VUV) emissions of the hydrogen Lyman series following the dissociative excitation of the Si-organic compounds tetraethoxysilane (TEOS), tetramethylsilane (TMS), and hexamethyldisiloxane (HMDSO) by controlled electron impact were analyzed. Absolute photoemission cross sections from threshold to 200 eV and appearance potentials were determined for the Lyman-k and Lyman-q emissions for each of the three targets. The measured apparent emission cross sections at 100 eV impact energy for the Lyman-k lines are 6,0 f 10 cm for TMS, 6,5 f 10 cm for HMDSO and 1.1 f 10 cm for TEOS. The apparent Lyman-k cross sections for all three targets contained significant cascade contributions from the H Balmer series. Detailed studies of the near threshold regions indicated that several break-up mechanisms of the parent molecules contribute to the emissions. The lowest onsets for HMDSO, and TEOS were attributed to the removal of a single excited atomic hydrogen atom in the ? '2 state. Additional onsets at higher energies could not be uniquely correlated in all cases with a particular break-up channel and (or) cascading, since the other fragments in these processes were not identified. The Lyman-q apparent emission cross sections were also measured and we found values of 1,3 f 10 cm for TMS, 1,6 f 10 cm for HMDSO, and 2,0 f 10 cm for TEOS at an impact energy of 100 eV.

Multiple ionization of gases by collisions with electrons

Multiple ionization cross sections (ICS) for atoms and molecules in gaseous phase have been studied in a linear time-of-flight (TOF) spectrometer, which was constructed in our laboratory. The feasibility of this instrument has been verified by determining multiple ionization reactions in the 90–3000 eV impact energy range. Emphasis is placed on the apparatus and techniques used. Absolute measurements on He, Ne and Ar electron impact ICS have been achieved by measuring the gas target density through an expanded-range thermocouple. The values are compared with the available data in the literature.

Fragmentation of clusters inuduced by collision with a solid surface: comparison of antimony and bismuth cluster ions

Mass-selected antimony cluster ions Sbn+ (n = 3-12) and bismuth cluster ions Bi{ntn}+ (n = 3-8) are allowed to collide with the surface of highly oriented pyrolytic graphite at energies up to 350 eV. The resulting fragment ions are analysed in a time-of-flight mass spectrometer. Two main fragmentation channels can be identified. At low impact energies both Sbn+ and Bin+ cluster ions lose neutral tetramer and dimer units upon collision. Above about 150 eV impact energy Sb3+ becomes the predominant fragment ion of all investigated antimony clusters. The enhanced stability of these fragment clusters can be explained in the framework of the polyhedral skeletal electron pair theory. In contrast, Bin+ cluster scattering leads to the formation of Bi3+, Bi2+ and Bi+ with nearly equal abundances, if the collision energy exceeds 75 eV. The integral scattering yield is substantially higher in this case as compared to Sbn+ clusters.

Vibrationally inelastic and elastic cross sections for collisions

Absolute elastic cross sections for electron impact on nitrogen trifluoride have been recorded for impact energies from 1.5 to 100 eV and scattering angles from to . Because of the small dipole moment, estimates of the integrated and momentum-transfer cross sections were obtained by extrapolation of the angular range with the help of a modified phase-shift program and subsequent numerical integration under the fits. Vibrational excitation function measurements show a 2 eV wide resonance at 3 eV impact energy. Vibrational loss spectra were measured near this resonance and resolved into their vibrational components and . From the angular behaviour of these fundamental modes, the resonance can be assigned to a temporary trapping of the impinging electron in the antibonding orbital of irreducible species `e', the LUMO orbit.

Generalized oscillator strengths for SF6 in the S 2p inner-shell region

Abstract The extrapolation to the optical oscillator strengths (OOSs) of the recently measured small-angle, θ ⩽ 10° generalized oscillator strengths (GOSs) for SF 6 in the S 2p inner-shell region at 1400 eV impact energy has been investigated using various theoretical approaches. These methods include the recent momentum dispersion formula which has the more reliable larger scattering angle measurements embedded in it, the universal formula and the Lassettre fitting formula. From the comparison between the measurements and the universal formula, it is concluded that dynamic effects are less significant for the T 1u (a 1g ) state for θ ⩽ 8°, but are important for the T 1u (t 2g ) and T 1u (e g ) states. We further conclude that the extrapolated OOSs depend sensitively upon the method of extrapolation. The momentum dispersion method produces extrapolated OOS values that are consistent with those of the experiment, thereby manifesting the importance of the accuracy of the larger scattering angle measurements in the extrapolation.

Electron scattering from laser-excited barium: studies of the to transition

We report on measurements of the parameter and absolute differential cross section for electron-impact excitation of the state in barium from the metastable, isotropically populated level at 20 eV impact energy. The determination of these scattering parameters was facilitated by the utilization of a novel laser excitation geometry and measurement scheme. Along with our previous measurements of electron-impact coherence parameters for this transition we now have available a complete set of the five parameters which characterize the excitation of a P state by the scattering of unpolarized electrons from an isotropic (metastable) D level. Comparison of measured values with recent calculations carried out in the unitarized distorted-wave approximation reveals good agreement.

Dissociative Excitation of Tetramethylsilane (TMS) and Hexamethyldisiloxane (HMDSO) by Controlled Electron Impact

AbstractThe optical emission spectra in the wavelength region 200–800 nm produced by electron impact on the silicon‐organic molecules TMS (tetramethylsilane) and HMDSO (hexamethyldisiloxane) under controlled single‐collision conditions have been analyzed. Absolute emission cross sections from threshold to 200 eV impact energy were determined for a variety of emission features. For both targets, the CH(A2Δ → X2Π) emission, the so‐called CH „4300 Å”︁ band, was found to have the largest emission cross section with values (at 100 eV) of 5.5 × 10−19 cm2 and 6.1 × 10−19 cm2 for TMS and HMDSO, respectively. Relatively high onset energies of 28.0 ± 1.5 eV (TMS) and 33.1 ± 1.5 eV (HMDSO) were measured for these emissions. Weaker emission features in both spectra were identified as CH bands corresponding to the B2∑− → X2Π transition (the CH „3900 Å”︁ system) and the C2∑ → X2Π transition, and as the atomic Si line emissions at 253 nm and 288 nm. Near‐threshold studies indicate an onset for the Si emissions of 29.0 ± 2.0 eV (TMS) and 44.6 ± 2.0 eV (HMDSO). Absolute cross sections and appearance energies were also determined for the strongest lines of the hydrogen Balmer series for both targets. The comparatively high onset energies and small emission cross sections for all observed emissions indicate that single‐step dissociative excitation processes are unlikely to play a dominant role in low‐temperature processing plasmas containing TMS and HMDSO.

Electronic and vibrational excitation of acrylonitrile by low and intermediate energy electrons

Electronic and vibrational excitation of acrylonitrile induced by 3–50 eV energy electrons has been investigated by the electron energy loss spectroscopy. Electronic excitation spectra have been recorded for 30 and 50 eV impact energies at a 10° scattering angle in the energy loss range from 5.5 to 11.5 eV, corresponding to the excitation of electrons belonging to the outermost-valence-shell molecular orbitals. We have reviewed the assignment of the valence excited states occurring in the 5.5–9 eV energy loss region. The vibrational patterns associated with the two lowest-energy singlet valence excited states have also been re-examined. Moreover, we have proceeded for the first time to the analysis and attribution of several Rydberg series converging to the ionic ground state and to its two lowest-energy electronic excited states. The study of the excitation function of the C—H stretching modes of acrylonitrile in the 3–11 eV electron impact energy range has shown evidence of a broad shape resonance built on the electronic ground state of the molecule and centered at 5.85 eV. This resonance contributes to a preferential excitation of the C—H stretching modes suggesting that the charge distribution of the additional electron is very likely that of a σ*(C—H) valence molecular orbital. A comparison has been made between the resonances observed in C2H4 and CH2CHCN, in order to discuss the symmetry of the resonant state and also to analyze the substitution effect of the cyanogen group.

Potential sputtering of lithium fluoride by slow multicharged ions.

Thin polycrystalline LiF films have been bombarded by slow $(\ensuremath{\le}1$ keV) multicharged ${\mathrm{Ar}}^{q+}$ ions $(q\ensuremath{\le}9)$, in order to study the resulting total sputter yields by means of a quartz crystal microbalance. More than 99% of sputtered particles are neutral and show yields, at given impact energy, in proportion to the potential energy of projectile ions. The respective ``potential sputtering'' process already takes place far below 100 eV impact energy. It can be related to defect production in LiF following electron capture by the multicharged ions, and removes about one LiF molecule per 100 eV of projectile potential energy.

Electron energy-loss spectroscopy in monosubstituted benzenes

The electron energy-loss spectra of toluene, monochlorobenzene, phenol, nitrobenzene and aniline vapours have been obtained at 70 eV impact energy and 0° scattering angle. Interpretation of the spectra has been attempted with the aid of previous experimental data and quantum chemical calculations in the literature. The excited states observed are generally in good agreement with those obtained in vacuum ultraviolet optical absorption studies. In extending the measurements beyond the first ionization potentials, unreported transitions have been obtained and are assigned to Rydberg states on the basis of term values with respect to ionization potentials determined by photoelectron spectroscopy. Apparent optical oscillator strength distributions were derived from the energy-loss data under the assumption that the Born approximation is valid at energies as low as 70 eV.

Momentum-space ionization spectroscopy of iso-dichloroethylene: A comparative (e,2e) study of electron momentum distributions of the frontier-orbital and innervalence many-body states in iso-dichloroethylene and its cis and trans isomers

The ionization energy spectra of iso-dichloroethylene (iso-C2H2Cl2) at electron momentum of ∼0 and 0.7 a.u. have been obtained by molecular (e,2e) spectroscopy in the symmetric noncoplanar geometry at 1200 eV impact energy. Momentum distributions (MDs) of selected ionic states, including the outervalence states X(3b1)−1 (frontier-orbital state) and G(6b2)−1, and the innervalence states H(9a1)−1, S1(8a1 )−1, S2(5b2)−1, and S3(7a1)−1 as well as an unassigned many-body feature, have also been determined and compared with ab initio calculations using self-consistent-field wave functions of 4-31G, 6-31G, and 6-31++G** basis sets. Combined with our earlier (e,2e) results on the cis- and trans-C2H2Cl2, these MD measurements can be used to investigate the effects of isomeric changes on the valence-shell electronic structure of these disubstituted ethylene derivatives. The sensitivity of the experimental MDs to chemical bonding details can be demonstrated. In addition, ‘‘new’’ many-body features at 19–22 eV and above ∼26 eV have been observed in our ionization energy spectra, which support the breakdown of Koopmans’ theorem as predicted by an earlier Green’s function calculation. Furthermore, our MD measurements can be used to attribute the new many-body features observed at 19–22 eV to (9a1)−1 satellite states and to characterize the many-body states above ∼26 eV predominantly as (7a1)−1 satellite states.

Differential ionization studies at forward angles

The energy distribution of the scattered positron and electron projectiles following ionization of Ar have been measured around 0‡ and 100 eV impact energy. At 30‡ an estimate of the absolute double differential cross-section of both outgoing particles (electron and positron) was determined at 100 eV impact energy.