Total Cross Sections For Electron Research Articles

Measurements of absolute total cross sections for electron scattering from triatomic polar molecules: SO 2 and H 2S

Absolute total electron-scattering cross sections (TCSs) have been measured in the energy range from 0.5 to 370 eV for SO 2 and from 6 to 370 eV for H 2S molecules. Measurements have been carried out using modified electron spectrometer working in a linear transmission mode. The present results are compared to published experimental TCS data. Some aspects important for achieving accurate absolute total cross sections in an electron transmission experiment are discussed.

Effective electron scattering cross-sections for zinc atoms

In the present paper, we show the possibility to measure in the same experiment the total cross section of electron scattering by Zn atoms, the ionization efficiency and low-lying level excitation as well as to provide their absolute calibration.

Total and elastic cross-sections of electron and positron scattering from C 3H 4 molecules (allene and propyne)

Total cross-sections (TCSs) for electron and positron scattering from C 3H 4 isomers (allene and propyne) were measured using the linear transmission method in the range of 0.8–600 eV for electrons and 0.7–600 eV for positrons. Elastic cross-sections for electron scattering were also measured by a crossed-beam experiment in the range of 2–100 eV. Sharp resonance-like peaks were observed in the electron TCSs for propyne around 3.6 and 8 eV, and 2.5 and 11 eV for allene. The positron TCSs for these molecules were found to be smaller and much smoother than the electron TCSs. Clear differences in resonance positions and magnitudes between the two isomers can be viewed as an isomer effect.

Total cross sections for electron scattering with some C3hydrocarbons

Absolute total cross sections (TCSs) for electron scattering from two C3H4isomers (allene and propyne) and from propane (C3H8) have been measured in alinear electron-beam transmission experiment for impact energy ranging from 0.5to 370 eV. Low-energy TCS functions for C3H4 are dominated by prominentstructures: a resonant-like enhancement of the cross section for allene peaks ataround 2.3 and 3.4 eV for propyne, while very broad enhancement is centred at9.5–10 and 8.0–8.5 eV for allene and propyne respectively. Some supplementaryweak features are also discernible. The general shape of cross sections for bothC3H4 isomers is similar except that the cross section enhancements are atdifferent locations. In addition, the TCS for propyne has been comparedwith cross sections for other open-chain hydrocarbons with three carbonatoms, i.e. propene (C3H6) and propane (C3H8), and the effect of themultiplicity of the C–C bond on the low-energy scattering is demonstrated.

Low-energy electron-hexafluoropropene(C3F6)scattering

Absolute total cross sections for electron scattering by ${\mathrm{C}}_{3}{\mathrm{F}}_{6}$ molecules in the energy range from 0.5 to 30 eV have been measured using a beam transmission method. Two resonant-like maxima in the total cross sections function are clearly visible: the first, small maximum is centered near 3.2 eV, the second one, much pronounced, is located at 9.5 eV. No previous experimental data have been found in the literature for comparison. The effect of fluorination is indicated.

Positron and electron total cross sections for collisions with C 2F 6 and C 2H 3F 3 molecules

The experimental total cross sections (TCSs) data for 0.7–600 eV positrons and 0.8–600 eV electrons colliding with hexafluoroethane (C 2F 6) and 1,1,1-trifluoroethane (C 2H 3F 3) molecules are presented. These data are compared with the data of ethane (C 2H 6) measured previously. On the positron TCS data below 5 eV, the TCS curves are quite different from each other. For energies 7–25 eV, the data for these three kinds of molecules are comparable, though above 60 eV the C 2F 6 TCS data are roughly twice those of C 2H 6. The C 2H 3F 3 TCS data have magnitudes almost midway between those of C 2F 6 and C 2H 6. The TCS curves for electrons show the same variation with energy as the positron data above 60 eV. In the low energy region, a few weak resonant peaks are observed in the C 2F 6 and C 2H 3F 3 electron TCS curves. On the electron TCS data in the range 5.5–14 eV, the C 2F 6 data are lower than the C 2H 6 data.

Total and elasticelectron scattering cross sections from ozone at intermediateand high energies

Total cross sections for electron scattering from O3molecules in the energy range 350-5000 eV have been measuredfor the first time. The experimental method used was based onthe measurement of the attenuation of a collimated electronbeam through an O3-O2 mixture in combination with useof an electron energy loss technique to determine the purity ofthe ozone sample. Differential and integral elastic crosssections have also been calculated using a scattering potentialin the framework of the independent-atom model. The presenttheoretical and experimental results are compared with earliercalculations available in the literature.

Energy dependence of the total cross section for electron scattering by N 2 and CO molecules at energies above 1 keV

Recently, an analytical formula to reproduce total cross section data for electron scattering by diatomic molecules has been published by Liu and Sun (Phys. Lett. A 222 (1996) 233). This formula was derived by fitting available experimental data for intermediate and high energy to a one-parameter expression that the authors state to be based on the Born approximation. However, as discussed in the present Letter, the energy dependence of the total cross section for high energies, above 1 keV, given by this formula is in contradiction with the Born–Bethe theory. We also present in this Letter a method to calculate differential and integral elastic cross sections for e–N 2 and e–CO collisions in the energy range (0.1–10 keV) that gives results in agreement with the available experimental data. By combining these results with inelastic cross section data existing in the literature has been confirmed the applicability of an alternative expression for the total cross section for electron scattering by diatomic molecules, which can be used in the energy range (1–10 keV). The energy dependence given by this expression shows an asymptotic behaviour for increasing energy in better agreement with the Born–Bethe theory.

Analytical Formulae for Total Cross Sections for Electron Scattering by Atoms (N, O, F, Ne, P, S, Cl, Ar, As, Se, Br, Kr) Between 0.5–10 keV

Analytical formulae for total cross sections for electron scattering by atoms which are close to the noble gases (Ne, Ar and Kr), for electron energies ranging from 0.5 to 10 keV, have been obtained in this study. We have shown, previously, that molecular total cross sections, at these energies, depend on target polarizability and the number of target electrons. A similar behaviour has been supposed for total cross sections of some atoms (N, O, F, P, S, Cl, As, Se and Br). The obtained expression depends on atomic parameters and it is based in some correlation derived from noble gases. The applicability of the formula has been checked by comparison with available data for atomic oxygen.

Absolute total cross sections for electron-stimulated desorption of hydrogen and deuterium from silicon(111) measured by second harmonic generation

Using second harmonic generation (SHG) as a sensitive measure of the amount of hydrogen or deuterium remaining on a silicon(111) surface, we have measured absolute total cross sections for electron stimulated desorption (ESD) over a range of incident energies from 50 to 300 eV. The measured cross sections for hydrogen are approximately a factor of 10 smaller than those found by Fuse [T. Fuse et al., Surf. Sci. 420, 81 (1999)] on the Si(100) surface and a factor of 10 greater than found for deuterium on the Si(111) surface by Matsunami [N. Matsunami et al., Surf. Sci. 192, 27 (1987)]. The measured cross sections for deuterium are consistent with the measurements of Matsunami et al. on the Si(111) surface. This indicates that while there is a significant isotope effect, the choice of surface also plays an important role. The details of the desorption spectrum are consistent with the multihole desorption model. Comparison to calculated ionization cross sections suggests that the $2s$ core hole excitation is more likely to lead to desorption than the $2p$ core hole excitation.

Energy dependence of the total cross section for electron scattering by chloromethanes in the energy range 0.5–10 keV

The energy dependence of the total cross section for electron scattering by methane and its chloride compounds has been studied by considering the latest experimental results and those predicted by the Born-Bethe theory. A simple analytical expression depending on the number of electrons and the polarizability of the target is proposed for these molecules. The energy dependence given by this formula shows an asymptotic behavior for increasing energy in better agreement with the Born-Bethe theory. The inconsistency with the Born-Bethe theory of the two-parameter formula given recently by Karwasz et al. has also been shown.

Intermediate-energy total cross sections for electron scattering onWF6

Total cross sections for electron scattering on ${\mathrm{WF}}_{6}$ molecules have been measured in the 75--3500-eV energy range by an absolute transmission method. The data are compared to other heavy gases, ${\mathrm{SF}}_{6},$ Xe, and ${\mathrm{GeH}}_{4}.$ Apparently different energy dependencies for these gases can be described by the same, simple four-parameter formula. A study of atomic total-cross-section parameters in the keV energy range for 15 atoms from H to W is presented.

Total and elastic cross-sections for electron and positron scattering from OCS molecule: A comparative study with CO2

We have carried out a joint experimental and theoretical study on determination of total and elastic cross-sections for electron and positron impact on OCS molecules. For total cross-section measurements, impact energies are from 0.7 to 600 eV for electron collisions and 0.8 to 600 eV for positron collisions. For elastic scattering, only electron impact has been studied from 1.5 to 100 eV. Our present measurements for total cross-sections for electron impact are found to agree extremely well with the measurements of Dababneh et al. (1982) in the entire energy region studied. The results by Szmytkowski et al. (1994) are consistently larger by 20% above 2 eV, and, in particular, are larger by a factor of two in the 1.2 eV resonance region. The present theory for elastic scattering is in good agreement with the present measurement, and has been employed for understanding the dynamics. The total cross-section recommended here is probably the best in accuracy as of today although further studies would be helpful.

Total cross sections of electron and positron collisions withC3F8andC3H8molecules and differential elastic and vibrational excitation cross sections by electron impact on these molecules

Total cross sections for electron ${(e}^{\ensuremath{-}})$ and positron ${(e}^{+})$ scattering from ${\mathrm{C}}_{3}{\mathrm{H}}_{8}$ and ${\mathrm{C}}_{3}{\mathrm{F}}_{8}$ have been measured from 0.8 to 600 eV and 0.7 to 600 eV, respectively. We have also investigated differential elastic cross sections by electron impact from 2.0 to 200 eV, and compared them with the present theoretical results. For ${e}^{\ensuremath{-}}$ scattering from ${\mathrm{C}}_{3}{\mathrm{H}}_{8},$ the cross sections are found to be larger by a factor of 2 than those of ${e}^{+}$ scattering below 20--30 eV. They show a large peak at 8 eV due to a shape resonance and a shoulderlike structure in the region of 20--40 eV. For ${e}^{\ensuremath{-}}$ scattering from ${\mathrm{C}}_{3}{\mathrm{F}}_{8},$ the cross sections are again larger by at least a factor of 2 than those of ${e}^{+}$ scattering below 50 eV, and they have two peaks at 4 and 8 eV, followed by a broad peak in the region of 20--40 eV. Some small structures overlie the broad hump. Both ${e}^{\ensuremath{-}}$ and ${e}^{+}$ impact cross sections for ${\mathrm{C}}_{3}{\mathrm{H}}_{8}$ and ${\mathrm{C}}_{3}{\mathrm{F}}_{8}$ quickly approach each other beyond 200 eV. From the differential cross section study, we have been able to provide more detailed information on shape resonances, and also we have carried out some analysis of resonances in vibrational excitation results. In general, the total and integrated elastic cross sections are in good qualitative and quantitative agreement.

Total cross sections for electron scattering on chloromethanes: Formulation of the additivity rule

Total cross sections for electron scattering on CH 3Cl, CH2Cl2 , and CHCl3 have been measured by an absolute method in the ~75‐4000!-eV energy range. The overall experimental error is below 5%. A formulation of the additivity rule is proposed. The molecular cross sections are approximated by a Born-like twoparameter formula. We show that the low-energy parameter is correlated to the molecular polarizability. We show also that the high-energy parameter for a given molecule can be expressed as the sum of the high-energy parameters of the constituent atoms. The model has been successfully verified for two groups of halomethanes: the CH4 ,C H 3Cl, CH2Cl2 , CHCl3 , and CCl4 series and the CF4 ,C F 3Cl, CF2Cl2 , CFCl3 , and CCl4 series. The model has been successfully extended to molecules containing Si and S atoms, such as H2S, SF6 , SiH4 , and SiF4 . @S1050-2947~99!10102-1# PACS number~s!: 34.80.Bm, 34.80.Gs Chlorine substituted methanes play an important role in atmospheric chemistry @1#; their time in the Earth’s troposphere is several tens of years. Free Cl radicals formed by ultraviolet-induced dissociation act as catalysts in the chain of reactions leading to the destruction of the stratospheric ozone @2#. In particular, chloromethane (CH3Cl), used in refrigerators and organic chemistry, is the most abundant halocarbon present in the atmosphere. CH3Cl is also the most frequently studied halocarbon in electron-scattering experiments.

Elastic and total cross sections for electron scattering by nitrogen molecule in the intermediate energy range

The Schwinger variational iterative method combined with the distorted-wave approximation is applied to the calculation of differential, integral and momentum transfer cross sections for elastic electron- scattering in the 20-800 eV energy range. In this study, a complex optical potential consisting of static, exchange, correlation-polarization plus absorption contributions, derived from a fully molecular wavefunction, is used for the electron-molecule interaction. It is shown that the introduction of absorption effects influences significantly the calculated differential cross sections in the intermediate and high incident energy range. Particularly, in the 50-300 eV range, the used model absorption potential has improved significantly the agreement between calculated results and the available experimental data.

Electron scattering by Ne, Arand Kr at intermediate and high energies,0.5-10 keV

Semi-empirical total cross sections for electron scattering of noble gases (Ne, Ar and Kr) in the energy range 0.5-10 keV have been obtained by combining transmission-beam measurements for impact energies up to 6 keV with an asymptotic behaviour at higher energies according to the Born-Bethe approximation. The influence of the forward electron scattering on the experimental system has been evaluated by means of a Monte Carlo electron transport simulation. Theoretical values have also been obtained by applying the Born approximation in the case of inelastic processes and by means of an atomic scattering potential for the elastic part. The results of these calculations show an excellent agreement with the semi-empirical values in the above-mentioned energy range.

Absolute total cross-section measurements for electron scattering from silicon tetrachloride, SiCl

The absolute total cross-sections for electron scattering on SiCl4 molecules have been measured in two distinct electron-transmission experiments, in Gdansk and in Trento laboratory, for impact energy ranging from 0.3 to 250 eV and from 75 to 4000 eV, respectively. The e--SiCl4 total cross-section function shows two very distinct resonant-like features: the strong peak at 1.9 eV and much broader main maximum centered near 10 eV with some additional substructure close to 5 eV. The present results are compared with low-energy total experimental data and elastic theoretical calculations for electron collisions with silicon tetrachloride molecules.

Total cross sections for electron scattering from well-known and exotic molecules

In this review paper, scattering of intermediate to high energy electrons on well-known as well as exotic molecular targets is considered. The ‘additivity rule’ and its modifications for calculating various total cross sections are discussed against the background of an extensive experimental data. The theory succeeds at high impact energies (E i>100 eV). Tentative upper and lower limits of e-molecule ionization cross sections are identified. Fitting formulas to represent total cross sections as functions of energy are also given.

Total cross-sections of electron and positron collisions with CHF 3 molecules: a comparative study with CH 4 and CF 4

Total cross-sections for electron (e −) and positron (e +) scattering from CHF 3 have been measured from 0.7 to 600 eV and compared with the theoretical elastic cross-section. For e − scattering, the cross-sections are found to be larger by 30% than the data compiled by Christophorou et al. [L.G. Christophorou, J.K. Olthoff, M.V.V.S. Rao, J. Phys. Chem. Ref. Data 26 (1997) 1] in the entire energy range measured, but their energy dependence is in good agreement. For e + scattering, the cross-sections are smaller by at least a factor of two than those for electron scattering below 20–30 eV, but quickly approach those of electron impact beyond 100 eV. A comparative study with CH 4 and CF 4 reported in our previous papers provides detailed information on the dynamics.