Electron-impact ionization of fluoromethanes – Review of experiments and binary-encounter models

Total ionization and attachment cross sections have been measured in C3F8 at 330 K using an electron beam and a total ion collection technique, calibrated by similar measurements on N2O and Xe. Our total ionization cross section is similar in general shape to a previous measurement of this type, but with typically half the magnitude. The ionization threshold cannot be accurately derived from these measurements, due to severe upward curvature immediately above threshold. The positive-ion signal rises above the background at 13.0±0.1 eV, to be regarded as a lower limit to the true threshold. An overall ionization cross section with a threshold at 13.3 eV is recommended, based on threshold data from photoelectron spectroscopy and the present data between 14 and 80 eV. The room temperature total attachment cross section peaks at 2.8 eV with a value of 1.75×10−17 cm2. This is 14 times smaller than the only other measurement of this type we are aware of. There is much better agreement with two more recently reported values unfolded from swarm experiments. The temperature dependence of the predominant dissociative attachment process, involving F− production, was studied in a different apparatus using a mass filter and ion pulse counting. At 730 K the peak cross section has increased by ∼60% and the threshold is lower by 1.1 eV. This second type of measurement was used to study the predominant dissociative attachment process in C2H3Cl, involving Cl− production. At 290 K this has a threshold at 0.85 eV and a peak at 1.35 eV of 3.2×10−17 cm2, in good agreement with recent work elsewhere. At 850 K the cross section at the peak is 2.6 larger, and lower in energy by 0.33 eV, while at 0 eV it has reached 6×10−18 cm2. At higher temperatures effects ascribed to thermal dissociation of the C2H3Cl were observed. The implications of the present results regarding the use of these gases in diffuse discharge switches are discussed.

(e,2e) ionization cross-sections for C4H4O, C4H8O and C6H8O

We report calculated total elastic cross sections Qel, total ionisation cross sections, Qion, summed total excitation cross sections ∑Qexc and total cross sections QT for CH3SH upon electron impact for energies from ionisation threshold to 5 keV. We have employed Spherical Complex Optical Potential (SCOP) formalism to calculate total elastic cross section Qel, and total inelastic cross section Qinel and used Complex Scattering Potential – the ionisation contribution (CSP-ic) method to extract the ionisation cross sections, Qion, from the calculated Qinel. The calculated total cross sections are examined as functions of incident electron energy and are compared with available data wherever possible and overall good agreement is observed. In this work Qel, Qion, and ∑Qexc are reported for the first time for CH3SH in this energy range.

This thesis presents a complete set of proton-impact ionization cross section data of DNA constituents determined at the Physikalisch-Technische Bundesanstalt (PTB). The cross section data describe the probabilities for the ionization of gas-phase tetrahydrofuran, pyrimidine and trimethyl phosphate that are structural analogues to the base, sugar, and phosphate residue of the DNA, respectively. Total, single-differential and double-differential ionization cross sections were determined, whereby the differential description refers to the energy or angular dependence of electron emission. Double-differential cross sections were measured for impact energies between 75 keV and 3000 keV over an angular range between 15бу and 150бу in 15бу intervals. Both single-differential and total ionization cross sections were derived by integration of the measured double-differential cross sections. The measurements of double-differential cross sections were carried out using a crossed-beam arrangement in a high-vacuum chamber. The proton beam, which perpendicularly crosses an effusive gas jet target of DNA constituents inside the chamber, was generated either by the 3.75 MV Van de Graaff accelerator of the PTB ion accelerator facility (PIAF) or a newly developed 155 kV ion accelerator. In the measurement a newly constructed electron spectrometer was used to detect the secondary electrons produced by proton collisions with the target molecules. The energy of the secondary electrons was selected by a hemispherical electron energy analyzer, which was mounted on a turntable to allow the spectrometer to rotate around the axis of the gas jet. Hence, the energy spectra could be recorded at defined observation angles with respect to the proton beam. These spectra were converted into absolute cross section values via a normalization procedure based on previously published absolute electron-impact cross section data.

Calculated total, differential, and momentum-transfer cross sections are reported for the vibrationally elastic scattering of electrons from ${\mathrm{H}}_{2}$O and ${\mathrm{NH}}_{3}$ molecules in the range of energy 0.5--20 eV. The total cross sections are approximated by a sum of the rotationally elastic ones and the rotational transition from ground to first excited state (00\ensuremath{\rightarrow}10). The first-order Born approximation with the rotating-molecule model is used in the calculation of the first rotational excitation cross section. The experimental value of the dipole moments of ${\mathrm{H}}_{2}$O and ${\mathrm{NH}}_{3}$ are used in this step of the calculation. Spherically approximate molecular wave functions are applied to calculate the spherical part of the interactions of the incident electron with the molecule and the rotationally elastic-scattering cross section, in which the static and exchange interactions are calculated exactly within the accuracy of the molecular wave function and a parameter-free correlation-polarization potential is used to account for the polarization effect. The agreement of the differential cross sections with the experimental data is good at small angles and reasonably good at intermediate and large angles. Good agreement with measured values is obtained for the momentum-transfer cross sections. Total cross sections are reported with theoretical results on the e-${\mathrm{NH}}_{3}$ system below 10 eV. The possible sources of the discrepancies between the current theoretical and experimental results are analyzed for the total cross sections at very low energies by a quantitative consideration of the contributions of very strong forward scattering at near-zero angles to the total cross sections.

The interactions of electrons with molecular systems under various conditions are essential to interdisciplinary research fields extending over the fundamental and applied sciences. In particular, investigating electron-induced ionization and dissociation of molecules may shed light on the radiation damage to living cells, the physicochemical processes in interstellar environments, and reaction mechanisms occurring in combustion or plasma. We have, therefore, studied electron-induced ionization and dissociation of the gas phase 3,4-dihydro-2H-pyran (DHP), a cyclic ether appearing to be a viable moiety for developing efficient clinical pharmacokinetics and revealing the mechanisms of biofuel combustion. The mass spectra in the m/z = 10-90 mass range were measured at several different energies of the ionizing electron beam using mass spectrometry. The mass spectra of DHP at the same energies were simulated using on-the-fly semi-classical molecular dynamics (MD) within the framework of the QCxMS formalism. The MD settings were suitably adjusted until a good agreement with the experimental mass spectra intensities was achieved, thus enabling a reliable assignment of cations and unraveling the plausible fragmentation channels. Based on the measurement of the absolute total ionization cross section of DHP (18.1 ± 0.9) × 10-16cm2 at 100eV energy, the absolute total and partial ionization cross sections of DHP were determined in the 5-140eV electron energy. Moreover, a machine learning algorithm that was trained with measured cross sections from 25 different molecules was used to predict the total ionization cross section for DHP. Comparison of the machine learning simulation with the measured data showed acceptable agreement, similar to that achieved in past predictions of the algorithm.

The total electron impact ionization cross sections of the acetylene molecule have been measured in the incident electron energy range from threshold to 1000 eV. These results are compared with other existing data, obtained by direct measurements of this molecular property or by indirect ones, via the partial ionization cross sections for the formation of different ions. Using three semiempirical equations, the total ionization cross sections were calculated and compared to available data, too. Direct and dissociative ionization cross sections were also calculated by a semiempirical equation and compared to existing data.

The molecular beam apparatus used for the measurements of total collision cross sections of potassium and rare gases is shown in detail. A new type of the scattering chamber with a variable length is used to eliminate the error of the effective scattering length. To measure the pressure of the rare gases in the scattering chamber an ionization gauge is connected directly to the scattering chamber. The sensitivities of the ionization gauge for the rare gases are calibrated by a McLeod gauge. To eliminate the mercury drag effect in the calibration, the cut-off position of the McLeod gauge is cooled with dry ice. The magnitudes of the effect are in good agreement with those calculated by Takaishi's theory. Effective total cross sections for the scattering of potassium by He, Ne, Ar, Kr, and Xe have been measured in the velocity range, 400-1200 m/sec, of potassium beam. Corrections for the angular resolution are calculated by a general method derived by the present authors. The correction values are compared with those calculated with Busch's formula. To estimate true total cross sections, the effective total cross sections are corrected for the thermal motion of the scattering gases by means of the method of Berkling et al. The velocity dependences of the total cross sections in the potassium-argon, potassium-krypton, and potassium-xenon systems are shown to be explained by the Van der Waals potential, —C/γ5, where s=6. Glory undulations are observed for the potassium-xenon system. For neon the velocity dependence can not be explained by s=6. For helium the correction factors for the thermal motion of the scattering gas are large and the value of s can not be determined unambiguously. To estimate the Van der Waals constants from the total cross sections, the SLL approximation formula is used and the values of 231, 337, and 515×10-60erg·cm6 are obtained for CK-Ar, CK-Kr, and CK-Xe, respectively. These values are in good agreement with the theoretical ones of Dalgarno and Davison. For the potassium-xenon system the potential parameters of L-J(12, 6)are estimated from the glory undulations, giving 2.3×10-14 erg and 4.7Aforeandγm, respectively.

We report electron impact total cross sections, Q(T), for e-N(2)O scattering over an extensive range of impact energies approximately from 0.1 eV to 2000 eV. We employ an ab initio calculation using R-matrix formalism below the ionization threshold of the target and above it we use the well established spherical complex optical potential to compute the cross sections. Total cross section is obtained as a sum of total elastic and total electronic excitation cross sections below the ionization threshold and above the ionization threshold as a sum of total elastic and total inelastic cross sections. Ample cross section data for e-N(2)O scattering are available at low impact energies and hence meaningful comparisons are made. Good agreement is observed with the available theoretical as well as experimental results over the entire energy range studied here.

We measured absolute cross sections for the electron-impact ionization and dissociative ionization of the C2F5 free radical from threshold to 200 eV using the fast-neutral-beam technique. The electron-impact ionization of C2F5 is dominated by two channels, the formation of the CF3+ fragment ion which has the largest partial ionization cross section with a value of 3.5 × 10-16 cm2 at 70 eV and the formation of the C2F5+ parent ion with a cross section of 1.3 × 10-16 cm2 at 70 eV. Both partial cross sections have error margins of about ±15%. All other partial ionization cross sections have maximum cross section values of less than 0.1 × 10-16 cm2. The total single C2F5 ionization cross sections at 70 eV (derived from the two measured partial ionization cross sections and an estimate for the sum of the cross sections for all weaker, unobserved ionization processes) was found to be about 5 × 10-16 cm2. A comparison of the measured total single-ionization cross sections with a calculated cross section, using a modified additivity rule, shows good agreement up to an electron energy of 50 eV, but poor agreement at higher impact energies. This behaviour is somewhat similar to that observed for the CFx (x = 1-3) free radicals, but it is very different from the excellent agreement between the calculated and measured total single-ionization cross sections that was observed for the stable molecules CF4 and C2F6.

We calculate electron impact partial and total ionization cross sections of R-carvone (C10H14O), 2-butanol (C4H10O), imidazole (C3H4N2), and 2-nitroimidazole (C3H3N3O2). We have used the Binary Encounter Bethe (BEB) model to obtain total electron impact ionization cross sections (TICSs). The modified BEB method in combination with mass spectrum data of the molecules is used to calculate the partial ionization cross section (PICS) of the cationic fragments dissociating from the parent molecule. Our PICS data for R-carvone and 2-butanol are in good agreement with the experimental data for all the cation fragments along with the TICS data. For imidazole and 2-nitroimidazole, the estimates of the PICS are reported for the first time in the present study. We have found that both the modified BEB method and the mass spectrum dependence method work effectively to estimate PICS if we have information about the appearance energies and relative abundance data of the target under investigation.

We present a theoretical study of the ionization of nitrogen atom by a singly charged sodium ion using the classical trajectory Monte Carlo method. Although we suffer from a lack of cross section data of this collision system, the knowledge of the basic cross sections is essential in fusion science, because this reaction has potential applications in the diagnostic of magnetically confined fusion plasmas. In our investigations, the Na+–N collision system is reduced to a three-body problem. The interaction between the collision partners is described by the Garvey-type model potential. The results of our study provide insight into the dynamics of singly charged sodium–nitrogen interactions. The total cross sections are presented in the impact energy range between 10 keV and 10 MeV and compared them with the available experimental data. The single and double differential cross sections are presented at 30, 40, 50 and 60 keV energies related to the energies of the plasma diagnostic used in the nuclear fusion.

A theoretical analysis of the collisions of particles with deuterons is carried out in the high-energy approximation. This approximation, which corresponds to a generalized form of diffraction theory, takes explicit account of double collision processes as well as single ones. It is used to express the amplitudes for elastic and inelastic scattering by deuterons in terms of the elastic-scattering amplitudes of the neutron and proton and the deuteron wave functions. The resulting expressions are used to evaluate the differential cross section for elastic scattering, and the angular distribution of inelastic scattering (i.e., the differential cross section for deuteron breakup integrated over final energies of the incident particle). The contributions to these cross sections of the various single and double scattering processes and the terms which represent their interference are exhibited individually. Expressions are derived for the total cross section of the deuteron and for its elastic and inelastic total scattering and absorption cross sections. The difference between the various types of deuteron cross sections and the sum of the corresponding cross sections for the free neutron and proton is explained in some detail. Spin-dependent interactions are treated, and for incident particles of spin \textonehalf{} an expression is given for the deuteron total cross section in terms of the general spin-dependent scattering amplitudes of the neutron and proton. The theory is applied to antiproton-deuteron collisions in the energy range from 0.13 to 17.1 BeV. The results for the total and absorption cross sections which are calculated for a variety of models of the deuteron wave function are found to be in good agreement with the measurements. The magnitudes of such effects as double scattering and the interference of single-and double-scattering amplitudes are seen to be appreciable.

We calculate total hadron-hadron cross sections at high energies using the Low-Nussinov two-gluon model of the Pomeron. The gluon exchange is represented by a phenomenological potential including screened color-Coulomb, screened confining, and spin-spin interactions. We use bound-state wave functions obtained from a potential model for mesons, and a Gaussian wave function for a proton. We evaluate total cross sections for collisions involving pion, kaon, rho, phi, D, J/psi, psi', Upsilon, Upsilon', and the proton. We find that the total cross sections increase with the square of the sum of the root-mean square radii of the colliding hadrons, but there are variations arising from the spin-spin interaction. We also calculate the total cross sections of a mixed-color charmonium state on a pion and on a proton. The dependence of the total cross section on the size and the color state of the charmonium is investigated.

Electron-impact cross sections for excitation of the 7 $^{2}$${\mathit{S}}_{1/2}$, 8 $^{2}$${\mathit{S}}_{1/2}$, 5 $^{2}$${\mathit{D}}_{3/2}$, and 6 $^{2}$${\mathit{D}}_{3/2}$ states of rubidium have been measured from threshold to 1000 eV. The optical-excitation-function method has been employed in a crossed atom- and electron-beam apparatus. Relative, total (cascade including) experimental cross sections are made absolute by comparison with the known total cross section of the Rb D1 line. Total excitation cross sections are compared with theoretical calculations employing first Born approximation and theoretical branching ratios. Born cross sections for the 7 $^{2}$${\mathit{S}}_{1/2}$ and 8 $^{2}$${\mathit{S}}_{1/2}$ states are obtained from the literature, while Born cross sections for the 5 $^{2}$${\mathit{D}}_{3/2}$, 6 $^{2}$${\mathit{D}}_{3/2}$, and all cascading states are calculated in this paper. At high energies, the measured total $^{2}$${\mathit{D}}_{3/2}$ state cross sections show 1/E behavior and converge to first Born theory; for Eg100 eV, experiment and theory agree within 6.7% for 5 $^{2}$${\mathit{D}}_{3/2}$ and within 3.7% for 6 $^{2}$${\mathit{D}}_{3/2}$. The total cross sections for the $^{2}$${\mathit{S}}_{1/2}$ states do not converge to Born theory even at 1000 eV, and it is shown that this cannot be attributed to cascading. At low energies, $^{2}$${\mathit{S}}_{1/2}$ and $^{2}$${\mathit{D}}_{3/2}$ state total excitation cross sections have similar shapes with sharply peaked maxima at about 0.9 eV above threshold. After cascading is corrected using first Born theory, estimated experimental cross sections for direct excitation of higher fine-structure states of rubidium are given.