Absolute Total Cross Sections Research Articles

RelativisticR-matrix close-coupling method based on the effective many-body Hamiltonian: Benchmarks on the electron-impact excitations of theKr6+ion

A relativistic $R$-matrix close-coupling method based on effective many-body Hamiltonians is employed to calculate the electron-impact excitations of intercombination transitions in the ${\text{Kr}}^{6+}$ ion as benchmarks in the quest for accurate representation of the target and collisional states in multivalence-electron ions. The effective Hamiltonian in relativistic multireference many-body perturbation theory accurately accounts for short-range many-body interactions unaccounted for by limited configuration-interaction representations of the basis states. The $R$-matrix method is successfully applied to the near-threshold electron impact excitation of the $4{s}^{2}\text{ }^{1}S\ensuremath{\rightarrow}4s4p\text{ }^{3}P$ intercombination transition in the zinclike krypton $({\text{Kr}}^{6+})$ ion, where the observed disagreement between the experimental absolute total cross sections dominated by dielectronic resonances and those predicted by using the Breit-Pauli and Dirac $R$-matrix methods reveals an inadequacy of the extant $R$-matrix close-coupling calculations in compact configuration-interaction representation of target states.

Absolute electron impact ionization cross-sections and polarisability volumes for C 2 to C 4 aldehydes, C 4 and C 6 symmetric ethers and C 3 to C 6 ketones

Absolute total electron impact ionization cross-sections from 15 to 285 eV are reported for the C 2 to C 4 aldehydes, the C 2 v symmetric C 4 and C 6 ethers and C 3 to C 6 ketones. The reproducibility of the measured cross-sections over the full energy range is better than ±4%, and absolute cross-sections measured for N 2 and SF 6 as reference species are in excellent agreement with values reported by several other groups claiming accuracies of around ±5%. Monotonic relationships previously shown to exist between maximum ionization cross-section and polarisability volume and between maximum ionization cross-section and the total number of carbon atoms in the skeletal chain are shown to apply to the aldehydes, ethers and ketones reported here. These relationships provide useful means for estimating unknown cross-sections with available literature polarisability volumes in good accord with values estimated from the measured maximum ionization cross-sections. The experimental data were also compared with calculations carried out using the binary encounter Bethe (BEB) method for the calculation of total ionization cross-sections.

Electron collisions with ethylene oxide molecules

The absolute total cross section (TCS) for electron–ethylene oxide (c-C2H4O) collisions was measured at electron-impact energies extending from 0.7 to 400 eV using the linear electron-transmission technique. In the present TCS energy function a distinct peak is visible near 4.6 eV which may be attributed to formation of a short-lived negative ion. Moreover, at intermediate and high energies the integral elastic (ECS) and ionization (ICS) cross sections were determined numerically. The calculated sum, ECS + ICS, is in good agreement with the experimental TCS in the overlapping energy range. Finally, the present experimental TCS for electron scattering from c-C2H4O is compared with that for its cyclic isoelectronic counterpart c-C3H6, and similarities and differences are pointed out and discussed.

Electron detachment of negative ions: The influence of the outermost electron and its neutral core atom in collision with He, Ne, and Ar

A systematic investigation of the absolute total detachment cross sections of atomic negative ions is presented and a simple semiclassical model developed based on the experimental findings. The total electron detachment process for several anion species, namely ${\text{C}}^{\ensuremath{-}}$, ${\text{O}}^{\ensuremath{-}}$, ${\text{F}}^{\ensuremath{-}}$, ${\text{S}}^{\ensuremath{-}}$, ${\text{Si}}^{\ensuremath{-}}$, ${\text{Cl}}^{\ensuremath{-}}$, ${\text{Ge}}^{\ensuremath{-}}$, and ${\text{Na}}^{\ensuremath{-}}$, in collisions with He, Ne, and Ar, in the velocity range of 0.2--1.8 a.u., were studied. In the intermediate-velocity regime the experimental data are well reproduced by the proposed model, which assumes independent contributions from the outermost quasifree electron and from the neutral core atom of the negative ion.

Absolute Total Photoionization Cross Section of C60in the Range of 25–120 eV: Revisited

The absolute total photoionization cross section σ abs,I of gaseous C 60 is measured in the photon energy h ν range from 25 to 120 eV by photoionization mass spectrometry with synchrotron radiation. The absolute detection efficiencies of photoions in different charge states are evaluated. The present σ abs,I curve is combined with the photoabsorption cross section curves of C 60 at h ν=3.5–26 eV in the literature, after appropriate alterations of the vapor pressure are taken into account. The oscillator strengths are computed from the composite curve to be 178.5 and 230.5 for the h ν ranges from 3.5 to 40.8 eV and from 3.5 to 119 eV, respectively. These oscillator strengths agree well with those expected from the Thomas–Kuhn–Reiche sum rule and 60 times the photoabsorption cross section of a carbon atom. Moreover, the present σ abs,I curve behaves similarly to the relative photoionization cross section curve reported by Reinköster et al.

Electron-impact dissociation cross sections for CHF3 and C3F8

Absolute total dissociation cross sections, σt,diss, by electron-impact are reported for CHF3 and C3F8 from 10 to 300 eV using the chemical gettering technique described by Winters and Inokuti (1982 Phys. Rev. A 25 1420). Data are concentrated in the near-threshold region (10–30 eV). The thresholds for dissociation of CHF3 and C3F8 are determined to be 10.4 eV and 11.9 eV, respectively. Ionization thresholds occur at 16 eV for CHF3 and 16.2 eV for C3F8. Neutral dissociation cross sections of both CHF3 and C3F8 are obtained by subtracting the ionization cross sections, σt,ion, from the total dissociation cross sections, σt,diss.

Dissociative ionization of JP-10 (C 10H 16) by electron impact

Ionization of JP-10 (C 10H 16, exo-tricycle [5.2.1.0 2,6] decane) by electron impact has been studied using Fourier transform mass spectrometry. Absolute total and partial ionization cross-sections have been measured as functions of the electron energy in the range of 10–200 eV. The major channel of parent ion fragmentation at low energies (<27 eV) produces C 9H 13 +, and at higher energies, C 5H 7 +. Possible fragmentation mechanisms are discussed.

Absolute metastable atom-atom collision cross section measurements using a magneto-optical trap

We present a new technique to measure absolute total collision cross sections from metastable neon atoms. The technique is based on the observation of the decay rate of trapped atoms as they collide with room temperature atoms. We present the first measurement of this kind using trapped neon atoms in the (3)P(2) metastable state colliding with thermal ground state argon. The measured cross section has a value of 556+/-26 A(2).

RelativisticR-matrix close-coupling method based on the effective Hamiltonian in many-body perturbation theory

A relativistic $R$-matrix close-coupling method is developed and implemented based on effective many-body Hamiltonians for accurate representation of the target and collisional states in multielectron ions. The effective Hamiltonian in relativistic multireference many-body perturbation theory accurately accounts for short-range many-body interactions unaccounted for in the extant nonrelativistic and Breit-Pauli $R$-matrix methods. The method is successfully applied to the near-threshold electron impact excitation of the $3{s}^{2}$ $^{1}S\ensuremath{\rightarrow}3s3p$ $^{3}P$ transition in Mg-like argon $({\mathrm{Ar}}^{6+})$ ion where the observed disagreement between the experimental absolute total cross sections and those predicted by using the Breit-Pauli $R$-matrix method reveals an inadequacy of the $R$-matrix methods based on the configuration-interaction representation of the basis states.

Dissociative capture and collision-induced dissociation of [formula omitted] in SF6

Absolute differential and total cross-sections for the production of H+ and H0 fragments in the energy range of 1–5keV and scattering angles from −4.6° to 4.6°, for the collision of H2+ on SF6 are reported. The differential and total cross-sections for dissociative capture (DC) and collision-induced dissociation (CID) were estimated. The differential cross-sections (DCS) of DC and CID for all acceleration energies show a monotonic decrease with increasing angle, showing an overall decrease of three and two orders of magnitude for DC and CID respectively. The total cross-section for the DC process is found to be in the range of 10.0–40.0Å2, while for the CID process it is between 0.8 and 2.3Å2. Both measured total cross-sections display a slowly increasing behavior as a function of the incident energy. The results show that the cross-section for the DC process is about 13.2 times higher in average than that for the CID process.

Electron capture by O3+ ions from He, H2O and CO2

Using the translational energy-gain spectroscopy technique, we have measured the energy-gain spectra and absolute total cross sections for single-electron capture in collisions of O3+ ions with He, H2O and CO2 at impact energies between 0.3 and 1.2 keV and scattering angles between 0° and 6°. At the lowest collision energy, 300 eV, the energy-gain spectrum for O3+ - He collisions indicates that single-electron capture into the 2s2p3 3P state of the product O2+ is the dominant reaction channel observed with smaller contributions from capture into the 2s2p3 1D, 3S and 1P states. For O3+ - H2O collisions, the dominant peak correlates with capture into the 2p3p state of O2+, with a significant contribution involving capture into the 2p3s state. In O3+ - CO2 collisions, the dominant reaction channel is due to capture into the 2p3s state of O2+, with contributions from capture into the 2p3p state. The measured cross sections are compared with the available measurements and theoretical results based on the multi-channel Landau-Zener (MCLZ) model.

Electron collision with B(CD3)3 molecules

Absolute total cross section (TCS) for electron–trimethylborane-d9 (B(CD3)3, TMB-d9) collisions has been measured at energies ranging from 0.4 to 370 eV using the linear electron-transmission technique. The most visible feature of the TCS energy function is a pronounced broad enhancement peaked between 5 and 10 eV. Some weak structures are also perceptible in the TCS curve. At intermediate energies the experimental results are compared with the cross section obtained as a sum of the integral elastic and ionization cross sections calculated in this work for the B(CH3)3 molecule. Similarities observed in cross sections for planar, boron-containing BX3 (X = F, Cl, CD3) molecules are also pointed out and discussed.

Electron impact dissociation cross sections for C2F6

Absolute total dissociation cross sections for electron impact, σt,diss, from 8 to 700 eV are reported for C2F6. A dense set of data was obtained in the technologically important 8–30 eV energy range relevant to modelling the type of plasmas used in both fundamental and applied scientific investigations. The threshold for dissociation was found to be 12.0 ± 0.2 eV and appears to be associated with a Rydberg state. Estimated values for the total neutral dissociation cross section, σneut,diss, were obtained by subtracting the ionization cross sections (all ionizing events cause dissociation) from the total dissociation cross section. It is shown that a calibration error in a paper by one of us (HFW) caused a distortion of several previous investigations. When the data from the present work are used to recalibrate data from swarm experiments, agreement becomes quite reasonable. There is now a consistent set of data obtained from several investigators which describe the dissociation of C2F6. Neutral dissociation cross sections are obtained from electron-impact excitation calculations and found to be in reasonable agreement with measurements over most of the energy range.

Mean excitation energies for the stopping power of atoms and molecules evaluated from oscillator-strength spectra

The mean excitation energy for the stopping power of matter, usually expressed by symbol I, is the only nontrivial material property in Bethe’s [Ann. Phys. 5, 325 (1930)] asymptotic stopping-power formula. It is therefore a crucial input for the evaluation of stopping power for swift charged particles. To calculate the I value of a material from its definition, it is necessary to know the oscillator-strength spectrum of the material in question over the entire range of the excitation energy. We evaluate the mean excitation energies of 32 atoms and molecules from the oscillator-strength spectra that were published by Berkowitz in 2002 [Atomic and Molecular Photoabsorption: Absolute Total Cross Sections (Academic, San Diego, 2002)]. We find that most of the present I values are consistent with those given in the literature. The I values of NO2, O3, and C60 in particular are evaluated in the present work. For buckminsterfullerene C60, an estimation of the I value is made also using the local-plasma approximation, in an attempt at understanding differences in the I values of carbon in a free atom, a C60 molecule, and graphite. Systematic trends of I values obtained in the present calculation are discussed, in the context of the Thomas-Fermi model.

Absolute total cross section measurements of the collision-induced dissociation of CO +

Absolute total cross sections for the dissociation of the CO + beam to C + and O + [CO + + H 2 (or N 2, SF 6) → C + (or C) + O (or O +)] in the energy range from 1.0 to 9.0 keV were measured. We found values of the order from 10 −19 to 10 −17 cm 2 for the dissociation to channel C + + O with all targets used herein. While for the dissociation through channel C + O + the absolute total cross section show overall decreases of at least one order of magnitude. The measurements reported here are not found in the literature and the increasing availability of the data on these systems may stimulate work in this direction.

Total cross-section measurements for electron collisions with α-tetrahydrofurfuryl alcohol (C 5H 10O 2)

The absolute total cross-section (TCS) for electron scattering from α-tetrahydrofurfuryl alcohol (C 5H 10O 2; THFA) has been measured using the linear transmission technique for energies ranging from 1 to 370 eV. The obtained TCS function has the relatively high magnitude over the whole energy range studied. That is also characterized by the broad enhancement spanned between 3.5 and 15 eV superimposed with two distinct resonant-like structures. Additional weak structures can be distinguished below 3 eV while two other features are discernible above ionization threshold. The present TCS is compared with available experimental and theoretical cross section data.

Kinetics of electron impact ionization and ion-molecule reactions of pyridine

Dissociative ionization of pyridine by electron impact is investigated using Fourier transform mass spectrometry (FTMS). Absolute total and partial ionization cross-sections are measured as functions of electron energy in the range of 10–200 eV. The parent ion and 15 fragment ions are observed, with the total cross-section reaching a maximum of 1.5 × 10 −15 cm 2 at ∼90 eV. Four important ions generated at 50 eV electron energy, C 5H 5N +, C 4H 4 +, C 4H 3 +, and C 4H 2 +, are found to react with pyridine via mechanisms of charge transfer and proton transfer. C 4H 4 + also undergoes condensation with pyridine followed by H elimination.

Absolute total cross-section measurements for electron collisions with tetrahydrofuran

The absolute total cross section (TCS) for electron collisions with tetrahydrofuran ${\mathrm{C}}_{4}{\mathrm{H}}_{8}\mathrm{O}$ in the gas phase has been measured using the linear transmission technique within $1--370\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ energy range. The total cross section function for ${e}^{\ensuremath{-}}\text{\ensuremath{-}}{\mathrm{C}}_{4}{\mathrm{H}}_{8}\mathrm{O}$ collisions is characterized by the relatively high magnitude and the broad enhancement spanned between 4 and $12\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, with some resonantlike structures. This enhancement can be explained as a result of overlapping core-excited resonances. At higher collision energies the magnitude of the measured total cross section monotonically decreases with the energy increase. At low impact energies some additional weak resonant features are visible near 1.9 and $3.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The reported TCS results are higher by 30% than recently published by Zecca et al. [J. Phys. B 38, 2079 (1995)]. They are also distinctly different from the recent $R$-matrix calculations. At intermediate energy range the present data agree well with the sum of calculated elastic and ionization cross sections.

Absolute partial cross sections and kinetic energy analysis for the electron impact ionization of propene

We present absolute partial electron impact ionization cross sections for propene in the electron energy range between threshold and 1000 eV measured with a two sector field double focussing mass spectrometer. Ion beam profiles have been measured at all electron energies by applying a deflection field method. In combination with a three-dimensional ion trajectory simulation of ions produced in the ion source ion kinetic energy distribution functions and discrimination factors can be derived form these data. Multiplication of these discrimination factors with the measured ion signals leads to relative partial cross sections. Normalization of the sum of the relative partial cross sections to an absolute total cross section gives absolute partial cross section values. The initial kinetic energy distributions of several fragment ions show the presence of two or more contributions exhibiting different electron energy dependencies. Differential cross sections with respect to initial kinetic energy of the ions are given and related to specific ion production channels. The high energetic parts of the initial kinetic energy distributions are assigned to the Coulomb explosion of doubly charged short-lived intermediates. The reaction C 3H 5 2+ → C 2H 2 + + CH 3 + has been studied quantitatively in the ion source, the first field free region and the second field free region of the two sector field mass spectrometer yielding a measured mean kinetic energy release of 4.99 ± 0.5 eV, 4.85 ± 0.5 eV and 5.5 ± 0.5 eV, respectively in good agreement with a theoretical estimation.

Electron collision with sulfuryl chloride (SO2Cl2) molecule

Absolute total cross section (TCS) for electron scattering from sulfuryl chloride (SO2Cl2) molecules was measured in a linear transmission experiment at energies ranging from 0.5 to 150 eV. The most distinct features found in the TCS are the deep minimum located near 1.8 eV and the broad enhancement peaked around 9.5 eV with a shoulder spanned between 3 and 5 eV. At intermediate energies, the present experimental TCS results agree well with our total cross-section estimations, based on calculations of elastic and ionization cross sections. In addition, the calculated total cross section for SO2F2 is reported. The results for e−–SO2Cl2 collisions are compared, for the energy dependence and magnitude, with experimental and computed cross sections for other molecules (SO2F2, SO2ClF) comprising a sulfone group.