Orbital alignment dependence of electron transfer cross sections

This paper reports experimental and theoretical results for the influence of target excitation and orbital alignment on the charge exchange process for the system H⁺-Na(3s, 3p). The experimental velocity range covers for the first time the region around and beyond the matching velocity of 0.47 a.u. of the Na(3p) state, i.e. the velocity characteristic of the orbital motion of the valence electron. The cross section parameters are found to depend sensitively on collision velocity, with a qualitatively different behaviour below and above the matching velocity. The results for the orbital alignment dependence support the intuitive picture that, when going beyond the matching velocity, electron transfer becomes increasingly favoured when the orbital velocity of the active electron has a component parallel to the collision velocity. Agreement with earlier experimental and theoretical results at lower velocities is good and allows conclusions about the quality of theoretical approximations in the various velocity regimes.     

On the cluster-size dependence of electron capture cross sections in ion-cluster collisions

The dependence of the electron capture cross section on cluster size in proton-cluster collisions is investigated within two different theoretical models. At low collision velocities a strong increase in the cross section with cluster size is found, whereas the size-dependency of the cross section becomes less pronounced at higher velocities.     

Energy dependence of rotational cross sections

A modified exponential model for rotational state-to-state cross sections is proposed which accounts for most of the observed features for Ar-N₂ at large [Δj] and large total energy E. This empirical form gives rise to non-linear surprisals, and suggests that the prior transition probability, usually taken as a constant, is proportional to E^?1.     

Energy dependence of reaction cross sections at high energy

The energy dependence of the total reaction cross sections for the reaction A + BC → AB + C is obtained in the sufficiently high energy regime, based on the Born approximation and the stationary phase approximation. The total reaction cross section (σ_t) which is a sum over all final vibrational and rotational states of the integral of the angular distribution over angles, is found to be in the form

where Q is the maximum exoergicity and positive. It is shown that, when adjusted to an experimental value, (σ_t) fits remarkably well the cross sections for the reaction K + CH₃I → Kl + CH₃, past the maximum of the cross sections reported by Gersch and Bernstein.     

Absolute cross sections for electron scattering from furan

We report results of measurements and calculations of absolute cross sections for electron scattering from furan molecules (C(4)H(4)O). The experimental absolute differential cross sections (DCSs) for elastic electron scattering were obtained for the incident energies from 50 eV to 300 eV and for scattering angles from 20[ordinal indicator, masculine] to 110[ordinal indicator, masculine], by using a crossed electron-target beam setup and the relative flow technique for calibration to the absolute scale. The calculations of the electron interaction cross sections are based on a corrected form of the independent-atom method, known as the screening corrected additivity rule (SCAR) procedure and using an improved quasifree absorption model. The latter calculations also account for rotational excitations in the approximation of a free electric dipole and were used to obtain elastic DCSs as well as total and integral elastic cross sections which are tabulated in the energy range from 10 to 10 000 eV. All SCAR calculated cross sections agree very well with both the present and previously published experimental results. Additionally, calculations based on the first Born approximation were performed to calculate both elastic and vibrationally inelastic DCSs for all the modes of furane, in the energy range from 50 eV to 300 eV. The ratios of the summed vibrational to elastic DCSs are presented and discussed. Finally, the present results for furan are compared with previously published elastic DCSs for the tetrahydrofuran molecule and discussed.     

Polarisation ratio velocity dependence: A novel polarisation-sensitive technique for atom-diatom rotational energy transfer cross sections

In this communication we describe the polarisation ratio velocity dependence technique, using polarisation-sensitive Doppler scan methods. Results from this technique allow us to report empirical evidence that the laboratory-fixed rate constants for different polarisation components (K = 0,1 and 2), describing rotational energy transfer between Li₂ and xenon or helium, are temperature dependent and each follow different functions of the Doppler-selected molecular velocity.     

Velocity dependence of collision broadening cross sections in NH3

The velocity dependence of collision broadening cross sections for both self-broadening and Xe broadening of an infrared transition in NH₃ has been studied by measuring laser saturation resonance linewidths for molecules with specified velocities along the laser propagation direction. For self-broadening, the velocity dependence is in accord with inelastic collisions due to a predominantly dipole-dipole interaction potential, with smaller contributions due to shorter range forces, whereas for Xe broadening, the magnitude and velocity dependence of the collision broadening cross section is in better agreement with velocity changing collisions in the pressure range studied.     

Total dissociative electron attachment cross sections of selected amino acids

Total dissociative electron attachment cross sections are presented for the amino acids, glycine, alanine, proline, phenylalanine, and tryptophan, at energies below the first ionization energy. Cross section magnitudes were determined by observation of positive ion production and normalization to ionization cross sections calculated using the binary-encounter-Bethe method. The prominent 1.2 eV feature in the cross sections of the amino acids and the closely related HCOOH molecule is widely attributed to the attachment into the -COOH pi* orbital. The authors discuss evidence that direct attachment to the lowest sigma* orbital may instead be responsible. A close correlation between the energies of the core-excited anion states of glycine, alanine, and proline and the ionization energies of the neutral molecules is found. A prominent feature in the total dissociative electron attachment cross section of these compounds is absent in previous studies using mass analysis, suggesting that the missing fragment is energetic H-.     

Elastic electron scattering cross sections of oriented and aligned molecules

Elastic differential electron scattering cross sections of oriented methyl iodide are calculated using the independent atom model. Results are presented for two specific orientations of the ICH₃ molecule for the purpose of comparison with the fictitious molecule IC, similarly oriented, at electron energies of 600 eV and 40 keV. Cross sections are also calculated for IC with a large angular momentum. In a comparison of the results for different orientations of the angular momentum vector, including random orientation, large differences between the cross sections are evident. This sensitivity to the plane of rotation of the molecule suggests the possibility of determining the degree of alignment of the angular momenta of a beam of such molecules by electron diffraction.     

Differential cross sections for the electron impact excitation of pyrimidine

We report on differential cross section (DCS) measurements for the electron-impact excitation of the electronic states of pyrimidine. The energy range of the present measurements was 15-50 eV with the angular range of the measurements being 10°-90°. All measured DCSs displayed forward-peaked angular distributions, consistent with the relatively large magnitudes for the dipole moment and dipole polarizability of pyrimidine. Excitations to triplet states were found to be particularly important in some energy loss features at the lower incident electron energies. To the best of our knowledge there are no other experimental data or theoretical computations against which we can compare the present results.     

Absolute cross sections for electron impact ionization of Ag2

The previously measured relative cross section function for electron impact ionization (EII) of neutral Ag₂ has now been calibrated quantitatively by combining the electron impact ionization with in situ non resonant two photon ionization (NR2PI). By comparing the NR2PI saturation intensities measured for Ag ₂ ⁺ and Ag⁺ with the corresponding EII intensities, the ratio between the electron impact ionization cross sections (EIICS) of neutral Ag₂ and Ag was determined to be σ_Ag2/σ_Ag=1.53 for an electron energy of 46 eV. This result agrees well with the geometricn ^2/3-rule \((\sigma X_n \sim n^{2/3} )\) commonly proposed for the dependence of the EIICS of clustersX _n on the cluster sizen.     

Dissociation cross sections of silane and disilane by electron impact

The total dissociation cross sections for silane and disilane are reported for electron energies above their ionization thresholds up to 110 eV. The measurements are derived from a kinetic analysis of silane and disilane dissociation in a constant-flow multipole dc plasma reactor. The methane dissociation cross section was also measured and found in agreement with published data. Maxima for silane and disilane, occurring around 60 eV, are respectively (1.2±3)×10^?15 cm² and (2.6±0.6)× 10^?15 cm². Total ionization cross sections are also measured and above 50 eV the ratios of dissociative ionization to dissociation cross sections are 0.5±0.1 and 0.25±0.10 respectively for silane and disilane. The probability for silane elimination in the disilane fragmentation reaches a maximum of 0.8 at 19 eV and decreases down to 0.5 at 100 eV. Dissociation processes of silane and disilane are discussed in comparison with methane and ethane     

KL 2,3 ionization in neon by electron impact in the range 1.5–50 keV: cross sections and alignment

We have measured the ratio of cross sections σ(KL _2,3)/σ(K) for neon for electron impact in the energy range ofE ₀=1.5 ... 50 keV via the intensity ofKL _2,3?LLL _2,3 Auger satellite lines relative to the intensity ofKL ₁ L _2,3 (³ P) diagram line. The experimental ratio decreases over the full range of energyE ₀ which is contrary to an earlier result by Carlson et al. We have also measured the alignment ofKL _2,3 ¹ P and³ P states via the angular distribution of Auger satellite intensity for the energy rangeE ₀=1.5 ... 4 keV, within experimental error we have found a zero alignment. The totalK Auger spectrum, measured forE ₀=40 keV and at the magic angle of emission ?=54.7°, has been decomposed into its components by using appropriate line shapes distorted by postcollision interaction. Finally, we discussed whether the lines observed at the high-energy side ofKL _2,3?LLL _2,3 Auger satellite lines can be interpreted as structures caused by an angular momentum exchange in the postcollision interaction predicted by Niehaus and Zwakhals.     

Nonresonant charge transfer: Semiclassical calculation of differential cross sections

Charge transfer and elastic scattering differential cross sections in Li⁺ + Na are described in a two state model by a consequent application of semiclassical methods. The coupling model used is Demkov coupling; coupling parameters and potential curves are taken from calculations of other authors. The theoretical results, which contain no adjustable parameters, are in good agreement with experiment.     

Total dissociative electron attachment cross sections for molecular constituents of DNA

Total cross sections for the dissociative electron attachment process are presented for the DNA bases thymine, cytosine, and adenine and for three compounds used as surrogates for the ribose and phosphate groups, tetrahydrofuran, 3-hydroxytetrahydrofuran, and trimethylphosphate, respectively. Cross section magnitudes are obtained by observation of positive ion production and normalization to ionization cross sections calculated elsewhere using the binary-encounter-Bethe method. The average cross section of the three bases is 3-10 times smaller than the effective cross section per nucleotide reported for single strand breaks in surface-bound supercoiled DNA. Consequently, damage to the bases alone does not appear to account for the major portion of the strand breaks. The presence of an OH group on the ribose surrogate considerably enhances its cross section. Model compounds in which protonation or OH groups are used to terminate bonds may therefore display larger cross sections than in DNA itself.     

Elastic and inelastic cross sections for low-energy electron collisions with pyrimidine

We present theoretical elastic and electronic excitation cross sections and experimental electronic excitation cross sections for electron collisions with pyrimidine. We use the R-matrix method to determine elastic integral and differential cross sections and integral inelastic cross sections for energies up to 15 eV. The experimental inelastic cross sections have been determined in the 15-50 eV impact energy range. Typically, there is quite reasonable agreement between the theoretical and experimental integral inelastic cross sections. Calculated elastic cross sections agree very well with prior results.     

Anomalous temperature-isotope dependence in proton-coupled electron transfer

Motivated by the experiments of Hodgkiss et al. [J. Phys. Chem. (submitted)] on electron transfer (ET) through a H-bonding interface, we present a new theoretical model for proton-coupled electron transfer (PCET) in the condensed phase, that does not involve real proton transfer. These experiments, which directly probe the joint T-isotope effects in coupled charge transfer reactions, show anomalous T dependence in k(H)k(D), where k(H) and k(D) are the ET rates through the H-bonding interface with H-bonded protons and deuterons, respectively. We address the anomalous T dependence of the k(H)k(D) in our model by attributing the modulation of the electron tunneling dynamics to bath-induced fluctuations in the proton coordinate, so that the mechanism for coupled charge transfer might be better termed vibrationally assisted ET rather than PCET. We argue that such a mechanism may be relevant to understanding traditional PCET processes, i.e., those in which protons undergo a transfer from donor to acceptor during the course of ET, provided there is an appropriate time scale separating both coupled charge transfers. Likewise, it may also be useful in understanding long-range ET in proteins, where tunneling pathways between redox cofactors often pass through H-bonded amino acid residues, or other systems with sufficiently decoupled proton and electron donating functionalities.     

Total ionization and electron attachment cross sections of CCl2F2 by electron impact

The absolute total ionization cross sections from threshold to 250 eV and dissociative attachment cross sections from zero to 10 eV have been measured for the CCl₂F₂ (dichloro-difluoro-methane) molecule by using a parallel plate condenser type ionization chamber. The maximum of the ionization cross-section curve was found to be at an energy of about 90 eV with a cross section of 1.44 × 10^?19 m². The attachment cross-section curve shows three peaks, the most intense being at zero electron energy with a cross-section value of 1.80 × 10^?20 m², and the other two at energies of 0.6 eV and 3.5 eV, respectively. The maximal relative error in cross-section values is 0.08, for electron energies larger than 0.4 eV.     

Emission cross sections from fragments produced by electron impact on silane

Optical emission spectroscopy in the visible and near UV of a silane plasma was performed in a low pressure hot cathode glow discharge bounded into a magnetized multipolar wall. Emissions from Si, Si⁺, SiH, SiH⁺ and H are shown to originate from the dissociative excitation of silane molecules by electron impact. The absolute cross sections for the various photoemissive processes were measured in the 17–68 eV range. The relevance of optical emission spectroscopy to silane plasma diagnostics is discussed.