Absolute Total Scattering Cross Sections Research Articles

Соударения медленных электронов с молекулами тимина

AbstractUsing a hypocycloidal electron spectrometer, the total scattering cross section of slow (0–9 eV) electrons and the dissociative electron attachment cross section for thymine molecules in the gas phase were measured. The ionization cross section for a thymine molecule was studied in the energy range of 9–32 eV. Some features were found in the scattering cross section, caused by the formation and decay of short-lived states of the molecular negative ion. Three of them ( E = 0.32, 1.71, and 4.03 eV) relate to shape resonances; the others, which are observed for the first time, refer to the Feshbach resonances (or core-excited resonances). In the total dissociative attachment cross section in the energy range of E < 4 eV, a clear structure is observed due to the formation of a negative ion (T–H)^–, and a less intense structure associated with the total contribution of fragment thymine ions is found above 4 eV. The correlation of the features found in the total scattering cross section and in the dissociative attachment cross section is assessed. The absolute total scattering cross section was obtained by normalizing the measured curve to the theoretical calculation. In the total ionization cross section, features are observed that are associated both with the effect of the formation of fr-agment ions and with ionization due to the ejection of electrons from the orbitals of the outer shell of the molecule.

Collisions of Slow Electrons with Thymine Molecules

Using a hypocycloidal electron spectrometer, the total scattering cross section of slow (0–9 eV) electrons and the dissociative electron attachment cross section for thymine molecules in the gas phase were measured. The ionization cross section for a thymine molecule was studied in the energy range of 9–32 eV. Some features were found in the scattering cross section, caused by the formation and decay of short-lived states of the molecular negative ion. Three of them (E = 0.32, 1.71, and 4.03 eV) relate to shape resonances; the others, which are observed for the first time, refer to the Feshbach resonances (or core-excited resonances). In the total dissociative attachment cross section in the energy range of E < 4 eV, a clear structure is observed due to the formation of a negative ion (T–H)–, and a less intense structure associated with the total contribution of fragment thymine ions is found above 4 eV. The correlation of the features found in the total scattering cross section and in the dissociative attachment cross section is assessed. The absolute total scattering cross section was obtained by normalizing the measured curve to the theoretical calculation. In the total ionization cross section, features are observed that are associated both with the effect of the formation of fr-agment ions and with ionization due to the ejection of electrons from the orbitals of the outer shell of the molecule.

An electrostatic slow positron beam apparatus for positron scattering experiments

An electrostatic slow positron beam system has been developed for positron scattering studies. The apparatus uses a remoderator in reflection geometry in conjunction with an electrostatic hemispherical energy analyzer for the brightness enhancement. It is possible to determine absolute total scattering cross section values under magnetic field-free conditions down to a few mG. Measurement of the energy distribution of the positrons that are reemitted from the remoderator and total cross sections for positron-He scattering are presented.

Ab initio potentials for the S(3Pj)–rare gas dimers: Implementation for elastic and inelastic collisions and comparison with scattering potentials

The interaction potentials between the ground state S(3P) atom and rare gas atoms Rg (He, Ne, Ar, Kr, and Xe) in Π3 and Σ−3 states are calculated ab initio using an unrestricted CCSD(T) level of theory and extended correlation consistent basis sets augmented by bond functions. For NeS, the effects of extending the basis set, of a more accurate treatment of triple excitations within the coupled cluster method, and of the frozen core approximation are analyzed. The spin–orbit interaction is taken into account by the commonly used atomic model, whose validity is verified by the direct ab initio calculations of spin–orbit coupling matrix elements. The ab initio potentials are tested in the calculations of the absolute total scattering cross sections measured in molecular beams and compared with the potentials derived from the same data. This comparison, along with an analysis in terms of correlation formulas, proves the high accuracy of ab initio potentials and characterizes the sensitivity of scattering cross sections to the properties of interaction potentials. Both ab initio and scattering derived potentials are implemented in the study of inelastic fine structure transitions in S+Rg collisions. The relaxation rate constants are calculated and compared with those for O+Rg collisions.

Total cross-section measurements for positrons and electrons scattered by sodium and potassium atoms.

Absolute total-scattering cross sections ({ital Q}{sub {ital T}}'s) have been measured for positrons and electrons colliding with sodium and potassium in the 3--102-eV range, using the same apparatus and experimental approach (a beam-transmission technique) for both projectiles. Comparing the present measured {ital Q}{sub {ital T}}'s for positron and electron scattering by each separate alkali-metal atom shows that (1) they are very similar in shape and magnitude over the entire energy range investigated, (2) they tend to merge near (and above) the relatively low energy of about 40 eV, and (3) the positron {ital Q}{sub {ital T}}'s become higher than the corresponding electron values as the projectile energy is decreased below about 40 eV. These positron and electron {ital Q}{sub {ital T}} comparison measurements differ markedly from the situation for room-temperature gases, but are supported by recent theoretical calculations both for the positron and electron {ital Q}{sub {ital T}} comparisons and for their respective absolute values.

The total cross section for low-energy electron scattering from krypton

Absolute total scattering cross sections have been measured for electron scattering from krypton in the energy range 0.175-20 eV. The measurements have been performed with a linear time of flight electron transmission spectrometer. Comparisons are made with the results of a number of other experiments and theoretical calculations and the agreement is generally good. However, at low energies, particularly in the region of the Ramsauer minimum, the present results and the recent measurements of Jost et. al. differ by more than 30%. A momentum transfer cross section is derived from the data by the use of the modified effective-range theory to enable direct comparison with swarm-derived cross sections.

Study of the interactions of atomic and molecular oxygen with O2 and N2 by scattering data

Absolute total scattering cross sections for the O2–O2, O2–N2, O–N2, and O–O2 systems are measured in the thermal energy range. A glory structure is present for O2–O2, O2–N2, and O–N2. For O–N2 this structure appears to be partially quenched due to the anisotropy of the 3P oxygen atom. For O–O2 the structure is almost completely quenched because of the presence of a manifold of interactions. From the analysis of the cross section data, information about the interaction potential of all these systems of atmospheric interest is obtained. For O2–O2 a potential able to reproduce also the spectroscopic and thermophysical data is proposed.

Electron-helium absolute total scattering cross sections from 0.5 to 50 eV

Absolute total cross sections for electron scattering from He have been measured over the energy range from 1 to 50 eV with an accuracy of 3% or better. A novel method employing time-of-flight monochromatization and energy determination of electrons from a pulsed secondary-emission source in transmission through a gas cell has been used. This method is contrasted with conventional ones and a comprehensive error discussion is given. The reported results are then compared to existing experimental and theoretical results. Discrepancies in existing experimental data are resolved and good agreement is obtained with some recent theoretical results.

Target density dependence of molecule-molecule absolute total scattering cross sections

The target density dependence of absolute total cross sections for LiF scattered by CO, CO2, CHF3, CH4, D2, and Ar was studied at a LiF velocity of 1.5 × 105 cm/sec and a target temperature of 297°K. The experiments were performed under conditions of high angular resolution in order to observe the full quantum mechanical cross section. For the systems LiF–CO, LiF–CO2, and LiF–CHF3 there was a substantial variation of observed cross section with density, while for LiF–CH4, LiF–D2, and LiF–Ar there was no observable variation. It is shown that the target transmission is the Laplace transform of the cross section distribution function. Information was obtained about the distribution of cross sections as a function of the internal state of LiF by fitting model distributions to the data. For the lower LiF rotational states the LiF–CO and LiF–CO3 cross sections are dominated by the dipole-quadrupole interaction and are determined at the outermost impact parameters by processes inelastic in the rotation of LiF, and inelastic and/or elastic in the targets CO of CO2; for the higher LiF rotational states the total cross section is determined mainly by elastic processes and is therefore dominated by induction and dispersion interactions. The dipole-quadrupole scattering effect, so important in microwave line broadening theories, thus has been identified in experiments of this type. For LiF–CHF3 the largest total cross sections are determined by dipole-dipole inelastic scattering. For those targets which showed no pressure dependence (Ar, D2, and CH4) the standard deviation of the cross sections was less than ∼15 percent of the mean as determined by the accuracy of the experiments and the range of target densities used. This is consistent with the symmetry of the interaction and the possible degree of anistropy. No rotational inelasticity could be inferred for these targets. A theoretical analysis of the distribution of cross sections based on dipole-quadrupole and dipole-dipole scattering processes is presented. Various fitted models for the distribution of cross sections can be evaluated in the light of these analyses.

Measurement of absolute total scattering cross sections for neon, argon, and oxygen

Absolute total cross sections for the Ne + Ne, Ar + Ar and 0/sub 2/ + O/ sub 2/ interactions have been measured in the thermal energy range using the crossed nozzle beam technique. Corrections to the cross section were made to account for the finite sizes of the detector and the beams. Scattering beam particle densities for each cross section were obtained using a capacitance manometer. (CES)

Absolute total cross sections for Li-molecule scattering

Absolute total scattering cross sections were measured for 7Li scattered by D2, N2, CO, CO2, CH4, and SF6 over a range of Li beam velocities of 0.75–4.75 × 105 cm/sec corresponding to a range of relative kinetic energies of 0.04–0.33 eV for Li–D2 to 0.02–0.79 eV for Li–SF6. In the observed cross sections random experimental errors were less than 1.5% and systematic errors were between −0.3% and +1.4%. Two spherically symmetric potentials, the Lennard-Jones (12,6) and the Buckingham-Corner were fit to deconvoluted and statistically smoothed data using a non-linear least squares fitting routine. For each Li-molecule system a satisfactory fit could be obtained with standard deviations of approximately 1%–2%. The best fit potentials gave accurate values for the average long range attractive part of the potentials over the range of limiting impact parameters. Evidence from high resolution differential cross section experiments is introduced that indicates that there should be relatively little damping of the glory features of the total scattering cross sections for the Li-molecule systems studied here, and hence the ability of spherical potentials to reproduce the data is not surprising.

Study of Total Scattering Cross Sections by Means of Molecular Beams: K-N2 and K-CO

The absolute total scattering cross sections for K-N2 and K-CO were measured by the atomic beam attenuation method with velocity-selected potassium beams. Care was taken of the mercury drag effect in pressure measurements of N2 and CO. The effective length of the scattering chamber was obtained experimentally by changing the length of the chamber. Corrections for the thermal motion of the target gas and the angular resolution were estimated with the Barkling et al.'s formula and von Busch's formula, respectively. It is confirmed that even in the atom-diatomic molecule system van der Waals forces are dominant for scattering at thermal energies, and van der Waals constants are 325×10-60 erg·cm6 for K-N2 and 328×10-60 erg·cm6 for K-CO. These values are compared with other experimental and theoretical ones and are in agreement with the theoretical results of Langhoff and Karplus within the limit of errors.

Absolute Total Scattering Cross Sections for 7Li on He, Ne, Kr, and Xe

Absolute total scattering cross sections were measured for the systems Li–He, Li–Ne, Li–Kr, and Li–Xe over a range of primary beam velocities of 0.75–4.75×105 cm/sec corresponding to a range of kinetic energies of 0.04–0.33 eV for Li–He to 0.02–0.79 eV for Li–Xe. The standard deviations of the cross sections were less than 1.2% of the observed cross section and the maximum systematic errors were between −0.3% and +1.4%. The data were deconvoluted for the effect of target motion and were statistically smoothed. The smoothed data had a standard deviation of 0.2%–0.4% which was about 4 times smaller than the standard deviation of the raw data points and which provided a criterion for judging the adequacy of fits for various model potentials. Model potentials with up to 5 variable parameters were fit to the smoothed cross sections by a least squares method. For Li–Kr and Li–Xe 4 potential parameters were determined simultaneously. These experiments were most sensitive to the potential value over a range of limiting impact parameters. For Li–Kr and Li–Xe this range was 6.5–9.2 Å and 7.8–9.7 Å, respectively, and in this region the potential was determined to an accuracy of 3%–4%, with the error arising primarily from systematic errors in the measurement of the cross section.

Calculations of Effusive-Flow Patterns. I. Knudsen-Cell Results

A simple and direct procedure has been used to calculate the flux gradients inside Knudsen cells and the related effusive-flow patterns. Knudsen cells of simple geometries with knife-edge orifices and flat sources are considered. The calculated value of the Motzfeldt factor (1. 020) is in very good agreement with the literature value (1. 020) and the Monte Carlo calculations (1. 020) for a cell of reduced height $H=4.0$ and the knife-edge orifice of reduced diameter=0.1064. It is suggested that an extension of these calculations has an importance in solving some of the problems encountered in determining the absolute total scattering cross section from scattering measurements.

Absolute Total Scattering Cross Sections of 7Li–Ar

The Li–Ar absolute total scattering cross section was measured to an accuracy of 1%–2% over the relative velocity range 1–4× 105 cm/sec, corresponding to relative energies of 0.03–0.5 eV. The absolute cross section determines the radial scale of the interatomic potential, by reference to the average distance between target molecules. The present study is intended as an exemplary system investigation of inferences that can be drawn from more accurate absolute total cross sections, since currently it is believed that this class of total cross section is known to an absolute accuracy of 5%–10% [R. B. Bernstein and J. T. Muckerman, Advan. Chem. Phys. 12, 414 (1967)]. The primary finding is the value of the interatomic potential with an accuracy of about 5% over a span of the relatively large interatomic distances 6–8 Å. At 7 Å the Li–Ar potential is −1.97 × 10−15 erg± 5%. In this region half of the error arises from the systematic error in the cross section and half from the ambiguity about the proper potential model to apply. The results should be useful for critically testing future model potential calculations and, in conjunction with differential scattering and other data, for giving a more exact and complete understanding of the potential. An apparatus was used that had been previously applied to molecule-atom scattering, where angular resolution corrections were negligible and where absolute pressures were measured with a McLeod gauge modified to physically eliminate trap pumping error. A deconvolution procedure that took into account the effects of target motion upon the glory structure of the total cross section was applied to the data. An economical method for applying nonlinear least squares fitting of model interatomic potentials is described and is applied to the data.

Absolute Total Scattering Cross Section of He–H2 over the Relative Velocity Range of 2000–3000 m/sec

Absolute helium-hydrogen molecule total cross sections for thermal energy collisions were measured using molecular beams derived from nozzle sources and crossed at right angles. Both nozzles were adjustable in temperature over limited ranges to provide variation in the relative velocities. The absolute density of the target gas (H2) was measured with a capacitance manometer. Over a relative velocity range of 2000–3000 m/sec the cross section, corrected for finite angular resolution, was found to be nearly constant in magnitude averaging 48 Å2. A spherical potential which best fits the experimental data [Lennard-Jones (12, 6), ε=0.7 meV, rm=3.57 Å] was obtained for atom-molecule separations of 2.7–3.3 Å. The range was determined from the response of the cross section to piecewise variation of the theoretical potential. The results are consistent with previously reported theoretical and experimental works.