Total Collision Cross Sections Research Articles

Positron scattering from hydrogen atom embedded in weakly-coupled plasmas

Elastic scattering of positrons from the hydrogen atoms in weakly-coupled plasmas has been studied using an expression for partial wave scattering amplitude that has been derived within the framework second order distorted wave Born approximation. The interactions among the charged particles in the plasma have been represented by Debye-Huckel potentials. A detailed study has been made on differential and total cross sections in the energy range 20–300 eV. To the best of our knowledge such a study on the differential and total cross sections for elastic positron-hydrogen collisions in a weakly-coupled plasma environment is reported for the first time in the literature.

Observation of quantum diffractive collisions using shallow atomic traps

We present measurements and calculations of the trap loss rate for laser-cooled $^{87}\text{R}\text{b}$ atoms confined in either a magneto-optic trap (MOT) or a magnetic quadrupole trap when exposed to a room-temperature background gas of Ar. We study the loss rate as a function of trap depth and find that copious glancing elastic collisions, which occur in the so-called quantum diffractive regime and impart very little energy to the trapped atoms, result in significant differences in the loss rate for the MOT compared to a pure magnetic trap due solely to the difference in potential depth. This finding highlights the importance of knowing the trap depth when attempting to infer the total collision cross section from measurements of trap loss rates. Moreover, this variation in trap loss rate with trap depth can be used to extract information about the differential cross section.

POSSIBLE POSITRONIUM FORMATION IN THE COLLISIONS OF POSITRONS WITH CALCIUM AND STRONTIUM POSITIVE IONS

The inelastic collisions of positrons with calcium and strontium positive ions are treated for the first time as a three-channel problem within the framework of the improved coupled-static and frozen-core approximations with the assumption that the elastic, ground positronium and excited-positronium formation channels are open. The calculations of the partial and total cross sections are carried out for eight values of the total angular momentum ℓ (0 ≤ ℓ ≤7), at 20 values of the incident energy lying above the excited positronium formation threshold and extending to a wide region above the ionization threshold of the target ion. The total collisional positronium formation cross sections of e+– Sr+scattering show a peak around the ionization threshold of Sr+but display a peak at 30 eV for e+– Ca+scattering. In both scattering problems the total excited Ps formation cross sections have considerable values in the energy range [Formula: see text].

Oscillatory Screening Effects on Elastic Collisions in Dense Electron-Ion Quantum Plasmas

Abstract The oscillatory screening effects on elastic electron-ion collisions are investigated in dense quantum plasmas. The eikonal method with the modified Debye-Hückel potential is employed to obtain the scattering phase shift and scattering amplitude. In addition, the total elastic collision cross section is obtained by the optical theorem with the forward scattering amplitude in quantum plasmas. It is shown that the modified Debye-Hückel screening in quantum plasmas produces the oscillatory behaviour of the scattering phase shift. In addition, the minimum position of the phase shift is receded from the target ion with decreasing the quantum wave number. It is also found that the oscillatory screening effect suppresses the differential cross section. The total cross section is also found to be decreased due to the oscillatory screening effect. In addition, it is shown that the total cross section decreases with an increase of the quantum wave number

Measurement of low-energy total absolute atomic collision cross sections with the metastableP23state of neon using a magneto-optical trap

We present total absolute collision cross sections for neon in the $^{3}P_{2}$ metastable state with ground state, thermal atoms and molecules using a recently developed experimental technique. The technique utilizes a magneto-optical trap (MOT) and involves the measurement of MOT population dynamics to determine the cross section. Collision cross section measurements are presented for metastable neon in the $^{3}P_{2}$ state with He, Ne, Ar, ${\mathrm{H}}_{2}$, ${\mathrm{O}}_{2}$, and ${\mathrm{N}}_{2}$. The average thermal energy of the collision ranges from $11\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}27\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. The measurements made using this technique have small errors, of the order of 10% of the measured cross section; the technique is capable of producing benchmark collision cross sections.

Measurements of the total charge-changing cross sections for collisions of target gases with a1−MeVK+-ion beam

The sum of ionization and charge-exchange cross sections of several gas targets (${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, He, Ne, Kr, Xe, Ar, and water vapor) impacted by a $1\text{\ensuremath{-}}\mathrm{MeV}$ ${\mathrm{K}}^{+}$ beam are measured. In a high-current ion beam, the self-electric field of the beam is high enough that ions produced from the gas ionization or charge exchange by the ion beam are quickly swept to the sides of the accelerator. The flux of the expelled ions is measured by a retarding field analyzer. This allows accurate measuring of the total charge-changing cross sections (ionization plus charge exchange) of the beam interaction with gas. Cross sections for ${\mathrm{H}}_{2}$, He, and Ne are simulated using classical trajectory Monte Carlo method and compared with the experimental results, showing good agreement.

Energy transfer of highly vibrationally excited 2-methylnaphthalene: Methylation effects

The methylation effects in the energy transfer between Kr atoms and highly vibrationally excited 2-methylnaphthalene in the triplet state were investigated using crossed-beam/time-sliced velocity-map ion imaging at a translational collision energy of approximately 520 cm(-1). Comparison of the energy transfer between naphthalene and 2-methylnaphthalene shows that the difference in total collisional cross section and the difference in energy transfer probability density functions are small. The ratio of the total cross sections is sigma(naphthalene): sigma(methylnaphthalene)=1.08+/-0.05:1. The energy transfer probability density function shows that naphthalene has a little larger probability at small T-->VR energy transfer, DeltaE(u)<300 cm(-1), and 2-methylnaphthalene has a little larger probability at large V-->T energy transfer, -800 cm(-1)<DeltaE(d)<-100 cm(-1). However, these differences are close to our experimental uncertainty. No significant difference in the probability of very large energy transfer, such as supercollisions defined arbitrarily as DeltaE(d)<-1500 cm(-1), was observed. The possible methylation effects due to the subsequent successive collisions were discussed.

Energy dependence of scattering ground-state polar molecules

We explore the total collisional cross section of ground-state polar molecules in an electric field at various energies, focusing on $\mathrm{RbCs}$ and $\mathrm{RbK}$. An external electric field polarizes the molecules and induces strong dipolar interactions, leading to nonzero partial waves contributing to the scattering even as the collision energy goes to zero. This results in the need to compute scattering problems with many different values of total $M$ to converge the total cross section. An accurate and efficient approximate total cross section is introduced and used to study the low-field temperature dependence. To understand the scattering of the polar molecules we compare a semiclassical cross section with the quantum unitarity limit. This comparison leads to the ability to characterize the scattering based on the value of the electric field and the collision energy. General and simple forms of the characteristic electric field and energy are given, enabling characterization of the scattering.

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).

First Measurements of the Index of Refraction of Gases for Lithium Atomic Waves

We report the first measurements of the index of refraction of gases for lithium waves. Using an atom interferometer, we have measured the real and imaginary parts of the index of refraction n for argon, krypton, and xenon as a function of the gas density for several velocities of the lithium beam. The linear dependence of (n-1) with the gas density is well verified. The total collision cross section deduced from the imaginary part of (n-1) is in very good agreement with traditional measurements of this quantity. Finally, the real and imaginary parts of (n-1) and their ratio rho exhibit glory oscillations, in good agreement with calculations.

DISCOVERED FUNCTION FOR POSITRON COLLISIONS WITH ALKALI-METAL ATOMS USING GENETIC PROGRAMMING

Genetic programming (GP) has been used to discover the function that describes the collisions of positrons with sodium, potassium, rubidium and cesium atoms at low and intermediate energies. The GP has been running based on experimental data of the total collisional cross sections to produce the total cross sections for each target atom. The incident energy and the static dipole polarizability of the alkali target atom have been used as input variables to find the discovered function. The experimental, calculated and predicted total collisional cross sections are compared. The discovered function shows a good match to the experimental data. We find that the GP technique is able to improve upon more traditional methods. To our knowledge, this is the first application of the GP technique to the data of positron collisions with alkali atoms at low and intermediate energies.

POSITRONIUM FORMATION CROSS-SECTIONS FOR POSITRON SCATTERING BY RUBIDIUM ATOMS

Cross-sections for positron-rubidium (37Rb) scattering have been calculated using the Clementi–Roetti wavefunctions and a combination of the coupled-static and frozen-core approximations. The total cross-sections, calculated with eight partial waves corresponding to the total angular momentum ℓ=0 to ℓ=7, are determined over a wide region of scattering energies ranging from 2.7 to 300 eV. The resulting total cross-sections are compared with experimental results and those calculated by other authors. Our total collisional cross-sections display a pronounced peak at 5 eV, nearly consistent with the measurements of Parikh et al. [Phys. Rev. A47, 1535 (1993)] and also reveal another peak at 7 eV, consistent with the experimental cross-section of Stein et al.23 in the neighborhood of 7 eV. The oscillating behavior of our total collisional cross-sections supports the possible existence of resonance, especially at low energy region. The effect of positronium formation on the total collisional cross-sections diminishes when the incident energy is larger than 100 eV.

GENETIC PROGRAMMING APPROACH FOR ELECTRON-ALKALI-METAL ATOM COLLISIONS

New technique is presented for modeling the total cross sections of electron scattering by Na, K, Rb and Cs atoms in the low and intermediate energy regions. The calculations have been performed in the framework of genetic programming (GP) technique. The GP has been running based on the experimental data of the total collisional cross sections to produce the total cross sections for each target atom. The incident energy and atomic number as well as the static dipole polarizability have been used as input variables to find the functions that describe the total collisional cross sections of the scattering of electrons by alkali atoms. The experimental, calculated and predicted total collisional cross sections are compared. The discovered functions show a good match to the experimental data.

Electron scattering and ionization of NO, N2O, NO2, NO3 and N2O5 molecules: theoretical cross sections

We have investigated the scattering of electrons from atmospheric molecules NO, N2O, NO2, NO3 and N2O5 at incident electron energy Ei from the ionization threshold (∼10 eV) to 2000 eV. Basically total cross sections for elastic and inelastic collisions are calculated and hence total ionization cross sections are deduced in the ‘complex scattering potential-ionization contribution’ method, developed earlier. Various calculated cross sections are compared extensively with experimental and theoretical data wherever possible. Recommended data can now be prepared for the well-known NO target. New results are presented for lesser known targets.

Reactive collisions of atomic antihydrogen with the H2 and H2+ molecules

The fermion molecular dynamics (FMD) method is used to determine the protonium (Pn) formation and total destruction cross sections for collisions of antihydrogen with the H2 molecule and the H2+ molecular ion at collision energies above 0.1 au in the centre-of-mass system. The cross sections and initial quantum numbers are compared with the analogous cross sections for and previously calculated. Like the projectile, the protonium-formation cross sections for the projectile are much larger and extend to higher energies with the molecular targets than with the atomic target. The possibility is considered that a relatively long-lived state of the molecule may be formed in rearrangement scattering of at low energies.

Single and double ionization of helium in heavy-ion impact

We present single- and double-ionization total cross sections for collisions of helium atoms with multiple charged ions. In our study we apply two different approaches, namely the ab initio coupled channel and the classical trajectory Monte Carlo (CTMC) method. We consider bare projectiles with charge states between two and eight with energies between 0.19 and 2.31 MeV/amu. We find that our coupled-channel calculations are very close to the experimental data even for the case when the Coulomb fields are strong. At the same time CTMC models, using different independent-particle approximations, show somewhat larger discrepancies compared to the experimental cross sections. We also present cross sections for 1.44 MeV/amu Oq+(q = 4–7) projectiles.

Calculated total cross sections of electron-impact ionization and excitations in tetrahedral(XY4)andSF6molecules

Various total cross sections for collisions of electrons in the energy range 10--2000 eV are calculated for the tetrahedral ${\mathrm{CH}}_{4},$ ${\mathrm{SiH}}_{4},$ ${\mathrm{GeH}}_{4},$ ${\mathrm{CF}}_{4},$ ${\mathrm{SiF}}_{4},$ and ${\mathrm{CCl}}_{4}$ targets and ${\mathrm{SF}}_{6}.$ Molecular total inelastic cross sections determined in the complex potential formalism are partitioned to account for the ionization and cumulative electronic-excitation channels. The present total cross sections and total ionization cross sections are found to be consistent with the previous data. The cumulative total excitation cross sections reported here indicate the relative importance of ionization together with electronic excitation channels including neutral dissociation of these molecules by electron impact.

Total hadron-hadron cross sections at high energies

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.

Theoretical description of fast proton scattering from steel surface under grazing incidence

The angle-energy distributions of fast protons scattered from a steel surface under grazing incidence are studied theoretically. The presented model is based on the Monte-Carlo method and on calculations of the total collision cross-sections using the Ziegler stopping tables. The energy distributions at a fixed scattered angle are characterized by certain parameters. The 300 and 500 keV proton calculation results for a 4° grazing angle are in qualitative agreement with the authors’ experimental data.

Entanglement-assisted coherent control in nonreactive diatom–diatom scattering

Intriguing quantum effects that result from entangled molecular rovibrational states are shown to provide a novel means for controlling both differential and total collision cross sections in identical particle diatom–diatom scattering. Computational results on elastic and inelastic scattering of para–H2 and para–H2 are presented, with the collision energy ranging from 400 cm−1 to the ultracold regime. The experimental realization and possible extension to other systems are discussed.