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Omidvar, K.

Publications and source records attributed to Omidvar, K..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Angular distribution of Auger electrons due to 3d-shell impact ionization of krypton

Cross sections for electron impact ionization of krypton due to ejection of a 3d-shell electron have been calculated using screened hydrogenic and Hartree-Slater wavefunctions for the target atom. While the total ionization cross sections in the two approximations are within 10% of each other, the Auger electron angular distribution, related to cross sections for specific magnetic quantum numbers of the 3d electrons, are widely different in the two approximations. The angular distribution due to the Hartree-Slater approximation is in excellent agreement with measurement. The physical reason for the discrepancies in the two approximations is explained.

Omidvar, K.↗
NASA NTRS1977

Angular distribution of Auger electrons due to 3d-shell ionization of krypton

Cross sections for electron impact ionization of krypton due to ejection of a 3rd shell electron have been calculated using screened hydrogenic and Hartree-Slater wave functions for target atom. While the total ionization cross sections in the two approximations are within 10% of each other, the Auger electron angular distribution, related to cross sections for specific magnetic quantum numbers of the 3rd electrons, is widely different in the two approximations. The angular distribution due to Hartree-Slater approximation is in excellent agreement with measurement. The physical reason for the discrepancies in the two approximations is explained.

Omidvar, K.↗
NASA NTRS1976

Minima in generalized oscillator strengths for initially excited hydrogen-like atoms

Generalized oscillator strengths for transitions from an initially excited state of a hydrogenic atom to final states (either discrete or continuum) have complicated structures, including minima and shoulders, as functions of the momentum transfer. Extensive calculations carried out in the present work have revealed certain systematics of these structures. Some implications of the minima to the energy dependence of the inner-shell ionization cross section of heavy atoms by proton impact are discussed.

Matsuzawa, M.↗
NASA NTRS1976

Differential cross section for positronium formation in positron-atomic-hydrogen collisions

The differential cross section for positronium formation in its ground state due to collision of a positron and a hydrogen atom is obtained on the basis of the combination of the L = 0 and 1 (L is the angular momentum quantum number) partial wave amplitudes with the L not less than 2 Born amplitudes. Minima at the scattering angles of 57 and 51 degrees were found for positron energies of 0.64 and 0.75 Ry. Total cross sections for positronium formation at low and intermediate impact energies are presented.

Drachman, R. J.↗
NASA NTRS1976

Scaling of cross sections for K-electron capture by high-energy protons and alpha-particles from the multielectron atoms

Electron capture by protons from H, He, and the K-shell of Ar, and alpha particles from He are considered. It is shown that when a certain function of the experimental cross sections is plotted versus the inverse of the collision energy, at high energies the function falls on a straight line. At lower energies the function concaves up or down, depending on the charge of the projectile, the effective charge and the ionization potential of the electron that is being captured. The plot can be used to predict cross sections where experimental data are not available, and as a guide in future experiments. High energy scaling formulas for K-electron capture by low-charge projectiles are given.

Omidvar, K.↗
NASA NTRS1976

Differential cross-section for positronium formation in electron-atomic hydrogen collisions

The L=0 and 1 partial wave amplitudes obtained by a two-state coupled static approximation with correlation with the L greater than or equal to 2 Born amplitudes were combined to obtain the differential cross section for positronium formation in electron-atomic hydrogen collisions. For positron energies of 0.64 and 0.75 ryd, minima at the scattering angles of 57 deg and 51 deg are found. Total cross sections for positronium formation for low and intermediate impact energies are given. Measurement of the differential cross section for the process positron + helium yields positronium + helium ion for the detection of possible minima is suggested.

Drachman, R. J.↗
NASA NTRS1976

Single-electron Born approximations for charge transfer from multielectron atoms to protons

The Born approximation, including the internuclear interaction, is used to compute cross sections for the transfer of one electron from a multielectron atom to an incident proton. When the full internuclear interaction is included, the results lie far above high-energy experimental K-shell data for protons and argon. However, when only enough internuclear interaction is included so that the total projectile-target interaction goes to zero asymptotically in accordance with plane-wave functions actually used, fair agreement at high energies is obtained. The latter form of the Born approximation is compared with data on K-shell capture from helium as well as argon and with the simpler approximation of Oppenheimer and of Brinkman and Kramers (OBK), where no internuclear interaction is included. The OBK results typically lie a factor of 2 to 8 above the data, while the present Born results are within a factor of 2 of most of the existing high-velocity K-shell data.

Omidvar, K.↗
NASA NTRS1975

Asymptotic form of the charge-exchange cross section in three-body rearrangement collisions

A three-body general-type rearrangement collision is considered in which the initial and final bound states are described by hydrogen-like wave functions. It is shown that the charge-exchange amplitude in the first Born approximation can be expanded at all incident energies in terms of the inverse powers of the principal quantum number (n). By expanding the exchange amplitude in this way, it is demonstrated conclusively that the cross section for capture into the s, p, and d states as well as for the sum over all the angular-momentum states is proportional to 1/n-cubed plus terms proportional to higher inverse odd powers of n. It is found that the low-lying levels cannot be scaled to the 1/n-cubed law irrespective of the value of the incident energy except in the case of capture into the s states in accordance with the Oppenheimer-Brinkman-Kramers approximation. Zeros and minima in the differential cross sections are given in the limit of high principal quantum number for electron capture by protons from atomic hydrogen and for positronium formation by proton-atomic hydrogen collisions.

Omidvar, K.↗
NASA NTRS1975

Asymptotic form of the charge exchange cross section in the three body rearrangement collisions

A three body general rearrangement collision is considered where the initial and final bound states are described by the hydrogen-like wave functions. Mathematical models are developed to establish the relationships of quantum number, the reduced mass, and the nuclear charge of the final state. It is shown that for the low lying levels, the reciprocal of n cubed scaling law at all incident energies is only approximately satisfied. The case of the symmetric collisions is considered and it is shown that for high n and high incident energy, E, the cross section behaves as the reciprocal of E cubed. Zeros and minima in the differential cross sections in the limit of high n for protons on atomic hydrogen and positrons on atomic hydrogen are given.

Omidvar, K.↗
NASA NTRS1973

Asymptotic form for the cross section for the Coulomb interacting rearrangement collisions.

It is shown that in a rearrangement collision leading to the formation of highly excited hydrogenlike states the cross section at high energies behaves as 1/n-squared, with n the principal quantum number, thus invalidating the Brinkman-Kramers approximation for large n. Similarly, in high-energy inelastic electron-hydrogenlike-atom collisions the exchange cross section for sufficiently large n dominates the direct excitation cross section.

Omidvar, K.↗
NASA NTRS1973

Asymptotic form for the cross section for the Coulomb interacting rearrangement collisions

It is shown that in a rearrangement collision leading to the formation of the highly excited hydrogenlike states the cross section in all orders of the Born approximation behaves as 1/n sq, with n the principal quantum number, thus invalidating the Brinkman-Kramers approximation for large n. Similarly, in high energy inelastic electron-hydrogenlike atom collisions the exchange cross section for sufficiently large n dominates the direct excitation cross section.

Omidvar, K.↗
NASA NTRS1972

Divergence of the total cross section for three body rearrangement collisions with coulomb interactions

Three charged particles 1, 2, 3 collide according to the reaction 1+(2+3) yields (1+3)+2, where (2+3) and (1+3) are hydrogenlike bound states. It is shown when (1+3) is in a highly excited state n, due to the repulsive potential, the cross section in the first Born approximation behaves as 1/n which makes the total cross section to diverge like ln n. The total cross sections in the higher orders of the Born approximation are similarly divergent logarithmically.

Omidvar, K.↗
NASA NTRS1972

Charge-impact excitation of the highly excited hydrogen atoms in the dipole form of the semiclassical impact-parameter and Born approximations.

Derivation of expressions for the excitation cross section in the dipole approximation of the semiclassical impact parameter and the Born approximations, making use of a formula given by Menzel (1968, 1969) for the asymptotic expansion of the oscillator strength of the hydrogen-like atoms. When only the leading term in the asymptotic expansion is retained, the expression for the cross section becomes identical with the expression obtained by the method of the classical collision and correspondence principle given by Percival and Richards (1970). Comparisons are made between the Bethe coefficients obtained by the author and the Bethe coefficients of the Born approximation for transitions where the Born calculation is available. Satisfactory agreement is obtained only for n yields n + 1 transitions, where n is the principal quantum number of the excited state.

Omidvar, K.↗
NASA NTRS1972

Ionization of multielectron atoms by fast charged particles.

Using plane waves to describe the incident and scattered particles, and screened hydrogenic and Coulomb functions to describe the atomic electrons before and after ejections, we have calculated the differential and total ionization cross sections of 11 atoms and one ion by electron impact, and ionization of helium by proton impact. The effective charges of the screened hydrogenic functions are fixed by the Hartree-Fock calculations. Calculations have been carried out for the atomic s, p, and d electrons. For low atomic numbers, we find reasonable agreement with the experimental data. For intermediate atomic numbers, we expect our results to overestimate the actual cross sections, since our choice of a unit charge for the Coulomb function of the ejected electrons will overestimate the atomic dipole potential strength, and in turn the high-energy cross sections. The advantage of the method presented here is that the ionization amplitude is given in analytic form. This may allow further analysis on this amplitude, and facilitates extension of the numerical integration for the cross section to high impact energies.

Omidvar, K.↗
NASA NTRS1971

Collisional excitation of the highly excited hydrogen atoms in the dipole form of the semiclassical impact parameter and Born approximations

Expressions for the excitation cross section of the highly excited states of the hydrogenlike atoms by fast charged particles have been derived in the dipole approximation of the semiclassical impact parameter and the Born approximations, making use of a formula for the asymptotic expansion of the oscillator strength of the hydrogenlike atoms given by Menzel. When only the leading term in the asymptotic expansion is retained, the expression for the cross section becomes identical to the expression obtained by the method of the classical collision and correspondence principle given by Percival and Richards. Comparisons are made between the Bethe coefficients obtained here and the Bethe coefficients of the Born approximation for transitions where the Born calculation is available. Satisfactory agreement is obtained only for n yields n + 1 transitions, with n the principal quantum number of the excited state.

Omidvar, K.↗