Zero Momentum Transfer limit of Generalized Oscillator Strengths from Ions.
It is well known that generalized oscillator strengths for optically allowed transitions converge to the optical oscillator strength (OOS) limit if the momentum transfer q tends to zero. According to the Lassettre’s theorem [1], they become equal to the OOS at q=0 regardless of the electron energy. We show that for scattering from ions the Lassettre’s theorem is not valid. We farther show, that the limit q=0 is given exactly by the Coulomb-Born GOS at all energies. Using the Coulomb-Born GOS we have obtained an analytical expression for the limit q=0, which strongly depends on energy and tends to the OOS only if the energy of the projectile electron tends to infinity. An analytical expression for the forward scattering is also obtained.
[1] Lassetre E N, Skerbele A and Dillon M A 1969 J Chem.
Phys. 50 1829.
[K2.002] Fully quantal (e,2e) calculations for hydrogen and alkali metals
J. Colgan, M. S. Pindzola (Auburn University, Auburn, AL), D Mitnik, D. C. Griffin (Rollins College, Winter Park, FL)
Total integral and ejected-energy differential cross
sections are calculated for the electron-impact ionization
of hydrogen and lithium using a combination of the
time-dependent close-coupling and time-independent
distorted-wave methods. Although the total integral cross
sections for hydrogen are in excellent agreement with the
experiment of Shah et al. (1987), the peak total cross
sections for lithium are 25% below the experiment of
Zapesochnyi and Aleksakhin (1969). Ejected-energy
differential cross sections for hydrogen and lithium are
compared with previous exterior complex scaling and
converged close-coupling methods. We plan to present
additional total integral and single differential cross
sections for electron ionization of 2p excited lithium and
3s ground and 3p excited sodium in support of current
crossed-beams and magneto-optical trap experiments. Work is
also in progress on the calculation of triple differential
cross sections for hydrogen and lithium.
[K2.003] Resonance Structures in Simultaneous Ionization--Excitation of Helium
Y. Fang, K. Bartschat (Drake University)
We have extended our recent work on simultaneous electron-impact ionization--excitation of helium\sim[1] to investigate resonance structures in the ejected-electron--residual-ion interaction. The calculations were performed using a second-order perturbative model for a ``fast" incident projectile together with a convergent R-matrix with pseudo-states (RMPS) close-coupling-type model for the initial bound state and the scattering of a ``slow" ejected electron in the field of the ion. Our predictions will be compared with experimental data and results from other theoretical models.
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[1] Y. Fang and K.\simBartschat (2001), J. Phys. B\simbf 34 L19.
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[K2.004] Investigation of spin dependent impact excitation of He^-(2s(2p)^2)^2D
B.G. Birdsey, T. Niessen, T.J. Gay (Behlen Laboratory of Physics, University of Nebraska, Lincoln, NE 68588-0111)
We have measured the integrated Stokes parameters P_1, P_2, and P_3 associated with the spin-polarized impact excitation of the (2s(2p)^2)^2D negative ion resonance of helium at 58.3~eV. The resonance auto-ionizes to a variety of excited states; we measured the 587.6~nm resonance fluorescence from the subsequent transition 3^3D \rightarrow 2^3P. We will comment on some relevant prior data of Batelaan(H.~Batelaan, Coherence in electron-impact excitation of helium), Doctoral thesis, University of Utrecht, Netherlands (1991) and a preliminary R-Matrix calculation by K.~Bartschat. This work was supported by NSF Grant PHY-9732258.
[K2.005] Low-energy electron scattering from atomic oxygen
Oleg Zatsarinny, Swaraj Tayal (Clark Atlanta University)
The electron scattering from neutral oxygen has been
investigated using a modified R-matrix method based on the
B-spline representation of the scattering orbitals. Integral
cross sections for the elastic scattering and for excitation
of the ^3P - ^1D, ^3P - ^1S, and ^1D - ^1S
transitions are calculated in the energy region from
threshold to 30 eV. Accurate representation of target wave
functions has been obtained with separate optimization of
each atomic state on the basis of non-orthogonal orbitals.
The close-coupling expansion consists of the 26
spectroscopic bound and autoionizing target states. A
calculation with 26 spectroscopic states plus 17
pseudo-states has also been carried out to check the effect
of coupling to the continuum. The present results for the
elastic scattering are in excellent agreement with the
measurement, but large discrepancies with the existing
experimental data are found for the ^3P - ^1D and ^3P
- ^1S transitions. The excitation cross sections agree
well with a recent R-matrix calculation with pseudo-states
except for lower energies close to the threshold where our
calculation does not confirm the large reduction in cross
sections due to the polarization effects.
[K2.006] Excitation of the D states of magnesium
R Srivastava (University of Roorkee, India), R P McEachran, A D Stauffer (York University, Toronto)
We have calculated the excitation of the 3^1,3D and
4^1,3D states of Mg by electron impact using the
Relativistic Distorted Wave method. Results are given for
differential cross sections and Stokes parameters for
electron impact energies of 20 and 40 eV. The effects of
configuration interaction in the representation of the
target states is investigated. Interesting behaviour is
noted in the Stokes parameters for fine-structure
transitions of the triplet states.
[K2.007] R-matrix with Pseudo-States Calculations for Electron Impact Excitation of Cesium.
K. Bartschat, Y. Fang (Drake University)
We present results of spin asymmetries and Stokes parameters for electron impact excitation of the 6s \to 6p transition in Cs atoms. These results were obtained in a convergent R-matrix with pseudo-states (RMPS) close-coupling-type calculation, with relativistic effects accounted for by the one-electron terms of the Breit-Pauli hamiltonian. The success of the model for elastic collisions~[1] and comparison with preliminary experimental data~[2] provides confidence in the accuracy of the results, indicating their potential to assist in the interpretation of ongoing experiments.
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[1] K.~Bartschat and Y. Fang (2000), Phys. Rev. A~62 052719.
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[2] G. Baum (2001), private communication.
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[K2.008] SUPERELASTICALLY SCATTERED ELECTRONS IN METASTABLE O^6+ AND C^4+ + H_2 COLLISIONS.
S. Hossain, A. S. Al-Naser, A. L. Landers, O. A. Haija, D. J. Pole, T. W. Gorczyca, J. A. Tanis (Western Michigan University), H. Knutson, E. Breza, R. Hebert (Kalamazoo Math Science Center.)
Superelastic scattering of quasi-free H_2 target
electrons was measured for metastable He-like ions having
sufficiently long lifetimes. In this process, the excited
ion transfers its electronic energy to the scattered
electron, thereby increasing the energy of the scattered
electron. We have measured electrons superelastically
scattered by the metastable (1s2s ^3S) component of 1.1
MeV/u O^6+ and 0.6 MeV/u C^4+ ions produced by the
Western Michigan University tandem Van de Graaff
accelerator. In the laboratory frame, loosely bound target
electrons scattered from the projectile ion were measured at
zero degrees. Energy gained by the electrons from the
de-excitation of the metastable projectile displays a broad
peak at the high-energy tail of the binary encounter peak.
Results will be compared with R-matrix calculations based on
the time-reversed process of inelastic impact excitation.
[K2.009] High resolution studies of electronic excitation and resonance formation in positron-molecule scattering
J.P. Sullivan, S.J. Gilbert, C.M. Surko (UCSD, La Jolla, CA), S.J. Buckman (ANU, Canberra, Australia)
The advent of high-brightness, high-resolution (\sim25 meV) positron beams, produced using positron trapping techniques, has opened new possibilities for spectroscopic studies in collisions of low energy positrons with atoms and molecules. Near threshold studies of vibrational excitation of diatomic and polyatomic molecules have recently been performed.(J. P. Sullivan et al., Phys. Rev. Lett., in Press (2001)) We demonstrate here that vibrationally-resolved \textitelectronic excitation of diatomic molecules such as CO, N_2 and O_2 by positron impact is achievable, including the measurement of absolute scattering cross sections for these processes. In addition, high resolution measurements of the energy dependence of total positron-molecule (atom) scattering cross sections are being undertaken in the search for Feshbach resonances predicted(M. A. P. Lima, Private Communication (2000))^,(C. P. Campbell et al., Nucl. Instr. and Meth. Phys. Res. B, \textbf143) 41 (1998) near the thresholds of the lowest-lying electronic states.
[K2.010] Multichannel scattering cross sections in e+ + H system
Chi Yu Hu (Dept. of physics, California State University at Long Beach), Modified Faddeev equation Collaboration
The modified Faddeev equation in bipolar and quintic spline
representation is solved for a range of energies between the
ps(n=2) and H(n=3) thresholds. The number of open channel is
six for S-state and eight for states with angular momentum
greater than zero. Interesting features in the cross section
will be reported. In this energy gap, the hydrogens are
formed predominately in excited states. The formation cross
sections reach several hundreds of square Bohr radii at some
energies.
[K2.011] Analysis of Doppler Broadening Spectra of Thermalization of Positronium in Gases
C.M. Nakamura, J.J. Engbrecht, M. Skalsey, D.W. Gidley (Dept. of Physics, The University of Michigan)
Time resolved Doppler broadening spectra have been used to
study the thermalization of positronium. The analysis of
these spectra is complicated due to the unknown nature of
the various backgrounds. We will present the effect of
various models and fitting techniques on our results.
[K2.012] Measurements of the Rotational Constants of the Helium Molecular Ion Using the Dissociative Recombination Reaction
Kenneth Hardy, Cristina Caraballo, Miguel Faxas, Abilio Pacheco, John Rojas, Adnan Salem, Libni Simons (Florida International University)
The dissociative recombination (DR) reaction of the helium molecular ion results in the production of two final state product atoms, one being in an excited energy state, and the other normally being in the ground state. The energies of these transitions are measured by time of flight (TOF) spectroscopy. Measurements of the velocities of these final products of the DR of He_2^+ from the v=3,4, and 5 vibrational states to the 3s^3S, 3s^1S, 3p^3P and the 3d^3D final product states have been made. The rotational constants obtained for the v=3 state of the molecule were B_v =6.46\pm0.2 cm^-1 and D_v =-7.1X10^- 4\pm0.0004 cm^-1, for the v=4 state, the constants obtained were B_v =6.01\pm0.2cm^-1 and D_v =9.5X10^- 5\pm0.00002 cm^-1, and for the v=5 state, B_v and D_v were 5.89\pm0.2 cm^-1 and 2.88X10^-3\pm0.007 cm^-1 respectively. Our measurements for the rotational constants agree with values reported by Huber and Herzberg(K.P Huber and G. Herzberg, in Molecular Spectra and Molecular Structure IV, Constants of Diatomic Molecules (Van Nostrand, New York, 1979)).