i r

Final report on Contract DE-AI05-91ER14184

with

The U. S. Department of Energy Office of Energy research

Division of Chemical Sciences

Electron-Atom Collision Studies Using Optically State-Selected Beams

National Institute of Standards and Technology Electron an8 Optical Physics Division

Physics Laboratory Gaithersburg, MD 20899

(15 May 1991 - 14 May 1994)

Electron Physics Group

michael-kelley @ nist.gov (301) 975-3722

. Ja& J. McClelland, Physicist Electron Physics Group

jabez-mcclelland@nist.gov (301) 975-3721

Submitted 15 March 1998

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement. m m - mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or.any agency thereof.

DISCLAIMER

Portions of this document may be illegible electronic image products. Images are produced from the best available original document.

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Finall Report

This report covers activities performed under research contract DE-AI05-9 1ER14184, entitled “Electron-atom collision studies using optically state-selected beams.” As stated in the original proposal, the goal of the project has been to perform electron-scattering experiments on a few model systems, with emphasis on resolving all the quantum-state variables possible. The purpose of these experimental studies has been to provide a set of measurements of unprecedented accuracy and completeness that can be used as benchmarks for comparison with theoretical calculations.

It is well established that for most electron scattering systems of immediate practical interest, heavy reliance on theoretical predictions is a necessity for obtaining cross section data and other relevant parameters. However, the theoretical methods that must be used in these calculations are by no means straightforward, and hence are subject to errors that are difficult to characterize. To provide a means of testing these calculations, a very thorough database of experimental results must be accumulated on the few experimental systems that are amenable to investigation. By fully resolving as much information as is allowed by quantum mechanics on these test systems, the most exacting tests possible can be applied to the theoretical methods, resulting in greater confidence in their predictive ability.

To resolve all the quantum-state variables in an electron scattering process, attention must be concentrated not only on the energies of the incident and scattered electron and the scattering angle, but also on the internal states of the electron and the atom. Thus experiments must have the capability of resolving the spin of the incident and scattered electrons, and also the spin and angular momentum of the electron(s) in the target atom. In our experiments, resolution of the spin of the incident electrons is achieved by using the GaAs polarized-electron source, developed in the Electron Physics Group at NIST. Resolution of the spin and angular momentum of the target atom is accomplished with the use of laser optical pumping methods, which allow the selection of pure spin and/or angular momentum states of the target, By resolving the states of the electron and target before collision in this manner, a detailed picture of the electron scattering process can be obtained. It is important to note that this type of experiment not only provides more information than can be obtained in a simple cross section measurement, but also results in more useful measurements because asymmetries can be reported. Asymmetries, such as the difference between spin-parallel and spin-antiparallel cross sections, divided by the sum, are not subject to the types of normalization errors encountered with cross sections. Thus pure numbers can be compared directly with theoretical predictions without the additional uncertainty associated with the difficult experimental problem of putting measurements on an absolute scale.

During the period covered by this report, our work has concentrated on measuring low-energy electron scattering from sodium and chromium. Sodium provides an ideal “one-electron” test case, since it has a single loosely bound valence electron, making it approachable by even the most complex electron scattering calculations. Ira addition, the atom has a strong optical transition from the 32S1,2 ground state to the 32P3,2 excited state whose wavelength (589 nm) matches the peak output of the laser dye rhodamine 6G. Thus optical pumping techniques can be readily applied in the laboratory, leading to either a population of ground state atoms in which the spin of the valence electron is oriented either “up” or “down” in the laboratory, or a spin-

polarized pure angular momentum state of the excited 32P3,2 state. Such an excited state makes possible superelastic scattering, where the internal energy of the atom is transferred to the electron during the collision. This turns out to be a very efficient way to study the inelastic scattering process.

Unlike sodium, chromium provides an extremely exacting test for theoretical methods because of its very complex electronic structure, not because it is simple. With a valence configuration consisting of five electrons in a half-filled 3d shell, plus another electron in a 4s shell, this atom provides a test case that can challenge even the simplest approximations. As in sodium, however, optical pumping methods can be applied using a dye laser, in this case tuned to the 7S3 to P4 transition at 425 nm. The result can be a spin-polarized ground state, in which all six valence electrons are oriented in the laboratory, or a spin-polarized pure angular momentum state of the excited P4 level. Thus state-resolved elastic and inelastic scattering can be investigated.

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Elastic Electron-Sodium Scattering

Our measurements on elastic electron-sodium scattering have concentrated on three distinct regimes: below, near and above the ionization threshold at 5.1 eV. In the first category, we have made measurements at 1.0 eV and 1.6 eV; in the second at 4.1 eV; and in the third, at 10, 12.1 , 20, and 54.4 eV[ 1,2,3,4]. All measurements were carried out over an angular range of 20"-140". The precise energies were chosen in part to correspond with superelastic measurements we have also made, as described below, and in part to correspond with existing calculations in the literature. These six sets of data constitute one of the most comprehensive studies of low energy electron scattering on an atom ever undertaken, and have led to major advances in the field of electron-atom scattering.

The measurements at 1 .O and 1.6 eV have provided a unique benchmark for theoretical comparison, since these incident energies are below the first excitation level of sodium, making elastic scattering the only open channel available. Theoretical calculations making use of the close-coupling approximation rely on ignoring some excitation channels, so this energy regime should be where they perform the best. Nevertheless, precise experimental tests of these calculations have not been available in this regime. Our results have shown excellent agreement with the 4-state close-coupling calculation of Moores and Norcross[S] , giving more confidence that at least in this energy regime this type of calculation is reliable.

Our higher energy measurements, on the other hand, have shown significant discrepancies with close-coupling calculations carried out with only a few states, as in the case of Moores and Norcross[5]. These discrepancies have stimulated a great deal of theoretical work, leading to the surprising discovery that if a large number of states are included in a close coupling calculation, and also if an optical potential is included, quite good agreement can be obtained between theory and experiment. This was demonstrated by Bray and McCarthy, who carried out a 15-state close-coupling calculation with optical potential that has shown remarkable agreement with our results[6].

“Complete” Scattering Amplitude Determination

One of the ultimate goals of a state-resolved electron-scattering experiment is the determination of all the information allowed by quantum mechanics. In the case of low energy electron scattering from a one-electron target, this corresponds to determining the absolute magnitude of the individual scattering amplitudes for the singlet and triplet channels, and also the relative phase between these two complex numbers. By combining our elastic scattering measurements at 4.1 and 12.1 eV with measurements of cross sections from one laboratory, and the spin polarization after scattering from another, we have been able to extract this information[2]. This determination represents a reduction of the quantum scattering problem to its most fundamental level, since all possible measurables are determined. By doing this, we have produced a set of data that provides the most stringent test of theory possible.

Superelastic Electron-Sodium Scattering

While our elastic scattering experiments have provided an unprecedented range of data for comparison with theory, they do not represent the whole story. There are also inelastic channels in the scattering process, and these must be investigated as well. To contribute measurements in this arena we have concentrated on the strongest inelastic channel in electron-sodium scattering, that is, the 3s-3P transition. By using optical pumping methods, we have a unique capability in that we can study the inelastic process in reverse, creating a population of 3P atoms and watching for electrons that have gained energy in the scattering process. This has the advantage that more signal-to-noise can be obtained-both because a large population of atoms is prepared in the desired state, and also because the observed electron has a higher energy than the primary beam so there are far fewer background electrons.

Because there are several angular momentum and spin channels available in the excitation of the 3P level in sodium by electron impact, this process is characterized by four complex amplitudes, corresponding to excitation of the A4 = f l levels in each of the singlet an triplet channels. Measurements of these individual components can be made by controlling the laser polarization used in the optical pumping that prepares the atoms for the superelastic scattering process. In this way, aligned and/or oriented angular momentum states of the atom and also spin-polarized states can be prepared. This allows the measurement of asymmetry-like parameters that can be measured without the necessity of placing a cross section on an absolute scale. For the 3s-3P transition in sodium, these are the angular momenta transferred to the atom in the singlet and triplet channels LlS and LIT, and the ratio r of triplet to singlet cross sections.

We have performed measurements of Lis, LIT, and r at incident energies of 2,7.9, 17.9,37.9, and 52.3 eV, corresponding to total energies of 4.1, 10,20,40, and 54.4 eV[7,8]. As with elastic scattering, extensive comparisons have been made with close-coupling calculations. Since the total energies in this case correspond to the energies measured in elastic scattering, each close- coupling calculation can be compared with a wide array of both elastic and inelastic measurements. Comparison with theory has shown some remarkable agreements and disagreements, providing significant new insights into why calculations cannot yet reliably predict scattering cross sections.

Electron-Chromium Scattering

In contrast with sodium, chromium has been the subject of very few electron scattering investigations. Nevertheless it presents an interesting and challenging subject for such studies. While in sodium it might be said that all the physics of the interaction is well characterized, this is not the case for chromium. For example, because there is an “inner” half-filled valence shell of 3d electrons, and an “outer” shell containing a single 4s electron, it is an open question what the relative roles of these two types of target electrons will be in a collision. With this type of question in mind we have undertaken a study of spin-polarized superelastic scattering from chromium.

To perform the desired measurements, it has been necessary to (a) refit our dye laser system for tuning to the 7S3 - P4 transition at 425 nm, which requires operation with stilbene 3 laser dye and operation of our argon ion pump in the ultraviolet; and (b) design and develop a UHV- compatible very high temperature (1600 “C) oven suitable for producing a high-flux, well collimated chromium atomic beam. This latter task has involved significant innovation, since very little precedent exists in the literature for this type of work.

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Our first achievement in this area has been to produce and diagnose a spin-polarized chromium beam by optical pumping[9]. By using circularly polarized laser light to optically pump the atoms, and detecting the degree of polarization of the fluorescence, we have determined that we are able to produce a beam of atoms with polarization essentially 1 .O, with uncertainty less than typical measurement errors in the scattering experiments.

This characterization of our polarized chromium beam has enabled us to proceed with two sets of superelastic scattering measurements over an angular range of 10” to 14O”--one set at total energy 6.8 eV and one at 13.6 eV (0.5 and 1.0 Ry, respectively)[lO]. As we have done in sodium, we have taken advantage of the atomic orientation capabilities of superelastic scattering with laser optical pumping to make measurements of the angular momentum transfer for each of the spin channels. While in sodium the scattering separates into the singlet and triplet spin channels, in chromium the scattering is divided into the sextet (S = 5/2) and octet (S = 7/2) channels. Thus we have obtained results for Lis, Elo, and Y, where S now stands for sextet, 0 stands for octet, and Y is the ratio of octet to sextet cross sections. These data represent the first measurements of spin-polarized electron scattering from a high-spin, open-shell atom, and thus provide a new benchmark for future electron scattering calculations. While we must wait for theoretical confirmation, we can already draw a tentative and somewhat surprising conclusion that the scattering is essentially alkali-like and the inner 3d electrons play little role, at least in the inelastic channel. We expect that this will not be the case in the elastic channel, but confirmation of this will have to wait for further experimental and theoretical work in the future.

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References

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S. R. Lorentz, R.E. Scholten, J. J. McClelland, M. H. Kelley, and R. J. Celotta, Phys. Rev. Lett. 67(27), 3761-3763 (1991).

J. J. McCleIland, S. R. Lorentz, R. E. Scholten, M. H. Kelley, and R. J. Celotta, Phys. Rev. A 46(9), 6079 (1992).

S. R. Lorentz, R. E. Scholten, J. J. McClelland, M. H. Kelley, and R. J. Celotta, Phys. Rev. A 47(4), 3000-3006 (1993).

J. J. McClelland, M. H. Kelley, and R. J. Celotta, Phys. Rev. Lett. 58(21), 2198 (1987).

D. L. Moores and D. W. Norcross, J. Phys. €3: At. Mol. Phys. 5, 1482-1505 (1972).

I. Bray and I. E. McCarthy, Phys. Rev. A 47,3 17 (1993).

R. E. Scholten, S. R. Lorentz, J. J. McClelland, M. H. Kelley, and R. J. Celotta, J. Phys. B. Lett. 24, L653 (1991).

6. J. McClelland, M, H. Kelley, and R. J. Celotta, Phys. Rev. A 40(5), 2321 (1989).

J. J. McClelland, R. E. Scholten, and R. J. Celotta, in Proceedings of the 18th International Conference on the Physics of Electronic and Atomic Collisions (ICPEAC XVIII), Aarhus, Denmark, July 21-27, 1993, (1993) p. 797.

G.F. Hanne, 9.J. McClelland, R.E. Scholten, and R.J. Celotta, J. Phys. €3 26, L753-E758 (1993).

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Publications of Contract-Supported Work

A. Publications in current three year contract period

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Spin-resolved Superelastic Electron Scattering from Laser Excited Chromium Atoms, @.Fa Hanne, J.J. McClelland, R.E. Scholten, md R.J. Celotta, J. Phys. B 26, L753-L758 (1993).

Production of a Spin-Polarized Atomic Cr Beam Using Optical Pumping Methods, J.J. McClelland, RE. Scholten, and W.J. Celotta, in Proceedings of the 18th International Conference on the Physics of Electronic and Atomic Collisions (ICPEAC XVIII), Aarhus, Denmark, July 21-27, 1993, (1993) p. 797.

Uncertainty Intervals for Polarized Beam Scattering Asymmetry Statistics, K.J. Coakley, J.J. McClelland, M.H. Kelley, and R.J. Celotta, Rev. of Sci. Instrum. 64(7), 1888-1894 (1993).

Spin-Resolved Elastic Scattering of Electrons from Sodium, S.R. Eorentz, R.E. Scholten, J.J. McCIelland, M.H. Kelley, and R.J. Celotta, Phys. Rev. A 47(4), 3000-3006 (1993).

Determination of Complex Scattering Amplitudes in Low-Energy Elastic Electron-Sodium Scattering, J. J. McClelland, S.R. Lorentz, R.E. Scholten, M.H. Kelley, and R.J. Celotta, Phys, Rev. A 46(9), 6079 (1993).

Spin-Polarized Electron-Sodium Scattering, S.R. Lorentz, R.E. Scholten, J.J. McClelland, M.H. Kelley, and R.9. Celotta, in Proceedings of the Seventeenth International Conference on the Physics of Electronic and Atomic Collisions (ICPEAC XVII), Brisbane, Australia, July 1991, ed. by LE. McCarthy and W.R. MacGillivray, (1991) p. 104.

Spin-resolved Superelastic Scattering from Sodium at 10 and 40 eV, R.E. Scholten, S.R. Lorentz, J.J. McClelland, M.H. Kelley, and R.J. Celotta, J. Phys. B. Lett. 24, L653 (1991).

Spin-Resolved Elastic Scattering of Electrons from Sodium Below the Inelastic Threshold, S.R. Lorentz, R.E. Scholten, J.J. McClelland, M.H. Kelley, and R.J. Celotta, Phys. Rev. Lett. 67(27), 3761-3763 (1991).

Scattering of Polarized Electrons from Optically Pumped Sodium Atoms, M.H. Kelley, J.J. McClelland, S.R. Lorentz, R.E. Scholten, and R.J. Celotta, in Proceedings of the VIth International Symposium on Correlation and Polarization in Electronic and Atomic Collisions and (e,2e) Reactions, Adelaide, Australia, July 18-21, 1991, ed. by P.J.O. Teubner and E. Weigold, (IOP Publishing, Bristol, 1992) p. 23.

B. Publications in previous contract years

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Polarized-Electron--Polarized-Atom Scattering, M.H. Kelley, J. J. McClelland, S .R. Lorentz, and R.J. Celotta, in Proceedings of the 1990 International Symposium on the Physics of Ionized Gases, Dubrovnik, Yugoslavia, September 3-7, 1990 (in press).

Spin Dependence in Electron-Atom Collisions, M.H. Kelley, J. J. McClelland, and R. J. Celotta, in Proceedings of the DOE Contractor’s Workshop on Atomic Physics, Berkeley, CA, LBL 2750 37, (1989).

Search for a Joint Spin-Orbit and Exchange Asymmetry in Elastic Electron Scattering from Spin-Polarized Sodium, J.J. McClelland, S.J. Buckman, M.H. Kelley, and R.J. Celotta, J. Phys. B: At. Mol. Opt. Phys. 23, L21 (1990).

Use of Thorium as a Target in Electron Spin Analyzers, J.J. McClelland, M.R. Scheinfein, and D.T. Pierce, Rev. Sei. Instrum. 60(4), 683 (1989).

Progress on Spin Detectors and Spin-polarized Electron Scattering from Na at NIST, J.J. McClelland, in Proceedings of the Fifth International Symposium on Polarization and Correlation in Electronic and Atomic Collisions, Stevens Institute of Technology, Hoboken, NJ, August 1989, (NIST Special Publication 789, 1990) p. 102.

Superelastic Scattering of Sgin-polarized Electrons from Sodium, J.J. McClelland, M.H. Kelley, and R.J. Celotta, Phys. Rev. A 40(5), 2321 (1989).

Spin-Polarized Electron Scattering from Optically Pumped Sodium at 20eV, S.J. Buckman, J.J. McClelland, MH. Kelley, and R.J. Celotta, in Proceedings of the Sixteenth International Conference on the Physics of Electronic and Atomic Collisions (ICPEAC XVI), New York, NY, July 1989, ed. by A. Dalgarno, R.S. Freund, M.S. Lubell, and T.B. Lucatorto, (1989) p. 149.

Increased Sherman Function in Electron Spin Analyzers Using a Bulk Thorium Target, J.J. McClelland, M.R. Scheinfein, and D.T. Pierce, in Proceedings of the Sixteenth International Conference on the Physics of Electronic and Atomic Collisions (ICPEAC XVI), New York, NY, July 1989, ed. by A. Dalgarno, R.S. Freund, M.S. Lubell, and T.B. Lucatorto, (1989) p. 805.

Improved Low-Energy Diffuse Scattering Electron-Spin Polarization Analyzer, M.R. Scheinfein, D.T. Pierce, J. Unguris, J.J. McClelland, R.J. Celotta, and M.H. Kelley, Rev. Sci. Instrum. 60(1), 1 (1989).

Spin Sensitivity of a Channel Electron Multiplier, R.E. Scholten, J.J. McClelland, M.H. Kelley, and R.J. Celotta, Rev. Sci. Instrum. 59(3), 506 (1988).

Mott Scattering Measurements of the Spin Polarization of the NBS GaAs Electron Source, J.J. McClelland and B.J. Waclawski, (Internal Report, April 6, 1987).

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Elastic Scattering of Spin-Polarized Electrons from Spin-Polarized Na Atoms, M.H. Kelley, J.J. McClelland, and R.J. Celotta, in Proceedings of the Fifteenth International Conference on the Physics of Electronic and Atomic Collisions (ICPEAC XV), Brighton, United Kingdom, July 1987, ed. by J. Geddes, H.B. Kilbody, A.E. Kingston, C.J. Latimer, and H.J.R. Walters, (1987) p. 150.

Superelastic Scattering of Spin-polarized Electrons from Optically Pumped Sodium, M.H. Kelley, J.J. McClelland, and R.J. Celotta, in Proceedings of the Fifteenth International Conference on the Physics of Electronic and Atomic Collisions (ICPEAC XV), Brighton, United Kingdom, July 1987, ed. by J. Geddes, H.B. Kilbody, A.E. Kingston, C.J. Latimer, and H.J.R. Walters, (1987) p. 152.

Analysis of Collisional Orientation and Alignment Studies using Spin-polarized Electrons and Laser Excited Atoms, I.V, Hertel, M.H. Kelley, and J.J. McClelland, in Proceedings of the Fifteenth International Conference on the Physics of Electronic and Atomic Collisions (ICPEAC XV), Brighton, United Kingdom, July 1987, ed. by J. Geddes, H.B. Kilbody, A.E. Kingston, @. J. Latimer, and H.J.R. Walters, (1987) p. 15 1

Large Angle Superelastic Electron Scattering from Na(3P), J.J. McClelland, M.H. Kelley, and R.J. Celotta, J. Phys. B 20, L385 (1987).

Analysis of Collisional Alignment & Orientation Studied by Scattering of Spin-polarized Electrons from Laser Excited Atoms, P.V. Hertel, M.H. Kelley, and J.J. McClelland, 2. Phys. D 6, 163 (1987).

Spin-orbit and Exchange Effects in Elastic Scattering of Spin-polarized Electrons from Spin-polarized Na Atoms, J.J. McClelland, M.H. Kelley, and R.J. Celotta, Phys. Rev. Lett. 58(21), 2198 (1987).

Spin Dependence in Superelastic Electron Scattering From Excited Sodium, M.H. Kelley, R.J. Celotta, and J.J. McClelland, in Proceedings of the DOE Atomic Physics Program Contractor’s Workshop, Boulder, CO, April 1986, (1986) p. 59.

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State Selection in Electron-Atom Scattering: Spin-Polarized Electron Scattering from Optically Pumped Sodium, J.J. McClelland, M.H. Kelley, and R.J. Celotta, in Proceedings of the XIII Symposium on Physics of Ionized Gases, Sibenik, Yugoslavia, September 1986, ed. by M.V. Kurepa, (1986) p. 15.

Spin Dependent Superelastic Scattering from Pure Angular Moments States of Na(3P), J.J. McClelland, M.H. Kelley, and R.J. Celotta, Phys. Rev. Lett. 56,2771 (1986) Erratum.

Spin Dependent Superelastic scattering from Pure Angular Momentum States of Na(3P), J.J. McClelland, M.H. Kelley, and R.J. Celotta, Phys. Rev. Lett. 56, 1362 (1 986).

Spin Dependent Interactions in Super Elastic Electron-Sodium Scattering, J.J. McClelland, M.H. Kelley, and R.J. Celotta, in Proceedings of the Atomic Physics Workshop, Chapel Hill, NC, May 1985, (1985) p. 10.

Spin Dependence in Superelastic Electron Scattering from Na(3P), J.J. McClelland, M.H. Kelley, and R.J. Celotta, Phys. Rev. Lett. 55(7), 688 (1985).

Observations of Spin Dependence in Superelastic Scattering of Polarized Electrons from Na(3P), J.J. McClelland, M.H. Kelley, and R.J. Celotta, in Electronic and Atomic Collisions, ed. by D.C. Lorents, W.E. Meyerhof, and J.R. Peterson, (North Holland, Amsterdam, 1986) g. 239.

Detailed Look at Aspects of Optical Pumping in Sodium, J.J. McClelland and M.H. Kelley, Phys. Rev. A 31(6), 3704 (1985).

Electron-Atom Collision Studies Using Optically State Selected Beams: Superelastic Scattering, J.J. McClelland, M.H. Kelley, and R.J. Celotta, in Proceedings of the Fourteenth International Conference on the Physics of Electronic and Atomic Collisions, (ICPEAC XIV), Palo Alto, CA, July, ed. by M.9. Coggiola, D.L. Huestis, and R.P. Saxon, (1985) p. 185.

Diagnosis of Spin Polarization in an Optically Pumped Sodium Beam, J.J. McClelland and M.H. Kelley, in Proceedings of the Fourteenth International Conference on the Physics of Electronic and Atomic Collisions (ICPEAC XIV), Palo Alto, CA, July, ed. by M.J. Coggiola, D.L. Huestis, and R.P. Saxon, (1985) p. 669.