Prof. Robyn Wilde

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Academic Career Length

12Years (2015-Present)

(Calculated once Scilit publication data is synced: current year − year of first publication + 1)

Representative Publications

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2 March 2016 in Journal of Physics B: Atomic, Molecular and Optical Physics

Journal Article

Positronium collisions with rare-gas atoms

We calculate elastic scattering of positronium (Ps) by the Xe atom using the recently developed pseudopotential method (Fabrikant and Gribakin 2014 Phys. Rev. A 90 052717) and review general features of Ps scattering from heavier rare-gas atoms: Ar, Kr and Xe. The total scattering cross section is dominated by two contributions: elastic scattering and Ps ionization (break-up). To calculate the Ps ionization cross sections we use the binary-encounter method for Ps collisions with an atomic target. Our results for the ionization cross section agree well with previous calculations carried out in the impulse approximation. Our total Ps–Xe cross section, when plotted as a function of the projectile velocity, exhibits similarity with the electron-Xe cross section for the collision velocities higher than 0.8 a.u., and agrees very well with the measurements at Ps velocities above 0.5 a.u.

17 October 2018 in Physical Review A

Journal Article

Positronium collisions with rare-gas atoms: Free-electron gas plus orthogonalizing pseudopotential model

Robyn Wilde

I. I. Fabrikant

Positronium collisions with rare-gas atoms are treated using the free-electron-gas approximation for exchange and correlation potential. The results confirm the absence of the Ramsauer-Townsend minimum in elastic scattering cross sections, but show lower cross sections in the lower-energy region when compared to previous pseudopotential calculations. This is explained by a more attractive ab initio correlation potential as compared to the previously used empirical potential. The results in the thermal-energy region agree very well with most swarm measurements for all rare-gas atoms. At higher energies, the results are compared with beam experiments and agreement for heavier rare-gas atoms Ar, Kr, and Xe is found to be very good. For He and Ne, some discrepancies with beam measurements are observed. This is explained by a poorer performance of the free-electron-gas potentials, based on the statistical Thomas-Fermi model, for systems with fewer electrons.

15 May 2018 in Physical Review A

Journal Article

Exchange and correlation in positronium-molecule scattering

I. I. Fabrikant

Robyn Wilde

Exchange and correlations play a particularly important role in positronium (Ps) collisions with atoms and molecules, since the static potential for Ps interaction with a neutral system is zero. Theoretical description of both effects is a very challenging task. In the present work we use the free-electron-gas model to describe exchange and correlations in Ps collisions with molecules similar to the approach widely used in the theory of electron-molecule collisions. The results for exchange and correlation energies are presented as functions of the Fermi momentum of the electron gas and the Ps incident energy. Using the Thomas-Fermi model, these functions can be converted into exchange and correlation potentials for Ps interaction with molecules as functions of the distance between the projectile and the target.

15 May 2018 in Physical Review A

Journal Article

Positronium collisions with molecular nitrogen

Robyn Wilde

I. I. Fabrikant

For many atomic and molecular targets positronium (Ps) scattering looks very similar to electron scattering if total scattering cross sections are plotted as functions of the projectile velocity. Recently this similarity was observed for the resonant scattering by the

N_{2}

molecule. For correct treatment of Ps-molecule scattering incorporation of the exchange interaction and short-range correlations is of paramount importance. In the present work we have used a free-electron-gas model to describe these interactions in collisions of Ps with the

N_{2}

molecule. The results agree reasonably well with the experiment, but the position of the resonance is somewhat shifted towards lower energies, probably due to the fixed-nuclei approximation employed in the calculations. The partial-wave analysis of the resonant peak shows that its composition is more complex than in the case of

e - N_{2}

scattering.

14 September 2015 in Physical Review A

Journal Article

Positronium collisions with molecular hydrogen

Robyn Wilde

I. I. Fabrikant

Positronium (Ps) collisions with molecular hydrogen are investigated theoretically. Elastic and Ps ionization cross sections are calculated. For elastic scattering the pseudopotential method, previously developed for rare-gas atoms, is applied. Ps ionization cross sections are calculated using the binary-encounter approximation. The results agree with swarm measurements at low collision energies and with beam measurements at higher energies. The total Ps-

H_{2}

cross section when plotted as a function of collision velocity is close to the

e^{-} − H_{2}

cross section at velocities above the Ps ionization threshold, confirming earlier observations [Brawley et al., Science 330, 789 (2010)]. However, below the threshold the two sets of cross sections are different because of the different nature of the long-range interaction between the projectile and the target, the polarization interaction in the case of

e^{-} − H_{2}

collisions and the van der Waals interaction in the case of Ps-

H_{2}

collisions.

6 August 2020 in Journal of Physics B: Atomic, Molecular and Optical Physics

Journal Article

Resonance scattering of positronium atoms by nitrogen molecules

Robyn Wilde

Ilya Fabrikant

We study theoretically the resonance scattering of positronium (Ps) atoms by nitrogen molecules and compare it with resonance e –N ₂ scattering in the Π _g symmetry. The velocity positions of the Ps–N ₂ resonances in Σ _u , Π _u , and Δ _g symmetries are very close to the velocity position of the e –N ₂ resonance which leads to the total Ps–N ₂ cross section being very similar to the e –N ₂ cross section when both are plotted as functions of the projectile velocity. However, in the Ps–N ₂ case the resonances are much wider and their positions vary much slower with increasing internuclear separation than in the e –N ₂ case. This makes the Ps-impact vibrational excitation cross section much smaller than the electron-impact vibrational excitation cross section, and no boomerang oscillations are observed in Ps–N ₂ scattering. We analyze the role of the static potential in the e –N ₂ case and compare exchange potentials for e –N ₂ and Ps–N ₂ interactions. This analysis allows us to explain the difference in the vibrational dynamics in these two cases.

About Me

I am a Professor of Physics at the Oregon Institute of Technology, where I combine over two decades of teaching experience with an active research agenda in atomic and molecular physics. I earned my Ph.D. in Physics from the University of Nebraska–Lincoln under the mentorship of Professor Ilya Fabrikant. My doctoral research focused on Dissociative Electron Attachment, exploring how low‐energy electrons induce fragmentation in molecules. This work provided me with a rigorous foundation in both experimental techniques and theoretical modeling of collision processes. More recently my research now extends to collisions involving electrons, positrons, and Positronium with atoms and molecules, including the processes of Positronium fragmentation and Positronium induced dissociative attachment of molecules. I have a deep interest in teaching and undergraduate education, in particular. I have taught a number of physics classes at the undergraduate level. Apart from the typical general physics classes, I have taught courses in introducing students to research, medical physics and the physics of ultrasound. I have involved a number of undergraduate students in my atomic and molecular physics research and am currently working developing research projects in medical imaging as well.

Prof. Robyn Wilde

Prof. Robyn Wilde

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