Prof. Francis Robicheaux
Dr. Robicheaux is a professor at Auburn University since 1993 and a Fellow of the American Physical Society since 2002. His research area is Theoretical Atomic Physics, mainly focusing on time dependent atomic phenomena, highly excited (Rydberg) atoms, electron scattering, strong fields and ultracold plasmas. He is a member of the ALPHA collaboration which was the first group to trap the antimatter version of the Hydrogen atom and the first group to quantitatively measure some of its properties.
Rydberg Atoms in Strong Fields(wikipedia link to Rydberg atoms)
For many years, we have had a strong effort in studying highly excited atoms and molecules. The interest in this area is that highly excited atoms can be understood using classical and quantum ideas. We are particularly interested in the behavior of these atoms in strong fields. The fields break the spherical symmetry of the atom and give rise to an interesting interplay between different types of motion. Also, specific and well controlled experiments can be performed on these systems.
For these calculations, we often solve both the classical equations of motion and the time dependent Schrodinger equation which governs the quantum behavior. Depending on the system being studied, the quantum calculations solve for the wavefunction can be converged for distances out to 10-6 m from the ion and include angular momentum up to 1000 hbar. Below is a brief description of results in two recent publications.
T. Topcu and F. Robicheaux, “Multiphoton adiabatic rapid passage: classical transition induced by separatrix crossing,” J. Phys. B 42, 044014 (2009). PDF (515 kB)
In this paper, we reinterpreted the recent experimental results from Tom Gallagher's group (H. Maeda, J.H. Gurian, D.V.L. Norum, and T.F. Gallagher, Phys. Rev. Lett. 96, 073002 (2006).) where they measured a 10 photon transition in Rb by chirping a microwave field. This appears to be a quintessential quantum process but we found that it was possible to obtain a classical explanation of the results. We also found that the final state contained many more angular momenta than could be explained by a 10 photon transtions.
This image shows the probability for finishing with angular momentum L after a 6 photon transition in hydrogen (triangles) and lithium (asterisks). Since the atom initially started with angular momentum L=1, the maximum L should be 7. Clearly the distribution extends to much higher values. The distribution extends to very high L because there are actually many stimulated absorption and emissions during the pulse.
There is a classical version of this process that, remarkably, gives a high localization of the final energy. The explanation is that the microwave field puts an island in the Poincare surface of section plot. By chirping the frequency of the microwaves, the island moves to lower action and the population swings through the separatrix and the chaotic sea to the upper side of the island. They finish with a small range of final action.
This image shows the Poincare surface of section plot with the microwaves just before the transition. The + show the population.
This image shows the population just as the population is entering the chaotic sea and shows a blow up of the region near the X point. Notice how some of the atoms are already to upper side of the chaotic sea.
This image shows the population just after the population has made the transition to the other side of the chaotic region. Notice how the final population has a very small range of final action. The horizontal lines in (a) and (c) show the position of the center of the island before and after the transition which shows how little the island has moved during the transition.
Y. Ni, S. Zamith, F. Lepine, T. Martchenko, M. Kling, O. Ghafur, H.G. Muller, G. Berden, F. Robicheaux, and M.J.J. Vrakking, “Above-threshold ionization in a strong dc electric field,” Phys. Rev. A 78, 013413 (2008). PDF (730 kB)
This paper was a collaboration with Marc Vrakking's experimental group to look at the strong field ionization of a highly excited state in a static electric field.
This image shows a schematic of the potential. The atom is put into a state just below the classical ionization threshold. The atom is then exposed to intense IR photons and can absorb several of them. By using a momentum camera, we measured the amount of energy the electron has perpendicular to the electric field.
This image shows the probability for detecting an electron with momenta p_x,p_y. The most prominent feature is rings which come from different number of photons absorbed. Amazaingly, there are a couple extra rings which do not simply correspond to the expected position.
This image shows shows the energy corresponding to each ring as a function of the energy of the initial state for a fixed photon energy. The black squares give the expected position from 1, 2, 3, etc photons absorbed; the solid line shows the calculated position of these rings. The open squares show the energy of the extra rings. They arise from a complicated electron trajectory while leaving the atom; the predicted position is given by the dashed lines.
Five Recent Publications
T. Topcu and F. Robicheaux, “Chaotic ionization of a highly excited hydrogen atom in parallel electric and magnetic fields,” J. Phys. B 40, 1925 (2007). PDF (2150 kB)
M. Strom, C. Sathe, M. Agaker, J. Soderstrom, J.-E. Rubensson, S. Stranges, R. Richter, M. Alagia, T.W. Gorczyca, and F. Robicheaux, "Magnetic-field induced enhancement in the fluorescence yield spectrum of doubly excited states in helium," Phys. Rev. Lett. 97, 253002 (2006). PDF (548 kB)
T. Topcu, M. S. Pindzola, C. P. Balance, D. C. Griffin, and F. Robicheaux, “Electron-impact ionization of highly excited hydrogenlike ions in a collinear s-wave model,” Phys. Rev. A 74, 062708 (2006). PDF (694 kB)
A. Wetzels, A. Gurtler, L.D. Noordam, and F. Robicheaux, "Far-infrared Rydberg-Rydberg transitions in a magnetic field: Deexcitation of antihydrogen atoms", Phys. Rev. A 73, 062507 (2006). PDF (269 kB)
C. Sathe, M. Strom, M. Agaker, J. Soderstrom, J.-E. Rubensson, R. Richter, M. Alagia, S. Stranges, T.W. Gorczyca, and F. Robicheaux, "Double excitations of helium in weak static electric fields," Phys. Rev. Lett. 96, 043002 (2006). PDF (897 kB)