Experimental Atomic, Molecular and Optical Physics

	Older Experimental Research


These stories were previously featured on our Emerging Research page. For the latest news on our research program, please see our experiment and theory groups' pages.


Antiproton on Hydrogen Collision Theory Emil Sidky and the JRML Atomic Theory group have produced an animation of antiproton on Hydrogen collisions. The video clip shows the 3D electron probability density projected onto the collision plane with: Antiproton Energy: 10 keV (v=0.63 au) Impact Parameter: 0.3 au Longitudunal Range Displayed: -10 au to +10 au Transverse Range Displayed: -10 au to +10 au The probability density was cut off at about 10% of maximum so that the weaker features would be visible. The probability density is set to zero near the proton and the anti-proton in order to see their positions.

This AVI-style animation is large, about 5 MB, so please be judicious in your downloads! The link at left will get you a media player to watch it in, should you need one.


	Charge Exchange at Very Low Collision Energies H⁺ + H(1s) and H⁺ + D(1s) are two of the most basic systems for studying electron transfer in slow collisions, but to date the most interesting energy range for both quantum effects and astrophysical significance (below 0.1 eV) has remained inaccessible to experiment. Fast impact ionization of the HD isotope, followed by the dissociation of the electronic ground state of HD⁺ (GSD)¹ is used to study very slow, "single pass" H⁺ + D(1s) collisions. Since the fragments produced in this process typically have below 250 meV of kinetic energy, we can use the molecular ion itself as our particle accelerator, and reach collision energies unattainable by current conventional methods. We use a COLTRIMS-style apparatus to image the momentum of the dissociating fragments in three dimensions and thereby measure the entire distribution of collision energies. ¹ I. Ben-Itzhak et al., J. Phys. B. 29, L21 (1996).

	Figure: Preliminary results for the slow D⁺ fragments produced in fast F⁴⁺ + HD collisions. Left: The 2D-momentum image shows the separation of the GSD from the other processes. Lower Right: The D⁺ yield as a function of kinetic energy release.

A paper from CAARI-98 in Denton, Texas is available in either Postscript or Acrobat format. More information on this topic is available from Professor Itzik Ben-Itzhak.

Electron Momentum Distributions


"Topview" electron momentum distributions from Ne⁺ on Ne at V_P=0.45 a.u. and P_r=15 a.u. (a) and He⁺ on He at V_P=0.9 a.u. and P_r=7 a.u. (b). Thin slices were cut out of these distributions along (c, d) and across (e, f) the beam axis and projected onto the horizontal and vertical axis, respectively. This spectrum shows the difference in continuum electron spectra generated by He⁺ on He (where the promotion of a pi state is expected to dominate ) and Ne⁺ on Ne (where the promotion of a sigma state is expected to dominate). Note especially in fig. e and f that the pi promotion produces a nodal line on the beam axis, while the sigma promotion produces a peak at this point. See "Observation of an impact-parameter window in low-velocity ionizing collisions of Ne⁺ on Ne proceeding through quasimolecular states", M.A. Abdallah, W. Wolff, H.E. Wolf, C.L. Cocke, M. Stöckli, Phys. Rev. A 58, 3379 (1998).
Further information on this work is available from Professor Lew Cocke.

Discovery of New Symmetries in Low-Energy Ionization of Atoms by Slow Projectiles

New recoil-ion electron emission symmetries are observed in the ionization of atoms by low velocity projectiles. The data presented here are obtained using cold-target recoil momentum spectroscopy (COLTRIMS) coupled with electron momentum imaging. The green plot shows a transverse recoil momentum distribution spectrum. The electron emission spectra observed in-plane (side view gate) and out-of-plane (top view gate) with the recoil ion emission are given to the right and to the top, respectively, of the recoil ion spectrum. The spectra show a clear pi-orbital type structure. The out-of-plane electron intensity oscillates relative to the internuclear axis as the ion-velocity is changed.

The ejection of continuum electrons from an atomic target by a slow-moving ionic projectile has been the center of considerable attention in the recent years, partially because it is the perfect testing ground for a recently developed "hidden crossing" approach to the description of ion-atom collisions, and partially because of its close connection to other processes which leave three Coulomb bodies with very little excess energy in the continuum. Remarkable new experimental insight into the low energy ionization process has recently been gained by using cold-target recoil momentum spectroscopy (COLTRIMS)¹ together with electron momentum imaging techniques. The electron spectra show a clear pi-orbital structure when viewed perpendicular to the collision plane, with an approximate nodal line located along the asymptotic internuclear axis^2,3. This is the first time that such an angular momentum structure has been so transparently revealed in the comprehensive momentum image of the continuum electrons. The distribution has an asymmetry in the electron density with respect to the internuclear line which oscillates back and forth with respect to this line as the collision velocity is increased². These observations will form a guide and a challenge to the theory of ionization⁴.

¹J. Ullrich, et al, Comments At. Mol. Phys., 30,285 (1994). ²R. Dörner, H. Khemliche, M.H. Prior, C.L. Cocke, J.A. Gary, R.E. Olson, V. Mergel, J. Ullrich and H. Schmidt-Böcking, Phys. Rev. Lett., 77, 4520 (1996). ³ M.A. Abdallah, W. Wolff, H. Wolf, C.L. Cocke and M. Stöckli, Bull. Am. Phys. Soc. 42, 1059 (1997). ⁴J.H. Macek and S. Yu. Ovichinnikov, Bull. Am. Phys. Soc. 42, 1094 (1997).

...And by Intermediate Velocity Projectiles Ionization through intermediate velocities has been similarly studied in Mohammad Abdallah's PhD dissertation. His findings are available as an on-line presentation entitled "Single Ionization in Highly Charged Ion-Atom Collisions at Low to Intermediate Impact Velocities". The figure at right depicts the recoil ion longitudinal momentum distribution in Ne⁺ on Ne collisions at v_p=0.35 au.
Further information on this work is available from Professor Lew Cocke.

Discovery of Narrow Electron Jets in the Forward and Backward Directions of Electron Spectra from Swift Ion-Solid Collisions
	Jet-like features emitted at the forward and backward directions in the electron emission spectra for ions traversing carbon foils are observed for the first time. These jets of electrons are interpreted in terms of the channeling of electrons in the plasma-like wake of the projectile track. These electron jets are observed in narrow cones at 0° (forward) and 180° (backward) with an angular divergence of ± 2.5°. The jets have velocities up to the ion velocity in the forward direction and up to twice the ion velocity in the backward direction. The result is interpreted as channeling of electrons in the plasma-like wake of the projectile track, which is corroborated by the observation that stronger jet emission takes place for ions with higher charge.
Further information on this work is available from Professor Siegbert Hagmann.

Discovery of Narrow Electron Jets in the Forward and Backward Directions of Electron Spectra from Swift Ion-Solid Collisions

Jet-like features emitted at the forward and backward directions in the electron emission spectra for ions traversing carbon foils are observed for the first time. These jets of electrons are interpreted in terms of the channeling of electrons in the plasma-like wake of the projectile track. These electron jets are observed in narrow cones at 0° (forward) and 180° (backward) with an angular divergence of ± 2.5°. The jets have velocities up to the ion velocity in the forward direction and up to twice the ion velocity in the backward direction. The result is interpreted as channeling of electrons in the plasma-like wake of the projectile track, which is corroborated by the observation that stronger jet emission takes place for ions with higher charge.

Further information on this work is available from Professor Siegbert Hagmann.

Last updated on Monday, 13-Feb-2006.