Bulletin of the American Physical Society
64th Annual Gaseous Electronics Conference
Volume 56, Number 15
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session ET3: Heavy-Particle Collisions |
Hide Abstracts |
Chair: Tom Kirchner, York University Room: 255F |
Tuesday, November 15, 2011 2:00PM - 2:30PM |
ET3.00001: Manipulating Atomic Fragmentation Processes by Controlling the Projectile Coherence Invited Speaker: Several years ago a surprising breaking of a symmetry strictly demanded by first-order theories was reported in measured fully differential cross sections (FDCS) for single ionization by ion impact even for small perturbation parameters $\eta $ (projectile charge to speed ratio) [1]. The data could not even qualitatively be reproduced by any fully quantum-mechanical calculation. In contrast, treating the projectile -- target nucleus interaction classically resulted in good agreement with the data [2]. This raises the question whether the fully quantum-mechanical calculations share a fundamental problem which has been overlooked so far. One feature which all of these calculations have in common is that they assume a de-localized projectile wave, i.e. a coherent projectile beam. This is an unrealistic assumption for fast ion impact since there the projectile wave packet usually has a width which is negligible compared to the size of the target atom. Here, we demonstrate that cross sections for atomic fragmentation processes can sensitively depend on the projectile coherence. We measured momentum-analyzed scattered projectiles in coincidence with the recoiling target ions for ionization in 75 keV p + H$_{2}$ collisions. From the data we extracted double differential cross sections (DDCS) for a projectile energy loss of $\varepsilon $ = 30 eV as a function of scattering angle $\theta $. The width of the projectile wave packet (i.e. the transverse coherence length $\Delta $r) is proportional to L$\lambda $/a, where L is the distance between the collimating slit and the target region, a is the slit width, and $\lambda $ the DeBroglie wave length of the projectile. The experiment was performed for L$_{1}$=50 cm and L$_{2}$=6.5 cm, which for a=0.15 mm corresponds to $\Delta $r $\approx $ 2 a.u. and 0.3 a.u., respectively [3]. In the DDCS for L$_{1}$ we observe a pronounced interference structure, which is absent for L$_{2}$. The interference is due to indistinguishable diffraction of the projectile wave from the two atomic centers in the molecule. However, it can only occur if the projectile wave packet is wide enough to illuminate both atomic centers simultaneously, i.e. if $\Delta $r$>$D (inter-nuclear separation). This explains why the interference is absent for L$_{2}$ since there $\Delta $r$<$D. These findings we recently confirmed by equivalent data for capture in p + H$_{2}$ collisions. We thus have to conclude that it is crucially important to properly account for the projectile coherence length in theoretical calculations. For atomic targets the unrealistic assumption of a coherent beam probably results in artificial path interference between two impact parameters leading to the same scattering angle. This could quite possibly explain the theoretical difficulties in reproducing measured FDCS for single ionization by fast ion impact [1]. \\[4pt] [1] M. Schulz et al., Nature \underline {422}\textbf{,} 48~(2003) \\[0pt] [2] M. Schulz et al., Phys. Rev. \underline {A76}, 032712 (2007) \\[0pt] [3] K. Egodapitiya et al., PRL \underline {106}, 153202 (2011) [Preview Abstract] |
Tuesday, November 15, 2011 2:30PM - 2:45PM |
ET3.00002: STUDENT AWARD FINALIST: Fully differential cross section for four-body charge transfer process Uttam Chowdhury, Allison Harris, Jerry Peacher, Don Madison Recently experimental fully differential cross sections (FDCS) have been reported for double capture, single capture and transfer excitation in proton helium collisions. In case of double capture, the proton captures both of the electrons from helium and leaves the collision as a H- ion. For single capture, the proton captures one electron from helium and leaves the other electron in the ground state. For the transfer excitation, it is similar to single capture except the target is excited to arbitary excited states. Recently experiments were performed for proton energies ranging from 25keV to 300keV. We introduce here a theoretical model for charge transfer processes which is fully quantum mechanical and takes all post collision interactions (PCI) between the particles into account exactly. Numerically, this requires a full nine-dimensional integral which is computationally expensive. The theoretical results will be compared with absolute experimental measurements. [Preview Abstract] |
Tuesday, November 15, 2011 2:45PM - 3:00PM |
ET3.00003: Multiple Differential Cross Sections for Dissociative Ionization of Molecular Hydrogen by 75 keV Proton Impact Kisra Egodapitiya, Sachin Sharma, Aaron Laforge, Michael Schulz, Ahmad Hasan Double differential cross sections (DDCS) were measured for fixed projectile energy losses as a function of the scattering angle for ionization accompanied by dissociation of molecular hydrogen by 75 keV proton impact. Earlier DDCS for non dissociative ionization have been measured. There, an oscillating pattern in DDCS was observed. We recently demonstrated that for such an oscillating pattern to be present the width of the projectile wave packet must be larger than the atomic separation of the hydrogen molecule, i.e. the projectile beam must be coherent. This particular experiment was also carried under similar conditions and indeed an oscillating pattern was observed. However when compared with the non-dissociative case, doubling of the oscillation frequency was observed. Possible causes for these will be discussed. [Preview Abstract] |
Tuesday, November 15, 2011 3:00PM - 3:30PM |
ET3.00004: Multiple Scattering Effects in Ionization Processes Invited Speaker: The momentum distributions of electrons emitted in the ionization of atoms and molecules by the impact of photons or massive particles usually present interference patterns similar to those of the demonstrations with light proposed by Thomas Young more than two centuries ago. Furthermore, these cross sections also display richer structures due to the same multiple-scattering effects that are at the origin of different techniques to probe atomic aggregates and solid samples. In this talk, I will review these effects and discuss some of their most important characteristics, showing that they lead to distortions that are not fully replicated by non-scattering or even single-scattering approximations. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700