#
64th Annual Gaseous Electronics Conference

## Volume 56, Number 15

##
Monday–Friday, November 14–18, 2011;
Salt Lake City, Utah

### Session ET3: Heavy-Particle Collisions

2:00 PM–3:30 PM,
Tuesday, November 15, 2011

Room: 255F

Chair: Tom Kirchner, York University

Abstract ID: BAPS.2011.GEC.ET3.1

### Abstract: ET3.00001 : Manipulating Atomic Fragmentation Processes by Controlling the Projectile Coherence

2:00 PM–2:30 PM

Preview Abstract
Abstract

####
Author:

Michael Schulz

(Missouri University of Science \& Technology)

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)

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2011.GEC.ET3.1