Transcript poster - atomic physics

Photo double ionization of fixed in Space Hydrogen
Th. Weber (a), R. Doerner (a), A. Czasch (a), A. Landers (b), T. Osipov (c), L. Cocke (c), O. Jagutzki (a), M. Prior (d), H. Schmidt-Boecking (a)
excess
ion energy
photon
energy
binding
energy
a) Institut fuer Kernphysik Frankfurt, Frankfurt am Main, Germany
b) Department of Physics, Western Michigan University, Kalamazoo, USA
c) Department of Physics, Kansas State University, Manhattan, USA
d) Lawrence Berkeley National Laboratory, Berkeley, USA
[email protected] / hsb.uni-frankfurt.de
Technical features of the experimental setup:
4 p solid angle
pulsed extraction for the recoil-ions:
3 V/cm
electron
double hit
60 V/cm
recoil-ion
double hit
Magnetic field to guide the electrons
• electrons up to 100 eV (for 10 Gauss)
Time of Flight and 2dim position
• 3dim momentum vector
High resolution for 0 eV
• < 10 meV electronic energy
electric field
magnetic field
HITEC
powered by
AOC & ROENTDEK
Using a magnetic field in order to prevent electrons leaving the
spectrometer, the electrons were spiraled up. These wiggles are a
good tool to determine the magnitude of the magnetic field. At the
same time they help calibrating the time of flight direction (see figure
above).
One of the central questions of today's atomic
physics concerns the dynamic-electron-electronDL80anode:
correlation of many electron systems. Stationary
many body systems are already examined in atomic
physics successfully with high precision and are
Dt < 100ns
theoretically very well described. However dynamical
multi-particle processes are not understood very well
today.
Electron and Recoil Ion Momen-tum Spectroscopy
was used in order to image the photo double
ionization of hydrogen (see figure above). The
electrons and the ions were guided on position
sensitive detectors applying an electrostatic and
magnetic field. In order to gain resolution for the
electrons the electric extraction field for the ions was
pulsed. From Time of Flight (TOF) and the position
on the detectors the momentum vector of all four
outgoing particles could be determined.
Dt < 8ns
See figure to the left: New kind of delay line anodes for the position
readout of Multi Channel Plate (MCP) detectors have been developed
in order to reduce multihit deadtime problems (Hexanode). Using three
layers instead of two, the spatial distance of two hits on the detector as
a function of time difference could be reduced to a circle with 10 mm in
diameter. For the common two layer anodes like the DL80 the blind
zone has a cross like shape dividing the detector by two perpendicular
lines with 10 mm strength (see upper row).
Dt < 10ns
Hexanode:
,
equal
energy
sharing
Helium
all orientations
H2
Measuring the outgoing momentum vectors of the two running out
cores in the final state of the reaction one can conclude directly to the
spatial orientation of the two centers of the molecule at the time of the
photoabsorption. The momentum vectors of the two electrons
represent the square of the wave function in the final state.
e1
See figure to the right:
While discerning the different molecular
orientations one can see that the
alignment of the polarization vector and
the internuclear axis in parallel results in
much more contribution along the nodal
axis (blue and red dashed lines on the
right) in proportion to the interval of polar
angles allowed in the case of helium.
H2
e
It was the aim to determine Triple Differential Cross Sections (TDCS) for a fixed in space electron
relative to the molecular axis. The intensity and the angular correlation can give explanation about
the influence of rotation statuses and about possible interference effects of the molecule. The nodes
in the distributions, which can be expected, can give a hint of the importance of symmetries (bparameter) in the molecule in contrast to the observed helium atom:
See figure above: Even when integrating over all molecular orientations, below an emission angle of
45 degree versus the polarization vector of electron 1, the polar angular distribution of electron 2
show distinct differences in comparison to helium. For the case of 25 degree one can see
contributions along the blue and red lines (lower row on the right) representing the nodes in case of
helium. Increasing the angle of electron 1 versus the polarization vector these contributions vanish
(not shown here). In order to survey the influence of the molecular orientation, for 25 degree the
distribution is split into three scenarios showing the results for hydrogen aligned parallel and
perpendicular to the polarization vector.
Forcing the second electron to be emitted in the molecular plane,
defined by the inter-nuclear axis and the polarization vector, one
can probe the molecule. Electron-electron “correlation” has been
switched off by orientating the first
electron rectangular to the molecular
plane. While varying the energy sharing
of the electrons the distribution changes
due to angular momentum transfer and
the influence of the polarization.
kin. energy of the nuclei
...which orientation is relevant ?
e1
H2
e1
e1+2
e2
H2
equal
energy
sharing
No angular
momentum
transfer
See left side: The
photo double ionization of hydrogen results in four free particles in the final
state. Which coordinates are relevant to
discover the interaction between the electrons and the protons ? The distributions are sensitive to
angular momentum
transfer (see figure to
the left - the lower
row is generated by
using Jacobi coordinates).
Scanning the sum energy of
the ions (e.g. the coulomb
explosion; see the inset in
the figures to the right), one
even can probe vibrational
states of the hydrogen molecule at the time of photoionization:
In terms of Legendre Polynoms, for the case
aligning the molecule perpendicular to the
polarization vector, one can see the angular
distribution of electron 2 changing from d- to f
like shape. This scenario shows a different
evolution for unequal energy sharing (not
shown here).
In the energy
spectrum the repulsive curve of
the H++H+state
maps directly the
initial state into
the continuum.
Going vice versa
is shown to the
left: