CESIUM ELECTRON IMPACT CROSS SECTIONS

1
CESIUM ELECTRON IMPACT CROSS SECTIONS
M. Lukomski1, J.A. MacAskill1, S. Sutton1, W.
Kedzierski1, T.J. Reddish1, J.W. McConkey1, P.L.
Bartlett2, I. Bray2, A.T. Stelbovics2, P.L.
Bartlett2, K. Bartschat3
1 Department of Physics, University of Windsor,
Ontario N9B 3P4, Canada 2 The ARC Centre for
Antimatter-Matter Studies, Murdoch University,
Perth 6150, Australia 3 Department of Physics,
Drake University, Des Moines, Iowa, 50311, USA
ELECTRON - MOT SPECTROSCOPY ( TRAP-LOSS TECHNIQUE)
Absolute electron impact cross sections are
measured using a magnet-optical trap (MOT) by
employing the trap-loss technique. This method
monitors the fluorescence decays of the trapped
atoms, with and without an electron beam present.
The loss rate of atoms from the trap due to
electron collisions, ?e, is related directly to
the cross section, ?, and electron flux, J,
through the trap. Hence measurements of ?e and J
yield ? directly knowledge of the absolute
target density is not required. We have used this
approach to measure absolute total cross sections
for both the ground, 62S1/2 and first excited
state, 62P3/2 of Cesium. By then altering the
timing sequence for the
pulsed magnetic field and electron beam, together
with the trapping and repumping lasers, we have
been able to determine the loss of atoms from the
trap due solely to ionization.
?e Ejection rate due to electron collisions?e
Electron scattering cross sectionJe
Electron current densitye Electron charge
COOLING and TRAPPING
Our conventional MOT creates optical molasses
using IR laser light that is slightly red detuned
(19MHz) to the cesium 62S1/2(F 4) ? 62P3/2(F
5) hyperfine transition. Three pairs (incident
and retroflected) of orthogonal lasers, 17mm in
diameter, are used so that within their overlap
region the atoms are cooled in all directions. A
second repumping laser is required to pump the
electrons out of the 62S1/2(F3) dark ground
state, so that the cooling/trapping laser can
access these atoms. If the repumper is turned
off the trap disappears in lt500 ns, as all the
atoms fall to the F 3 ground state, and cannot
partake in the cooling sequence. A spherical
quadrupole magnetic field is used to provide a
position-dependent force which, in conjunction
with the circularly polarised trapping laser
beams, causes the atoms to be pushed towards the
trap centre. Relevant Zeeman sub-levels are shown
in the simplified diagram for a two level atom
moving in the direction of increasing z, away
from the trap centre. The B field was produced by
a pair of in-vacuum
ELECTRON BEAM PRODUCTION PROFILING
A 7-element electron gun with a BaO disc cathode
is used to produce a near parallel 10 mm
diameter electron beam of uniform current
density over the entire 7-400 eV energy range.
Two orthogonal, thin wire probes are used to
measure the spatial current distribution of the
beam. These 0.010 inch diameter wire probes are
micrometer-driven on linear motion feedthroughs
and are arranged to both intersect the electron
beam at right angles and pass through the trap
centre. During trapping they are retracted from
the laser paths. The e-beam energy was calibrated
by detecting the threshold production of He ions
using a time-of-flight system.
Fig. 4electron gun
Fig. 7electron beam probes
Fig. 5electron beam simulation
Fig. 6Typical beam profiles
anti-Helmholtz coils, each of 40 turns, 3 cm in
radius and separated by 4.5 cm, and whose
centre coincides with that of the bichromatic
orthogonal laser beams. The B field gradient is
approximately 10 G/cm in the horizontal plane and
20 G/cm in the vertical plane.
Fig. 21D MOT for asimple two level atom

Fig. 3B - field contours
CROSS SECTION RESULTS
TIMING SCHEMES AND TRAP FLUORESCENCE MEAUSREMENTS
Total Cross Section for Electron Scattering from
Cesium Theoretical and Experimental Results
(62P3/2 excited state)
Total Cross Section for Electron Scattering from
Cesium Theoretical and Experimental Results
(62S1/2 ground state)
Fig. 9Typical scan and measured loses
Fig. 8Beam pulsing arrangement for 62S total ?
measurements
Fig. 10Beam pulsing arrangement for ionisation
measurements
Fig. 11Typical scan and measured loses
The ratios of the 2P / 2S ionization cross
sections for Cesium and Rubidium
The measured TICS out of the Cs 6 2P3/2 state
compared to SICS from CCC, RMPS and Born
calculations
ExCITED STATE POPULATION
In the ionisation experiment, the atoms in the
trap are in a mixture of 62S1/2 and 62P3/2
states, as both trapping and repumping lasers are
present in the electron-atom interaction. The
excited state
fraction, ?e, can be estimated using the given
two level atom approximation. We used a Pockels
cell to rapidly rotate the polarization of the
trapping laser beam to control the intensity
while maintaining a constant number of atoms in
the trap and a constant detuning ? ( 19 MHz).
From this method we estimate the trap to contain
26(1) of excited 62P3/2 state cesium, with the
remaining 74 in the 62S1/2 ground state.
? 32.7686 MHz I Laser intensity, Is
Saturation Intensity
ACKNOWLEDGEMENTS
SUMMARY AND CONCLUSIONS

• A broad 7- 400eV incident electron energy range
  is covered in these experiments. Excellent
  agreement is found for total electron scattering
  cross sections from 62S Cs with earlier
  experimental work obtained by very different
  methods. CCC and RMPS calculations appear to
  overestimate ? below 75 eV.
• A novel feature of our MOT method is in
  utilizing Doppler cooling of neutral atoms to
  determine total ionization cross sections (TICS)
  via fluorescence-monitoring techniques.
• The trap contained a known mixture of cesium
  atoms of in the 62S1/2 ground and 62P3/2 excited
  states. From subsequent data analysis, the TICS
  out of excited Cs atoms was obtained.
• We demonstrate that autoionisation, core and
  multiple ionization make a significant
  contributions to the ground and excited state
  TICS. We also identify a significant and as yet
  unexplainable discrepancy between theory and
  experiment below 12eV in the excited state
  ionisation cross section.

PUBLICATIONS
1 J. A. MacAskill, W Kedzierski, J W McConkey,
J Domyslawska, and I Bray, J Elect Spect Rel Phen
, 123, 173, (2002). 2 M. Lukomski, J. A.
MacAskill, D P Seccombe, C McGrath, S Sutton, J
Teeuwen, W Kedzierski, T J Reddish, J W
McConkey, and W A van Wijngaarden, J. Phys. B. 38
3535 (2005) 3 M. Lukomski, S. Sutton, W.
Kedzierski, T.J. Reddish, K. Bartschat, P.L.
Bartlett, I. Bray, A.T. Stelbovics and J.W.
McConkey, Phys Rev. A. (submitted 2006)
www.uwindsor.ca/physics/atomic