1
Photodetachment in Parallel Electric and Magnetic
Fields
J.N. Yukich, Davidson College, Davidson, North
Carolina
Abstract
We investigate photodetachment from negative ions
in a homogeneous 1.0 Tesla magnetic field and a
parallel electric field of 15 V/cm.
Calculations show that an electric field of 10
V/cm or more should considerably diminish the
Landau structure in the detachment cross
section.8 The ions are produced and stored in a
Penning ion trap. We present preliminary results
showing roughly a 30 decrease in the modulation
at the first Landau level with addition of the
electric field. We also discuss future
experiments.
Background
Detachment in Magnetic Fields
Ion trap apparatus, showing UHV vacuum, 2.0
Tesla electromagnet and magnet power supply.
Optical apparatus, showing diode laser MOPA
in foreground and wavemeter electro-optics.
Preliminary data

• X- photon ? X e-
• Considered as ½ of an electron-atom collision.
• Minimum energy needed to detach is called the
• electron affinity, analogous to photoelectric
  effect.
• Electron detaches as plane wave into continuum.

• Departing electron executes cyclotron motion in
  field.
• Motion in plane perpendicular to B is quantized
  to
• Landau levels separated by cyclotron ?
  eB/me.
• For typical B 1.0 Tesla, ? 30 GHz, period
  36 ps.
• Electron revisits atomic core once every
  cyclotron period.
• Motion along axis of field is continuous,
  non-quantized.
• Quantized Landau levels add structure to
  detachment
• cross section. Structure results from electron
  wave
• function interfering with itself as it revisits
  core.

Motivation

• Photodetachment in combined E, B fields has
  received extensive theoretical attention, but
  little experimental attention.8-12
• Effect of a parallel E field pushes the
  electron away from the atomic core as it executes
  cyclotron motion diminishes
• or eliminates the wave function interference,
  and thus the Landau structure in the cross
  section.
• Similar effect found with motional Stark field
  of a thermally energetic ion. Such fields
  diminish resolution of magnetic
• field structure and spectroscopy.
• Calculations (both full quantum-mechanical and
  semi-classical) predict 10 V/cm parallel to 1.0
  Tesla should
• considerably diminish Landau structure, 30
  V/cm should almost completely eliminate Landau
  structure.

Preliminary data showing ratio of S- ions
surviving laser illumination near the 2P3/2 ?
3P2 threshold (electron affinity). Structure
observed at the first Landau level is diminished
when the electric field is added (B 1.0 Tesla).
 
 
Conclusions

• Modulation structure at the first Landau level
  observed to be diminished by 30 when electric
  field
• of 14 V/cm is added parallel to the 1.0 Tesla
  field.
• Observations are consistent with theory
  predictions, but more detailed observations/analys
  is are needed.
• Motional Stark fields present in the ion trap (
  8 V/cm) may play a significant role in
  diminishing
• magnetic field structure in the detachment
  cross section.

Experimental technique

• Ions produced by dissociative attachment from a
  carrier gas, using hot tungsten filament.
• Ions trapped and stored in Penning ion trap (see
  figures below), with B 1.0 Tesla.3
• Relative detachment cross section probed with
  highly-tunable, single-mode, amplified diode
  laser (see MOPA below).
• Parallel electric field achieved by
  superimposing a 1 MHz radio frequency on the
  trap endcaps. On time scale of one
• cyclotron period, electric field appears
  static to the ions.

Active Layer
Future Work

• Current and future work will investigate
  identical phenomena in O-, which is easily
  accessible with the
• diode laser MOPA.
• To investigate What happens at higher E fields?
  What field is necessary to completely eliminate
  the
• magnetic field structure? How is this
  condition approached with increasing electric
  field?
• Evaporative cooling of trapped ion population
  does a reduced motional Stark effect enhance the
  magnetic
• field structure? Can we improve spectroscopic
  resolution of Landau levels?
• Replace hot tungsten filament with cold
  field-emission electron source to reduce further
  the trapped ion
• population temperature.
• Possible time-domain Ramsey interferometry of
  cyclotron wavepackets, with and without electric
  fields.
• Possible investigations with THz radiation
  momentum kick given to electron by a half-cycle
  pulse may
• yield further insight into the detached
  electrons interaction with the neutral core.

Apparatus
References
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• Diode laser master oscillator power amplifier
• Commercial diode laser seeds a high-gain tapered
  diode amplifier.
• Highly-tunable, single-mode output.
• Monitored by Fabry-Perot spectrum analyzer
  measured by
• traveling Michelson-interferometer wavemeter.

• Penning ion trap system
• Trap consists of three hyperbolic electrodes
  coaxial with B field.
• Biased trap endcaps form nearly-harmonic axial
  potential well.
• Heterodyne detection system measures relative
  trapped ion
• population before and after laser
  illumination.

Acknowledgements

• This work has been supported by
• Research Corporation
• Davidson College
• University of Virginia
• John D. and Catherine T. MacArthur Foundation