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
Photodetachment
-
-
-
+
-
Detachment cross section in B field
-
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
1.1
e-
1.0
• Departing electron executes cyclotron motion in field.
• Motion in plane perpendicular to B is quantized to
Landau levels separated by cyclotron ω = eB/me.
• For typical B = 1.0 Tesla, ω ≈ 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.
Fraction of ions surviving
•
+ photon → X +
• 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.
X-
Without E field
With E field (14 V/cm)
0.9
0.8
0.7
0.6
0.5
Motivation
0.4
• 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.
-20
0
20
40
60
80
Relative frequency (GHz)
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).
• 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.
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.
Experimental technique
• Observations are consistent with theory predictions, but more detailed observations/analysis are needed.
• Ions produced by dissociative attachment from a carrier gas, using hot tungsten filament.
• 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.
Active Layer
• 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.
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?
Apparatus
• 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?
Diode seed
MOPA:
• Replace hot tungsten filament with cold field-emission electron source to reduce further the trapped ion
population temperature.
Diode amplifier
250 mW single-mode tunable
• Possible time-domain Ramsey interferometry of cyclotron wavepackets, with and without electric fields.
Wavemeter
to 0.02 cm-1
• Possible investigations with THz radiation: momentum kick given to electron by a half-cycle pulse may
yield further insight into the detached electron’s interaction with the neutral core.
Spectrum
Analyzer
8 GHz FSR
References
Ion
trap
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.
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.
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Acknowledgements
This work has been supported by:
• Research Corporation
• Davidson College
• University of Virginia
• John D. and Catherine T. MacArthur Foundation