Photodetachment of the negative oxygen atom was observed.

1
Photodetachment Spectroscopy at the lowest O- ion
threshold
Robert Mohr, Davidson College, Davidson, North
Carolina
Abstract
Photodetachment from the negative oxygen ion in a
magnetic field is a well-studied phenomenon at
the transition known as the electron affinity.
However, the goal of this work is to study the
spectroscopy of the lowest energy detachment
transition, which occurs approximately 20 meV
below the electron affinity. A Penning ion trap
was used to trap the ions and photodetachment was
achieved using a continuous wave tunable diode
laser. High-resolution spectroscopy has allowed
us to resolve the energy of the lowest detachment
threshold.
Background

• The negative oxygen ion has 2 bound states
  2P3/2 and 2P1/2
• The ground state of the neutral oxygen atom is
  part of an inverted triplet 3P2 (lowest state),
  3P1, 3P0.

Computer
The electronics used to detect the image current
produced by the oscillating oxygen ion cloud.
848.5 nm
Results
Zeeman Effect

• In the absence of a magnetic field, the energy
  levels which make up the 2P1/2 and 3P2 states are
  degenerate.
• In the presence of an external magnetic field the
  2P1/2 and 3P2 states split into levels.
• Instead of a single transition there are several
  transitions between these states. Which
  transitions are allowed is determined by
  conservation of momentum.

Apparatus
A plot showing the fraction of ions surviving
detachment as a function of photon energy. The
threshold transition can be observed
approximately around a photon energy of 11607.75
cm-1.

• The ions are held in a Penning ion trap located
  in an ultra high vacuum (UHV) at a pressure of 6
  x 10-8 Torr.
• A continuous wave tunable diode laser is used to
  provide photons for photodetachment. Light with p
  polarization was used in this investigation.
• The fraction of ions surviving detachment can be
  measured by using a radio frequency (RF)
  potential to cause the ion ensemble to oscillate
  in the trap. This generates an image current
  which can be measured and compared to the
  pre-detachment current magnitude to find the
  fraction surviving.
• A photodiode allows the same amount of light of
  to be used for each run.

A second scan showing the fraction of ions
surviving detachment as a function of photon
energy. The threshold transition here can be
observed around a photon energy of 11607.80 cm-1.
Conclusions

• Photodetachment of the negative oxygen atom was
  observed.
• Further work will permit a precision measurement
  of the lowest threshold energy.
• The magnetic Zeeman transitions were, contrary to
  what was predicted, not observed.

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Acknowledgements
Thanks to Dr. John Yukich and the Davidson
Physics Department