Transcript Document

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 wellstudied 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),
3P , 3P .
1
0
861.5 nm
Computer
The electronics used to detect the image current produced by the oscillating oxygen ion cloud.
848.5 nm
Results
Scan 6: 8/9/07
10500
Zeeman Effect
10000
9500
R/R
•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.
9000
8500
Apparatus
8000
11606
11607
11608
11609
11610
11611
photon energy (cm-1)
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.
Scan 5: 8/7/07
10500
10000
R/R
9500
•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.
9000
• A continuous wave tunable diode laser is used to provide photons for
photodetachment. Light with π polarization was used in this investigation.
8500
•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 predetachment current magnitude to find the fraction surviving.
8000
11606
11607
11608
11609
11610
11611
photon energy (cm-1)
• 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