1. Measurement of Binding Energy of Negative Gallium Ion using Infrared Photodetachment Threshold
Spectroscopy
Wae Nakayama, Achbold Sergelen Jugdersuren
Denison University Department of Physics and Astronomy
Research Advisor: C. Wesley Walter & N. Daniel Gibson
Introduction
What is Photodetachment?
Photodetachment refers to a process where a photon is absorbed by a
negative ion to knock off an electron.
Project Methodology
Ions are generated by a Cs sputter source and then mass selected by a
magnetic field. After the selected ions enter the interaction region, the
wavelength of a tunable pulsed laser Optical Parametric Oscillator (OPO)
beam is scanned across a range around the thresholds to knock off the
electrons from the ions. Ions which are photodetached turn into neutral
atoms that are sensed by a neutral atom detector at the end.
Figure 2. Diagram of the experimental apparatus
Results
We observed thresholds at 277.9 (3) meV, 300.9 (3) meV, 341.2 (3) meV, each
corresponding to the Ga- (4p2 3P1) to Ga (4p2 2P1/2) transition, the Ga- (4p2 3P0) to Ga (4p
2P1/2) transition, and the Ga- (4p2 3P2) to Ga (4p 2P3/2) transition. We obtained precise
measurements that support the theoretical values but significantly revise the previous
experimental value.
Figure 6. Red line is fit of the s-wave Wigner Law with d-wave and p-wave leading term correction.
Discussion
A simulation is conducted using the Wigner threshold law with
leading term correction (Farley, 1989) and calculated relative
intensity of transitions (Engelking, 1979). The following graph
shows a good agreement between the experimental data and the
simulation.
Wigner’s threshold law: 𝜎∼(E-ET) 𝓁+½
(𝜎: cross section E: photon energy ET: threshold energy ℓ: angular momentum)
Figure 7. Comparison between experimental data and simulation
Figure 8. Current result compared with previous experiments and theory
Acknowledgements
• National Science Foundation
• Anderson Summer Research Endowment
• Dave Burdick
Works Cited
1. Pegg, D. J. (2004). Structure and dynamics of negative ions. Rep. Prog. Phys. Reports on
Progress in Physics, 67(6), 857-905.
2. Farley, J. W. (1989). Photodetachment cross sections of negative ions: The range of
validity of the Wigner threshold law. Phys. Rev. A Physical Review A, 40(11), 6286-6292.
3. Engelking, P. C., & Lineberger, W. C. (1979). Laser photoelectron spectrometry of Fe − :
The electron affinity of iron and the "nonstatistical" fine-structure detachment intensities
at 488 nm. Phys. Rev. A Physical Review A, 19(1), 149-155.
4. Sundholm, D. et al., J.Phys. B 32,5853 (1999).
5. Williams, W. W. , et al. J. Phys. B 31, L341 (1998).
Figure 1. Energy diagram of Gallium Ion
Why Gallium?
• Important semiconductor element for
electronics and industrial applications
• Large disagreement for the electron
affinity (the amount of energy required
to add an electron to a neutral atom to
form a negative ion) between
theoretical values and previous
experimental results
Why Negative Ions?
• A negative ion refers to an atom possessing an extra electron.
• Due to protective screening of the nucleus by the atomic electrons,
negative ions show intensified sensitivity to correlation.
• Since negative ions show the interactions between electrons
around the nucleus, they are broadly studied and well suited for us
to develop a deeper understanding of the importance of electron
correlation in the structure and dynamics of many-electron
systems. (Pegg, 2004)
AutoBoltz
Figure 4. A picture of AutoBoltz
Figure 5. A diagram of AutoBoltz
This system, named after
Ludwig Boltzmann, one of
the faces of
thermodynamics, was
developed to automatically
control the intensity by the
rotation of a polarizing filter.
The key components are a
stepper motor controlled by
a driver, a lacrosse ball that
uses friction to rotate the
polarizer, and a smart
computer algorithm (in
LabView) that takes in
desired laser pulse energy
range and continuously
assesses whether to move or
not. AutoBoltz maximizes
the data collection efficiency.
Figure 3. Algorithm of the program
running AutoBoltz