Photodetachment of the negative oxygen atom was observed.

1. 861.5 nm 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 π 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. References [1] C. Blondel, W. Chaibi, C. Delsart, and C. Drag, J. Phys. B. 39, 1409 (2006). [2] C. Blondel, C. Delsart, C. Valli, S. Yiou, M.R. Godefroid and S. Van Eck, Phys. Rev. A. 64, 052504 (2001). [3] P. R. Bevington, Data Reduction and Error Analysis for the Physical Sciences(McGraw-Hill, New York, 1969). [4] W. A. M. Blumberg, “Laser Photodetachment Spectroscopy of Negative Ions in a Magnetic Field” (PhD Thesis, Harvard University, 1979). [5] W. A. M. Blumberg, R. M. Jopson, and D. J. Larson, Phys. Rev. Lett. 40, 1320 (1978). [6] W. A. M. Blumberg, W. M. Itano, and D. J. Larson, Phys. Rev. A. 19, 139 (1979). [7] C. W. Clark, Phys. Rev. A. 28, 83 (1983). [8] H. G. Dehmelt and F. L. Walls, Phys. Rev. Lett. 21, 127 (1968). [9] D. J. Griffiths, Introduction to Quantum Mechanics 2nd Ed (Pearson Education, Upper Saddle River, NJ, 2005). [10] C. Heinemann, W. Koch, G. G. Lindner and D. Reinen, Phys. Rev. A. 52, 1024 (1995). [11] G. Herzberg, Atomic Spectra and Atomic Structure (Dover Publications, New York, 1945). [12] A. K. Langworthy, D. M. Pendergrast, and J. N. Yukich, Phys. Rev. A. 69, 025401 (2004). [13] D. J. Larson and R. Stoneman, J. de Physique. 43, C285 (1982). [14] D. J. Larson and R. Stoneman, Phys. Rev. A. 31, 2210 (1985). [15] D. M. Pendergrast and J. N. Yukich, Phys. Rev. A. 67, 062721 (2003). [16] B. M. Smirnov, Physics of Atoms and Ions (Springer, New York, 2003). [17] E. P. Wigner, Phys. Rev. 73, 1002 (1948). [18] J. N. Yukich, “Electron Wave Packets and Ramsey Interference in a Magnetic Field” (PhD Thesis, University of Virginia, 1996). [19] J. N. Yukich, C. T. Butler, and D. J. Larson, Phys. Rev. A. 55, 3303 (1997). [20] J. N. Yukich, T. Kramer, and C. Bracher, Phys. Rev. A. 68, 033412 (2003). Acknowledgements Thanks to Dr. John Yukich and the Davidson Physics Department