Conclusions

1. 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 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 • X- + photon → X + e- • 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. • 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. Motivation • 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. • 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. 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). 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. • Observations are consistent with theory predictions, but more detailed observations/analysis are needed. • 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. Experimental technique • Ions produced by dissociative attachment from a carrier gas, using hot tungsten filament. • 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. Active Layer 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? • 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? • Replace hot tungsten filament with cold field-emission electron source to reduce further the trapped ion • population temperature. • Possible time-domain Ramsey interferometry of cyclotron wavepackets, with and without electric fields. • 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. Apparatus References 1. M.C. Baruch, W.G. Sturrus, N.D. Gibson, and D.J. Larson, Phys. Rev. A 45, 2825 (1992); N.D. Gibson, B.J. Davies, and D.J Larson, Phys. Rev. A 48, 310 (1993); M.C. Baruch, T.F. Gallagher, D.J. Larson, Phys. Rev. Lett. 65, 1336 (1990). 2. C.H. Bryant et al, Phys. Rev. Lett. 58, 2412 (1987).  3. W.A.M. Blumberg, R.M. Jopson, D.J. Larson, Phys. Rev. Lett. 40, 1320 (1978); W.A.M. Blumberg, W.M. Itano, D.J. Larson, Phys. Rev. A 19, 139 (1979).  4. I. Yu. Kiyan and D.J. Larson, Phys. Rev. Lett. 73, 943 (1994); J.N. Yukich, C.T. Butler, and D.J. Larson, Phys. Rev. A 55, 3303 (1997).   5. H.F. Krause, Phys. Rev. Lett. 64 1725 (1990). 6. C.H. Greene, Phys. Rev. A 36, 4236 (1987), H. Crawford, Phys. Rev. A 37, 2432 (1988). 7. M.L. Du and J.B. Delos, Phys. Rev. A 38, 5609 (1988). 8. M.L. Du, Phys. Rev. A 40, 1330 (1989); I.I. Fabrikant, Phys. Rev. A 43, 258 (1991). 9. Q. Wang and A.F. Starace, Phys. Rev. A 48, R1741 (1993); Q. Wang and A.F. Starace, Phys. Rev. A 51, 1260 (1995); Q. Wang and A.F. Starace, Phys. Rev. A 55, 815 (1997). 10. A.D. Peters, C. Jaffe, and J.B. Delos, Phys. Rev. Lett. 73, 2825 (1994); A.D. Peters, C. Jaffe, and J.B. Delos, Phys. Rev. A 56, 331 (1997).  11. Z.Y. Liu and D.H. Wang, Phys. Rev. A 55, 4605 (1997); Z.Y. Liu and D.H. Wang, Phys. Rev. A 56, 2670 (1997).  12. M.L. Du and J.B. Delos, Phys. Rev. Lett. 58, 1731 (1987); M.L. Du and J.B. Delos, Phys. Rev. A 38, 1931 (1988); W.P. Reinhardt, J. Phys. B 16, L635 (1983). • 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. • 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. Acknowledgements • This work has been supported by: • Research Corporation • Davidson College • University of Virginia • John D. and Catherine T. MacArthur Foundation