Photodetachment spectroscopy from cooled negative ions

1. Photodetachment spectroscopy from cooled negative ions Summer research in the AMO lab* June – August 2005 James Wells * Support from Davidson College and the American Chemical Society

2. - - - - - + + - - - - Photodetachment - • X- + photon → X + e- • Equivalent to latter half of an electron-atom collision.

3. Effects of ions’ random motion • Photon frequency is Doppler broadened • Causes uncertainty ΔE in any energy-dependent measurement • Typical experimental goal: measure probability of detachment as f(Ephoton) • ΔE blurs experimental results: fewer details, less contrast/structure.

4. Evaporative cooling • Ions trapped in an ion trap: electrostatic potential well. • Cooling applet

5. Ion trap apparatus

6. Ring dye laser

7. Laser LabVIEW control code (Screen shot)

9. - - - - - + - + - - - - Negative Ion Formation • Short-range attractive potential (~ 2 eV deep and a few Å wide) • Electron correlation effects – partly responsible for covalent bonds

10. Energy Levels (Oxygen)

11. Photodetachment with B-Fields • departing electron executes cyclotron motion in field • motion in plane perpendicular to B is quantized to cyclotron levels • cyclotron states separated by ω = eB/me • motion along axis of field is continuous, non-quantized • for typical B = 1.0 Tesla, ω ≈ 30 GHz, period = 36 ps • quantized Landau levels add structure to detachment cross section

12. Trap electronics

13. Detachment cross section in B field