Photodetachment Photodetachment spectroscopy from cooled spectroscopy from cooled

negative ionsnegative ions

James WellsJames Wells

Summer research in the AMO lab*June – August 2005

* Support from Davidson College and the American Chemical Society

PhotodetachmentPhotodetachment

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• X- + photon → X + e-

• Equivalent to latter half of an electron-atom collision.

Effects of ions’ random motionEffects of ions’ random motion

Photon frequency is Doppler Photon frequency is Doppler broadenedbroadened Causes uncertainty Causes uncertainty ΔΔE in any energy-E in any energy-

dependent measurementdependent measurement Typical experimental goal: measure Typical experimental goal: measure

probability of detachment as f(Eprobability of detachment as f(Ephotonphoton)) ΔΔE blurs experimental results: fewer E blurs experimental results: fewer

details, less contrast/structure.details, less contrast/structure.

Evaporative coolingEvaporative cooling

Ions trapped in an ion trap: electrostatic Ions trapped in an ion trap: electrostatic potential well.potential well.

Cooling appletCooling applet

Ion trap apparatusIon trap apparatus

Ring dye laserRing dye laser

Laser LabVIEW control codeLaser LabVIEW control code

(Screen shot)

Negative Ion FormationNegative Ion Formation

• Short-range attractive potential (~ 2 eV deep and a few Å wide)

• Electron correlation effects – partly responsible for covalent bonds

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Energy Levels (Oxygen)Energy Levels (Oxygen)

Photodetachment with B-FieldsPhotodetachment 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

Trap electronicsTrap electronics

Detachment cross section in B Detachment cross section in B fieldfield