Optical and NIR Photodetachment Spectroscopy

in External Fields

Charlotte Chapter of the OSAMarch 15, 2001

John YukichDavidson College

Department of Physics

Negative Ion Formation

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• Short-range attractive potential ( ~ 2 eV by a few Å )

• Electron correlation effects – responsible for covalent bonds

• Only one or two stable, bound states of the ion

Photodetachment

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• X- + photon X + e-

• ½ of electron-atom collision

• minimum photon energy necessary is known as the “electron affinity”

• Why study photodetachment in fields?

Photodetachment with B-Fields

• departing electron executes cyclotron motion in field

• motion in plane perpendicular to B is quantized to cyclotron or Landau levels separated by the cyclotron frequency 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

Detachment cross section in B field

Optical Apparatus

Diode seed Diode amplifier

Ion trap

MOPA: 250 mW single-mode tunable

SpectrumAnalyzer

8 GHz FSR

Wavemeter to 0.02 cm-1

Detachment scan in 1.0 Tesla

Time-domain spectroscopy

• Short pulse excites multiple cyclotron levels simultaneously.

• Wave packet of cyclotron states orbits atomic core with uniform cyclotron frequency.

• Subsequent short pulse probes the detached portion of the electron wave function

• Alternately: second pulse creates additional wave packet

Ramsey interferometry

• Multiple path interferometry

• Phase information of first pulse is stored in the ions

• Phase information of second pulse is then compared with that of first pulse

• Optical memory!

What about electric fields?

Photodetachment with E-Fields

Ion creation

Energy levels of O-

Detachment cross section, field-free

Ion trap

Ion trap detection electronics

Half of the wavemeter

Ultrafast apparatus