Transcript Photodetachment of O- ions in Magnetic and Electric Fields

Optical and NIR
Photodetachment Spectroscopy
in External Fields
Charlotte Chapter of the OSA
March 15, 2001
John Yukich
Davidson 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 
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½ of electron-atom collision
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minimum photon energy necessary is known
as the “electron affinity”
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X + e-
Why study photodetachment in fields?
Photodetachment with B-Fields
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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
MOPA:
Diode amplifier
250 mW single-mode tunable
Wavemeter
to 0.02 cm-1
Spectrum
Analyzer
8 GHz FSR
Ion
trap
Detachment scan in 1.0 Tesla
Time-domain spectroscopy
• Short pulse excites multiple cyclotron levels
simultaneously.
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Wave packet of cyclotron states orbits atomic core with
uniform cyclotron frequency.
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Subsequent short pulse probes the detached portion of
the electron wave function
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Alternately: second pulse creates additional wave packet
Ramsey interferometry
• Multiple path interferometry
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Phase information of first pulse is stored in the ions
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Phase information of second pulse is then compared with
that of first pulse
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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