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Interactions in Ultracold Gases Heidelberg 2002
Photoexcitation and Ionization of Cold Helium
Atoms R. Jung1,2 S. Gerlach1,2 G. von Oppen1 U.
Eichmann1,2 1Technical University of Berlin 2
Max-Born-Institute
this work is partially supported by DFG
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photoexcitation and ionization of cold atoms
Two regimes of interest excitation shortly
above the ionization threshold observation of
plasma generation recombination into Rydberg
states excitation of Rydberg levels below
ionization threshold redistribution into
long-lived states spontaneous formation of a
plasma
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ultracold neutral plasmas
Creation of an ultracold neutral plasma first
observed by NIST group on metastable
xenon. (Killian, Phys.Rev.Lett., 83, 4776
(1999))
characteristics of cold plasmas well-known
initial conditions trapping of electrons due to
Coulomb interaction very low temperatures stro
ngly coupled systems studying recombination
processes, especially three body recombination
(large temperature dependence)
Formation of Rydberg atoms in an expanding
ultracold plasma. (Killian, Phys.Rev.Lett.,
86, 3759 (2001))
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cold Rydberg gases

• Studying cold dense sample of Rydberg atoms
• - evolution of cold Rydberg atoms into cold
  plasma
• (Robinson et. al., Phys.Rev.Lett. 85 , 4466
  (2000))
• observation of unusual long-lasting electron
  emission signal from a
• cold Rydberg gas
• redistribution into high angular momentum states
  and thermal
• ionization
• (Dutta et.al., Phys.Rev.Lett. 86 ,3993
  (2001))

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cold metastable helium

• How do we get cold metastable He atoms ?
• laser-cooling of helium atoms by the means of
  the Stark effect
• - deceleration of the atoms in inhomogeneous
  electric fields
• - comparable short cooling section (1,5 m)
• - alternative to the usual Zeeman-technique
• trapping of He atoms in an ordinary MOT
• (We plan to replace the MOT by a electric trap to
  study cold collisions)

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level scheme of metastable Helium
continuum
200000
longitudinal cooling transition at 389
nm transversal cooling transition at 1083
nm pulsed laser at 260 nm
190000
33P
33S
180000
energy cm-1
389 nm
260 nm
170000
23P
1083 nm
160000
23S
polarizability a (33P) 4,3 MHz/(kV/cm)2 a
(23P) 0,08 MHz/(kV/cm)2
gas-discharge
11S
0
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Stark slower - scheme
- resonant atom-light interaction during the
deceleration
frequency
deceleration length
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experimental setup - cooling section -
LN2-cooled He-source (gas-discharge)
diode laser l 1083 nm
transversal cooling
- Stark-Slower - longitudinal cooling section
He-deflection
MOT
aperture
deflection collimation of the He beam
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results of Stark slowed He
vstart 1000 m/s
fixed applied voltage on the first two field
plates U1 12,1 kV U218,6kV
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setup - magneto-optical trap -
laser-cooled Helium atoms (v lt 10 m/s )
MCP-detector
cooling section
MOT-laser l 1083 nm
l/4-plate
gold-coated mirror
cooling laser l 389 nm
compensation coil
MOT-coils (anti-Helmholtz-configuration)
pair of field plates
l/4-plate
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characteristics of the magneto-optical trap
estimation of the temperatur of the trapped
helium sample
measured tof - spectrum simulation
MCP-signal arb. units
T 4 mK
time of flight s
parameters of the trap number of trapped
atoms ca. 105 trap lifetime 250
ms density 108-109cm-3
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setup - ionization experiments
ADC
MCP (ion detection)
fixed voltage (-160 V)
l 1083 nm
Nd-YAG laser (30Hz system, 10ns pulses)
He-MOT
l 389 nm
He
UV-pulse (trigger)
l 260 nm
10ms
Dye-Laser (frequency doubling unit)
Ufp
pulsed field plates
delay
data aquisition switching logic
fast photodiode (trigger)
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Rydberg spectrum of helium
field strength F 125 V/cm
ionization threshold (Eion 38461,5 cm-1, lion
260,004 nm)
n 40
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delayed detection of Rydberg spectra
- delay time 100 ns
- delay time 1 ms
field ionization threshold (F 170 V/cm)
field ionization threshold (F 47 V/cm)
n 37
n 28
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time evolution of the signal at n 70
field pulse amplitude above field ionization
threshold
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fixed Rydberg state
time evolution of the signal for excitation to n
42 and field strength below the field
ionization threshold
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strong ion signal at short delay times
excitation of the n 18 - state
F 10,5 V/cm F 44,7 V/cm F 143,0 V/cm
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photoionizing metastable helium atoms
- varying field strength
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conclusion and outlook

• An apparatus was build to study photoexcitation
  of cold helium atoms.
• First measurements of Rydberg states show a
  redistribution
• to long-lived levels
• reason redistribution due to blackbody radiation
  into higher Rydberg levels or
• collisional redistribution to levels with high
  angular momentum
• strong ion signal observed at short time scales
  (independent of n)
• - also observable above ionization threshold
• - (independent of excess energy)
• - no explanation yet
• Detection of ions not sufficient to identify
  unambigiously a cold plasma
• Further experiments will concentrate on electron
  detection, and
• refinement of the trapping parameters