Photoexcitation and Ionization of Cold Helium Atoms
R. Jung1,2
S. Gerlach1,2
G. von Oppen1
U. Eichmann1,2
1Technical University of Berlin2 Max-Born-Institute
Interactions in Ultracold GasesHeidelberg 2002
this work is partially supported by DFG
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
photoexcitation and ionization of cold atoms
Creation of an ultracold neutral plasma first observed byNIST 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
strongly coupled systems
studying recombination processes, especiallythree body recombination (large temperature dependence)
Formation of Rydberg atoms in an expanding ultracold plasma.
(Killian, Phys.Rev.Lett., 86, 3759 (2001))
ultracold neutral plasmas
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))
cold Rydberg gases
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)
cold metastable helium
level scheme of metastable Helium
160000
170000
180000
1083 nm
gas-discharge
389 nm
0
en
erg
y [
cm
-1]
190000
200000
260 nm
continuum
33S33P
23P
23S
11S
longitudinal cooling transition at 389 nm
transversal cooling transition at 1083 nm
pulsed laser at 260 nm
polarizability(33P) = 4,3 MHz/(kV/cm)2
(23P) = 0,08 MHz/(kV/cm)2
Stark slower - scheme
Atom - Laser
- resonant atom-light interaction during the deceleration
field strength [kV/cm]
way of cooling [m]
field plate 1 field plate 2 field plate 3
calculatedexperimentalconditions
- spatial electric field strength
deceleration length
freq
uen
cy
LN2-cooledHe*-source(gas-discharge)
MOT
aperture
transversalcooling
He*-deflection
diode laser = 1083 nm
- Stark-Slower -longitudinal cooling section
Bz = 0,1 mT
experimental setup - cooling section -
0 1 2 3
0
2
4
6
8
10
12
MC
P-s
ign
al [a
rb.
un
its]
time of flight [ms]
vp=2100m/s
vp=1000m/s
precooling of the He*-source
0 5 10 15 20
without laser beam deflection + transversal cooling
MC
P-s
ign
al
[arb
. u
nit
s]
time of flight [ms]
deflection + collimation of the He* beam
fixed applied voltage on the first two field platesU1 = 12,1 kV; U2=18,6kV
fixed applied voltage on the first two field platesU1 = 12,1 kV; U2=18,6kV
results of Stark slowed He*
vstart ~ 1000 m/s
MCP-detectorcooling section
laser-cooled Helium atoms(v < 10 m/s )
gold-coatedmirror
MOT-coils(anti-Helmholtz-configuration)
compensationcoil
MOT-laser = 1083 nm/4-plate
cooling laser = 389 nm
pair of field plates/4-plate
setup- magneto-optical trap -
MOT-parameters (coils)- turns: 2 x 77- diameter: 19 cm- vertical distance: 10 cm- maximum current: 40-50A
parameters compensation coil- turns: 27- diameter: 12 cm- maximum current: 12 A
0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,16 0,18 0,20
0
2
4
6
8
10
12
14 Expansion der Heliumwolke
gemessenes TOF-Spektrum Simulation
MC
P-S
ign
al /
sim
ulie
rte
Te
ilch
en
anz
ahl
[w.E
.]
Flugzeit [s]
0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,16 0,18 0,20
0
2
4
6
8
10
12
14 Expansion der Heliumwolke
gemessenes TOF-Spektrum Simulation
MC
P-S
ign
al /
sim
ulie
rte
Te
ilch
en
anz
ahl
[w.E
.]
Flugzeit [s]
parameters of the trap:number of trapped atoms: ca. 105 trap lifetime: ~250 msdensity: 108-109cm-3
characteristics of the magneto-optical trap
estimation of the temperatur of the trapped helium sample
T ~ 4 mK
MC
P-s
ign
al
[arb
. u
nit
s]
time of flight [s]
measured tof - spectrumsimulation
Nd-YAG laser(30Hz system,10ns pulses)
Dye-Laser(+frequency doubling unit)
pulsed field plates
fast photodiode(trigger)
MCP(ion detection)
He*
ADC
data aquisitionswitching logic
+Ufp
~10sUV-pulse(trigger)
delay
= 260 nm
= 389 nm
= 1083 nm
He*-MOT
setup - ionization experiments
fixed voltage (-160 V)
n = 40
field strength F = 125 V/cm
ionization threshold(Eion = 38461,5 cm-1, ion = 260,004 nm)
Rydberg spectrum of helium
260,0 260,1 260,2 260,3 260,4 260,5 260,60
500
1000
1500
2000
2500
inte
nsi
ty [
arb
. un
its]
wavelength [nm]
260,0 260,2 260,4 260,6 260,8 261,00
500
1000
1500
2000
inte
nsi
ty [
arb
. un
its]
wavelength [nm]
- delay time: 100 ns - delay time: 1 ms
field ionization threshold(F = 170 V/cm)
field ionization threshold(F = 47 V/cm)
delayed detection of Rydberg spectra
n = 37
n = 28
field pulse amplitude above field ionization threshold
time evolution of the signal at n ~ 70
long storage period of high excited helium atoms trapped in the MOT
requirement for producingultracold plasmas
fixed Rydberg state
0 200 400 600 800 1000
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
inte
nsi
ty [
arb
. un
its]
delay time [s]
time evolution of the signal for excitation to n = 42 and field strength below the field ionization threshold
0 1 2 3 4 5 6 70
5000
10000
15000
20000
25000
30000
35000
40000
inte
nsi
ty [
arb
. un
its]
delay time [s]
excitation of the n = 18 - state
strong ion signal at short delay times
0 1 2 3 4 5 6 70
5000
10000
15000
20000
25000
30000
35000
40000
inte
nsi
ty [
arb
. un
its]
delay time [s]
n ~ 250
F = 10,5 V/cmF = 44,7 V/cmF = 143,0 V/cm
0 1 2 3 4 5 6 7 8 9 10
0
5000
10000
15000
20000
25000
Dye = 259 nm
inte
nsi
ty [
arb
. un
its]
delay time [s]
- varying field strength
1E7 1E8 1E9
0
5
10
15
20
25
30 plasma evolution free expansion of ion cloud
tim
e o
f fl
igh
t [
s]density [atoms/cm3]
photoionizing metastable helium atoms
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
• 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
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