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