Photoexcitation and Ionization of Cold Helium Atoms R. Jung 1,2 S. Gerlach 1,2 G. von Oppen 1 U. Eichmann 1,2 1 Technical University of Berlin 2 Max-Born-Institute Interactions in Ultracold Gases Heidelberg 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 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 strongly 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)) 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 nm gas-discharge 389 nm 0 energy [cm -1 ] nm continuum 33S33S 33P33P 23P23P 23S23S 11S11S longitudinal cooling transition at 389 nm transversal cooling transition at 1083 nm pulsed laser at 260 nm polarizability (3 3 P) = 4,3 MHz/(kV/cm) 2 (2 3 P) = 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 1field plate 2field plate 3 calculated experimental conditions - spatial electric field strength deceleration length frequency

LN 2 -cooled He*-source (gas-discharge) MOT aperture transversal cooling He*-deflection diode laser = 1083 nm - Stark-Slower - longitudinal cooling section experimental setup - cooling section MCP-signal [arb. units] time of flight [ms] v p =2100m/s v p =1000m/s precooling of the He*-source deflection + collimation of the He* beam

fixed applied voltage on the first two field plates U 1 = 12,1 kV; U 2 =18,6kV fixed applied voltage on the first two field plates U 1 = 12,1 kV; U 2 =18,6kV results of Stark slowed He* v start ~ 1000 m/s

MCP-detector cooling section laser-cooled Helium atoms (v < 10 m/s ) gold-coated mirror MOT-coils (anti-Helmholtz-configuration) compensation coil 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

parameters of the trap: number of trapped atoms:ca trap lifetime:~250 ms density: cm -3 characteristics of the magneto-optical trap estimation of the temperatur of the trapped helium sample T ~ 4 mK MCP-signal [arb. units] time of flight [s] measured tof - spectrum simulation

Nd-YAG laser (30Hz system, 10ns pulses) Dye-Laser (+frequency doubling unit) pulsed field plates fast photodiode (trigger) MCP (ion detection) He* ADC data aquisition switching logic +U fp ~10 s UV-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 (E ion = 38461,5 cm -1, ion = 260,004 nm) Rydberg spectrum of helium

- 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 producing ultracold plasmas

fixed Rydberg state time evolution of the signal for excitation to n = 42 and field strength below the field ionization threshold

excitation of the n = 18 - state strong ion signal at short delay times n ~ 250 F = 10,5 V/cm F = 44,7 V/cm F = 143,0 V/cm

- varying field strength 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