State evolution in cold helium Rydberg gas S

State evolution in cold helium Rydberg gas S. 1 Gerlach , R. 2 Jung , G. von 2 Oppen , U. 1, 2 Eichmann Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born-Str. 2 A, D-12489 Berlin 2 Institut für Atomare Physik und Fachdidaktik, Technical University Berlin, D-10623 Berlin 1 Overview Cold metastable helium Rydberg atoms – experimental setup Why helium ? § H-like system with very small quantum defect for l>0 § light atoms : new effects through different dynamics § „condensation effect“ : fast ions act as condensation seed for cold metastable helium Data acquisition MCP (ion/electron detection) l = 1083 nm Investigation : § cold helium Rydberg gas at low density § Excitation below ionisation threshold with and without electric fields § Study the redistribution of a (n, l) state as a function of time He*- MOT Nd-YAG Laser (30 Hz system, 10 ns pulses) l = 260 nm Dye laser (+ frequency doubling) Properties of Helium UV-puls (Trigger) switched field plate ~10 ms Delay +Ufp Logics fast photo detector (Trigger) Energy level of metastable helium with excitation scheme Continuum Helium Rydberg spectra in electric fields 200000 260 nm 190000 Energy [cm-1] 33 S 389 nm 170000 23 P 1083 nm Increasing delay times frequency doubled pulsed Dye laser : Rydberg excitation + Ion creation 180000 160000 Increasing electric field 33 P frequency doubled Ti. Sa-Laser : Lasercooling 23 S gas discharge 0 laser diode (1083 nm) : MOT 11 S Stark l-mixing Lasercooling of metastable helium He*-source (gas discharge) delay = 10 µs electric field of 10 V/cm Above field ionization threshold transversal cooling laser diode = 1083 nm Below field ionization threshold short time scales : all states detected long time scales : only long-lived (high l) states survive - “Stark-Slower” cooling section short time scales : medium times : long times : Helium Stark manifold only low stark-states (not l=0!) all states detected high l states (long lifetime) MOT 1 meter Parameter of the He* MOT: number of atoms : density : lifetime : temperature : ~ 106 108 -109 cm-3 up to 900 ms no s-states at short times! ca 1 m. K n-state lifetimes time resolved level redistribution with a puls ramp lifetime up to 1. 5 ms Increased up to n~60 Theoretical occupation Decreasing over n~70 because of ionization outlook Ø clarify the role of the permanent dipole(p- vs. s-states) Ø higher density cloud with more atoms Ø Ø investigate collisional effects : Ø three body recombination Ø “condensation” plasma effects (back and forth helium Rydberg plasma)