International Workshop on Atomic Interactions in Laser Fields - Abstracts

Observation of Vibrational Levels in Cold He*

G.R. Woestenenk, J.W. Thomsen, M. van Rijnbach, P. van der Straten, A. Niehaus

Debye Institute, Utrecht University Postbus 80.000, 0508 T.4 Utrecht, T1

 

With the development of laser cooling techniques it has become possible to study interactions between atoms at ultralow temperatures. Introducing near resonant laser light into the sample of cold atoms makes it possible to control and dramatically alter the rate coefficient of collision processes [1-4]. The study of optical collisions, where during the collision of two cold atoms a photon is absorbed, gives detailed information on the long range interactions of the molecular complex.

We study optical collisions between cold metastable helium atoms, He(23S). A cold sample of He* atoms with a temperature of about 1 mK is trapped in a Magneto-Optical Trap (MOT). A probe laser is used to excite the quasimolecule to the first excitet state He(23S)-He(23P), where Penning Ionization (PI) and Associative Ionization (AI) rates are investigated as a function of probe laser detuning and intensity. Ionization rates are found to dep end sensitively on laser detuning and intensity.

We have observed that at small laser detunings the AI yield is dramatically enhanced by a factor of 20 [4, 5]. At large laser detunings the AI rate can be related to ionization resulting from collisions between ground state atoms, where only the lowest order partial wave can contribute to the cross section. In the excited state, however about 10 partial waves contribute, which explains the dramatic enhancement of the AI rate near resonance. Good agreement is found between theoretical models and measurements [1-4].

Recently, we have studied in more detail PI and AI rates in the presence of a strong probe laser [6]. Apart from the enhancement of the total ionization rate near resonance, we now observe pronounced structures as a function of detuning, depending on the intensity of the laser beam. We belive that these structures are associated with vibrational levels in the quintet potentials of the excited state potential. This would then be the first time vibrational levels of the quintet state of He* are observed in optical collisions.

References

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  2. K.-A. Suominen, J. Phys. B, 29, 5981 (1996).
  3. F. Bardou, O. Emile, J.-M. Courty, C.T. Westbrook, A. Aspect, Europhys. Lett. 20, 681 (1992).
  4. H. Mastwijk, J.W. Thomsen, P. van der Straten and A. Niehaus, Phys. Rev. Lett. 80, 5516 (1998).
  5. H. Mastwijk, M. van Rijnbach, J.W. Thomsen, P. van der Straten and A. Niehaus, Eur. Phys. J. D., 4, 131 (1998).
  6. Work in collaboration with W. Vassen and W. Hogervorst, Laser Center Vrije Universiteit, University of Amsterdam.