Helium - metrology at

¹ Laser Centre Vrije Universiteit - De Boelelaan 1081 1081 HV Amsterdam, The Netherlands
² Department of Applied Physics, Eindhoven University of Technology P.O. Box 513, 5600 MB Eindhoven, The Netherlands

Received: 7 July 2006
Accepted: 8 September 2006

Abstract

An experiment is proposed to excite the “forbidden” - magnetic dipole () transition at in a collimated and slow atomic beam of metastable helium atoms. It is demonstrated that an excitation rate of can be realised with the beam of a 2 W narrow-band telecom fiber laser intersecting the atomic beam perpendicularly. A Doppler-limited sub-MHz spectroscopic linewidth is anticipated. Doppler-free excitation of 2% of trapped and cooled atoms may be realised in a one-dimensional optical-lattice geometry, using the 2 W laser both for trapping and spectroscopy. The very small (8 Hz) natural linewidth of this transition presents an opportunity for accurate tests
of atomic structure calculations of the helium atom. A measurement of the ³He-⁴He isotope shift allows for accurate determination of the difference in nuclear charge radius of both isotopes.