Helium

An experiment is proposed to excite the “forbidden” $1s2s\,^3S_1$ - $1s2s\,^1S_0$ magnetic dipole ($M1$) transition at $1.557\ {\rm \mu m}$ in a collimated and slow atomic beam of metastable helium atoms. It is demonstrated that an excitation rate of $5000\ { \rm s^{-1}}$ 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