Large numbers of cold metastable helium atoms in a magneto-optical trap

Abstract

We report loading of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{9}$ metastable triplet helium atoms in a large magneto-optical trap, using far-red-detuned laser beams. We fully characterized this trap by measuring trap losses and absorption of a probe beam. From the highly nonexponential trap decay we derive Penning ionization loss rate coefficients for two detunings: $5.3(9)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}/\mathrm{s}$ at $\ensuremath{-}35\mathrm{MHz}$ and $3.7(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}/\mathrm{s}$ at $\ensuremath{-}44\mathrm{MHz}.$ Also, we find that the loss rate is maximum at $\ensuremath{-}5\mathrm{MHz}$ detuning, where the rate is $1.3(3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}{\mathrm{cm}}^{3}/\mathrm{s},$ much larger than recent theoretical and experimental values. In the absence of light the $S\ensuremath{-}S$ ionization rate constant is measured to be $1.3(2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{\mathrm{cm}}^{3}/\mathrm{s}.$