  We present the results of a search for dark matter WIMPs in the mass range
below 20 GeV/c^2 using a target of low-radioactivity argon. The data were
obtained using the DarkSide-50 apparatus at Laboratori Nazionali del Gran Sasso
(LNGS). The analysis is based on the ionization signal, for which the
DarkSide-50 time projection chamber is fully efficient at 0.1 keVee. The
observed rate in the detector at 0.5 keVee is about 1.5 events/keVee/kg/day and
is almost entirely accounted for by known background sources. We obtain a 90%
C.L. exclusion limit above 1.8 GeV/c^2 for the spin-independent cross section
of dark matter WIMPs on nucleons, extending the exclusion region for dark
matter below previous limits in the range 1.8-6 GeV/c^2.
