  Direct searches for low mass dark matter particles via scattering off target
nuclei require detection of recoiling atoms with energies of ~1 keV or less.
The amount of electronic excitation produced by such atoms is quenched relative
to a recoiling electron of the same energy. The Lindhard model of this
quenching, as originally formulated, remains widely used after more than 50
years. The present work shows that for very small energies, a simplifying
approximation of that model must be removed. Implications for the sensitivity
of direct detection experiments are discussed.
