  We study the non-contact friction between an atomic force microscope tip and
a metal substrate in the presence of bias voltage. The friction is due to
energy losses in the sample created by the electromagnetic field from the
oscillating charges induced on the tip surface by the bias voltage. We show
that the friction can be enhanced by many orders of magnitude if the ads orbate
layer can support acoustic vibrations. The theory predicts the magnitude and
the distance dependence of friction in a good agreement with recent puzzling
non-contact friction experiment \cite{Stipe}. We demonstrate that even an
isolated adsorbate can produce high enough friction to be measured
experimentally.
