  The dynamics of ultrafast energy transfer to water clusters and to bulk water
by a highly intense, sub-cycle THz pulse of duration $\approx$~150~fs is
investigated in the context of force-field molecular dynamics simulations. We
focus our attention on the mechanisms by which rotational and translational
degrees of freedom of the water monomers gain energy from these sub-cycle
pulses with an electric field amplitude of up to about 0.6~V/{\AA}. It has been
recently shown that pulses with these characteristics can be generated in the
laboratory [PRL 112, 213901 (2014)]. Through their permanent dipole moment,
water molecules are acted upon by the electric field and forced off their
preferred hydrogen-bond network conformation. This immediately sets them in
motion with respect to one another as energy quickly transfers to their
relative center of mass displacements. We find that, in the bulk, the operation
of these mechanisms is strongly dependent on the initial temperature and
density of the system. In low density systems, the equilibration between
rotational and translational modes is slow due to the lack of collisions
between monomers. As the initial density of the system approaches 1~g/cm$^3$,
equilibration between rotational and translational modes after the pulse
becomes more efficient. In turn, low temperatures hinder the direct energy
transfer from the pulse to rotational motion owing to the resulting stiffness
of the hydrogen bond network. For small clusters of just a few water molecules
we find that fragmentation due to the interaction with the pulse is faster than
equilibration between rotations and translations, meaning that the latter
remain colder than the former after the pulse.
