  We study the effects of ions on the reorientation dynamics of liquid water
with polarization-resolved femtosecond mid-infared spectroscopy. We probe the
anisotropy of the excitation of the O-D stretch vibration of HDO molecules in
solutions of NaCl, NaI and tetra-alkylammonium bromide salts in 8 percent
HDO:H2O. We find that the reorientation O-D groups of HDO molecules hydrating
the Cl- and I- anions occurs on two different time scales with time constants
of 2pm0.3 ps and 9pm2 ps. The fast component is due to a wobbling motion of the
O-D group that keeps the hydrogen bond with the halogenic anion intact. For
solutions of tetra-alkylammonium bromide salts we observe a very strong slowing
down of the reorientation of water that is associated with the hydration of the
hydrophobic alkyl groups of the tetra-alkylammonium ions.
