We report the Sum Frequency Generation (SFG) spectra of aqueous
sodium iodide interfaces computed with the methodology outlined by
Morita and Hynes (J. Phys. Chem. B, 2002, 106, 637), which is based
on molecular dynamics simulations.  The calculated spectra are in
qualitative agreement with experiment.  Our simulations show that
the addition of sodium iodide to water leads to an increase in SFG
intensity in the region of 3400 cm-1, which corresponds to an
increase in ordering of hydrogen-bonded water molecules.
Depth-resolved orientational distribution functions suggest that
the ion double layer orders water molecules which are about one
water layer below the Gibbs dividing surface.  We attribute the
increase in SFG intensity to these ordered subsurface water
molecules which are present in the aqueous sodium iodide/air
interfaces, but are absent in the neat water/air interface.