Sum Frequency Generation of O−H Vibrations on the Surface of H2O/HNO3Solutions and Liquid HNO3

The surfaces of aqueous HNO 3 solutions and liquid HNO 3 are examined using sum frequency generation (SFG). A molecular-level picture of these atmospherically relevant systems is developed. Consistent with previous interpretations, an electric double layer comprised of subsurface anions and cations develops in 0.005 x and 0.01 x HNO 3 solutions, where x = mole fraction. Compared to pure water, these solutions generate more SFG signal in the hydrogen-bonded region as water molecules respond to the subsurface electric field by aligning with the surface normal. At higher concentrations, 0.05 x and 0.4 x HNO 3, ionic complexes or molecules sufficiently approach the surface to disrupt the hydrogen-bonding network and perturb the first water layer. Neither liquid nitric acid nor its solutions show a clear O−H.