Sum frequency generation orientation analysis of molecular ammonia on the surface of concentrated solutions

Ammonia–water complexes have been detected with sum frequency generation (SFG) at the liquid/vapor interface of concentrated ammonia solutions (0.3x NH3, x=bulk mole fraction). SFG spectra taken with the ssp polarization combination (s-polarized sum frequency signal, s-polarized visible light, p-polarized infrared beam) are dominated by the N–H symmetric stretch (ν1) at 3312 cm−1 and a weaker deformation mode (2ν4) at 3200 cm−1. The dangling (free) OH peak due to water at 3700 cm−1 is suppressed at this concentration, indicating that water molecules are complexed through hydrogen bonds to ammonia at the interface. The polarization dependence of ν1 indicates that the NH3 C3 molecular axis is tilted with respect to the surface normal by θ⩽38°. Variation of the antisymmetric stretch, ν3, sum frequency signal intensity with polarization restricts the lower limit for surface ammonia tilt angles such that 25°⩽θ⩽38°. Based on this orientation, the structure of the ammonia–water complex is discussed.