Ammonia–water complexes on the surface of aqueous solutions observed with sum frequency generation

Sum frequency generation (SFG) is used to obtain the vibrational spectrum of aqueous ammonia solution. The SF spectrum of concentrated solutions is dominated by the N–H symmetric stretch (ν1) at 3312 cm−1 and a weaker deformation mode (2ν4) at 3200 cm−1. The free OH peak of water is suppressed, compared to pure water, and the hydrogen-bonded region (3000–3450 cm−1) has interference in the presence of ammonia molecules at the interface. In dilute solution, ν1 is less intense and the free OH peak of water appears. These observations confirm the existence of an ammonia–water complex at the liquid/vapor interface.