| Sum Frequency Generation (SFG) is used to probe vibrational modes of ammonia at the liquid/vapor interface. SFG is a surface specific technique that has been used to examine liquid surfaces on a molecular level. The ssp vibrational spectra of aqueous ammonia, NH3 (aq), are obtained from 0.005x to 0.3x NH 3 (x = mole fraction) and are characterized by the N-H symmetric stretch ( n1 ) and bend overtone ( 2n4 ). The antisymmetric stretch, n3 , is observed at −25°C with ppp polarization. Suppression of the free OH peak in SFG spectra of concentrated NH3 solutions indicates that H2O molecules are complexed through hydrogen bonds to ammonia at the interface. In dilute solutions, features due to water appear and the free OH peak emerges. The average molecular orientation of NH3 is determined from the polarization dependence of the n1 and n3 sum frequency intensities, ISFG. The results indicate an average orientation of 25° ≤ &thetas; ≤ 38° and a twist angle of &phis; ≥ 10° (&thetas; = tilt angle, &phis; = twist angle). The temperature dependence of ISFG is used to calculate the adsorption energy, ΔHads, of NH3 to free OH groups at the aqueous surface (−4.8kJ/mol). The surface coverage (xammonia, surface) of NH 3 is determined at each temperature. The temperature dependence is due to the Raman scattering cross section of the N-H n1 Q band, and the partitioning of bulk-phase NH 3 to the interface. The Raman and infrared spectra of NH3(aq) have been obtained from 25°C to −25°C. The Raman spectrum is characterized by the NH3 Q, O/ P, and R/S rotational bands of n1. The infrared absorption spectrum of NH3-H2O in CCl4 is characterized by the NH3 n1 band and several peaks attributed to hydrogen-bonded water. With decreasing temperature, the Raman intensity increases for all bands, but n1 is unaffected in the infrared spectrum. Finally, the SFG spectrum of neat liquid ammonia at −60°C is obtained with the ssp and ppp polarization combinations. The ssp spectrum is characterized by two strong peaks at 3200 and 3300 cm−1. Peak assignments are proposed based on Fermi resonance, hydrogen-bonding interactions, and different NH3 orientations at the liquid surface. |
