Temperature Dependence For The Relative Raman Cross Section Of The Ammonia/Water Complex

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Danielle Simonelli and Mary Shultz Department of Chemistry, Tufts University, Medford, Massachusetts 02155 Journal of Molecular Spectroscopy, 205, 221 - 226, 2001.
The Raman spectra of aqueous ammonia solutions have been obtained between -40^o and 25^oC. The Raman spectrum of neat water was also obtained at 25^oC and is characterized by two broad peaks observed at 3200 and 3400 cm^-1. The spectrum due to water is subtracted to determine the NH₃ spectrum at all temperatures. In ammonia-water solutions, the spectrum shows three features at measured displacements of 3250, 3316, and 3400 cm^-1. The feature at 3316 cm^-1 is assigned to the Q branch of the symmetric stretch. The broad, weak features at 3250 and 3400 cm^-1, previously assigned to rotational bands, are assigned to combination band. The NH₃combination bands are assigned by comparing with sum frequency generation (SFG) experiments, monitoring changes with temperature, and analyzing the polarization data. The rotational structure of the Q band is also discussed. As the temperature is lowered from 25 to -40^oC, an increase in the Raman intensity is observed for all bands. The relative Raman scattering cross section is determined from the numerically integrated area of the NH₃ Q branch at each temperature.


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