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C. Lund Myhre, H. Grothe, A Gola, C. Nielsen:
"Optical constants of HNO3/H2O and H2SO4/HNO3/H2O at low temperatures in the infrared region";
Journal of Physical Chemistry A, 109 (2005), 7166 - 7171.

The complex index of refraction of liquid HNO3/H2O and H2SO4/HNO3/H2O has been obtained at different
temperatures and acid concentrations. FT-IR specular reflectance spectra were obtained for 30, 54, and 64 wt
% aqueous HNO3 and for four different H2SO4/HNO3/H2O mixtures in the temperature region from 293 to
183 K. The complex index of refraction was obtained from the reflectance spectra with the Kramers-Kronig
transformation. The optical constants of the binary and ternary mixtures vary with the acid concentration and
the temperature. The results demonstrate that vibrational bands originating from the sulfate species are more
sensitive to changes in temperature than the bands originating from vibrations in the nitrate species; only
minor changes in the nitrate vibrational bands are observed as the temperature decreases below 248 K.

http://pubs.acs.org/cgi-bin/asap.cgi/jpcafh/asap/pdf/jp0508406.pdf