REFRACTIVE-INDEXES OF AMORPHOUS AND CRYSTALLINE HNO3/H2O FILMS REPRESENTATIVE OF POLAR STRATOSPHERIC CLOUDS

Optical interference techniques were used to measure the real index of refraction of nitric acid/ice films representative of type I and type II polar stratospheric clouds (PSCs). Possible candidates for type I PSCs include amorphous HNO3/H2O mixtures as well as crystalline nitric acid trihydrate (NAT), dihydrate (NAD), and monohydrate (NAM). Amorphous and crystalline model PSC films were grown in vacuum by vapor deposition on single-crystal Al2O3 substrates at low temperatures. The real indices of refraction at lambda = 632 nm were measured for these films using the time-dependent optical interference during film deposition. The stoichiometries of the HNO3/H2O films were determined using laser-induced thermal desorption (LITD) techniques. For the amorphous films at 130 K, the refractive indices increased with increasing nitric acid content. The values ranged from n = 1.31 +/- 0.01 for pure ice to n = 1.47 +/- 0.01 for nearly pure nitric acid. A Lorentz-Lorenz analysis was in good agreement with the measured refractive indices of the amorphous HNO3/H2O films as a function of HNO3 mole fraction. Growth of HNO3/H2O films at 175 K resulted in the formation of either crystalline NAM or NAD. The crystalline indices were substantially higher than their amorphous analogs. The crystalline refractive indices at 175 K were n 1.52 +/- 0.01 for NAD and n 1.54 +/- 0.01 for NAM. Attempts to measure the refractive index of crystalline NAT were unsuccessful because NAT films would not nucleate under allowable temperature and pressure conditions.