The refractive indices of vapor‐deposited ice films were measured versus substrate temperature from 35–140 K and H2O pressure from 1.9 × 10−7–9.5 × 10−5 Torr using optical interference techniques. The refractive indices were measured for ice films deposited on a cooled, single‐crystal Al2O3 substrate in vacuum. Ice densities were calculated from these refractive indices using the Lorentz‐Lorenz relationship. Above 120 K, the ice films had a refractive index of n=1.31 and a density of ρ=0.93 g/cm³. Significantly lower refractive indices and densities were found at lower temperatures and higher H2O fluxes that were consistent with microporous ices. A refractive index of n=1.24 and density of ρ=0.68 g/cm³ were determined at 35 K and an H2O pressure of 4 × 10−6 Torr. Above 35 K, the refractive index and density progressively increased with increasing temperature and decreasing H2O flux. The ice densities observed can be explained qualitatively using a ballistic deposition model. Estimates are also obtained for the surface diffusion of H2O on vapor‐deposited ice.
Optical interference techniques were used to measure the real index of refraction of nitric acid/ice films representative of type I and type I1 polar stratospheric clouds (PSCs). Possible candidates for type I PSCs include amorphous HNO3/H20 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 A1203 substrates at low temperatures. The real indices of refraction at X = 632 nm were measured for these films using the time-dependent optical interference during film deposition. The stoichiometries of the HNO3/H20 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 f 0.01 for pure ice to n = 1.47 f 0.01 for nearly pure nitric acid.A Lorentz-Lorenz analysis was in good agreement with the measured refractive indices of the amorphous HNO3/H20 films as a function of HN03 mole fraction. Growth of HNOs/H20 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 f 0.01 for NAD and n = 1.54 f 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.
The infrared spectra of nitric‐acid/ice films representative of polar stratospheric clouds (PSCs) were collected with simultaneous optical interference measurements to determine the real refractive indices at λ = 632 nm. Ice and amorphous nitric‐acid/ice films were prepared by condensation of water and nitric acid vapors onto a wedged Al2O3 substrate. The real refractive indices of these films were determined from the optical interference of a reflected helium‐neon laser during film growth. The indices of the amorphous films varied smoothly from n = 1.30 for ice to n = 1.49 for nitric acid, similar to observations in previous work. We were unable to obtain the refractive index of crystalline films during adsorption because of optical scattering caused by surface roughness. Therefore crystalline nitric acid hydrate films were prepared by annealing amorphous nitric‐acid/ice films. Further heating caused desorption of the crystalline hydrate films. During desorption, the refractive indices for ice, NAM (nitric acid monohydrate), α‐ and β‐NAT (nitric acid trihydrate) films were measured using the optical interference technique. In agreement with earlier data, the real refractive indices for ice and NAM determined in desorption were n = 1.30±0.01 and n = 1.53±0.03, respectively. The real refractive indices for α‐ and β‐NAT were found to be n = 1.51±0.01 and n ≥ 1.46, respectively. Our measurements also suggest that the shape of crystalline nitric acid particles may depend on whether they nucleate from the liquid or by vapor deposition. If confirmed by future studies, this observation may provide a means of distinguishing the nucleation mechanism of crystalline PSCs.
The reactive uptake of chlorine nitrate (ClONO 2 ) on ice surfaces (ClONO 2 + H 2 O f HOCl + HNO 3 ) was studied with surface sensitivity using laser-induced thermal desorption (LITD) techniques. Thin films of vapor-deposited ice were exposed to ClONO 2 vapor at substrate temperatures from 75 to 140 K. The reactive uptake of ClONO 2 was directly measured by monitoring the hydrolysis reaction products, HOCl and HNO 3 , on the ice surface in real time. At low temperatures from 75 to 110 K, the HOCl coverage initially increased rapidly with ClONO 2 exposure, indicating an efficient hydrolysis reaction. After longer ClONO 2 exposures, the rate of HOCl production decreased and the HOCl reached a constant coverage. A reaction probability of γ ) 0.03 was calculated for the reactive uptake of ClONO 2 on ice and was independent of temperature from 75 to 110 K. At temperatures greater than 110 K, the reaction probability decreased with increasing temperature and reached a value of γ ) 0.005 at 140 K. This decrease in the reaction probability with increasing substrate temperature is consistent with a precursor-mediated adsorption model. The good fits to the precursor-mediated adsorption model indicate that the ClONO 2 hydrolysis reaction has a low activation barrier. The precursormediated adsorption model extrapolated to stratospheric temperatures predicts a reaction probability that is significantly lower than the accepted literature value of γ ∼ 0.3. This discrepancy may be caused by the higher pressures and the dynamic ice surface at stratospheric conditions that could enhance the reaction probability. The constant HOCl coverage reached after longer ClONO 2 exposures is attributed to the poisoning of the ice surface by product HNO 3 . Calibrated HNO 3 signals at 86 and 140 K revealed that the ClONO 2 hydrolysis reaction is inhibited at a nitric acid coverage of ∼7.5 × 10 14 molecules/cm 2 or ∼ 1 monolayer. This coverage suggests that the hydrolysis reaction is limited to the surface or near-surface region of ice at these low temperatures.
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