Desorption kinetics of model polar stratospheric cloud films measured using Fourier Transform Infrared Spectroscopy and Temperature‐Programmed Desorption

This study combines Fourier transform infrared (FTIR) spectroscopy and temperature‐programmed desorption to examine the evaporation kinetics of thin films of crystalline nitric acid hydrates, solid amorphous H₂O/HNO₃ mixtures, H₂O–ice, ice coated with HCl, and solid HNO₃. IR spectroscopy measured the thickness of each film as it evaporated, either at constant temperature or during a linear temperature ramp (temperature‐programmed infrared, TPIR). Simultaneously, a mass spectrometer measured the rate of evaporation directly by monitoring the evolution of the molecules into the gas phase (temperature‐programmed desorption, TPD). Both TPIR and TPD data provide a measurement of the desorption rate and yield the activation energy and preexponential factor for desorption. TPD measurements have the advantage of producing many data points but are subject to interference from experimental difficulties such as uneven heating from the edge of a sample and sample‐support as well as pumping‐speed limitations. TPIR experiments give clean but fewer data points. Evaporation occurred between 170 and 215 K for the various films. Ice evaporates with an activation energy of 12.9 ± 1 kcal/mol and a preexponential factor of 1 × 10^32±1.5 molec/cm² s, in good agreement with the literature. The beta form of nitric acid trihydrate, β–NAT, has an E_des of 15.6 ± 2 kcal/mol with log A = 34.3 ± 2.3; the alpha form of nitric acid trihydrate, α–NAT, is around 17.7 ± 3 kcal/mol with log A = 37.2 ± 4. For nitric acid dihydrate, NAD, E_des is 17.3 ± 2 kcal/mol with log A = 35.9 ± 2.6; for nitric acid monohydrate, NAM, E_des is 13 ± 3 kcal/mol with log A = 31.4 ± 3. The α–NAT converts to β–NAT during evaporation, and the amorphous solid H₂O/HNO₃ mixtures crystallize during evaporation. The barrier to evaporation for pure nitric acid is 14.6 ± 3 kcal/mol with log A = 34.4 ± 3. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 295–309, 2001