H2O and HCl adsorption on single crystal α-Al2O3(0001) at stratospheric temperatures

The adsorption of H2O and HCl on single crystal alpha -Al2O3(0001) at stratospheric temperatures was studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The alpha -Al2O3(0001) surface was used to model stratospheric Al2O3 exhaust particles from solid rocket motors. H2O and HCl coverages may influence heterogeneous chemistry on Al2O3 particles at stratospheric temperatures of similar to 200-270 K. Compared with earlier measurements at 298-300 K, the saturation coverages of both H2O and HCl on alpha -Al2O3(0001) were approximately two times higher at 210 K. The saturation coverages of H2O and HCl at 210 K were Theta (H2O) = 4.4 x 10(14) molecules/cm(2) and Theta (HCl) = 2.0 x 10(14) molecules/cm(2). The sticking coefficients for both H2O and HCl on alpha -Al2O3(0001) at 210 K were also higher than the corresponding sticking coefficients at 298-300 K. The sticking coefficients (S) for both H2O and HCl at 210 K were similar to0.08 at the lowest coverages. The TPD spectra revealed that H2O and HCl desorption occurred over a broad range of temperatures. These results indicate that H2O and HCl adsorb onto a wide distribution of binding sites on the alpha -Al2O3(0001) surface. The LITD and TPD results predict that alpha -Al2O3 rocket exhaust particles will be covered with chemisorbed H2O and HCl under stratospheric conditions. Additional H2O and HCl exposure can also replace the adsorbed HCl and H2O species, respectively, by exchange reactions. (C) 2001 Elsevier Science B.V. All rights reserved.