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

The adsorption of H2O and HCl on single crystal alpha -Al2O3(0001) at strat ospheric temperatures was studied using laser-induced thermal desorption (L ITD) and temperature programmed desorption (TPD) techniques. The alpha -Al2 O3(0001) surface was used to model stratospheric Al2O3 exhaust particles fr om solid rocket motors. H2O and HCl coverages may influence heterogeneous c hemistry on Al2O3 particles at stratospheric temperatures of similar to 200 -270 K. Compared with earlier measurements at 298-300 K, the saturation cov erages of both H2O and HCl on alpha -Al2O3(0001) were approximately two tim es higher at 210 K. The saturation coverages of H2O and HCl at 210 K were T heta (H2O) = 4.4 x 10(14) molecules/cm(2) and Theta (HCl) = 2.0 x 10(14) mo lecules/cm(2). The sticking coefficients for both H2O and HCl on alpha -Al2 O3(0001) at 210 K were also higher than the corresponding sticking coeffici ents at 298-300 K. The sticking coefficients (S) for both H2O and HCl at 21 0 K were similar to0.08 at the lowest coverages. The TPD spectra revealed t hat H2O and HCl desorption occurred over a broad range of temperatures. The se results indicate that H2O and HCl adsorb onto a wide distribution of bin ding sites on the alpha -Al2O3(0001) surface. The LITD and TPD results pred ict that alpha -Al2O3 rocket exhaust particles will be covered with chemiso rbed H2O and HCl under stratospheric conditions. Additional H2O and HCl exp osure can also replace the adsorbed HCl and H2O species, respectively, by e xchange reactions. (C) 2001 Elsevier Science B.V. All rights reserved.