Interaction of NH₃ with Re(0001): An optical second harmonic generation study

The adsorption desorption kinetics of ammonia on Re(0001) single crystal surface was studied in the crystal temperature range of 80-300 K., utilizing optical second harmonic generation (SHG) and temperature programmed desorption (TPD) measurements. Four different binding states, denoted as α₁, α₂, β and γ were detected by TPD. The α₁ and α₂it states represent a direct intera NH₃ with the metal while β and γ are the second and multilayer adsorption states, respectively. A coverage independent sticking probability of 0.75 ± 0.15 is measured at an adsorption temperature of 80 K. It does not decrease during the population of the second layer (β state). Isothermal desorption and equilibrium measurements were performed by employing the SHG ability to directly probe, nondestructively the surface coverage as a function of crystal temperature and ambient NH₃ pressure. Very strong coverage dependence was found for both the activation energy for desorption (E_d)it and for the pre-exponential factor (A). E_d decreases from 21 ± 1 to 8.5 ± 1 kcal mol and A changes from 10^{17 ± 1.5} to 10^{12 ± 1.0} as coverage increases from 0.2 of the saturation coverage (θ_s) to 0.9. E_d and A are found to be coverage independent in the range θ/θ_s = 0.4-0.8. A comparison is made betwe kinetic parameters obtained from TPD lineshape analysis, isothermal desorption and the equilibrium measurements. It is found that while TPD and isothermal desorption are preferred methods for the low coverage, equilibrium measurements provide more reliable kinetic parameters and their coverage dependence at the high coverage regime.