Isothermal measurements of NH3 and ND3 desorption from Cu(001).

The kinetics of NH3 and ND3 desorption from Cu(001) surface have been measured by time-resolved electron energy loss spectroscopy. The desorption precess is observed to be characterized by pseudo-first-order kinetics. The desorption energy and the prefactor have been determined for both NH3 and ND3. The prefactors are unusually low as compared to those observed for simple diatomic molecules. Moreover, we observed an unusual behavior consistent with an isotope kinetic compensation effect. Both of these observations can be explained within the context of transition state theory by the interplay between isotopic masses and the shifts of frustrated vibrational modes of adsorbed molecules. Our results imply that the conventional Arrhenius form would not be precisely applicable to molecular desorption, if a large enough temperature range could be measured, particularly for hydrogen-containing molecules. [ABSTRACT FROM AUTHOR]