Abstract
It is well-established that three features are resolved in the thermal desorption spectra of H2O from clean Ru(001): one at low temperature due to desorption from ice multilayers, and two at higher temperatures due to desorption from a chemisorbed bilayer. However, we find that for D2O the highest-temperature state is strongly suppressed. By varying the heating rate during H2O desorption, we obtain evidence that the highest-temperature state can be formed by conversion from the intermediate state. Conversion is much slower for D2O than for H2O, so that direct desorption is kinetically favored over conversion and the high-temperature state is suppressed for D2O. The rate of conversion is about 3 to 8 times slower for D2O than for H2O, which is consistent with a model in which the rate-determining step for conversion is rotation of one or more water molecules within a hydrogen-bonded cluster. This work provides the first evidence for an isotope effect in thermal desorption of water. An important ramification is that adsorbed D2O is not always interchangeable with H2O for chemical studies of isotopic mixing or for spectroscopic studies of vibrational frequency shifts, as is commonly assumed.
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