Abstract
The dynamics of H2 O adsorption on Pt {110} - (1×2) is studied using supersonic molecular beam and temperature programed desorption techniques. The sticking probabilities are measured using the King and Wells method at a surface temperature of 165 K. The absolute initial sticking probability s0 of H2 O is 0.54±0.03 for an incident kinetic energy of 27 kJmol. However, an unusual molecular beam flux dependence on s0 is also found. At low water coverage (θ<1), the sticking probability is independent of coverage due either to diffusion in an extrinsic precursor state formed above bilayer islands or to incorporation into the islands. We define θ=1 as the water coverage when the dissociative sticking probability of D2 on a surface predosed with water has dropped to zero. The slow falling H2 O sticking probability at θ > 1 results from compression of the bilayer and the formation of multilayers. Temperature programed desorption of water shows fractional order kinetics consistent with hydrogen-bonded islands on the surface. A remarkable dependence of the initial sticking probability on the translational (1-27 kJmol) and internal energies of water is observed: s0 is found to be essentially a step function of translational energy, increasing fivefold at a threshold energy of 5 kJmol. The threshold migrates to higher energies with increasing nozzle temperature (300-700 K). We conclude that both rotational state and rotational alignment of the water molecules in the seeded supersonic expansion are implicated in dictating the adsorption process.
Original language | English |
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Article number | 114717 |
Journal | Journal of Chemical Physics |
Volume | 128 |
Issue number | 11 |
DOIs | |
Publication status | Published - 2008 |
Externally published | Yes |