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Dynamics of water adsorption on Pt{110}-(1x2)

We have generated a supersonic molecular beam of water using the room temperature water vapour pressure (30 mbar) and an overpressure of an inert gas (i.e. the carrier gas) to give a total pressure of 1.5-3 bar necessary for the supersonic expansion. This gives a very diluted mixture with 1%-2% water in the beam and no cluster formation is found in the beam.

We have measured the sticking probability of water on Pt{110}-(1x2) using the King and Wells method at surface temperature of 165 K, for which the lifetime of the water multilayer is short on this surface. Hence, the focus is on the first water layer chemisorbed on Pt{110}-(1x2). A typical King and Wells experiment is shown in Figure 1 for a water molecular beam possessing 27 kJ/mol incident kinetic energy.

The advantage of using a supersonic molecular beam is that the incident translational energy dependence of the initial sticking probability, s0, can be measured. This is done by varying the content and type of the carrier gas in the beam mixture and/or changing the nozzle temperature.

Figure 2 shows how s0 changes with incident translational energy. It is essentially a step function going from 0.1 at thermal energy to 0.6 at high energies, with a sharp threshold energy of 5 kJ/mol. We believe that the threshold energy may be ascribed to the collisional parameter that involves the molecular orientation at the point of impact. In other words, we interpret this low energy barrier (5 kJ/mol) as a result of non-optimal orientations of the water molecule approaching the surface.

We have also investigated the coverage, beam flux and internal energy dependence of the water sticking probability. For more details the reader is referred to the article below.

Figure 1: Water adsorption on Pt{110}-(1x2) using the King and Wells method at 165 K surface temperature.

Figure 2: The initial sticking probability of water as a function of translational energy at Ts = 170 K and a nozzle temperature Tn = 300 K. Data points with the highlighted gas represent H2O seeded in the respective pure carrier gas. The others represent those with a fraction of He mixed in with the respective carrier gas. The line through the data points is a guide to the eye only.

Related publications

"Influence of the rotational degrees of freedom on the initial sticking probability of water on Pt{110}-(1 x 2): A molecular dynamics study"
T. Panczyk, V. Fiorin and T.P. Warzocha
J. Chem. Phys. 133 (2010) 034708

"Dynamics of water adsorption on Pt{110}-(1x2): A molecular dynamics study"
T. Panczyk, V. Fiorin, R. Blanco-Alemany and D.A. King
J. Chem. Phys. 131 (2009) 064703

"Adsorption dynamics of water on Pt{110}-(1x2)"
F.R. Laffir, Fiorin V and D.A. King
J. Chem. Phys. 128 (2008) 114717

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