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A study of the dissociation dynamics of D₂O on zirconium surfaces using resonance enhanced multiphoton ionization spectroscopy Musa, Mohamed O.

Abstract

Atomic hydrogen adsorbates on Zr surfaces have much higher probability for dissolution into the bulk than recombinative desorption even when the surface is heated at rates as high as 10¹⁰ K/s. On the other hand, water dissociation on hot Zr surfaces (which takes place at temperatures as low as 80 K) produces molecular hydrogen very efficiently at temperatures above 230 K. To help resolve these apparently contradictory observations and to generally understand the dynamics of heavy water (D₂O) dissociation on Zr, we built an UHV chamber equipped with pressure and surface diagnostic tools to probe the rotational, vibrational and translational energy distributions of the evolving deuterium (D₂) using resonance enhanced multiphoton ionization (REMPI) spectroscopy and time-of-flight mass spectrometry. The rotational population distributions of the first two vibrational states (υʹʹ = 0, 1) of the D₂ product were measured and found to fit Boltzmann-like distributions at Trot < or = 500 K while the surface was held at 800 K. In addition the total population of the υʹʹ = 1 was found to be four times higher than the expected value at the surface temperature, corresponding to temperature of Tvib = 1070 ± 50 K. These distribution are consistent with previous observations of hydrogen recombination on metal surfaces. Regarding the translational populations, the population of the υʹʹ = 0 was measured and was found to fit a thermal beam model at 1200 K. This indicates the existence a very small energy barrier to desorption. To verify that the observed hydrogen was due to surface recombination, we probed the rotational population distributions of the υʹʹ = 1 of hydrogen scattered from or released (through slow thermal desorption of hydrogen dissolved in the bulk) by Zr at T = 800 K. These distributions fitted at a Boltzmann-like distribution at Trot < or = 400 K. From these observations, we conclude that hydrogen release from the dissociation of water arises from recombinative desorption occurring at the surface. The efficient production of hydrogen when water reacts with hot Zr is explained in terms of competition between hydrogen and oxygen adsorbates for subsurface/bulk sites.

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