SURFACE CATALYSIS OF THE EQUILIBRIUM $2Na \leftrightarrow; Na_{2}$ STUDIED BY LASER SPECTROSCOPY
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Date
1980
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Ohio State University
Abstract
Dissociation and recombination reactions at a surface have been studied for a long time (see e.g. $^{1}$). We will show here that laser spectroscopy offers a new entrance to this field. For demonstration we have chosen the system $(Na, Na_{2})$. In principle, the equilibrium $2Na \leftrightarrow; Na_{2}$ can be established in the volume, by three-body collisions, or at the surface, by wall collisions. The equilibrium is probed by applying a temperature gradient across a sodium vapor cell. This leads to gradients of the densities [Na] and $[Na_{2}]$ due to thermalizing and reactive collisions. The [Na] gradient is measured by detecting near-resonant Rayleigh scattering from a dye laser beam and the $[Na_{2}]$ gradient by detecting $B \to $X fluorescence excited by an argon laser. Cells with various surface-to-volume ratios have been used. Preliminary results in the Knudsen regime are as follows: (i) The chemical equilibrium is catalyzed efficiently by the surface; equilibrium is established within a few wall collisions. (ii) The surface reaction enthalpy is equal to the spectroscopic value of the $Na_{2}$ dissociation energy. (iii)Volume recombination is negligible; this agrees with a rough estimate of three-body volume collision processes.
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$^{1}$ K.J. Laidler, Chemical Kinetics (Mc Graw-Hill, 1950) Ch. 6.
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