Knowledge Bank

Dissociation and recombination reactions at a surface have been studied for a long time (see e.g. $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$). We will show here that laser spectroscopy offers a new entrance to this field. For demonstration we have chosen the system $(Na,Na2)$. In principle, the equilibrium $2Na\leftrightarrow ;Na2$ 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 $[Na2]$ due to thermalizing and reactive collisions. The [Na] gradient is measured by detecting near-resonant Rayleigh scattering from a dye laser beam and the $[Na2]$ 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 $Na2$ dissociation energy. (iii)Volume recombination is negligible; this agrees with a rough estimate of three-body volume collision processes.