The chemical etching of silicon in F-2 ambient is considered. The desorption activation energy for an SiF2 molecule is evaluated using an experimentally measured dependence of etching rate on concentration of F-2 molecules. It is found that the desorption activation energy of SiF2 molecules is equal to E-d = (0.815 +/- 0.010) eV. This corresponds to a value of the mean lifetime of adsorbed molecules on the surface of tau = (26 +/- 3) ms at temperature T = 376 K. (c) 2005 Published by Elsevier B.V.