The equilibrium concentration of passivating adatoms in the system of
Si crystal plus the gas of H-2 (or D-2) molecules is calculated The ef
fect of adatom localized vibrations on the equilibrium concentration i
s taken into account. It is shown that the substitution of D for H loa
ds to an increase of the adatom concentration by 1-1.5 orders of magni
tude. (C) 1998 American Institute of Physics. [S0021-8979(98)00402-2].