At low pressures, diffusion plays an important role in determining the
number density n(r) of metastable particles created in an electrical
discharge. The appropriate surface boundary condition for solving the
metastable diffusion equation is known to be of the form n(r) is-propo
rtional-to s . deln, where s is the unit vector normal to the surface.
In this paper a new expression for the proportionality constant is de
rived using the arguments of gas kinetics. This expression is used to
estimate the reflectivities of a variety of types of metastable partic
le.