Heteroepitaxial growth of Pb on the Ge(001) surface has been studied b
y He atom scattering. For low substrate temperatures, Pb is found to g
row layer by layer with (111) orientation. A detailed analysis of the
specular peak profile as a function of the He wave vector reveals that
the step height of the growing monatomic terraces oscillates with the
him thickness. This variation, initially as large as +/-15% around th
e value of the Pb(Ill) bulk interlayer spacing, gradually dampens out
after the deposition of a dozen monolayers. This is direct evidence of
quantum size effects affecting the interlayer distance of a growing m
etal film.