  A Rashba type spin-orbit splitting is found for quantum well states formed in
ultrathin Pb films on Si(111). The resulting momentum splitting is comparable
to what is found for semiconductor heterostructures. The splitting shows no
coverage dependency and the sign of the spin polarization is reversed compared
to Rashba splitting in the Au(111) surface state. We explain our results by
competing effects at the two boundaries of the Pb layer.
