In this paper, we give a geometric realization of discrete series repr
esentations for unimodular Lie groups on the spaces of harmonic spinor
s by using Connes-Moscovici's L(2)-Index theorem. Our work is a contin
uation of Atiyah-Schmid's geometric realization of discrete series rep
resentations for semisimple Lie groups and Connes-Moscovici's realizat
ion of square-integrable representations for nilpotent Lie groups. (C)
1997 Academic Press.