  We study the optically induced spin polarization, spin dephasing and
diffusion in several high-mobility two-dimensional electron systems, which are
embedded in GaAs quantum wells grown on (110)-oriented substrates. The
experimental techniques comprise a two-beam magneto-optical spectroscopy system
and polarization-resolved photoluminescence. Under weak excitation conditions
at liquid-helium temperatures, we observe spin lifetimes above 100 ns in one of
our samples, which are reduced with increasing excitation density due to
additional, hole-mediated, spin dephasing. The spin dynamic is strongly
influenced by the carrier density and the ionization of remote donors, which
can be controlled by temperature and above-barrier illumination. The absolute
value of the average electron spin polarization in the samples is directly
observable in the circular polarization of photoluminescence collected under
circularly polarized excitation and reaches values of about 5 percent. Spin
diffusion is studied by varying the distance between pump and probe beams in
micro-spectroscopy experiments. We observe diffusion lengths above 100 $\mu$m
and, at high excitation intensity, a nonmonotonic dependence of the spin
polarization on the pump-probe distance.
