  The spectral properties of a one-dimensional (1D) single-chain Mott insulator
Sr$_2$CuO$_{3}$ have been studied in angle-resolved photoemission and optical
spectroscopy, at half filling and with small concentrations of extra charge
doped into the chains via high oxygen pressure growth. The single- particle gap
is reduced with oxygen doping, but the metallic state is not reached. The
bandwidth of the charge-transfer band increases with doping, while the state
becomes narrower, allowing unambiguous observation of separated spinon and
holon branches in the doped system. The optical gap is not changed upon doping,
indicating that a shift of chemical potential rather than decrease of
corelation gap is responsible for the apparent reduction of the photoemission
gap.
