  Films of cerium-doped LaMnO$_3$, which has been intensively discussed as an
electron-doped counterpart to hole-doped mixed-valence lanthanum manganites
during the past decade, were analyzed by x-ray photoemission spectroscopy with
respect to their manganese valence under photoexcitation. The comparative
analysis of the Mn 3s exchange splitting of La$_{0.7}$Ce$_{0.3}$MnO$_3$ (LCeMO)
films in the dark and under illumination clearly shows that both oxygen
reduction and illumination are able to decrease the Mn valence towards a mixed
2$+$/3$+$ state, independently of the film thickness and the degree of CeO$_2$
segregation. Charge injection from the photoconductive SrTiO$_3$ substrate into
the Mn e$_g$ band with carrier lifetimes in the range of tens of seconds and
intrinsic generation of electron-hole pairs within the films are discussed as
two possible sources of the Mn valence shift and the subsequent electron
doping.
