  The strain effect from a substrate is an important experimental route to
control electronic and magnetic properties in transition-metal oxide (TMO) thin
films. Using hard x-ray photoemission spectroscopy, we investigate the strain
dependence of the valence states in LaNiO$_{3}$ thin films, strongly correlated
perovskite TMO, grown on four substrates: LaAlO$_{3}$,
(LaAlO$_{3}$)$_{0.3}$(SrAl$_{0.5}$Ta$_{0.5}$O$_{3}$)$_{0.7}$, SrTiO$_{3}$, and
DyScO$_{3}$. A Madelung potential analysis of core-level spectra suggests that
the point-charge description is valid for the La ions while it breaks down for
Ni and O ions due to a strong covalent bonding between the two. A clear x-ray
photon-energy dependence of the valence spectra is analyzed by the density
functional theory, which points to a presence of the La 5$p$ state near the
Fermi level.
