  Ejected material from neutron star mergers give rise to electromagnetic
emission powered by radioactive decays of r-process nuclei, which is so called
kilonova or macronova. While properties of the emission are largely affected by
opacities in the ejected material, available atomic data for r-process elements
are still limited. We perform atomic structure calculations for r-process
elements: Se (Z=34), Ru (Z=44), Te (Z=52), Ba (Z=56), Nd (Z=60), and Er (Z=68).
We confirm that the opacities from bound-bound transitions of open f-shell,
Lanthanide elements (Nd and Er) are higher than those of the other elements
over a wide wavelength range. The opacities of open s-shell (Ba), p-shell (Se
and Te), and d-shell (Ru) elements are lower than those of open f-shell
elements and their transitions are concentrated in the ultraviolet wavelengths.
We show that the optical brightness can be different by >2 mag depending on the
element abundances in the ejecta such that post-merger, Lanthanide-free ejecta
produce brighter and bluer optical emission. Such blue emission from
post-merger ejecta can be observed from the polar directions if the mass of the
preceding dynamical ejecta in these regions is small. For the ejecta mass of
0.01 Msun, observed magnitudes of the blue emission will reach 21.0 mag (100
Mpc) and 22.5 mag (200 Mpc) in g and r bands within a few days after the
merger, which are detectable with 1m or 2m-class telescopes.
