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 M _ { \odot } , 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 .