  We present a simple and well defined prescription to compare absorption lines
in supernova (SN) spectra with lists of transitions drawn from the National
Institute of Standards and Technology (NIST) database. The method is designed
to be applicable to simple spectra where the photosphere can be mostly
described by absorptions from single transitions with a single photospheric
velocity. These conditions are plausible for SN spectra obtained shortly after
explosion. Here we show that the method also works well for spectra of
hydrogen-poor (Type I) superluminous supernovae (SLSNe-I) around peak. Analysis
of high signal to noise spectra leads to clear identification of numerous
spectroscopic features arising from ions of carbon and oxygen, that account for
the majority of absorption features detected in the optical range, suggesting
the outer envelope of SLSN-I progenitors is dominated by these elements. We
find that the prominent absorption features seen in the blue are dominated by
numerous lines of OII, as previously suggested, and that the apparent
absorption feature widths are dominated by line density and not by doppler
broadening. In fact, we find that while the expansion velocities of SLSNe-I
around peak are similar to those of normal SNe, the apparent velocity
distribution (manifest as the width of single transition features) is much
lower (~1500 km/s) indicating emission from a very narrow photosphere in
velocity space that is nevertheless expanding rapidly. We inspect the
controversial case of ASASSN-15lh, and find that the early spectrum of this
object is not consistent with those of SLSNe-I. We also show that SLSNe that
initially lack hydrogen features but develop these at late phases, such as
iPTF15esb and iPTF16bad, also differ in their early spectra from standard
SLSNe-I.
