Infrared line emissions from atoms and atomic ions in NGC 7027: improved
  wavelength determinations for infrared metal lines and a probable detection
  of Zn$^{5+}$
  An infrared L- and M-band spectral survey, performed toward the young
planetary nebula NGC 7027 with the iSHELL instrument on NASA's Infrared
Telescope Facility (IRTF), has revealed more than 20 vibrational lines of the
molecules HeH$^+$, H$_2$, and CH$^+$ and more than 50 spectral lines of atoms
and atomic ions. The present paper focuses on the atomic line emissions, the
molecular lines having been discussed in two previous publications (Neufeld et
al. 2020, 2021). The atomic lines detected with high confidence in the 2.951 -
5.24 micron region covered (incompletely) by this survey comprise (1) six
collisionally-excited lines of metal ions that had previously been identified
in astrophysical nebulae but for which the present observations provide the
most accurate wavelength determinations obtained to date; (2) a spectral line
at 4.6895 micron, not previously reported, for which the probable
identification is the $^4F_{7/2} - $ $^4F_{9/2}$ fine structure transition of
[Zn VI]; (3) 39 recombination lines of H and He$^+$, with upper states of
principal quantum number up to 38 (H) or 24 (He$^+$); (4) 10 recombination
lines of the multielectron species He, C$^{2+}$, and C$^{3+}$.
