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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$^{3+}$.