  Two papers recently reported the detection of gaseous nickel and iron in the
comae of over 20 comets from observations collected over two decades, including
interstellar comet 2I/Borisov. To evaluate the state of the laboratory data in
support of these identifications, we re-analyzed archived spectra of comet
C/1996 B2 (Hyakutake), one of the nearest and brightest comets of the last
century, using a combined experimental and computational approach. We developed
a new, many-level fluorescence model that indicates that the fluorescence
emission of Fe I and Ni I vary greatly with heliocentric velocity. Combining
this model with laboratory spectra of an Fe-Ni plasma, we identified 22 lines
of Fe I and 14 lines of Ni I in the spectrum of Hyakutake. Using Haser models,
we estimate the nickel and iron production rates as Q(Ni) = 2.6 - 4.1 x 10^22
s^-1 and Q(Fe) = 0.4 - 2.8 x 10^23 s^-1. From derived column densities, the
Ni/Fe abundance ratio log10[Ni/Fe] = -0.15 +/- 0.07 deviates significantly from
solar abundance ratios, and it is consistent with the ratios observed in solar
system comets. Possible production and emission mechanisms are analyzed in
context of existing laboratory measurements. Based on the observed spatial
distributions, excellent fluorescence model agreement, and Ni/Fe ratio, our
findings support an origin consisting of a short-lived unknown parent followed
by fluorescence emission. Our models suggest that the strong heliocentric
velocity dependence of the fluorescence efficiencies can provide a meaningful
test of the physical process responsible for the Fe I and Ni I emission.
