  Recent observations have shown that the atmospheres of ultra hot Jupiters
(UHJs) commonly possess temperature inversions, where the temperature increases
with increasing altitude. Nonetheless, which opacity sources are responsible
for the presence of these inversions remains largely observationally
unconstrained. We used LBT/PEPSI to observe the atmosphere of the UHJ KELT-20 b
in both transmission and emission in order to search for molecular agents which
could be responsible for the temperature inversion. We validate our methodology
by confirming previous detections of Fe I in emission at $16.9\sigma$. Our
search for the inversion agents TiO, VO, FeH, and CaH results in
non-detections. Using injection-recovery testing we set $4\sigma$ upper limits
upon the volume mixing ratios for these constituents as low as
$\sim1\times10^{-9}$ for TiO. For TiO, VO, and CaH, our limits are much lower
than expectations from an equilibrium chemical model, while we cannot set
constraining limits on FeH with our data. We thus rule out TiO and CaH as the
source of the temperature inversion in KELT-20 b, and VO only if the line lists
are sufficiently accurate.
