  We present high-resolution dayside thermal emission observations of the
exoplanet WASP-18b using IGRINS on Gemini South. We remove stellar and telluric
signatures using standard algorithms, and we extract the planet signal via
cross correlation with model spectra. We detect the atmosphere of WASP-18b at a
signal-to-noise ratio (SNR) of 5.9 using a full chemistry model, measure H2O
(SNR=3.3), CO (SNR=4.0), and OH (SNR=4.8) individually, and confirm previous
claims of a thermal inversion layer. The three species are confidently detected
(>4$\sigma$) with a Bayesian inference framework, which we also use to retrieve
abundance, temperature, and velocity information. For this ultra-hot Jupiter
(UHJ), thermal dissociation processes likely play an important role. Retrieving
abundances constant with altitude and allowing the temperature-pressure profile
to freely adjust results in a moderately super-stellar carbon to oxygen ratio
(C/O=0.75^{+0.14}_{-0.17}) and metallicity ([M/H]=1.03^{+0.65}_{-1.01}).
Accounting for undetectable oxygen produced by thermal dissociation leads to
C/O=0.45^{+0.08}_{-0.10} and [M/H]=1.17^{+0.66}_{-1.01}. A retrieval that
assumes radiative-convective-thermochemical-equilibrium and naturally accounts
for thermal dissociation constrains C/O<0.34 (2$\sigma$) and
[M/H]=0.48^{+0.33}_{-0.29}, in line with the chemistry of the parent star.
Looking at the velocity information, we see a tantalising signature of
different Doppler shifts at the level of a few km/s for different molecules,
which might probe dynamics as a function of altitude and location on the planet
disk. Our results demonstrate that ground-based, high-resolution spectroscopy
at infrared wavelengths can provide meaningful constraints on the compositions
and climate of highly irradiated planets. This work also elucidates potential
pitfalls with commonly employed retrieval assumptions when applied to UHJ
spectra.

Earth and Planetary Astrophysics