  Accurate absorption models for gases at high pressure and temperature support
advanced optical combustion diagnostics and aid in the study of harsh planetary
atmospheres. Developing and validating absorption models for these applications
requires recreating the extreme temperature and pressure conditions of these
environments in static, uniform, well-known conditions in the laboratory. Here,
we present the design of a new gas cell to enable reference-quality absorption
spectroscopy at high pressure and temperature. The design centers on a
carefully controlled quartz sample cell housed at the core of a pressurized
ceramic furnace. The half-meter sample cell is relatively long compared to past
high- pressure and temperature absorption cells, and is surrounded by a
molybdenum heat spreader that enables high temperature uniformity over the full
length of the absorbing gas. We measure the temperature distribution of the
sample gas using in situ thermocouples, and fully characterize the temperature
uniformity across a full matrix of conditions up to 1000 K and 50 bar. The
results demonstrate that the new design enables highly uniform and precisely
known conditions across the full absorbing path length. Uniquely, we test the
new gas cell with a broadband, high-resolution dual frequency comb spectrometer
that enables highly resolved absorption spectroscopy across a wide range of
temperature and pressure conditions. With this system, we measure the spectrum
of CO$_2$ between 6800 and 7000 cm$^{-1}$ at pressures between 0.2 and 20 bar,
and temperatures up to 1000 K. The measurements reveal discrepancies from
spectra predicted by the HITRAN2016 database with a Voigt line shape at both
low- and high-pressure conditions. These results motivate future work to expand
absorption models and databases to accurately model high-pressure and
-temperature spectra in combustion and planetary science research.
