  Polaritonic states arise when a bright optical transition of a molecular
ensemble is resonantly matched to an optical cavity mode frequency. Here, we
lay the groundwork to study the behavior of polaritons in clean, isolated
systems by establishing a new platform for vibrational strong coupling in
gas-phase molecules. We access the strong coupling regime in an intracavity
cryogenic buffer gas cell optimized for the preparation of simultaneously cold
and dense ensembles, and report a proof-of-principle demonstration in gas-phase
methane. We strongly cavity-couple individual rovibrational transitions and
probe a range of coupling strengths and detunings. We reproduce our findings
with classical cavity transmission simulations in the presence of strong
intracavity absorbers. This infrastructure provides a new testbed for benchmark
studies of cavity-altered chemistry.
