Buffer gas cooling of large polyatomic molecules

Cryogenic helium buffer gases have cooled a diversity of molecules to temperatures near 1 K. Motivated by collisional studies as well as mixture and chiral analysis, we are now cooling larger (often biological) molecules. A detailed understanding of helium-molecule sticking is currently unknown, as is any possible molecular size limit of buffer gas cooling. We report on experiments with polyatomic molecules that explore He-molecule collisions at energies well below their binding energy, in which the pair would form a dimer (or a larger cluster) under equilibrium conditions. Molecules from an oven source (300-500 K) flow into a cold (5 K) cell, where they undergo a few hundred collisions with He at a density of $\sim10^{14}$ cm$^{-3}$. We observe translational and internal cooling of both trans-stilbene and the significantly larger molecule Nile Red. Narrow spectral lines in both molecules indicate the gas-phase monomer. In Nile Red, we also see broad spectral features suggestive of molecule-molecule dimer formation. We see no evidence of high He-molecule dimer formation rates and conclude that for these molecules (and smaller molecules\footnote{Patterson, \textit{Molecular Physics} (110), 1757, 2012}) the lifetime of possible helium-molecule dimers is below $\sim$1 microsecond.