Hydrogen clusters that remained fluid

\textit{Para}-H$_{2}$ may constitute the only other superfluid besides helium. The superfluid transition temperature is predicted to be around 2 K, well below freezing of H$_{2}$ at 13.8 K. Numerous attempts to supercool macroscopic H$_{2}$ samples proved to be unsuccessful. Our approach includes formation of H$_{2}$ clusters in a pulsed cryogenic nozzle beam expansion of a neat $p$H$_{2}$ gas as well as \textbf{\textit{X}}\textbf{{\%}} of $p$H$_{2}$ diluted in He and interrogation via Coherent Anti-Stokes Raman Scattering. At \textbf{\textit{X}}\textbf{ = 2 -- 100 {\%}} the frequency of the vibrational Q$_{1}$(0) line in clusters remains constant at about $\nu $ = 4149.7 cm$^{-1}$ very similar to 4149.6 cm$^{-1}$ as in solid $p$H$_{2}$ and lower than in liquid $p$H$_{2 }$at 18 K (4151.9 cm$^{-1})$. The rotational S$_{0}$(0) transition show some characteristic crystal field splitting having magnitude of about 6 cm$^{-1}$. The splitting pattern is different from that in the \textit{hcp} solid, suggesting different structure in solid $p$H$_{2}$ clusters. At \textbf{\textit{X}}\textbf{ $\le $ 2 {\%}}, the frequency of the Q$_{1}$(0) line increases to about 4150.5 cm$^{-1}$, which is consistent with that expected in the supercooled liquid. The S$_{0}$(0) transition in these clusters, consisting of about 5 x 10$^{4}$ molecules, appears as a single line at the same frequency as in liquid $p$H$_{2}$. The temperature of these supercooled clusters is estimated to be less than about 1 K. Possible superfluidity of the clusters is discussed.