Hydrogen Clusters that Remain Fluid at Low Temperature

Superfluidity of hydrogen, predicted three decades ago, continues to elude experimental observation, due to the high freezing temperature of H₂ at T=13.8 K. Here, large para-H₂ clusters are obtained in a cryogenic pulsed free jet expansion and are studied via nonlinear Raman spectroscopy. Clusters formed from neat pH₂ are solid as evidenced by the characteristic splitting of the rotational S₀(0) line. However, clusters formed of highly diluted pH₂ (≤1%) in He have a single S₀(0) line and remain liquid at the estimated superfluid transition temperature of T=1--2 K.