Probing the Superfluid Response of <i>para</i>-Hydrogen with a Sulfur Dioxide Dopant

We recently presented the first attempt at using an asymmetric top molecule (<i>para-</i>water) to probe the superfluidity of nanoclusters (of <i>para</i>-hydrogen) [Zeng, T.; Li, H.; Roy, P.-N. J. Phys. Chem. Lett. 2013, 4, 18−22]. Unfortunately, <i>para-</i>water could not be used to probe the <i>para</i>-hydrogen superfluid response. We now report a theoretical simulation of sulfur dioxide rotating in <i>para</i>-hydrogen clusters and show that this asymmetric top can serve as a genuine probe of superfluidity. With this probe, we predict that as few as four <i>para</i>-hydrogen molecules are enough to form a superfluid cluster, the smallest superfluid system to date. We also propose the concept of “exchange superfluid fraction” as a more precise measurement. New superfluid scenarios brought about by an asymmetric top dopant and potential experimental measurements are discussed