What is the Smallest Atom as a Probe for Characterizing Nanostructures?

In gas adsorption techniques for characterizing the surface structure of solids and the porosity of porous materials, a smaller adsorptive is expected to be useful as a probe to detect finer structures. According to the periodic table, the smallest noble gas atom is helium. However, in cryogenic systems, light atoms such as helium behave as atoms larger than the size estimated from their electron shell structures because of the uncertainty in the position of the atom arising from quantum effects. Here, to unveil the smallest atom as a probe, we perform path integral grand canonical Monte Carlo simulations for the adsorption of noble gases on pore and solid surface models at the respective boiling temperature of each noble gas. First, we show that helium gas behaves exactly as an atom larger than its classically estimated size at low temperatures; in pores, however, it is smaller than in the bulk phase because quantum delocalization is suppressed by the pore walls and helium anomalously acts as a spheroidal atom in confined space. Second, a systematic comparison of the simulation results reveals that neon can enter an extremely small space helium is unable to enter, suggesting that the effective size of the atom at cryogenic temperatures is determined by the size of the electron cloud and wave nature of an atomic nucleus; consequently, neon is the smallest and most suitable noble gas as a probe to access more in-depth information on atomistic scale nanostructures.