Quench-condensed hydrogen films studied by cryogenic time-of-flight secondary ion mass spectrometry

Surface melting of solidified hydrogen has attracted attention in the field of superfluidity, but the existence of surface melting of solid hydrogen itself is still controversial. In the present study, we developed cryogenic time-of-flight secondary ion mass spectrometry (TOF-SIMS) capable of detecting surface melting by selectively analyzing hydrogen on the outermost surface. Combined with low-energy ion scattering for well-defined film growth, we successfully investigated the surface structural transition of the quenched condensed hydrogen film grown on polycrystalline tungsten substrate below the triple point. It was found that the TOF-SIMS intensity variation of H⁺ ions by increasing the temperature of the solid hydrogen film at a constant ramp rate (temperature-programmed TOF-SIMS) shows two prominent features: the increase accompanied by sublimation, and the decrease due to the elimination of the hydrogen admolecule from the tungsten surface. Both features are well explained by the desorption of hydrogen molecules from the solid surface. We observed no evidence of surface melting.