Formation of nanoscale vacuum films governing thermal resistance in amorphous ice

Rapid crystallization prevents equilibrium studies of liquid water in the deeply supercooled regime. Although ultrafast X-ray experiments enable simultaneous heating and structural probing of water and ice, the heating dynamics of amorphous solid water (ASW), especially how heat propagates across nanoscale films under non-equilibrium conditions, remain poorly understood. Here, we combine X-ray free-electron laser pump–probe diffraction, continuum heat-transfer modeling, and molecular dynamics (MD) simulations to unravel the transient thermal response of ASW films on a Pt substrate at a base temperature of 110 K. Picosecond laser pulses induce a nanosecond-long temperature increase of the Pt surface to 700 K. Surprisingly, the hundreds-nanometer-thick ASW layer remains unchanged within tens of ns, revealing a striking thermal decoupling between metal and ASW. Continuum modeling indicates that the inferred interfacial resistance cannot be explained by conventional equilibrium models. MD simulations identify the spontaneous formation of a nanometric vapor film at the interface as the microscopic origin of this decoupling, which suppresses thermal contact and insulates the ice. The experimentally observed out-of-plane diffraction peak splitting matches the signature obtained in simulations only when a vapor-nucleated layer is present at the interface, demonstrating that the diffraction anisotropy originates from interfacial separation rather than bulk porosity.