  We present a microscopic mechanism that accounts for the outward burst of
"cold" ion species (IS) in a high-energy particle track due to coupling with
''hot" non-ion species (NIS). IS refers to radiolysis products of ionized
molecules, whereas NIS refers to non-ionized excitations of molecules in a
medium. The interaction is mediated by a quantized field of acoustic phonons, a
channel that allows conversion of thermal energy of NIS to kinetic energy of
IS, a flow of heat from the outer to the inner core of the track structure. We
demonstrate the coexistence of "hot" NIS with "cold" IS in the radiation track
structures right after their generation. NIS, concentrated within nano-scales
volumes wrapping around IS, are the main source of intensive heat-waves and the
outward burst of IS due to femto-second time scale IS-NIS coupling. By
comparing the transport of IS coupled to NIS with identical configurations of
non-interacting IS in thermal equilibrium at room temperature, we demonstrate
that the energy gain of IS due to the surrounding hot nanoscopic volumes of NIS
significantly increases their effective diffusion constants. The much higher
diffusion constants predicted in the present model suggest higher inter-track
chemical reaction rates at FLASH-UHDR, as well as lower intra-track reaction
rates.
