  We show theoretically that the photoionization process in a hollow-core
photonic crystal fiber filled with a Raman-inactive noble gas leads to a
constant acceleration of solitons in the time domain with a continuous shift to
higher frequencies, limited only by ionization loss. This phenomenon is
opposite to the well-known Raman self-frequency red-shift of solitons in
solid-core glass fibers. We also predict the existence of unconventional
long-range non-local soliton interactions leading to spectral and temporal
soliton clustering. Furthermore, if the core is filled with a Raman-active
molecular gas, spectral transformations between red-shifted, blue-shifted and
stabilized solitons can take place in the same fiber.
