  We investigate refractive index changes caused by femtosecond photoionization
in a gas-filled hollow-core photonic crystal fiber. Using spatially-resolved
interferometric side-probing, we find that these changes live for tens of
microseconds after the photoionization event - eight orders of magnitude longer
than the pulse duration. Oscillations in the megahertz frequency range are
simultaneously observed, caused by mechanical vibrations of the thin-walled
capillaries surrounding the hollow core. These two non-local effects can affect
the propagation of a second pulse that arrives within their lifetime, which
works out to repetition rates of tens of kilohertz. Filling the fiber with an
atomically lighter gas significantly reduces ionization, lessening the strength
of the refractive index changes. The results will be important for
understanding the dynamics of gas-based fiber systems operating at high
intensities and high repetition rates, when temporally non-local interactions
between successive laser pulses become relevant.
