  We experimentally and numerically investigate flat supercontinuum generation
in gas-filled anti-resonant guiding hollow-core photonic crystal fiber. By
comparing results obtained with either argon or nitrogen we determine the role
of the rotational Raman response on the supercontinuum formation. When using
argon, a supercontinuum extending from 350 nm to 2 {\mu}m is generated through
modulational instability. Although argon and nitrogen exhibit similar Kerr
nonlinearity and dispersion, we find that the energy density of the continuum
in the normal dispersion region is significantly lower when using nitrogen.
Using numerical simulations, we find that due to the closely spaced rotational
lines in nitrogen, gain suppression in the fundamental mode causes part of the
pump pulse to be coupled into higher-order modes which reduces the energy
transfer to wavelengths shorter than the pump.
