  The solar transition region satisfies the conditions for presence of
non-Maxwellian electron energy distributions with high-energy tails at energies
corresponding to the ionization potentials of many ions emitting in the EUV and
UV portions of the spectrum. We calculate the synthetic Si IV, O IV, and S IV
spectra in the far ultra-violet (FUV) channel of the Interface Region Imaging
Spectrograph (IRIS). Ionization, recombination, and excitation rates are
obtained by integration of the cross-sections or their approximations over the
model electron distributions considering particle propagation from the hotter
corona. The ionization rates are significantly affected by the presence of
high-energy tails. This leads to the peaks of the relative abundance of
individual ions to be broadened with pronounced low-temperature shoulders. As a
result, the contribution functions of individual lines observable by IRIS also
exhibit low-temperature shoulders, or their peaks are shifted to temperatures
an order of magnitude lower than for the Maxwellian distribution. The
integrated emergent spectra can show enhancements of Si IV compared toO IV by
more than a factor of two. The high-energy particles can have significant
impact on the emergent spectra and their presence needs to be considered even
in situations without strong local acceleration.
