  In this letter we have made a comparative study of degradation of solar EUV
radiation and EUV-generated photoelectrons in the inner comae of comets having
different gas production rates, Q, with values 1x10^28, 7x10^29, 1x10^31, and
1x10^32 s^-1. We found that in higher-Q comets the radial profile of H2O+
photo-production rate depicts a double-peak structure and that the differences
in sunward and anti-sunward photoionization rates are pronounced. We show that
photoelectron impact ionization is an order of magnitude larger than
photoionization rate near the lower photoionization peak in comets with Q >~
1x10^31 s^-1. The present study reveals the importance of photoelectrons
relative to solar EUV as the ionization source in the inner coma of high-Q
comets.
