Measurements of spark source emission, transmission, and photoionization spectra in a CO2 laser mixture seeded with a low-ionization-threshold additive are presented. The results show a photoelectron density enhancement in excess of 3 orders of magnitude contained within a narrow transmission band at 1200 Å. Enhancement is also observed at wavelengths between 1700 and 2100 Å. The long-wavelength effect, characterized by an increased photon mean free path, may identify a two-step photoionization process.