Measurement of the cross sections for electron-impact excitation into the 5p⁵6p levels of xenon

Electron-impact emission cross sections for transitions from the ten 5p⁵6p levels of xenon to the lower levels have been measured at different gas pressures between 0.1 and 2.0 m Torr, and with incident electron energy from threshold to 150 eV. The exceptionally strong pressure dependence of the measured emission cross sections reported previously is confirmed. The optical emission cross sections yield the apparent excitation cross sections for the 5p⁵6p levels which vary significantly with pressure. Use of a Fourier-transform spectrometer makes it possible to measure previously uninvestigated infrared cascade transitions into the 5p⁵6p levels. Our measured cascade cross sections increase with pressure because of radiation trapping effects, and their pressure dependence is similar to that of the apparent excitation cross sections. By subtracting the cascade contribution from the apparent cross sections, we obtain the direct excitation cross sections which are found to be independent of the pressure. The peak direct cross sections for the 2p₁-2p₄ levels (Paschen’s notation) are much smaller than those of the 2p₅-2p₁₀ group. This is explained on the basis that the ionization energies for the former group are about only one-half of those for the latter group. Within each of these two groups (2p₁-2p₄ and 2p₅-2p₁₀), the levels with even values of the total angular momentum J have larger cross sections than the levels with odd J.