Electron-impact excitation and collisional transfer into the nF levels of helium

Through the use of Fourier-transform spectroscopy, the electron-impact optical cross sections for the infrared emissions from the 4F, 5F, and 6F levels of the helium atom have been measured for gas pressures between 3 and 50 mTorr and incident electron energies from threshold to 200 eV. The very strong pressure dependence of these measured emission cross sections at different energies is in excellent quantitative agreement with the mechanism of excitation transfer from the n¹P levels through collisions with ground-state helium atoms. From our data, we determine the electron-impact cross sections for excitation into the 4F, 5F, and 6F levels. The effects of the singlet-triplet mixing in the nF states on the excitation cross sections are discussed. The excitation data of the 5F level are compared with those of 5¹S and 5¹D. We have also obtained the n¹P⃗nF collisional excitation transfer cross sections for n=4–7.