Electron-impact excitation and collisional transfer into thenFlevels of helium

Abstract

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 ${n}^{1}P$ 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 $5{}^{1}S$ and $5{}^{1}D.$ We have also obtained the $n{}^{1}\stackrel{\ensuremath{\rightarrow}}{P}\mathrm{nF}$ collisional excitation transfer cross sections for $n=4--7.$