Distribution of Excitation Transfer in Helium

Abstract

The distribution of atom-atom collisional excitation among various levels of the helium atom is determined. The determination is made by analyzing absolute measurements of the optical electron excitation cross sections of 63 levels at 100 eV electron energy and 63 mTorr pressure. Factors pertaining to our method of obtaining absolute optical cross sections are discussed in some detail. Collisional-excitation transfer on the $n=3\mathrm{}$ level is found to be slight with a cross section for $3{}_{}{}^1{}_{}{}^{}P$ to $3{}_{}{}^1{}_{}{}^{}D$ transfer of less than 1.2 × ${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$. A detailed analysis is made on the $n=4\mathrm{}$ level and collisional excitation transfer cross sections are found to be (4.7±1.5) × ${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$ for $4F$ to $4D$ transfer and (2.3±0.6) × ${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$ for $4{}_{}{}^1{}_{}{}^{}P$ to $4F$ transfer. The pressure dependence of the apparent electron excitation cross sections of 47 levels is also presented at 100 eV electron energy. Our work shows that the apparent cross sections of levels with large principal quantum numbers decrease as the pressure increases, an effect opposite to that observed for levels of low principal quantum number. A short discussion explains these phenomena in terms of a cyclic energy-exchange process which includes electron excitation, imprisonment of resonance radiation, collisional excitation transfer, and radiative transfer. In the course of this investigation it was necessary to obtain values for the electron excitation cross sections of the $n{}_{}{}^1{}_{}{}^{}P$ series up to $n=10\mathrm{}$. The values showed good agreement with Born-approximation theoretical cross sections.