Excitation Transfer in Helium

Abstract

The apparent lifetimes of the $n^{1}P$, $3^{1}D$, $4^{1}D$, and $3^{3}D$ levels of helium are studied as a function of pressure. The collisional transfer of excitation from $n^{1}P$ levels to $\mathrm{nF}$ levels is conclusively shown to be an important mechanism in populating $D$ levels. It is found that the Wigner spin rule inhibits collisional transfer between $4^{1}P$ and $4^{3}F$ levels. For $n>4$, the $n^{1}P$ transfer to $n^{3}F$ levels is about three times greater than to the $n^{1}F$ levels. For $n>4$, the cross sections for collisional transfer from $\mathrm{nF}$ to $n^{1}P$ levels are found to be about 10% of those for the reverse process. This result is in agreement with the principle of detailed balance. The collisional $n^{1}P\ensuremath{\rightarrow}\mathrm{nF}$ transfer cross sections are measured and appear to increase approximately as ${n}^{4}$. The pressure dependence of the apparent $3D$ cross sections are calculated, using the above measurements in conjunction with the $n^{1}P\ensuremath{\rightarrow}\mathrm{nF}$ transfer model. The results are found to be in excellent agreement with experimental measurements.