Excitation Transfer in Helium

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 $n>\mathrm{}4\mathrm{}$, the $n{}_{}{}^1{}_{}{}^{}P$ transfer to $n{}_{}{}^3{}_{}{}^{}F$ levels is about three times greater than to the $n{}_{}{}^1{}_{}{}^{}F$ levels. For $n>\mathrm{}4\mathrm{}$, 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 $n{}_{}{}^1{}_{}{}^{}P\to \mathrm{nF}$ 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 $n{}_{}{}^1{}_{}{}^{}P\to \mathrm{nF}$ transfer model. The results are found to be in excellent agreement with experimental measurements.