Radiative Lifetimes and Excitation Mechanisms in Helium

The radiative decay rates of the $n{}_{}{}^1{}_{}{}^{}S\mathrm{}(n=4,5\mathrm{})\mathrm{}$, $3{}_{}{}^1{}_{}{}^{}P$, $n{}_{}{}^1{}_{}{}^{}D\mathrm{}(n=3,4,5\mathrm{})\mathrm{}$, $n{}_{}{}^3{}_{}{}^{}S\mathrm{}(n=4,5\mathrm{})\mathrm{}$, $n{}_{}{}^3{}_{}{}^{}P\mathrm{}(n=3,4,5\mathrm{})\mathrm{}$, and $n{}_{}{}^3{}_{}{}^{}D\mathrm{}(n=3,4\mathrm{})\mathrm{}$ states of helium have been measured using pulsed, variable-energy electron-impact excitation and a sampling method of detection. The results agree well with previously published measurements. Experimental evidence is presented for the mechanisms involved in the collisional-transfer reactions where atoms in ${}_{}{}^1{}_{}{}^{}P$ states excite ${}_{}{}^3{}_{}{}^{}D$ levels; $\mathrm{nF}$ levels ($n\ge 5$) are populated by collisional transfer from $n{}_{}{}^1{}_{}{}^{}P$ levels and in turn cascade to the ${}_{}{}^3{}_{}{}^{}D$ levels. Possible laser action in the $3{}_{}{}^1{}_{}{}^{}P\to 3{}_{}{}^1{}_{}{}^{}S$ and $5F\to 3{}_{}{}^3{}_{}{}^{}D$ transitions is suggested.