Radiative Lifetimes and Excitation Mechanisms in Helium

Abstract

The radiative decay rates of the $n^{1}S(n=4,5)$, $3^{1}P$, $n^{1}D(n=3,4,5)$, $n^{3}S(n=4,5)$, $n^{3}P(n=3,4,5)$, and $n^{3}D(n=3,4)$ 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 ($n\ensuremath{\ge}5$) are populated by collisional transfer from $n^{1}P$ levels and in turn cascade to the $^{3}D$ levels. Possible laser action in the $3^{1}P\ensuremath{\rightarrow}3^{1}S$ and $5F\ensuremath{\rightarrow}3^{3}D$ transitions is suggested.