Low-energy measurements of electron-photon angular correlation in electron-impact excitation of the2P1state of helium

Electron-photon angular correlations between electrons which have excited the $2{}_{}{}^1{}_{}{}^{}P$ state of He and photons from the $2{}_{}{}^1{}_{}{}^{}P\to 1{}_{}{}^1{}_{}{}^{}S$ transition have been studied for 27-, 30-, 35-, and 40-eV incident electrons. Values of $\lambda$ and $\left|\chi \right|$ obtained from these measurements are compared to values obtained in distorted-wave and $R$-matrix calculations. The values of $\lambda$ and $\left|\chi \right|$ have been combined to examine the behavior of $\left|O_{1-}^{\mathrm{col}}\right|$ [$\lambda (1-\lambda )sin\left|\chi \right|$], the nonvanishing component of orientation. At 27 eV, a substantial decrease was observed in the values of $\lambda$ and $\left|O_{1-}^{\mathrm{col}}\right|$, compared with their values for $E>~30$ eV.