Temperature dependence of alignment production in He i by beam-foil excitation

We have measured the dependence upon target-foil temperature of the linear polarization fraction ($\frac{M}{I}$) of the $2s{}_{}{}^1{}_{}{}^{}S-3p{}_{}{}^1{}_{}{}^{}P$, 5016-Å transition in He i for ion energies between 60 and 180 keV. The thin carbon exciter foils were heated externally by Nichrome resistance elements. The measurements of Hight et al. are duplicated; the energy and current dependencies of $\frac{M}{I}$ are the same, assuming correspondence between beam heating and external heating. We also observe that $\gamma$, the number of secondary electrons produced per incident ion, decreases with increasing foil temperature. These two effects, in conjunction, offer a plausible explanation for the variation of polarization with beam-current density. The temperature of the foil is shown to depend on beam current to the one-fourth power, indicating that radiation is the primary energy-loss mechanism.