Effective Mass of the Normal Negative-Charge Carrier in Bulk He II

The first acceleration measurements are reported for negative ions in bulk superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. Data recorded at 70 mK, 25 bars for electric fields in the range $80<E<\mathrm{}150\mathrm{}$ V ${\mathrm{m}}^{-1\mathrm{}}$ are consistent with an ionic dispersion relation identical to that of a free particle with mass $(87\mathrm{}\pm 5)m_4$, where $m_4$ is the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atomic mass.