Observation of a striking departure from velocity proportionality in low-energy electronic stopping

By time-of-flight spectrometry we have measured the energy loss of hydrogen projectiles in helium gas. We show that for projectile velocities ${\mathit{v}}_1$ below the Bohr velocity the presence of an appreciable minimum excitation energy leads to a significant deviation of the electronic stopping cross section from the ${\mathit{v}}_1$ proportionality generally assumed; at 4 keV a dependence on roughly the third power of ${\mathit{v}}_1$ is found.