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 upsilon-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 upsilon-1 proportionality generally assumed; at 4 keV a dependence on roughly the third power of upsilon-1 is found.