Proton-stopping cross sections and mean excitation energies for gaseous Cl2 and Br2

First measurements of proton stopping powers in gaseous Cl2 and Br2 are reported for the energy range between 50 and 750 keV with statistical and systematical errors of about 1% each. In the stopping-power maximum the experimental values are higher than those predicted by the Andersen-Ziegler tables; moreover, lower peak energies are found. Experimental shell corrections were deduced from the proton stopping cross sections and adjusted to the theoretical predictions of Bonderup, whereby higher-order Z1 correction terms were included. Within this procedure semiempirical values for the mean ionization potentials of 174 eV for Cl2 and 363 eV for Br2 were obtained