Bragg additivity was explored through measurement of Balmer-α emission cross sections produced by the collision of 100–350-keV ${\mathrm{He}}^{+}$ incident on ${\mathrm{H}}_2$, ${\mathrm{CH}}_4$, ${\mathrm{C}}_2$${\mathrm{H}}_2$, ${\mathrm{C}}_2$${\mathrm{H}}_4$, and ${\mathrm{C}}_2$${\mathrm{H}}_6$. The cross-section data show a linear relation on a Fano plot, thus indicating that the Bethe-Born theory may provide an appropriate description of the collision. Evidence for Bragg additivity would be a direct dependence of the value of the emission cross section on the number of hydrogen atoms in the target molecule. Results indicate a failure of strict additivity. Once molecular structure had been taken into account by a calculation of approximate electron densities, the cross sections followed an additive rule demonstrating a dependency not only on the number of hydrogens present, but also on the number of electrons available per hydrogen atom.

• Received 9 August 1990

DOI:https://doi.org/10.1103/PhysRevA.42.6423