Rotational and Vibrational Excitation ofH2by Electron Impact at 4.5 eV: Angular Distributions

The differential cross sections for rotational, vibrational, and rotational-vibrational excitation by electron impact at 4.5 eV in ${\mathrm{H}}_2$ have been measured with a resolution of 18-22 meV. The cross-section ratio of pure vibrational excitation ($\Delta j=0\mathrm{}$) to rotational-vibrational ($j=1\mathrm{}\to 3$) excitation for $v=1, 2, \mathrm{and} 3$ shows the expected $p$-wave character. However, at a given angle of observation, this ratio decreases as the vibrational quantum number increases. This effect has not been predicted by theory.