Angular Distribution of Protons and Deuterons Produced by Dissociative Ionization ofH2andD2near Threshold

From observations of the kinetic-energy distribution of protons and deuterons at corresponding forward and backward angles with respect to the electron-beam direction, the forward momentum imparted to the dissociating ${\mathrm{H}}_2$ (${\mathrm{D}}_2$) molecule by the incident electron has been determined for electron energies up to 300 eV. The momentum transfer along the beam was found to remain nearly constant at all energies above threshold, although above 100 eV the values obtained for ${\mathrm{D}}_2$ were systematically higher than the values for ${\mathrm{H}}_2$. When corrected to the center-of-mass system, the angular distribution of ${\mathrm{H}}^{+}$ (${\mathrm{D}}^{+}$) near threshold was found to contain a large isotropic component and an anisotropic component which deviates, in a manner suggested by Zare, from the ${cos}^2\Theta$ dependence predicted from a simple dipole Born approximation.