Study of ${\mathrm{H}}_2^{+}$ + ${\mathrm{H}}_2$ → ${\mathrm{H}}_3^{+}$ + H Using Merging Beams

A merging-beams technique was used to determine cross sections for the reaction ${\mathrm{H}}_2^{+}$ + ${\mathrm{H}}_2$ → ${\mathrm{H}}_3^{+}$ + H as a function of interaction energy $W$. Cross-section measurements at $W$ were accompanied by measurements at 1 eV so that we obtained ratios of cross sections, $\frac{Q_W}{Q_1}$. These ratios were determined for $0.1 \mathrm{eV}\le W\le 10 \mathrm{eV}$. Some information about the reaction was also obtained for $W\le 0.03$ eV. An absolute cross section was determined for $W=1\mathrm{}$ eV. Its estimated error is considerably larger than errors associated with the cross-section ratios. Gioumousis and Stevenson (GS) predict a linear dependence of the cross section on $W^{-\frac{1}{2}}$. On a $\frac{Q_W}{Q_1}$ versus $W^{-\frac{1}{2}}$ plot, a straight line can be fitted, within experimental error, to our values for $0.1 \mathrm{eV}\le W\le 1 \mathrm{eV}$. This line is in fair agreement with that predicted by GS. On either side of this range, however, our ratios fall below the $W^{-\frac{1}{2}}$ fit. Between 5 and 7 eV our results indicate that, within experimental error, $\frac{Q_W}{Q_1}$ vanishes. On the high-energy side, such a falloff could perhaps be explained by a spectator stripping model. Our cross-section ratios are in general agreement with those obtained from a tandem mass-spectrometer experiment by Giese and Maier, and in rather poor agreement with the results of a single-state mass-spectrometer experiment by Reuben and Friedman. Lower $W\text{'}\mathrm{s}$ were obtained with the merging-beams technique than with the mass-spectrometer methods.