Energy distribution ofH−from the collision-induced three-particle breakup ofH3+

We have measured the laboratory energy distribution of ${\mathrm{H}}^{\mathrm{-}}$ produced by the collision-induced dissociation of ${\mathrm{H}}_3^{+}$ from He targets for incoming beam energies ranging from 2.417 to 7.0 keV. An approximate projectile frame energy distribution of ${\mathrm{H}}^{\mathrm{-}}$ is also obtained. The most probable ${\mathrm{H}}^{\mathrm{-}}$ projectile frame energy is estimated to be approximately 0.75 eV. We have also determined, from the shift in the position of the ${\mathrm{H}}^{\mathrm{-}}$ laboratory peaks, an inelastic energy loss of 60±12 eV for the excited ${\mathrm{H}}_3^{+}$ ions producing near zero kinetic energies of ${\mathrm{H}}^{\mathrm{-}}$ in the projectile frame. Our results suggest that ${\mathrm{H}}^{\mathrm{-}}$ is produced by the electronic excitation of ${\mathrm{H}}_3^{+}$ with subsequent dissociation into ${\mathrm{H}}^{+}$+${\mathrm{H}}^{+}$+${\mathrm{H}}^{\mathrm{-}}$. In this case the motion of the two ${\mathrm{H}}^{+}$ ions is highly correlated. For the most probable c.m. energy of ${\mathrm{H}}^{\mathrm{-}}$, we put a lower limit of 163° on the correlation angle between the protons in the c.m. of dissociating ${\mathrm{H}}_3^{+}$.