Secondary-electron yields from a clean surface of Mo have been measured for ${\mathrm{H}}^{+}$, ${\mathrm{H}}_2^{+}$, ${\mathrm{H}}_3^{+}$, ${\mathrm{H}}^{-}$, ${\mathrm{O}}^{+}$, ${\mathrm{O}}_2^{+}$, ${\mathrm{O}}^{-}$, ${\mathrm{O}}_2^{-}$, ${\mathrm{N}}^{+}$ and ${\mathrm{N}}_2^{+}$ in the kinetic energy range 40 eV to 2 keV. These measurements have been made under ultrahigh vacuum of order ${10}^{-10\mathrm{}}$ Torr. The rate of variation of the electron yield $\gamma$ with kinetic energy of the incident ion decreases as the ion mass increases, consistently from ${\mathrm{H}}^{+}$ to ${\mathrm{O}}_2^{+}$. Evidence is presented to show that kinetic emission is much less affected by monolayer coverage of the surface than potential emissions is. For clean surfaces, the negative-ion yields represent most closely the kinetic-emission characteristics of the bombarding particles.

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