Velocity Dependence of the Ionization of Ar, Kr, and Xe on Impact of Metastable Neon Atoms

Measurements of the velocity dependence of the total ionization cross section of argon, krypton, and xenon on impact of thermal‐energy metastable neon (Ne^*) atoms have been made using a velocity‐selected beam of the metastable atoms. The composition (³P₂/³P₀ ratio) of the Ne^* atomic beam has been measured using an inhomogeneous‐field deflecting magnet and found to be about 5/1, assuming equal detection efficiency for the two metastable species. The ionization measurements were of sufficient precision to allow simultaneous determination of both the secondary‐electron‐ejection efficiency and the cross section. The secondary‐electron‐ejection efficiency from an argon‐covered gold surface on impact of metastable neon atoms was found to be $\text{0.27± 0.14}$ . In the relative velocity range investigated (g=320–1700 m/sec) the cross section Q was found to vary as g^−m below g=650 m/sec and to rise at higher relative velocities. The value of m obtained is 0.622 for ${\mathrm{Ne}}^{*}\mathrm{–Ar}$ , 0.728 for ${\mathrm{Ne}}^{*}\mathrm{–Kr}$ , and 0.874 for ${\mathrm{Ne}}^{*}\mathrm{–Xe}$ . A simple semiempirical model based on a two‐state impact parameter treatment is used to fit the experimental results at low velocities. The model indicates that the interaction leading to ionization is short ranged.