Electron removal fromH0(n) in fast collisions with multiply charged ions

The cross sections for electron removal from highly excited ($n=9-24\mathrm{}$) hydrogen atoms in fast collisions with multiply charged ($q=1-5\mathrm{}$) N, O, and Ar ions were investigated in an ion-atom crossed-beams experiment. The ion-atom collisions occurred inside a deflector where a moderate electrostatic field of up to 1.8 kV/cm was applied. The range of collision velocity ($v_c$) investigated is $v_c=1.0\mathrm{}{v}_1-2.0\mathrm{}{v}_1$, where $v_1=2.2\mathrm{}\times {10}^8$ cm/s is the Bohr velocity. The electron-removal cross section was found to be independent of ion species for a given $q$ and $v_c$, to increase as $q^2$ for a given $v_c$, and to decrease as $v_c^{-2\mathrm{}}$ for a given $q$. These $q$ and $v_c$ dependences of the experimental cross section are in accord with classical Coulomb ionization theories. The experimental $n$ dependence of the cross section differs significantly from the theoretically predicted dependence, but the difference can be accounted for if we assume the presence of the external electric field in the collision volume reduces the ionization energy.