Resonant coherent excitation of Mg(sup 11+): Electronic collisions of state specified short-lived excited states in a crystal channel

摘要

Hydrogenic ions passing through axial and planar channels can be excited from n = 1 to n = 2 when the frequency of perturbation by the atoms in the crystal spaced a distance d apart comes into resonance with the spacing between eigenstates i and j (Delta)E(sub ij) = hK(v(sub i)/d) where K is a harmonic 1,2,3(hor ellipsis) of the (v(sub i)/d) frequency. The degeneracy in the n = 2 levels is removed; first by the assymetry in the crystal field and second by Stark mixing of 2s with 2p(sub x) which is caused by the wake field. Thus, the resonant frequency, and hence velocity, for excitation to 2p(sub x,y) is different than that for 2p(sub x) and they can be excited selectively. In the present work we used Mg(sup 11+), where the n = 2 ionization cross section is small enough to permit escape of some of the excited ions from the crystal without being ionized by subsequent collisions and with the subsequent emission of radiation. Since we can excite different orientations of the ion selectively by varying the velocity we can measure the separate ionization cross sections for these states by determining the yields of totally stripped ions compared to those which emit a Ly (alpha) x-ray. A comparison of the two channels shows that the probability of escape from the crystal without ionizations is greater for ions in the 2p(sub x) state than those in the 2p(sub x,y) state. These RCE data and are presented as proof of principal for experiments which measure electron bombardment ionization cross sections for short lived excited states with specific polarization.