Resonant coherent excitation of channeled ions

The hydrogenlike ions ${\mathrm{B}}^{4+}$, ${\mathrm{C}}^{5+}$, ${\mathrm{N}}^{6+}$, ${\mathrm{O}}^{7+}$, and ${\mathrm{F}}^{8+}$, and the heliumlike ion ${\mathrm{F}}^{7+}$ have been shown to exhibit resonant coherent excitation from the atomic level $n=1$ to $n=2$, caused by the periodic potential oscillations which occur as the ion moves in axial and nearly axial planar channels in thin crystals of Au and Ag. Various harmonics of the lattice-string periodicity are effective, at certain critical ion velocities, in reducing the number of one-electron ions which survive passage through the crystal without electron loss. Resonances, appearing as minima in the one-electron charge-state fraction versus velocity, have been found for several axial channels and for several different harmonics of the string frequencies. In some cases the resonances can be split by small crystal tilts which produce sidebands upon the string frequency. Certain harmonics have been shown to produce tilt-insensitive double minima which measure Stark splittings of the $n=2$ levels.