Observation of the Double resonant coherent excitation of highly charged ions in crystals

An ion passing through a crystal feels the periodic Coulomb potential of the target like a fast oscillating electromagnetic field with frequencies in the x-ray range. If the field frequency matches the energy difference between two electronic states a resonant excitation of the ion became possible. The onset of the process can be steered by tuning the relative orientation of the incoming ion velocity and the crystallographic orientation. If the crystal orientation permits simultaneously the frequencies for two different electronic transitions, a double resonant excitation process became possible. This contribution present experimental evidence of the double excitation process in relativistic H-like and He-like Ar ions observed by means of resonance coherent excitation (RCE) using a mono-crystalline Silicon target. The experiment was performed by using beams of Ar ions at few hundreds MeV/u incident energy and a 1 μm thick crystal. The double resonance was detected by measuring the charge state distribution of the ions after the interaction with the crystal and the yield of the x-rays emitted during the decay of the excited states as function of the relative orientation of the target to the beam direction. Resonance spectra for transitions from n=1 to n=3,4,5 levels were obtained.