Basic Concept and Experimental Realization

The crucial parameter for the resolution in fast-beam laser experiments is the kinetic energy spread of the ions. For low-energy beams ( 100 keV) it is determined by the ion source and typically of the order 1 eV, if the acceleration voltage is well stabilized. To meet this value, plasma sources have to be operated with special care with regard to the pressure and potential distribution within the source. Surface ionization sources may almost reach the thermal energy spread of fcT, but this requires very homogeneous and clean surfaces. 

In estimating the velocity spread of an extracted beam, we assume an energy spread 8E, not considering details of the distribution which strongly depend on the particular ion source. The dimensionless parameter j8 = t)/ c is used for the velocity. For its dependence on the acceleration voltage U and the atomic mass m, it is sufficient to take the nonrelativistic expression 

Of course, the superimposed-beam geometry involves a considerable Doppler shift of the transition frequency in the rest frame of the atoms. In the laboratory frame this is given by 

many collinear fast-beam experiments have been performed on neutral atoms. For this purpose, the ions are neutralized by charge transfer, preferably in passing them through an alkali vapor cell. The charge-transfer cross sections for the reaction 

Moreover, the missing energy A is converted completely into additional kinetic energy of the fast beam. If several states of the atom B are populated in the electron capture, the beam is composed of atoms with several discrete velocities that can be observed in the Doppler-shifted optical resonances. These velocity spectra give direct access to the final-state 

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