Lifetime Measurements in Fast Atomic Beams

This method, which is a modernized version of the old Wien method, uses fast atomic, molecular, or ion beams with kinetic energies in the keV to MeV range. The atoms which move into the x direction are excited at a well-defined small interval ax around x = 0. The excitation source may be a laser or collisions with other atoms in foils or gas chambers. The subsequent fluorescence Ip- (x) is measured as a function of the distance x from the point of excitation (see Fig.11.17). The transformation to a time scale Ip (t) uses the relation x = vt where the velocity v=(2e U/m) of the ions is determined by the acceleration voltage U and the mass m of the ions. Neutral atoms or molecules can be produced from ions by charge exchange. 

The energy loss during charge exchange is negligible in most cases. 

The accuracy of the method is in principle limited only by the accuracy of distance measurement, and many lifetimes of highly excited atoms or ions have been measured this way (beam foil spectroscopy) [11.18b]. However, a severe drawback of this method with nonselective simultaneous excitation of many upper levels results from cascade effects. The level population excitation 

Influence of cascade transitions on measured decay curves. 

N (t) decays by fluorescence to lower levels but is simultaneously fed by radiative transitions from upper levels (Fig.11.18a). We obtain the rate equation 

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