Recoil shift

In addition to the Rydberg constant a number of different quantities, all based on intrinsically accurate frequency measurements, are needed. Experiments are under way in Stanford in S. Chu s group to measure the photon recoil shift free = fmh/2mcs( of the cesium Di line [48]. Together with the proton-electron mass ratio mp/me, that is known to 2 x 10-9 [49] and even more precise measurements of the cesium to proton mass ratio mcs/mp in Penning traps, that have been reported recently [50], our measurement has already yielded a new value of a [45]. 

In the optical region the recoil shift Aco is extremely small and well below the natural linewidth of most optical transitions. Nevertheless, it has been measured for selected narrow transitions [1113, 1114]. 

Since such small splittings can only be observed if the width of the Lamb peaks is smaller than the recoil shift, all possible broadening effects, such as pressure broadening and transit-time broadening, must be carefully minimized. This can be achieved in experiments at low pressures and with expanded laser beam diameters [1117], An experimental example is displayed in Fig. 9.3. 

Since the Doppler broadening is large compared to the recoil shift and the homogeneous linewidth, the probability that the emitted photon is absorbed by any atom is PtoM = n R a = 1. 

In the optical region the recoil shift is extremely small because of the... 

Fig, 13,8. Recoil shift of Bennet holes in the lower state population (b) and of Bennet peaks in the upper state population (a). Recoil doublet in the output of a laser with internal absorption cell, generated by the shift of the Lamb dip against the Lamb peak... 

Nonetheless, even these relatively crude measurements permitted a determination of the H-D isotope shift to within about 6 MHz or 1 part in 10, sufficient to obtain first qualitative experimental evidence for the 11.4 MHz relativistic recoil shift of the hydrogen ground state. And a comparison of the 1S-2S interval with the n=2 to 4 interval, observed by Doppler-free polarization spectroscopy of the Balmer-g line, provided an experimental value of the ground state Lamb shift accurate to within 0.4 percent. 

Owing to these recoil shifts resonant absorption of Y-rays by nuclei cannot generally take place. Those nuclei which demonstrate the Mossbauer effect form an exception to this rule. 

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