Resonance fluorescence collision broadening

The powerful technique of two-photon absorption, which permits limitations of Doppler broadening to be overcome (see Chapter 1), has been used in a study on sodium atoms in which the Stark effect in the 5s 2Si and 4d D and D levels was observed.188 The radiative lifetimes of the S and D Rydberg levels of Na,189 the use of laser-induced resonance fluorescence for the measurement of small concentrations of Na vapour,170 the quenching of Na(32P) and K(42P) by N2, 02, H2, and HaO,171 the chemi-ionization reactions of photoexcited atoms,172 and excitation of the K(42P ) level in collisions with rare-gas atoms173 have been the subjects of recent reports. 

The relatively simple apparatus, high sensitivity, and the selective nature of the optical excitation used in many resonance fluorescence experiments combine to make this method the most accurate method for measuring atomic lifetimes. In many cases the experimental measurements may be made at densities which are so low that the effect of resonance trapping and collision broadening are completely absent. In these cases the experimental results are usually quoted with errors in the range 3-5 per cent. The method is suitable for precision lifetime measurements and may for this reason allow a set of relative oscillator strengths obtained by the absorption or emission methods to be placed on a reliable absolute basis. 

Comment on collisional depolarization studies. The resonance fluorescence experiments described above have served to stimulate and test the development of recent sophisticated theories of collision broadening. However, the hope that new information on interatomic forces would be obtained by these experiments has not been realized. This is largely because the observed broadening is the mean effect of collisions averaged over all possible relative orientations and over the thermal velocity distribution of the ensemble. It may be that in the future more detailed information on the interatomic potentials could be obtained by combining tunable dye laser excitation and atomic beam scattering techniques. 

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