Observation of Transient Effects

An excited atom located at the point ta can be considered as an oscillating dipole )/(rfl,f v) given by Eqs (7.20) or (7.21) depending on the sign of Vz- Its radiation field is given by a fwo-dimensional Fourier integral 

Obviously, for such waves we have 9 9c with 9c the critical angle. The evanescent waves transmitted across the interface can be observed at 9 9c (Fig. 7.3). Due to the decaying character of the evanescent field, only atoms at distances 

These results can be understood as follows. In the absence of phase correlations between different atoms, the intensity of the radiation field is determined by the squared modulus of (7.33) averaged over the atomic velocities, i.e.. 

The fluorescence signal observed below the critical angle originates from a large vapor volume of a thickness given by the absorption length, L. The contribution of the transient term in Eq. (7.21) which is essential at 2 Wr/7 L can be neglected. The spectrum is then determined as 

The fluorescence registered above the critical angle results from radiation of atoms located at 2 Vj/j- At negative detunings from resonance (the laser frequency is less than the atomic transition frequency) only the atoms moving to the interface are excited due to the Doppler effect. They are in the laser field for a long time L/vt) and their excited states are highly populated. At pos- 

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The observation of transient effects is closely connected to the feature of Doppler switching by sudden changes of the ion velocity. Applications include cascade-free lifetime and quantum beat measurements by observation of the free decay after a short excitation region. Following the first experiments by Andra/ the alternative to produce a short excitation pulse by crossing the ion and laser beams has been applied extensively (cf. Part B, Chapter 20 by H. J. Andra). 

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