Polarisation and negative ions of clusters

The first term represents the usual mechanism for radiative capture, while the second results from the polarisation mechanism. The ratio r/ of the partial cross sections of the two mechanisms of capture is approximately p 4 arf(w) 2 jir2R3f, where Rf is the radius of the final state orbit of the electron. Within a giant resonance, the dipole dynamical polarisability is mainly determined by its imaginary part Jmad(oj) = cct (u )/4-k, where cr7(co) is the photoabsorption cross-section of the cluster. 

This mechanism for capture is a very natural one for the formation of negative ions, since it provides a means of radiating the excess energy of the system which is directly associated with the dipole field of the target, and this dipole field is also responsible for attachment. 

From equation (12.10), one finds that interference between the direct bremsstrahlung process and polarisation radiation results in an asymmetry of the giant resonance profile observed in fluorescence with electron excitation even when the corresponding photoabsorption profile is symmetrical. This is exactly analogous to the Fano resonances (chapter 6) bremsstrahlung plays the role of the continuum, while the resonant chan- 

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