Harmonic beam divergence

Fig. 3.25. The degree of order induced by poling can be determined using the results above to analyse the variation in second harmonic intensity as the poled sample is rotated about an axis normal to the fundamental beam. Generally, the low field limit, given by Equation (3.106), is found to apply and the degree of orientation is low. However, for molecules with large dipole moments th,e divergence from the low field limit shown in Fig. 3.25 can be observed, anil much higher orientation is obtained. The experimentally determined non-linearities are usually the electro-optical (r) and the second harmonic generation (d) coefficients. These are related to the hyperpolarisabilities by ...

At the insertion device machines such as the ESRF and the APS an undulator with a fundamental or third harmonic emission in the 1.0 A region is realistic and will provide exceptional low divergence beams in this beneficial short X regime. An even shorter wavelength than 1.0 A would be attractive for the user but could be achieved only in higher harmonics of the undulator emission. Some possibilities are put forward in the next and final section of this chapter. Table 6.3 provides a compilation of solved structures based on short wavelength data collection in macromolecular crystallography. 

The beautiful combination of properties of SR is so often what is important in making an experiment work. In this context the undulator harmonic would provide a very high intensity at ultra-short wavelength (0.3 A) with a very small divergence beam. 

Beamlines. The greatest advantage of SR beamlines over conventional sources, when dealing with soft condensed matter research, is maybe not even the larger amoimt of photons which can be thrown onto the sample, but more so the fact that these photons are in less divergent beams. This means that optical elements can be used to condition the beam, which is not feasible with conventional sources. Obviously there are some drawbacks as well, but apart from the higher harmonic problem, which is discussed later, these mainly have consequences for the constmction of the beamline and should not concern users of these beamlines. However, it is a fact of synchrotron life that most experimenters using these facilities will one day encounter some problems due to technical failures connected... 

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