Comparison with photon scattering cross sections

4 Comparison with photon scattering cross sections 

Whilst photons have wavelengths similar to interatomic distances for crystallographic work, or energies similar to molecular vibrational energies for spectroscopic work, a neutron has both, i.e. neutrons are suitable for simultaneous elastic and inelastic measurements. 

Photons are scattered by the electrons around an atom heavier atoms, with more electrons, scatter much more strongly than light atoms. To emphasise this trend some X-ray scattering cross sections (based on the Thomson free electron cross section, oj == 0.7 bam) are given below. These can be compared with the neutron cross sections given in Appendix 1. 

From one vibration to another there can be considerable variation and, generally, the displacement of electron rich atoms will influence the spectral intensities more than electron poor atoms. Thus, since polarisability increases with atomic size, Raman spectra involving the heavier atoms are easier to observe, as demonstrated by the weakness of hydrogen s vibrational features in metal hydridocarbonyls ( 11.2.6). It also follows that the optically strong features of a spectmm may mask weaker vibrations of interest, a common problem in intercalate and catalytic systems. 

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