Geometry asymmetrical reflection

When reflection geometries are set up in modern scattering applications to study the structure of thin layers, the simplifying assumption of infinite sample thickness is not allowed, and the absorption correction becomes more difficult. Moreover, symmetrical-reflection geometry is utilized less frequently than asymmetrical-reflection geometry with fixed incident angle. Thus both cases are of practical interest. 

For very thin sample thickness t and a scattering angle 29 that is well above the critical angle of total reflection, the exponential factor is approximately unity and a simple background subtraction without consideration of absorption is allowed. Symmetrical-reflection geometry is only a special case of asymmetrical-reflection geometry. 

For asymmetrical-reflection geometry the absorption factor is changed as well, as the primary beam is illuminating the complete sample surface. In this case... 

For asymmetrical-reflection geometry the relation is more complicated. Considering the geometry sketched in Fig. 7.5, the true tilt angle is... 

According to [18,19] and our experience, it is usual that the smaller FWHM for symmetrical reflections (measuring mostly tilt mosaicity) corresponds to a larger FWHM for asymmetrical reflections (measuring together twist and tilt mosaicity). For example, for the MOCVD GaN layer which possessed a 00.2 RC of only 40 arc sec, the 10.2 reflection exhibited an FWHM of 740 arc sec [18], Similarly, for a 0.76 pm MBE layer, Amano et al [19] reported 48 arc sec for the 00.2 reflection and 5226 arc sec for the 10.0 reflection (grazing incidence geometry). The other sample (1.7 pm) possessed 365 arc sec and 1581 arc sec, respectively, for those two reflections. 

Figure 7.5. Relationship between symmetrical (

reflection geometry. Bold bars symbolize the sample in symmetrical (dashed) and asymmetrical (solid) geometry. Incident and scattered beam are shown by dashed-dotted arrows, the incident angle is a = 0 + scattering vector s. For the tilted sample the sample-fixed scattering vector S3 is indicated (after [84])... 

In the heterocycle 16, P and Se high-resolution solid-state NMR spectroscopy was used to study structural properties. Both P CP/MAS and Se CP/ MAS experiments revealed that the asymmetric unit consists of two independent molecules with a different geometry around phosphorus and selenium centres. The established values of anisotropy and asymmetry parameters reflected the distortion of the phosphorus environment, and correlated with X-ray diffraction data. P NMR spectroscopy has been used to follow a new type of P-decomposi-tion in diphosphorylated amines and to characterise new organophosphorus compounds with -N-P(0)-N- linkages. A series of dioxaphosphocin-6-oxides (17), of varying substituent X, have been characterised by using P NMR and... 

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