Kirchhoff s diffraction theory

The beam-defect interaction is modelled using Kirchhoff s diffraction theory applied to elastodynamics. This theory (see [10] for the scattering by cracks and [11] for the scattering by volumetric flaws) gives the amplitude of the scattered wave in the fonn of coefficients after interaction with defects and takes account of the possible mode-conversion that may occur. 

In principle, the diffraction patterns can be quantitatively understood within the Fraunhofer approximation of Kirchhoff s diffraction theory as described in any optics textbook (e.g., [Hecht 1994]). However, Fraunhofer s optical diffraction theory misses an important point of our experiments with matter waves and material gratings the attractive interaction between the molecule and the wall results in an additional phase of the molecular wavefunction [Grisenti 1999], Although the details of the calculations are somewhat involved2, the qualitative effect of this attractive force on far-field diffraction can be understood as a narrowing of the real slit width to an effective slit width [Briihl 2002], For our fullerene molecules the reduction can be as big as 20 nm for the unselected molecular beam and almost 30 nm for the slower, velocity selected beam. The stronger effect on slower molecules is due to the longer and therefore more influential interaction between the molecules and the wall. 

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