Diffraction-limited value

The 500 nm size is a limit value crystallites below this size tend to broaden the diffraction peaks in a spectrum, while size distributions above this value produce particularly sharp signals whose half width is a function only of the wavelength of the X-ray beam and the equipment. Signal broadening is at its maximum in materials known as X-ray amorphous substances, featuring particle size distributions below 8 nm. These afford flattened, washed-out spectra of little analytical value. 

Figure 7.4 The crosses are computed from Mie theory. Scalar diffraction theory and geometrical optics predict the limiting value 1.067. From Bohren and Herman, 1979.

The heat conductance through the sample and in the plasma is responsible for the fact that with the Nd YAG lasers available today, the crater diameters are still much wider than the values determined by the diffraction limitations. When using conventional lasers with pulses in the ns and ps range the plasma shields the radiation, whereas with the femtosecond lasers that are now available a free expanding plasma is obtained, where the heating of the plasma appears to be less supplemented by the laser radiation. This leads to less fractionated volatilization of the solid sample and differences in crater shape, which need to be investigated further [229]. 

Fully hydrated samples of dipalmitoylphosphatidylcholine (DPPC, 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine), the lipid used in the first studies with synchrotron radition show a series of three well-defined transitions in the temperature range between 15 and 45 °C which are termed sub- (around 17 °C), pre- (around 33 °Q and main transition (41,4 °C) for a detailed investigation of the static X-ray diffraction behaviour of this system, see Refs. The underlying structures are schematically depicted in Figure 6a. The transition parameters, temperature and enthalpies, are constant only above a limiting value of full hydration (25 moles H O per mol DPPC, i.e. approximately 38 weight- % H O), and are sensitive also to the ionic environment. 

The independence of the maximum adsorption (or the lowest possible area per molecule in the dense adsorption layer. smin = 5,) of the surfactant chain length can only be explained if we assume that as the adsorption values reach Tmax, the surfactant molecules are closely packed and oriented normal to the surface. Estimates of the limiting values of adsorption, Tmax b / R T, from the experimental a(c) dependencies, with the successive evaluation of minimum area per molecule, s, = 1/NA Tmax, for carboxylic acids, are -0.21 nm2, which agrees with the values established by other methods, e.g. by X- ray diffraction on surfactant crystals. 

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