Wavelength extrapolation to zero extinction: a gamma-ray case. The quantitative effect of the extrapolation function on the kinematical limit value.
To ensure that a true zero-extinction kinematical limit value has been attained by extrapolation of a series of measurements on one reflection, the proper dependence of a function of F versus the function of the physical variable involved in the measurements has to be identified. To demonstrate this point, the multiwavelength gamma-ray data on seven reflections of NiF(2) reported by Palmer & Jauch [Acta Cryst. (1995), A51, 662--667] have been utilized. A new physical component has been introduced into the relationship between diffracted intensity and wavelength--that due to the decrease in angular divergence of diffraction from crystallites with decrease in wavelength. For gamma-rays, this leads to a function of F(2) in respect of wavelength, viz F(2) = F(0)(2) - alphalambda + betalambda(2), which is different from that derived from Zachariasen-type models, viz F(2) = F(0)(2) - klambda(2). Comparison of the limit values according to Palmer & Jauch and according to Mathieson & Stevenson demonstrates the advantage of the functional dependence proposed in this study.
Mathieson
A McL
AM
Chemistry Department, La Trobe University, Bundoora, Victoria 3083, Australia. s.mathieson@latrobe.edu.au
Stevenson
A W
AW