Dialog Box: IXS Parameters [Settings menu, Control Window]
This sets parameters for the calculation of Inelastic X-Ray Scattering intensities. The calculated spectrum appears in the Spectra Window, and the output may be sent to a file with the Spectral Data Text File option in the File Menu of that window.
You must specify the location of the calculation in reciprocal space, that is the Q-point or k-point. This can be done by giving the indices of a central zone (uvw) and a 2-theta value, or actual indices, which may be fractional. The choice is made in the radio box to the left, and this determines which of the sets of indices is used.
If you select the Zone and 2-theta, then the actual indices will be given in the Spectra Window; or if you select the actual Indices, then the 2-theta will be calculated and shown in the Spectra Window.
The Base frequency (in cm-1) determines where the calculations starts. The two width values are for the half-width at half height for either Gaussian or Lorentzian lines. You set widths for zero frequency and 1000 cm-1 and the width of each mode is interpolated based on the location of the peak. If the width at 1000 cm-1 is zero it will be set equal to the width at zero cm-1.
The calculation uses a single isotropic Debye-Waller (temperature) factor for all atoms - this is not affected by the Temperature value.
Lineshape. The theoretical lineshape is Lorentzian, but broadening of various kinds typically produces a lineshape that is effectively Gaussian. The Voigt lineshape is a Gaussian convolution of Lorentzian lines - an approximation to it is used in many real-life situations in which the lineshape must be fitted rather closely. If Voigt is chosen the intensity contribution from a given mode is simply a linear combination of Gaussian and Lorentzian terms, with the line width in common between the two terms. Gaussian in Voigt is the fraction applied to the Gaussian term - this fraction is applied to all modes. See P. Thompson, D.E. Cox, J.B. Hastings, J. Appl. Cryst. 1987, 20, 79, for details of this approximation to the Voigt profile.
The Read Observed button calls up the Read Observed Spectra Data dialog.