If function in peak shape

Przemek_Rzepka

Dear Topas users,

I have data with a highly asymmetric (instrumental effect) first peak, and I would like to fit it with a different function than the remaining peaks. I attempted to use an "if" function defined as follows:

gauss_fwhm = If(2Th < 3, prm1, prm2 * Tan(Th) + prm3 / Cos(Th));
lor_fwhm = If(2Th < 3, prm4, prm5 * Tan(Th) + prm6 / Cos(Th));
circles_conv = If(2Th < 3, prm7, 0);

However, Topas does not recognize the "if" statement and attempts to fit the first parameter (prm1, prm4 and prm7) across the entire 2Theta range. Is there any way to force the program to use a different profile function for the first peak (to a specific 2Theta range < 3)?

I would appreciate any help.

johnsoevans

It's a quick fix answer as I only have a couple of seconds. If you can't get the "if" commands working, you could always split the data. Have two xdd sections, one for low 2-theta and one for high. Then fit the same structural model to both. I know that isn't very elegant!

Przemek_Rzepka

Thank you. I guess the only thing I can do (although it seems bizarre 'if' doesn't work) is to split xdd and define for xdds { for strs {}}

deppaa

An option could also be to use a HKL-dependence instead of the Theta dependence in the If-statement I guess. For example, if it is only the first peak, and lets say it has indices 001, your peak shape could be written as:

gauss_fwhm = If(And(H==0, K==0, L==1), prm1, prm2 * Tan(Th) + prm3 / Cos(Th));
lor_fwhm = If(And(H==0, K==0, L==1), prm4, prm5 * Tan(Th) + prm6 / Cos(Th));
circles_conv = If(And(H==0, K==0, L==1), prm7, 0);

Przemek_Rzepka

Thanks, Ola. That solved the problem.

rowlesmr

Check your units. I think Th is in radians.

gauss_fwhm = If(Th < 1.5 Deg, prm1, prm2 * Tan(Th) + prm3 / Cos(Th));
lor_fwhm = If(Th < 1.5 Deg, prm4, prm5 * Tan(Th) + prm6 / Cos(Th));
circles_conv = If(Th < 1.5 Deg, prm7, 0);

should work.