Hi ouss,
There are a few minor mistakes (e.g., cannot use [...], but (...) can be; need '*' for multiplication), and the following function form should work in the Fitting Function Builder tool:
Function Type: Origin C
Parameters: a0,a1,a2,W
Function body:
y=(a0*(1.0-exp(-2.0*(x/W)^2 ))+a1*(1.0-exp(-2.0*x^2 )*(1.0+4.0*(x/W)^4))+a2*(1.0-exp(-2.0*(x/W)^2 )*(1.0+8.0*(x/W)^4-8.0*(x/W)^6+4.0*(x/W)^8)))
Hope this helps.
--Hideo Fujii
OriginLab