How do fit a custom model to my data?

I have the following equation: I(theta)=I_0*(cos(beta))^2*(sin(alpha))^2 where: beta=(pi*d/lambda)*sin(theta) alpha=(pi*a/lambda)*sin(theta) I am supposed to fit this function to several sets of data for a nonlinear fit and obtain the values of the constants "a" and "d". I have tried using the line fit application from matlab but I do not get what I want. I am trying to constraint the coefficients to an estimate of what they are supposed to be, but the R-square just becomes worse. This is my data: "xdata = [-0.0050,-0.0045,-0.0040,-0.0035,-0.0030,-0.0025,-0.0020,... -0.00150,-0.0010,-0.0005,0.0001,0.00050,0.0010,0.0015,0.0020,... 0.0025,0.0030,0.00350,0.0040,0.0045,0.0050]; ydata = [1.53,2.36,2.0,2.43,1.56,2.50,2.23,3.03,2.23,2.96,6.26,4.86,... 2.20,4.63,4.83,2.60,2.86,3.13,2.36,2.70,2.73];" Are there any other ways in which I can do this, is there a specific script I can use to fit the function showed above for which I could get a good R-square with close coefficients? I would be most thankful if you could take me through the process with detailed explanations, I am very new at matlab.

[José-Luis]

It might mean that your function is not suited to fit your data. Impossible to say without looking at it.

[Ramiro]

This is the model that I am trying to fit to the data.