Switch point and piecewise function

Hi, I want to fit my data, here is my data

The function is


when R < R1, psi = 0, psi starts to deviate at R1 < R < R2, and psi equals psi maximum at R = R2, with psi_maximum is set at a certain value.

I want to fit the data with these equation and need to know the best value of R1, R2, and alpha.

Can you give me advice or some references to study.


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