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Gaussian Fitting with an Exponential Background

This example fits two poorly resolved Gaussian peaks on a decaying exponential background using a general (nonlinear) custom model.

Fit the data using this equation

$y (x) = a e^{- b x} + a_{1} e^{- {(\frac{x - b_{1}}{c_{1}})}^{2}} + a_{2} e^{- {(\frac{x - b_{2}}{c_{2}})}^{2}}$

where a_i are the peak amplitudes, b_i are the peak centroids, and c_i are related to the peak widths. Because unknown coefficients are part of the exponential function arguments, the equation is nonlinear.

Load the data and open the Curve Fitter app.
```
load gauss3
curveFitter
```
The workspace contains two new variables:
- xpeak is a vector of predictor values.
- ypeak is a vector of response values.
In the Curve Fitter app, on the Curve Fitter tab, in the Data section, click Select Data. In the Select Fitting Data dialog box, select xpeak as the X data value and ypeak as the Y data value. Enter Gauss2exp1 as the Fit name value.
On the Curve Fitter tab, in the Fit Type section, click the arrow to open the gallery. In the fit gallery, click Custom Equation in the Custom group.
In the Fit Options pane, replace the example text in the equation edit box with these terms:
```
a*exp(-b*x) + a1*exp(-((x-b1)/c1)^2) + a2*exp(-((x-b2)/c2)^2)
```
The fit is poor (or incomplete) at this point because the starting points are randomly selected and no coefficients have bounds.
Specify reasonable coefficient starting points and constraints. Deducing the starting points is particularly easy for the current model because the Gaussian coefficients have a straightforward interpretation and the exponential background is well defined. Additionally, as the peak amplitudes and widths cannot be negative, constrain a₁, a₂, c₁, and c₂ to be greater than 0.
1. In the Fit Options pane, click Advanced Options.
2. In the Coefficient Constraints table, change the Lower bound for a₁, a₂, c₁, and c₂ to 0, as the peak amplitudes and widths cannot be negative.
3. Enter the StartPoint values as shown for the specified coefficients.
  Coefficients Start Point
  a 100
  a1 100
  a2 80
  b 0.1
  b1 110
  b2 140
  c1 20
  c2 20
  As you change the fit options, the Curve Fitter app updates the fit.
Observe the fit and residuals plots. To create a residuals plot, click Residuals Plot in the Visualization section of the Curve Fitter tab.

Coefficients	Start Point
`a`	100
`a1`	100
`a2`	80
`b`	0.1
`b1`	110
`b2`	140
`c1`	20
`c2`	20