What function should I fit to this data?

[gothloli]

Homework Statement

I'm supposed to find the first ionization energy of a gas (Argon), and to do that I used mass spectrometer. The data I plotted was amplifier voltage (acts as Ion current) vs. electron volts. The graph appears in the attachment as data2.jpeg.
To find the ionization energy I'm supposed to take the first and second derivative of the data, and the max x-value of the data is the ionization energy. When I do that the plot of the derivatives is not smooth; the points fluctuate. So I decided to fit a function to the data, and then take the derivative.

Homework Equations

I fitted using curve_fit on numpy, python 2.7. and tested the goodness of fit using chi-squared/dof. The fit was bad.

The Attempt at a Solution

I used the logistic function defined here http://en.wikipedia.org/wiki/Logistic_function, with parameters, (0.026, 0.745, 19.0, 0.0005). The fitted function is shown here. Since the fit was not good, is there a better function to fit this kind of data, or is there a better way to find the derivatives of the data?

 

[Quantum Defect]

Homework Statement

I'm supposed to find the first ionization energy of a gas (Argon), and to do that I used mass spectrometer. The data I plotted was amplifier voltage (acts as Ion current) vs. electron volts. The graph appears in the attachment as data2.jpeg.
To find the ionization energy I'm supposed to take the first and second derivative of the data, and the max x-value of the data is the ionization energy. When I do that the plot of the derivatives is not smooth; the points fluctuate. So I decided to fit a function to the data, and then take the derivative.

Homework Equations

I fitted using curve_fit on numpy, python 2.7. and tested the goodness of fit using chi-squared/dof. The fit was bad.

The Attempt at a Solution

I used the logistic function defined here http://en.wikipedia.org/wiki/Logistic_function, with parameters, (0.026, 0.745, 19.0, 0.0005). The fitted function is shown here. Since the fit was not good, is there a better function to fit this kind of data, or is there a better way to find the derivatives of the data?
View attachment 80813

View attachment 80813

Why not do some smoothing first? Numerical Recipes has some methods for doing this.

 

[gothloli]

Why not do some smoothing first? Numerical Recipes has some methods for doing this.

would smoothing the data give a clean first and second differential plot? If so which smoothing algorithm would work best? Since the data was originally plotted with time, would exponential smoothing work?

 

[Quantum Defect]

would smoothing the data give a clean first and second differential plot? If so which smoothing algorithm would work best? Since the data was originally plotted with time, would exponential smoothing work?

You have high frequency noise, with a very clean underlying signal. I would just try some different ways to smooth (moving window is easiest) and see what you get. It will get rid of the high frequency noise, and should clean up the derivatives.

 

[gothloli]

Actually each data point shown above, was an average of an even noisier data set. Since I changed the electron volts every 2 seconds, I took the average of the voltage every two seconds. So using moving average would make the data even more cleaner? Do you know any smoothing functions from numpy?