King Analysis

In this project, I wrote a program which automatically performs the so-called King analysis of molecular desorption spectra.

king-0The background for this work is the development of new methods which will allow theoretical physicists to make ever more precise predictions of the properties of chemical compounds that haven’t even been synthesized. In the course of developing new theoretical methods, these methods need to be tested by comparing their predictions to experimental results.

More specific, an important benchmark of the theoretical models in question is the strength of the bond between a molecule that is in contact with a surface, and that surface. This quantity can be measured experimentally through temperature-programmed desorption. I am not going into the details of this technique but suffice it to say that the relevant information cannot be extracted from a single spectrum but only from a series of spectra. The procedure to do so is known as the King analysis.

The following graphic shows the experimental data set:

king-1Using Igor Pro’s internal scripting language I wrote a program that analyzes each of these spectra and performs the rest of the following procedure automatically.

In a first step, each of the spectra is integrated. The figure below shows the result, which is a set of smooth curves.


Now, the intersection of each curve with a set coverage (on the y axis) is determined. The corresponding temperature (on the x axis) is plotted together with the associated intensity value from the original spectrum (inset in the top right corner). Plotting data points in a logarithmic plot gives a linear relation, as evident from the below left plot. Each color represents one coverage.


Finally, each of these distributions (of one color) is fitted with a linear model, the slope of which is plotted in the above right diagram. The axis intersection of that plot gives us the strength of the bond between molecule and substrate.

Experiments analyzed with this program were published in Physical Chemistry Chemical Physics (article freely available on arXiv) and Journal of Physics: Condensed Matter.