I would like to calculate the dispersion relation (dielectric constants $\varepsilon'$ and $\varepsilon''$) from two spectra: refractive index $n$ and absorption (in %). I tried to use the relation $$\alpha = \frac{4\pi}{\lambda} \kappa$$ to extract the extinction coefficient $\kappa$ from it and with this calculate $\varepsilon$ from the formulas $$\varepsilon' = n^2 - \kappa^2$$ $$\varepsilon'' = 2n\kappa$$

This results in very wrong-looking numbers, which makes me suspect something is wrong here, probably the first step.

Alternatively, I first calculated $- \log (A)$ from the data to get a value that is proportional to the extinction coefficient $\epsilon$ in Lambert-Beer's law, but I am not sure if this is applicable here. Either way, the values that result from this are even stranger.

How can I find the extinction coefficient (or at least an estimate for $\kappa$) from the absorption spectrum?


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