# How can I assign to pure metal reflectance spectra a precise transition to every absorption band?

I am dealing with some reflectance spetra of pure or mixed metal, so no ion complex usually. I have some reflectance spectra that show bands with lower reflectance thay my be linked with absorptions. I would like to understand which transition could lead to that absorption.

For istance in the case of copper in the NIST website I have found all the energies levels of the pure copper Neutral Copper ( Cu I ) (I guess that ( Cu I ) is not a Stock notation otherwise oxidation state would be 0) but I can't understand how to derive the transition from the energy level, theoretically if a energy level it at a certian energy $E_{1}$ and another level it is at another energy $E_{2}$ transition should occur when the photon has energy $\Delta E$. How can I figure out the possible $\Delta E$ where transitions may occur? What are the selection rules?