As is explained in the answers of this question: Why are most metals gray/silver? nicely, the responsible transition occurs between the 5d orbital and the 6s orbital, which is relativistically shrunk and has therefore a smaller energy gap than, e.g. Silver, to the 5d orbital. But I still have problems to wrap all the other things I believe to know around this, for me "new information": ;-)
(A) Obviously, this effect is already observed in bulk gold. I always thought that for bulk materials we need to consider bands. As this was not discussed in those answers: are here actually transitions between the bands of the 5d and 6s orbitals meant?
(B) If so, where does the metallic properties of gold come from? I.e. the high electrical and thermal conductivity and metallic sheen of gold at longer wavelengths must surely come from electrons that are "swimming" in a conduction band. Clearly, the not fully occupied 5d band would fulfill these conditions. But I know that there are cases where there is energetival overlap between a full valence band an an unoccupied band that lead to metallic properties.
(C) Last but not least, the optical transition from the 5d to the 6s band is an optical one. That means that also the reverse should be possible, as e.g. in a kind of spontaneous or stimulated emission especially, when intense radiation at wavelengths below 500 nm would be supplied. However, I haven't heard or read up to this moment about anything such as stimulated emission from gold, Rabi oscillations or gold fluorescence. Are there any processes that prevent this? E.g. radiationless relaxations? Or I am simply not aware of respective publications?
Thanks a lot!
