>So in non-metals, does the band gap between the valence and conduction bands get crossed by electrons excited by incoming photons?

It is an active area of research and they started exploring 'metallic ammonia' last year. recent paper looking at metallic ammonia using synchrotron x-ray PES in a refrigerated liquid microjet setup: https://science.sciencemag.org/content/368/6495/1086

They are currently known as 'metastable' nanostructures exhibiting plasmonic effects similar to noble nanoparticles.