Credit: SLAC National Accelerator Laboratory
In experiments at SLAC, intense laser light (red) shining through a magnesium oxide crystal excited the outermost “valence” electrons of oxygen atoms deep inside it. When these electrons hit neighboring atoms, the collisions generated light of much higher energy and shorter wavelengths (blue) through a process called high harmonic generation. Rotating the crystal and analyzing the generated light revealed the density of the neighboring atom’s valence electrons, which could not be directly seen before.