Alternating-sign photostimulated resonant electrical conductivity of chromium ions in ruby
Basun S. A., Kaplyanskii A. A., Feofilov S. P.
A current observed during the excitation of Al2O3:0.4% Cr in the R lines changes sign as the frequency is scanned over the lines. This photocurrent results from a hopping of excess charge along Cr3 + ions excited into the 2 Ε state. The sign of the current changes because of a spectrally dependent, spatially asymmetric excitation of Cr3+ ions in the Coulomb field of the charge carriers (Cr2+ or Cr4 +).