Multiphoton Transitions in Ionic Crystals

Photoconductivity measurements in uncolored single crystals of NaCl, KCl, and KI are reported. The exciting source is the beam of a $Q$-switched ruby laser ($\hslash \omega =1.78\mathrm{}$ eV) and its second harmonic ($\hslash \omega =3.56\mathrm{}$ eV). The experimental results show clear evidence of multiphoton absorption processes. The nonlinear cross sections ${\gamma }_n$ of two-, three-, four-, and five-photon transitions have been measured and compared with the cross sections predicted by the existing theories.