Laser-induced energy gap transitions in lead iodide crystals

Single crystals of the common polytype 2H of lead iodide were grown by zone melting and subjected to argon ion laser irradiation. Both time and intensity of radiation were systematically varied to assess their effect on the magnitude of band gap of the crystals. The band gap was determined by UV spectrophotometry. When the beam intensity was varied, the band gap value steadily decreased from 2.53 to 2.02 eV, but the value drastically decreased from 2.53 to 1.04 eV when the irradiation time was varied. The results are explained in terms of changes in polycrystallinity, surface roughness, amount of absorption, and anisotropic nature of thermal conductivity of the material.