Two-Photon Mechanism of the Excitation of Exciton States in Noble Gas Cryocrystals

The results of an experiment on the two-photon excitation of exciton states in krypton cryocrystals by the fifth harmonic of 209-nm laser radiation have been presented. It has been shown that the two-photon excitation mechanism allows reaching a density of 1016 cm−3 of free excitons with an excitation energy of ~8.4 eV at a focal radiation intensity of ≈ 10^11 1pt W/cm2. The observed broad photoluminescence line arising from the relaxation of excited exciton states covers the 3/2⁺(8.4 1pt \;eV) → 5/2⁺(0.0) low-energy nuclear isomeric transition in 229Th. Analytical estimates showing that two-photon laser generation of exciton states in noble gas crystals can be used as a new method for the excitation of the nuclear isomeric state of thorium at a pump intensity of ~1011 W/cm2 have been provided.