Transient optical absorption of hole polarons in ADP (NH4H2PO4) and KDP (KH2PO4) crystals

A study of transient optical absorption of the ADP (NH4H2PO4) and KDP (KH2PO4) nonlinear crystals in the visible and UV spectral regions is reported. Measurements made by absorption optical spectroscopy with nanosecond-time resolution established that the transient optical absorption (TOA) of these crystals originates from optical transitions in the hole A and B radicals and the optical-density relaxation kinetics is rate-controlled by interdefect tunneling recombination, which involves these hole centers and the electronic H-0 centers representing neutral hydrogen atoms. At 290 K, hole polarons and the H-0 centers undergo thermally stimulated migration, which is not accompanied by carrier ejection into the conduction or valence band. The slow components of the TOA kinetics with characteristic times from a few tens of milliseconds to a few seconds can be assigned to diffusion-controlled annihilation of hydrogen vacancies associated with impurity or structural defects. (C) 2002 MAIK "Nauka/Interperiodica".