Hydrogen atoms in KH2PO4 crystals

Hydrogen atoms have been produced at 77 K in single crystals of potassium dihydrogen phosphate (KH2PO4 or KDP) using either 60-kV x rays or the fourth harmonic (266 nm) of a pulsed Nd:YAG (yttrium aluminum garnet) laser. The electron-paramagnetic-resonance spectrum from these hydrogen atoms exhibits a characteristic 500-G hyperfine splitting and is easily saturated with microwave power. These atoms occupy interstitial sites and are electron traps. The corresponding hole trap is the well-known (HPO4)(-) hole center identified by its 31-G phosphorus hyperfine splitting. Both the hydrogen atoms and the hole centers thermally decay in the temperature range between 80 and 200 K. The observed displacement of protons by 266-nm photons provides direct evidence in support of the proton-transport mechanism recently proposed by Davis, Hughes, and Lee [Chem. Phys. Lett. 207, 540 (1993)] to explain laser-induced transient optical absorption at room temperature in KDP.