Hydrogen atoms in KH 2 PO 4 crystals S. D. Setzler, K. T. Stevens, and L. E. Halliburton Department of Physics, West Virginia University, Morgantown, West Virginia 26506 M. Yan, N. P. Zaitseva, and J. J. DeYoreo Lawrence Livermore National Laboratory, Livermore, California 94551 Received 31 July 1997 Hydrogen atoms have been produced at 77 K in single crystals of potassium dihydrogen phosphate KH 2 PO 4 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 character- istic 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 (HPO 4 ) - 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. S0163-18299803906-X Potassium dihydrogen phosphate (KH 2 PO 4 ), better known as KDP, is an important nonlinear optical material with many applications in the ultraviolet. Among these are the generation of the third 355 nm and fourth 266 nm harmonics of pulsed Nd:YAG yttrium aluminum garnet la- sers. Problems can arise, however, when unwanted defect- related optical-absorption bands are produced by pump lasers operating at high peak powers. Despite their importance, little is presently known about the optically active point de- fects in KDP, including possible defect formation mecha- nisms. Thus, fundamental studies are being undertaken to identify and characterize those defects that limit device per- formance. We anticipate that a better understanding of the point defects will lead to improvements in the power- handling capability of KDP crystals i.e., minimization of optical damage and will permit the growth of more uniform KDP crystals. A recent paper by Davis, Hughes, and Lee 1 reported the formation of a broad transient optical-absorption band cov- ering much of the visible and near-uv region when KDP crystals were exposed to an intense 266-nm laser beam (GW/cm 2 ) at room temperature. The absorbing defects de- cayed nonexponentially over periods approaching 20 s. Al- though their experimental techniques did not allow them to directly identify the defects involved, these investigators noted that the spectral dependence of the transient absorption matched that of a previously reported x-ray-induced spectrum 2–4 in KDP. The defect responsible for this absorp- tion is the (HPO 4 ) - center, which consists of a ‘‘proton vacancy’’ with a hole trapped on the nearest oxygen ion in the adjacent PO 4 unit. These observations led Davis, Hughes, and Lee 1 to suggest that the transient optical absorption was initiated by a two-photon absorption event quickly followed by the transport of a proton or a hydrogen atom and the simultaneous formation of an (HPO 4 ) - center. Marshall et al. 5 also studied the broad transient absorption induced in KDP by intense 266-nm laser beams. These latter investiga- tors determined experimental values for the various param- eters needed to characterize the phenomenon and then used their results to successfully model the effect of the transient absorption on the efficiency of fourth-harmonic conversion in KDP. PHYSICAL REVIEW B CONDENSED MATTER AND MATERIALS PHYSICS THIRD SERIES, VOLUME 57, NUMBER 5 1 FEBRUARY 1998-I BRIEF REPORTS Brief Reports are accounts of completed research which, while meeting the usual Physical Review B standards of scientific quality, do not warrant regular articles. A Brief Report may be no longer than four printed pages and must be accompanied by an abstract. The same publication schedule as for regular articles is followed, and page proofs are sent to authors. 57 0163-1829/98/575/26434/$15.00 2643 © 1998 The American Physical Society