John P. Spoonhower

Time-resolved spectroscopy of BaFBr:Eu2+

Abstract The time and wavelength dependence of the Eu 2+ 5d-4f emission in BaFBr was measured over the extended temperature range 10–506 K. Excitation was provided by either a nitrogen laser (337 nm)or the third harmonic output of a Nd + -YAG laser (355 nm). At 10 K, the measured emission lifetime is (500±5) ns; at 506 K, the lifetime increases to (968±5) ns. In our kinetic analysis, three parameters are required to fit the temperature dependence of the emission lifetime. We determine the low-temperature lifetime limit, the difference in energy for two relatively close-spaced excited state levels and the degeneracy ratio g -1 , for the two levels. Our best fit to the data yields (508±7) ns, (-476±8) cm -1 , and (3.5±0.1), respectively, for these parameters. Corrected cw emission spectra show that the lineshape is asymmetric at low temperatures; this asymmetry is greatly reduced as the temperature is increased. The spectral dependence of the emission lifetime, however, shows no such asymmetry. There is no variation of the emission lifetime with wavelength, throughout the band, regardless of temperature, as expected for direct substitution of Eu 2+ for the Ba 2+ in this lattice.

Trapped holes in silver halides

Abstract The properties of holes in silver halides are reviewed with emphasis on trapped holes. The chemical and electronic structure of the self-trapped hole in AgCl has been well documented. In contrast, the nature of the intrinsic hole trap in AgBr is still speculative and could benefit from more experimentation. A variety of trapped-hole species, induced by doping of the silver halides, have been identified. Magnetic resonance methods have been the most successful techniques for elucidating the structure of these defects. Little is known about holes trapped in AgF and AgI or in many of the mixed halide crystals.

Radiative recombination in iridium-doped silver bromide single crystals

Abstract The infrared emission spectrum of iridium-doped silver bromide results from a process that is competitive with the visible emission due to iodide centers in pure AgBr crystals. The emitting species is Ir 3+ , and experiments are described which help elucidate the nature of the radiative processes involved.

Site symmetry for Rh3+ in AgBr

Abstract Raman spectroscopy has been used to elucidate the structure of the rhodium(III) defect center in silver bromide. A trigonal ([111] axis of distortion) site symmetry has been deduced from an analysis of polarized Raman spectra of oriented single crystals. There is evidence for a multiplicity of rhodium sites in silver bromide. Plausible models for the defect site observed in the Raman experiments are discussed.

Radiative recombination at Ir3+ sites in doped AgBr

Abstract Low-temperature (≲25 K) photoluminescence spectra of Ir3+-doped AgBr single crystals were obtained. These spectra are highly structured and show a progression in a low-frequency (∼38 cm−1) resonance vibrational mode and the participation of other localized vibrations. A sharp zero-phonon (ZP) line was observed at 15622.9 cm−1. The temperature dependence of the luminescence intensity was determined. The emission is rapidly quenched with temperature so that above 25 K no emission bands are observed. The results of a number of experiments suggest that the emission originates at trivalent iridium sites in the doped crystal. Several physical models are considered as likely descriptions of the emission process: trapped electron/free hole recombination, trapped electron/trapped hole (pair) recombination, or deeply bound exciton recombination. Each is discussed in light of the experimental evidence.

Radiative recombination at Ir3+ sites in doped AgBr. visible luminescence

Abstract Low-temperature ( ⪷ 25 K) photoluminescence spectra of Ir 3+ -doped AgBr single crystals were obtained. These spectra are highly structured; they show a progression in a low- frequency ( ∼ 38 cm -1 ) resonance vibrational mode and the participation of other localized vibrations. A sharp zero-phonon (ZP) line was observed at 15622.9 cm -1 . The assignment of this line as the spectral origin of the luminescence band is based upon photoluminescence excitation spectra obtained with a CW dye laser. In crystals doped with ∼ 80 molar ppm Ir 3+ , the linewidth of the ZP line is ⪷ 2.0 cm -1 . A Zeeman effect on the ZP line was observed. The temperature dependence of the luminescence intensity was obtained. The emission is rapidly quenched with temperature, so that no emission bands are observed above 25 K. The results of several experiments suggest that the emission originates at iridium sites in the doped crystal. Several physical models are considered as likely descriptions of the emission process. These include trapped carrier/free carrier recombination, trapped electron/trapped hole (pair) recombination, and deeply bound exciton recombination. Each is discussed in light of the experimental evidence.

Spectroscopic frequency calibration of tunable dye lasers

A simple, inexpensive technique for optogalvanic frequency calibration of tunable dye lasers is presented. Its feasibility is demonstrated in the acquisition of the photoluminescence excitation (PLE) spectrum of a weakly emitting species. This experiment reveals a number of problems inherent in the use of tunable dye lasers in low-light-intensity investigations, for example, in obtaining PLE spectra or in obtaining resonance Raman enhancement profiles.

Multiple localized exciton species in sulfied-doped AgBr detected by luminescence and ODMR

Abstract The low temperature photoluminescence of sulfide-doped AgBr has been examined with both steady state and pulsed excitation. Emission bands peaking at about 495, 566, 596, and 725 nm are observed, depending upon excitation conditions. Subband-gap excitation resonances at 464, 496, 530, and 558 nm are observed, depending upon monitoring wavelength. Excitation at 496 nm preferentially enhances 596 nm emission, which gives rise to an ODMR spectrum. ODMR data indicate that at least three species are contributing to the 596 nm emission. These species are interpreted as triplet excitons. A highly antisotropic species observed in the ODMR spectra is interpreted as a triplet exciton bound to a substitutional divalent sulfur ion/interstitial silver ion complex with approximate [111] symmetry.

Characterization of an intermediate case exciton in the emission of Cd-doped and pure AgBr

Abstract Pulsed emission data and ODMR spectra of the 580 nm band provide evidence for an intermediate case exciton in pure and Cd2+-doped AgBr. This interpretation is based on the observed g-factor and the absence of a wavelength shift of the emission after pulsed excitation in pure AgBr. It is supported by the response of the ODMR spectra as the microwave modulation frequency is changed. The two central ODMR lines observed in AgBr (g = 1. 78) are assigned to transitions from the ms=±1 levels to the ms = 0 level of an intermediate case triplet exciton, which has an exchange coupling (singlet-triplet splitting) of -1. 9 ± 0. 2 cm−1.