V. A. Pustovarov

Luminescence of F and F^+ centers in corundum upon excitation in the interval from 4 to 40 eV

Abstract Time-resolved excitation spectra of luminescence from F + - and F-centers in α -Al 2 O 3 were analyzed with subnanosecond resolution in the interval from 4 to 40 eV at different orientations of the C 3 optical axis relative to the intensity vector of an electric field E of the exciting light. A new excited state of the F + -center near 6 eV was examined. The time-resolved excitation spectra had a doublet structure at the edge of the fundamental absorption. The structure was interpreted as the manifestation of a Frenkel exciton. It was found that the luminescence decay kinetics of F + -centers changed considerably with increasing excitation energy ( E ⩾ E g , where E g is the band gap energy).

Luminescence VUV spectroscopy of cerium-and europium-doped lithium borate crystals

The paper reports on a study of the luminescence of lithium borate crystals (Li6Gd(BO3)3 doped by Eu3+ and Ce3+ ions, Li5.7Mg0.15Gd(BO3)3: Eu, and Li6Eu(BO3)3) initiated by selective excitation by synchrotron radiation at excitation energies of 3.7–27 eV at 10 and 290 K. Efficient energy transfer between the rare-earth ions Gd3+ → Ce3+ and Gd3+ → Eu3+ was found to proceed by the resonance mechanism, as well as by electron-hole recombination. Fast decay kinetics of luminescence of the Ce3+ activator centers was studied under intracenter photoexcitation and excitation in the interband transition region. The mechanisms involved in luminescence excitation and radiative relaxation of electronic states of rare-earth ions are analyzed, and the energy transfer processes operating in these crystals are discussed.

Self-trapped excitons in LiB3O5 and Li2B4O7 lithium borates: Time-resolved low-temperature luminescence VUV spectroscopy

Results are reported of a coordinated investigation of the dynamics of electronic excitations in LiB3O5 and Li2B4O7 crystals by low-temperature luminescence VUV spectroscopy performed with subnano-second resolution under synchrotron photoexcitation. Data on the photoluminescence (PL) decay kinetics, time-resolved PL and PL excitation spectra, and reflectance spectra obtained at 295 and 9.6 K are reported for the first time; the PL of the borates in the 3.5-eV region caused by radiative annihilation of self-trapped excitons (STE) has been established to have an intrinsic nature; the σ and π STE luminescence bands originating from singlet and triplet radiative transitions have been isolated; the shift of the STE σ band relative to the π band has been interpreted; the LBO recombination luminescence band has been isolated; and the creation and decay channels of relaxed and unrelaxed excitons in lithium borates are discussed.

Low-temperature time-resolved vacuum ultraviolet luminescent spectroscopy of KH2PO4 crystals with defects

Low-temperature photoluminescence (PL) of unactivated KDP crystals under selective synchrotron excitation is for the first time measured with subnanosecond time resolution. Time-resolved PL (2–6 eV) and PL excitation (4–35 eV) spectra, as well as PL kinetics, are measured at 7 K. From the acquired experimental data, luminescent bands related to intrinsic defects of the KDP lattice are identified; in particular, the long-wave band at 2.6 eV is assigned to L defects, and the band at 3.5–3.6 eV is attributed to D defects. An efficient energy transfer over the hydrogen sublattice is shown to take place in KDP at low temperatures. It results in the efficient excitation of L and D center photoluminescence in the fundamental absorption region, at electron transitions to the bottom levels of the conduction band, corresponding to the states of the hydrogen atom. The band gap Eg is evaluated to be 8.0–8.8 eV.

Electronic excitations and luminescence in CsLiB6O10 crystals

The paper presents the results of a complex investigation into the dynamics of electronic excitations in the CsLiB6O10 crystal (CLBO) by low-temperature luminescence VUV spectroscopy with subnanosecond time resolution under photoexcitation by synchrotron radiation. Strong broad-band low-temperature photoluminescence (PL) of the CLBO crystal has been revealed. Data on the PL decay kinetics, time-resolved PL and PL excitation spectra, and reflectance spectra at 9.3 and 295 K are obtained. It is shown that the intrinsic PL of CsLiB6O10 in the 3.5-eV range is caused by radiative annihilation of self-trapped excitons. The channels of creation and decay of relaxed and unrelaxed excitons in cesium lithium borate are discussed. The band gap of CLBO is estimated as Eg≈8.5 eV. A monotonic increase in the excitation efficiency of intrinsic CLBO luminescence at exciting photon energies above 19 eV is identified as the photon multiplication process.

Relaxation of electronic excitations in beryllium oxide: A time-resolved vacuum-UV spectroscopy study

Beryllium oxide crystals are studied by time-resolved optical and luminescence vacuum-UV spectroscopy. The low-temperature luminescence spectra and the luminescence decay kinetics (2.5–10 eV, 1–500 ns) upon selective photoexcitation, and also the luminescence excitation and reflectivity spectra (8–35 eV), are analyzed for BeO crystals with the optic axis aligned parallel and perpendicular to the electric vector of exciting polarized synchrotron radiation. It is found that the radiative relaxation of electronic excitations proceeds through a large number of channels. The excited states of self-trapped excitons are characterized by different multiplicity depending on the excitation energy and the sample orientation.

Electronic excitation dynamics and energy transfer in lithium-gadolinium borates doped by rare earths

This paper reports on luminescence studies of lithium borate Li6Gd(BO3)3 doped with Eu3+ and Ce3+ and Li6Eu(BO3)3 crystals upon selective excitation by synchronous radiation in the pump energy region 3.7–27 eV at temperatures of 10 and 290 K. The effective energy transfer between the rare-earth ions Gd3+ → Ce3+ and Gd3+ → Eu3+ is found to operate by the resonant mechanism, as well as through electron-hole recombination. A study is made of the fast decay kinetics of the Ce3+-center activator luminescence under intracenter photoexcitation and excitation in the interband transition region. The mechanisms underlying luminescence excitation and radiative relaxation of electron states of rare-earth ions are analyzed and energy transfer processes active in these crystals are discussed.

Low-temperature time-resolved vacuum ultraviolet spectroscopy of self-trapped excitons in KH2 PO4 crystals

A complex investigation of the dynamics of electronic excitations in potassium dihydrophosphate (KDP) crystals is performed by low-temperature time-resolved vacuum ultraviolet optical luminescence spectroscopy with subnanosecond time resolution and with selective photoexcitation by synchrotron radiation. For KDP crystals, data on the kinetics of the photoluminescence (PL) decay, time-resolved PL spectra (2–6.2 eV), and time-resolved excitation PL spectra (4–24 eV) at 10 K were obtained for the first time. The intrinsic character of the PL of KDP in the vicinity of 5.2 eV, which is caused by the radiative annihilation of self-trapped excitons (STEs), is ascertained; σ and π bands in the luminescence spectra of the STEs, which are due to singlet and triplet radiative transitions, are resolved; and the shift of the σ band with respect to the π band in the spectra of the STEs is explained.

Luminescence excitation of pure and impure BeO single crystals using synchrotron radiation

Abstract Absorption, reflection, UV and VUV luminescence excitation and thermoluminescence excitation spectra have been measured for BeO and BeOZn crystals in the energy range from 8 to 35 eV, using synchrotron radiation. The nature of electronic excitations in the fundamental absorption edge region is discussed; the value of the forbidden-gap energy, E g , is estimated as 10.6 eV for BeO. Intrinsic luminescence with a 6.7 eV maximum for BeO and impurity luminescence with a 6.0 eV maximum for BeOZn are due to the relaxation of both optically created self-trapped or impurity-trapped excitons. The multiplication effect of electronic excitations with E >2 E g (or E ≥2 E ex for 6.7 eV luminescence) for beryllium oxide is due to the inelastic scattering of hot photoelectrons or hot photoholes.

Low-temperature time-resolved spectroscopy of APb2X5 crystals (A ≡ K, Rb; X ≡ Cl, Br)

The paper reports on a low-temperature (8 K) time-resolved spectroscopic study of excitonic states and radiative relaxation of electronic excitations in undoped APb2X5 crystals (A ≡ K, Rb; X ≡ Cl, Br) performed under selective photoexcitation by synchrotron radiation. The study has revealed a variety of channels of radiative relaxation of intrinsic electronic excitations, which should be primarily assigned to specific features of the electronic structure of the crystal.