Radiative decay of self-trapped excitons in CaMoO₄and MgMoO₄crystals

Abstract

Spectroscopic properties of CaMoO 4 and MgMoO 4 crystals were studied in view of their application to cryogenic scintillation detectors. Luminescence spectra and the luminescence decay kinetics were measured over a wide range of temperatures (8-300 K). For the first time we measured time-resolved luminescence spectra of CaMoO 4 . In addition to the green emission arising from the triplet state of self-trapped excitons (STEs), a new band at around 430 nm with a decay time constant 10 ± 3 ns was observed at T = 8 K. This emission is assigned to the radiative decay of a singlet STE.The relaxation of electronic excitations in the crystals under study is discussed on the basis of our current understanding of their electronic structures and a configuration coordinate model for the radiative decay of STEs. The model includes adiabatic potential energy surfaces (APESs) associated with singlet and triplet states and explains the variation of the luminescence kinetics with temperature as a result of a re-distribution in the population of these states. Thus, judging from the change of the singlet STE emission due to temperature variation, we infer the existence of an energy barrier between the singlet and triplet APESs. The multi-exponential character of the decay of the triplet emission can be understood assuming that the relevant radiative transitions originate from different minima of the triplet APES. Non-radiative energy transfer processes control the population of these states, resulting in thermal variation of the intensities of the different emission components.

Extensive studies of CaMoO4 crystals for dark matter experiments

“…It makes the crystal to be "yellow" and expose additional experimental problems for absolute light yield measurements. The shape of the measured spectrum (filled rectangles) coincides well with spectrum (line) reported in [11][12][13]. To register enough photons one should use a proper photodetector with sensitivity matching the crystal's light emission spectrum.…”

“…Waveforms of output signals of both PMTs are digitized and processed to measure the total charge and arrival times of scintillation photons. Preliminary results demonstrates that the crystal scintillation paraneters temperature dependences differ substantially from previously reported in [11,12,13], for scintillation light yield in particular. In Fig.…”

“…So far it has been claimed in a number of publications that CaMoO 4 crystals have just a simple single exponential scintillation emission with only one decay time constant of 15-17 s [9,10,11,12]. Only in reference [13] the existence of a fast scintillation component with a decay time constant of 10 ns was reported. However, it was observed at low, cryogenic temperature of ~10 K. In our measurements we observed for the first time the fast scintillation emission components of CaMoO 4 crystal at room temperature seen after irradiation by -particles with E  =5.5 MeV from a 238 Pu and by -quanta with E  =662 keV from 137 Cs.…”

Pulse shape analysis with scintillating bolometers

“…the production of scintillation light and phonons and their collection in the sensor) can not be considered instantaneous even in slow detectors such as the bolometers. Indeed, at low temperature the averaged scintillation decay time of molybdates 11,12 is of the order of hundreds of µs and therefore it is comparable to the typical rise time of the heat signal of our scintillating bolometers. In the following a possible simplified model to explain the pulse shape discrimination observed in scintillating bolometers will be presented.…”

Studies of scintillation properties of CaMoO4 at millikelvin temperatures

“…At low temperatures, it exhibits a broad band with a maximum at around 540 nm. 26,27 The emission transitions originate from the two triplet levels 3 T 2 and 3 T 1 and terminate at the 1 A 1 ground state. Thus, both triplet levels contribute to the emission that results in the spectrum and decay kinetics composed of few components.…”