Nonradiative relaxation and ionization of theFcenter in NaBr studied with picosecond optical pulses

Abstract

With a picosecond pump-probe laser technique the recovery of the ground-state population after optical excitation of the F center in NaBr is studied. Two decay components are observed, the fastest one dominating at low temperatures. Its time constant is interpreted as the lifetime of the relaxed excited state and is established to be 6+1 ns at 10 K. This value is perfectly consistent with the experimental emission efEciency and the expected radiative lifetime, provided that the nonradiative transition to the ground state is assumed to occur after the relaxed excited state is reached. Within the same assumptions a nonradiative lifetime of 27 ns at zero temperature is predicted for the F center in NaI. A theoretical expression for vibronic tunneling to the ground state from a thermalized excited state is applied to the F center in alkali halides and is shown to agree equally well with the experimental emission efBciencies as the Dexter-Klick-Russell criterion. The same expression also accounts for the strong temperature dependence of the nonradiative relaxation process observed in NaBr below 100 K. At higher temperatures an accurate analysis of the radiationless transition rate is encumbered by the infiuence of ionization on the lifetime of the excited state. The relative contribution of the second, much slower decay component increases rapidly above 70 K and is related to retrapping of electrons released by ionization of F and F' centers.

Triplet luminescence of cadmium centres in alkaline-earth fluoride crystals

“…Definitely, the relative Stokes shift 0.19 is too small for luminescence to be observed. Another model, which also well explains the existence and absence of F-centre luminescence, is based on the assumption that the system always reach the minimum of the excited configuration curve and the absence of luminescence is due to horizontal vibronic tunnelling to the ground configurational curve [14]. The correspondence between the two different models is not surprising: lowering the crossover point implies an increased overlap between the vibrational wave functions of the ground and the excited state with the same energy, which is a crucial factor in the horizontal vibronic tunnelling rate [14].…”

Electronic, vibrational, and configurational relaxation of theFH(OH−Gustin1, Leblans2, Bouwen3 et al. 1996Phys. Rev. BSelf Cite10010View full textAdd to dashboardCite

“…For a simpler system, such as the pure F center in NaI, one can assume that the lattice relaxation is the ratedetermining process for the ground-state recovery, and identify the measured time constant with this lattice relaxation. 11,12 The F H (OH Ϫ ) center is a more complicated system, because each configuration of the F H center possesses its own set of electronic levels, coupled to the vibrational levels of the impurity. If one solves the set of rate equations of a model for this center, such as those presented in Fig.…”

“…Measurements on the intrinsic luminescence quenching of the F center in NaBr revealed a relaxation component, attributed to the conversion of F centers to F Ϫ centers and recapture of conduction electrons. 12 Even in undoped KBr, maximally 10% of the F centers can be converted to F Ϫ centers below 90 K at low F-center concentrations. 37 The presence of OH Ϫ impurities reduces the ionization efficiency, even before optical aggregation.…”

“…[9][10][11] The two processes can be distinguished by time-resolved measurements of the ground-state recovery, because the crossover process must occur on the time scale of the lattice relaxation ͑100 fs-10 ps͒, while the horizontal vibronic tunneling process may be much slower. 11,12 The F-center luminescence can also be quenched by aggregation of the F center to an impurity with a highfrequency intramolecular vibration, such as OH Ϫ , OD Ϫ , and CN Ϫ . In hosts with the NaCl structure the presence of CN Ϫ impurities does not completely suppress the luminescence, 13 but aggregation of the F center to OH Ϫ and OD Ϫ does.…”

Nuclear wave-packet dynamics on nearly degenerate two adiabatic potential energy surfaces in the excited state of KIFcenters

“…[16][17][18] In order to understand the NWP behavior, it is necessary to perform real-time measurements. Time-resolved spectroscopy measurements with time resolutions of picoseconds or a few hundred femtoseconds have been done for the F centers in KCl, [19][20][21][22] NaBr, 23 or NaI. 24 Ultrafast pump-and-probe experiments have been performed for KBr F centers with ϳ10 fs laser pulses under the so-called degenerate pump-probe conditions.…”