M. Leblans

Influence of OH- impurities on the relaxation of F centers studied with picosecond optical pulses

Abstract The nonradiative electronic relaxation of the F h (OH - ) center in KBr is studied with a pump-probe technique for induced transparency. Relaxation components on pico-and nanosecond time scale are observed. The slowly-decaying contribution is possibly related to conversion between the two different configurations of thid defect.

Femtosecond time-resolved photoelectron spectroscopy of annealed and sputtered GaP(110)

Abstract The relaxation of photoexcited electrons and holes at surfaces of GaP(110) was studied by means of two- and three-photon photoelectron emission under 3.16 eV photon excitation. With 200 fs time resolution, only the relaxation of surface states within the bulk band gap could be resolved occurring with time constants of 0.3 and 2.5 ps.

Femtosecond photolysis of KCl with 6 eV photons

Abstract We have measured transient absorption spectra following creation of electron-hole pairs by 600 fs pulses in the 5.7–6 eV photon energy range in potassium chloride. This is the highest excitation photon energy yet used in sub-picosecond absorption spectroscopy of defect formation in insulators. It allows two-photon excitation of electron-hole pairs in materials including the alkaline earth fluorides. Compared to previous studies on familiar samples such as KCl and KBr, a different distribution of electron-hole pair states is populated initially, and the excitation pulse interacts with a different spectrum of early photo-induced transient absorption.

Ultrafast spectroscopy of defects

The intrinsic luminescence quenching of the F center in NaI and NaBr has been the subject of discussion for many years. The key question was whether the nonradiative electronic transition to the ground state after optical excitation occurs during or after lattice relaxation in the excited electronic state. Ultrafast time resolved techniques showed that in the case of NaBr the electronic transition predominantly occurs from the relaxed excited state (RES), whereas in NaI the electronic transitions during and after lattice relaxation have a comparable efficiency. A similar question raises when the luminescence is quenched by aggregation of the F-center with a molecular impurity. In addition one wants to characterize the electronic-vibrational (e-v) energy transfer associated with the nonradiative F-center relaxation by measuring the population of the vibrational levels after the transfer. Ultrafast spectroscopy can contribute to a better understanding of the e-v transfer process in particular in the case of the OH - perturbed F center : Because of the stronger quenching of electronic luminescence and the much faster (nonradiative) vibrational decay for OH - , much fewer information is available yet than in the case of CN - .

Influence of OH^{-} impurities on the relaxation of **F** centers studied with picosecond optical pulses

Abstract The relaxation of the two configurations of the FH(OH−) center in KBr is studied with a time-resolved induced transparency technique. Multiple decay components are observed. The slowest of these is interpreted as conversion in the ground state between the red and blue configuration of the FH center, on a longer than nanosecond time scale. A contribution of the order of 100 ps might be related to non-radiative electronic relaxation, but cannot be identified with certainty. An unresolved transient, faster than 10 ps, might be related to a cross-over process.

Radiationless electronic relaxation of the F-center in NaI

Abstract The temperature dependence of the ground-state recovery of the F-center in NaI after optical excitations is studied with a pump-probe laser technique. At 10 K the lifetime of the relaxed excited state (tens of ns) is identified together with a much smaller 10 ps contribution possibly related to the Dexter-Klick-Russell cross-over process.