Cz. Koepke

Excited-state absorption in excimer-pumped CaWO4 crystals

Abstract We report excited state absorption (ESA) in a CaWO 4 under excimer laser excitation (308 nm). The ESA spectrum proves to be very broad and intense. We calculate the stimulated emission cross-section and show that it is at least one order of magnitude smaller than the broad-band ESA cross-section. This precludes laser action in an undoped CaWO 4 crystal. The ESA spectra can be assigned to the transitions between terms of (t 5 1 , 2e) and (t 5 1 , 4t 2 ) configurations of the WO 2- 4 complex. Using experimental parameters deduced from the emission and ESA spectra, we construct a configuration coordinate diagram including a higher excited state. The diagram predicts that the initial and terminal states of the ESA transition are characterized by different equilibrium positions and, consequently, different charge distribution. It is suggested that this effect can be explained by contribution of p orbitals of the ligand ions to the 4t 2 state. The overall ESA is of a complex nature, being a combination of conventional ESA, decaying like luminescence and another, long lasting component possibly connected with phosphorescent traps. This behavior is reproduced by a simple kinetic model.

Stoichiometric cerium compounds as scintillators. I. CeF/sub 3/

The authors present a study of the scintillation cerium trifluoride, CeF/sub 3/. They have investigated spectroscopy of CeF/sub 3/ using optical and ionizing radiation excitations. It was shown that rapid quenching of Ce emission in this material was responsible for the loss of about 50% of the light output and generated the so-called ultra-fast component. This ultra-fast component provided evidence of nonradiative quenching, and therefore limited the efficiency. Additional competition came from unidentified centers, probably due to perturbed Ce ions. Although the light output of CeF/sub 3/ was only about 50% of BGO, the potential for large improvement was demonstrated. >

Excited-state absorption in ZnWO4 crystal

Abstract We report the first measurement of Excited-State Absorption (ESA) in a zinc tungstate (ZnWO4) crystal. The ESA is assigned to the 2e→4t2 transitions and, in the simplest picture, can be related to the 10Dq energy in Td symmetry. The breadth and height of the ESA spectrum, compared with stimulated emission spectrum, precludes any possibility of laser action in this material. The very high strength of ESA transitions suggests a triplet character of the terminal ESA term, originating in (t51,4t2) configuration, and breaking the Laporte selection rule. The latter is consistent with considerable contribution of p-orbitals of the ligands and the tungsten ion in the terminal ESA state.

Excited state absorption in CaMoO4

Abstract We report the first measurement of excited state absorption (ESA) in a calcium molybdate (CaMoO 4 ) crystal, and compare the ESA spectrum with a stimulated emission spectrum calculated in terms of the McCumber theory of phonon terminated lasers. It appears that the magnitude of ESA would preclude laser action in this crystal. The temporal ESA dependence has been measured and discussed in terms of the most likely assignment of energy levels involved in ESA.

Cerium compounds as scintillators

Stoichiometric Ce-materials with negligible Ce-Ce interactions should have superior scintillator properties. The authors present two materials: CeF/sub 3/ and Ce/sub x/La/sub 1-x/P/sub 5/O/sub 14/. While cerium trifluoride is a known scintillator, pentaphosphate is of a limited usefulness, except as a remarkable model material. It is shown that quenching in fluoride is responsible for loss of 50% of the light output and is the cause of the so-called ultrafast component (2 ns). The light output of fluoride (about 50% of BGO) could be significantly improved. It is concluded that deeper understanding of Ce systems is needed to fully exploit their potential.<<ETX>>

Glasses containing closed-shell transition metal complexes: 1. Luminescence

Abstract We report the first spectroscopic evaluation of several new glasses, containing molecular complexes of transition metal ions. Under 308 nm excitation these glasses show broad and efficient luminescence, peaking in the blue and green. We have measured the temperature dependence of the total light yield and decay time. The thermal quenching of the luminescence appears to be very small for zirconate glasses between low and room temperature.

Glasses containing closed-shell transition metal complexes: 2. Excited-state absorption

Abstract We have evaluated several new glasses, containing molecular complexes of d 0 transition metal ions, from the point of view of their suitability as possible tunable laser materials. Using as excitation source the 308 nm line of an excimer laser, we found that in spite of the relatively large quantum efficiency, these glasses have very strong excited-state absorption (ESA) of cross-sections about three order of magnitude greater than the stimulated emission cross-sections. The ESA spectra are very broad and their maxima are shifted to the violet or ultraviolet region with respect to luminescence, however in view of its high level, ESA still precludes any laser action in these materials. Nevertheless, one of these glasses (niobate-doped) suggest a direction for further development and possibly reduced ESA.

Excited state “photochromism” in bismuth germanate

Abstract We report first time observation of transient excited state absorption (ESA) whose magnitude increases with the intensity of the probe beam. The effect is observed in bismuth germanate (Bi 4 Ge 3 O 12 ) and behaves as a sort of an excited state photochromism (ESP). In this material ESA increases from ≈ 20% up to ≈ 76% when the probe beam intensity increases from 1 to 16 W / cm 2 . It has been measured with two different methods and occurs even with a Xe flash lamp used as a probe beam source. Several possible mechanisms of the ESP effect are considered.

Excited state absorption in Bi4Ge3O12

Abstract Excited state absorption (ESA) spectra can sometimes be distorted by the dependence of ESA on probe beam intensity. An example of such a case is encountered in bismuth germanate, where severe distortion of ESA spectrum is caused by the strong increase of ESA with the intensity of the probe beam. We describe a method which extracts the true ESA spectrum and allows to ascribe the effect to concrete states. The method may be useful in other cases when strong dependence on probe beam intensity occurs.

Thermal effects in excimer-excited bismuth germanate

An interesting thermal effect associated with intense ultraviolet excitation of bismuth germanate (Bi4Ge3O12) crystal is found to increase the excitation rate approximately 2 orders of magnitude beyond that determined from excitation intensity and absorption. The effect involves a nonlinear increase of absorption of thermal origin owing to a shift of the absorption edge and indicates the need to take this shift into account when dealing with excited-state absorption and gain measurements.