Excited-state absorption in excimer-pumped CaWO4 crystals

We report excited state absorption (ESA) in a CaWO4 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 CaWO4 crystal. The ESA spectra can be assigned to the transitions between terms of (t51, 2e) and (t51, 4t2) configurations of the WO2-4 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 4t2 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.