The magnetic field and the temperature dependence of both guest and host fluorescence in pentacene-doped tetracene single crystals is studied. From a n analysis of the experimental results i t is concluded that pentacene enters substitutionally into the host lattice sites and that thermally activatedheterofission is site-dependent with activation energies =z (0.13 & 0.01) eV and 5 0.06 eV.From a kinetic fit t o the temperature dependence of the green and red fluorescence, a thermally activated host singlet exciton diffusion coefficient of = 2 x loe2 cm2/s is inferred; its activation energy is estimated to be (0.018 f 0.001) eV. The heterofission rate of Y (4.7 f 0.5) x 1O-IO cm3 s-1 is determined and location of the pentacene triplet level is found to be at (0.86 rf 0.03) eV.Es wird die Abhangigkeit sowohl der Gast-als auch der Wirts-Fluoreszenz in pentazendotierten Tetrazeneinkristallen vom Magnetfeld und yon der Temperatur untersucht. Aus einer Analyse der experimentellen Ergebnisse wird geschlossen, daB Pentazenzentren substitutionell auf Wirtsgitterplatze eingebaut werden und daO die thermisch aktivierte Heterospaltung gitterplatzabhan-

Room-temperature pulsed laser action has been obtained in chromium-activated forsterite (Cr:Mg2SiO4) for excitation of the near-infrared absorption band of the system by the 1064 nm radiation from a Nd:YAG laser. The characteristics of laser emission are similar to those observed for 532 nm pumping. It is suggested that the laser action is due to a ‘‘center’’ other than the trivalent chromium (Cr3+), presumably the tetravalent chromium (Cr4+).

Laser action of Rhodamine 640 perchlorate in highly scattering media consisting of TiO(2) nanoparticles in methanol pumped by a 3-ns 532-nm laser pulse is observed by use of spectral and temporal measurements. The threshold for stimulated emission is dependent on the concentration of both laser dye and scattering particles. For a 2.5 x 10(-2) M dye concentration, the lasing threshold was found to be reduced by more than 2 orders of magnitude when the density of scattering particles was increased from 5 x 10(9) to 2.5 x 10(12) cm(-3). However, for a concentration of 5 x 10(-4) M of Rhodamine 640 the lasing threshold was higher in scattering media than for the methanol host. The pulse duration of stimulated emission for Rhodamine 640 perchlorate in these random systems is ~50 ps.

Spatial distribution of two-photon-excited fluorescence (TPF) for dye beneath the surface of a highly scattering medium was investigated with picosecond laser pulses at 1064 nm. The active scattering media consisted of a suspension of polystyrene particles in a solution of Rhodamine 590 Tetrafluoroborate dye. With the increase of scattering strength of the medium, the location of the maximum TPF intensity was found to move closer to the surface away from the focal region; the intensity of TPF was not confined to the focal region as in the case of nonscattering medium but was more evenly distributed. The spatial resolution of nonlinear optical microscopy for probing a scattering medium is degraded. Taking into account the scattering of the medium qualitatively explains the observed TPF spatial distribution.