Ph. Pee

Spectra of direct excitation of phosphorescence in an isotopic mixed naphthalene crystal

(1978). Spectra of direct excitation of phosphorescence in an isotopic mixed naphthalene crystal. Molecular Physics: Vol. 35, No. 2, pp. 595-599.

Triplet exciton transfer and fusion in isotopically mixed molecular crystals

We present experimental results on triplet energy transfer from traps to super‐traps and on triplet exciton fusion in substitutionally disordered naphthalene crystals, monitored by the phosphorescence and by the delayed fluorescence of the traps and of the super‐traps. The concentration dependence of these emissions at fixed temperatures has been discussed abundantly by other authors. In this work, we present new results on their dependence on temperature and on the source of excitation, which have, respectively, been little or not at all reported on before. Comparison of experiments done with selective and with nonselective excitation of the traps shows that the results depend on the method of preparation of the trap excitons. The existence of threshold temperatures for these emissions is interpreted as evidence of phonon assisted processes, for which mechanisms are proposed. A significant drop in transport efficiency at high temperatures is observed for trap concentrations from 4% to 8%. This drop with ...

Characterisation of species of triplet miniclusters in the system N-h8/N-d8. Cluster—cluster interactions and model inhomogeneous distributions in a two-dimensional lattice

Abstract A full spectroscopic characterisation of triplet miniclusters of N-h8 in a crystai of N-d8 and their interactions via the lattice states is presented in a two-dimensional lattice. Triplet mini-exciton energy levels, transition moments and polarisations are calculated. A randon trial model is used to calculate the effective (spectroscopic) concentration of the different clusters and their topology (spatial distribution and orientation in a two-dimensional 100 × 100 lattice, Spectra, and cluster—cluster couplings leading to transfer or exciton fusion are calculated for local concentrations. It is shown that the average concentration is a very poor parameter for predicting cluster-cluster interactions or cluster spectra lineshapes. The knowledge of inhomogeneity of traps is also important when direct excitation of triplet traps is performed.

Spectra of direct excitation of phosphorescence in an isotopic mixed naphthalene crystal. High resolution excitation of the two Davydov components of the N-d8 host crystal

Abstract Direct excitation spectra of the triplet exciton band of deuterated naphthalene are presented. The high resolution spectra (0.1 cm −1 ) of the two Davydov components show a substructure that we attribute to the presence in the crystal of natural traps and local strains. A model based on the calculation of the eigenstates of a finite plane of molecules is provided for lineshape simulations. In this work the artificial edge effects introduced by the finite size of the model crystal are eliminated by appropriate boundary conditions. Our method allows us to introduce a random distribution of traps, with variable energies, located inside and outside the exciton band, thus featuring a real crystal. Comparison between calculations and experimental results accounts for the salient effects of the traps on the spectroscopic properties of a mixed crystal.

Calculation of phosphorescence excitation spectra of clusters in isotopically mixed crystals of deuteronaphthalene

Abstract Using a model of generating random numbers for 100 × 100 sites of a two-dimensional lattice and the approximation of deep traps, we calculate phosphorescence excitation spectra for N h 8 /N- d 8 mixed crystals. We show that observed spectra are consistent with couplings between states AB + and AB − , and AA + and AA − which give strong intensity to the weak peak AB − and makes observable the electronically forbidden state AA − . A strong variation in the local concentration of Clusters is discussed.

Spin—spin and spin—orbit interactions in the triplet states of naphthalene clusters

Abstract The phosphorescence lineshape of triplet clusters is analysed using the direct spin—orbit coupling mechanisms. Triplet spin-quantisation and spin—orbit routes are calculated for any configurations of cluster. This and the consideration of molecules with one substituted 13 C allow us to understand better the napthalene phosphorescence lineshape.

Resonant secondary emission and spectral diffusion in isotopically mixed naphthalene crystals

Abstract Resonant secondary radiation (zero-phonon lines) flourescence spectra of isotopic 3D aggregates of traps (NH 8 ) in a host crystal (ND 8 ), as well as transfer-induced fluorescence at trap concentrations between 0.01 and 10% and at temperatures between 1.6 and 10 K, are presented. Laser-excited (0–1) resonant secondary (zero-phonon) emission is used to resolve close-lying electronic resonances, corresponding to monomers, aggregates and to their host and trap satellite lines, in a quasi-continuum of 3D trap states. Such states are also calculated by numerically diagonalizing a mixed-crystal 3D hamiltonian. Measured and calculated delocalization indices are given for all trap species. In addition, low-temperature spectral diffusion of photoselected trap species is measured by plotting the attenuation of the zero-phonon lines. We give a simple discussion on the modulation of the resonant secondary radiation spectra by concentration, excitation wavelength and temperature. We show that low-temperature exciton diffusion, assited by acoustical phonon creation, occurs above a very low concentration threshold ( C > 0.05% for monomers at T = 1.6 K). Thermally activated transfer of the photoselected excitons is also plotted for temperatures up to 10 K above which the zero-phonon lines are quenched.

Direct and delayed emissions from a naphthalene crystal doped with pyrene molecules

Abstract Direct and delayed emissions from a naphthalene crystal doped with pyrene molecules are investigated at temperatures between -196 and +70°C. Data are obtained for different optical excitations and for different concentrations of the host molecules. It is shown that these observed emissions are due to the deformation of the crystalline cell by the presence of the guest molecules.

Fusion and Transfer of Triplet Excitons in Isotopically Mixed Naphthalene Crystals

Results on temperature and concentration dependences of triplet exciton fusion and triplet energy transfer in isotopically mixed naphthalene crystals are presented. Exciton fusion in the trap states is investigated by delayed fluorescence emission. Energy transfer efficiency is analyzed by the relative transfer yield to a supertrap acting as a sensor. The temperature dependences observed on delayed fluorescence and on energy transfer are similar, with a threshold at T ∿ 5K, suggesting a common origin for both processes. Numerical solutions of a master equation in a two-dimensional crystal are provided. The results may be understood by considering a two optical phonon mechanism (with a phonon energy equal to 40 cm-1) and by taking into account the long-range dipolar interaction between traps due to the spin-orbit coupling of triplet states to upper singlet states.

Guest distrubution and the effect of guest-guest transfer on the excitation phosphorescence spectrum of Nh8/Nd8 at 2K

Abstract The observed phosphorescence excitation spectrum of traps in Nh 8 /Nd 8 cannot properly be accounted for by simulations using no more than the Boltzmann populations of the levels in question. We propose an energy transfer model causing broadening of the lines limited in this paper to monomers and dimers.