New studies of non-photochemical holes of dyes and rare-earth ions in polymers. II. Laser-induced hole filling

The phenomenon of laser-induced hole filling (LIHF) is reported and discussed in detail for rhodamine 640, Nd³⁺, Pr³⁺ and the mixed system of rhodamine 560 and cresyl violet. Importantly, LIHF does not show any spectral diffusion (i.e. broadening). Possible mechanisms (thermal heating, site reversion and energy transfer) involved in LIHF are discussed and are argued to have insignificant contribution. A tentative model is developed to explain these results. The model basically invokes a connectivity between spatially removed extrinsic (impurity) two-level systems (TLS) via an ensemble of intrinsic (host) TLS (the glassy state). Additionally, a correlation between impurity excitation energies and absolute glassy state energies is imposed.     

Quantum study of electronically non-adiabatic collinear reactions. II. Influence of spin-orbit transitions on the F + HH reaction

Electronic transitions in the collinear reactive FHH system are investigated using two models of electronic potentials with different spin-orbit coupling strengths. One is based on the DIM method and the other one uses the Muckerman's MV reactive surface and a modified Blais-Truhlar repulsive potential. The hyperspherical electronuclear potential diagram permits to predict two kinds of spin-orbit transitions: (1) vibrationally adiabatic spin transitions are induced in the reactant valley where the two electronic surfaces are nearly degenerate; (2) electrovibrational transitions are induced in the complex zone at the crossings between hyperspherical potentials. A close-coupling procedure based on hyperspherical coordinates allows one to compute inelastic and reactive probabilities up to 1.8 eV. The first mechanism induces reactive or inelastic probabilities from the electronically excited state by multi-step processes. The second mechanism is responsible for local enhancement of the reactivity on several probabilities involving vibrationally excited products. Resonances are found at the same total energies as in the single-surface calculation, but their intensities are partly controlled by the second mechanism.     

Ground and excited state prototropic reactions in 2-(o-hydroxyphenyl)benzimidazole

A solvent dependence study of the steady state absorption, excitation and fluorescence spectra of 2-(o-hydroxyphenyl) benzimidazole has indicated the presence of different tautomers in the S₁ state which have the same precursor in the S₀ state. A similar study as a function of hydrogen ion concentration has identified the presence of dication, monocation, zwitterion, phototautomer and monoanion species. The respective pK_a and pK^*_a values have been determined with fluorimetric titrations and Förster cycle methods and compared.     

Density and temperature dependence of the relaxation of the 1Δg state of highly compressed O2 fluid

The electronic relaxation of O₂ is investigated by an absorption excitation and fluorescence detection technique. The relaxation rate constant of O₂(¹Δ_g) is measured in the density range from 10²¹ to 3 × 10²² cm⁻³ at temperatures between 90 and 295 K. The experimental results are compared with theoretical models based on the pair distribution functions of the fluid. The effects of intermolecular potentials with hard or soft cores are discussed.     

Vibrational population distributions in infrared multiple-photon excitation probed by coherent anti-stokes Raman spectroscopy

Coherent anti-stokes Raman spectroscopy has been used as a probe for molecules pumped by high-intensity CO₂ laser radiation. Ozone, sulfur hexafluoride, and chloroethane have been investigated. The intensity of the ground-state CARS signal is decreased by IR multiphoton excitation, but signals at new vibrational origins do not appear. The observations on chloroethane are interpreted in terms of a rate-equation model for IRMPE, modified to take vibrational redistribution into account.     

Factorizable and meson-pole contributions to hyperon nonleptonic decays

All vacuum-insertion contributions to hyperon nonleptonic decays can be identified within the framework of current algebra. The contribution to the s-wave amplitude has been identified by Donoghue, Golowich, Holstein, and Ponce. His pointed out that the p-wave factorizable term for the penguin operator which is not normal-ordered emerges from the disconnected contributions to the baryon poles. This separable amplitude is shown to be equivalent to the K-meson pole in the normal-ordering representation. The inclusion of a kaon pole is thus mandatory in the normal ordering scheme, but is negligible if the weak hamiltonian is not normal-ordered.     

Low-energy antiproton-neon interaction

By means of a streamer chamber exposed to the LEAR antiproton beams, the total reaction cross sections, the charged-prong multiplicity distributions and lower limits for the production of negative pions and K_s⁰ in the p̄-Ne interaction were measured at 19.6, 48.7 and 179.6 MeV. Annihilation was found to be dominant over all other non-elastic p̄-Ne processes. An analysis of Ne and other nuclei data in the frame of the Glauber theory allowed us to determine the ratio between the p̄-n and the p̄-p cross sections. An analysis in the light of INC model predictions allowed us to show up events which can be interpreted as annihilations having occurred deeply inside the nuclei.     

Angular distributions in quasi-fission reactions

Angular distributions for fission-like fragments were measured in the systems ⁵⁰Ti, ⁵⁶Fe + ²⁰⁸Pb by applying an off-line K X-ray activation technique. The distributions d²σ/dθdZ exhibit forward-backward asymmetries that are strongly Z-dependent. They result from a process (quasi-fission) which yields nearly symmetric masses in times comparable to the rotational period of the composite system. A method for obtaining the variance of the tilting angular momentum, K₀², from these skewed, differential angular distributions is described. The results indicate that the tilting mode is not fully excited in quasi-fission reactions. The results are compared to the sum of the variances of all statistical spin components, measured via γ-multiplicities. Integration of the angular distributions d²σ/dθdZ over all values of Z yields the integral angular distributions dσ/dθ and dσ/dΩ symmetric around 90°. The associated unusually large anisotropies do not at all provide an adequate basis for tests or modifications of the transition-state theory. A deconvolution of d²σ/dθdZ is performed with gaussian distributions depending on rotational angles Δθ extending over a range of up to 540°. From the mean values 〈Δθ〉 a time scale for the evolution of K₀ is calculated.