Decay Kinetics of Benzophenone Triplets and Corresponding Free Radicals in Soft and Rigid Polymers Studied by Laser Flash Photolysis

The kinetics of transients formed under photoexcitation of benzophenone (B) dissolved in three different polymers was studied by ns laser flash photolysis. These polymers were the soft rubbers poly (ethylene‐co‐butylene) (EB), polystyrene block‐poly(ethylene‐ran‐butylene)‐block‐polystyrene (SEBS) and hard polystyrene (PS). We monitored the decay kinetics of triplet state ³B^*and of ketyl radicals BH^●. We observed exponential decay of ³B^* and two‐stage decay kinetics of BH^●in EB. The first stage is a fast cage recombination of a radical pair (BH^●, radical of polymer R^●). The second slow stage of BH^● decay follows the second‐order law with a relatively high rate constant, which corresponds to recombination of BH^● in a homogeneous liquid with a viscosity of only ~0.1 P (about five times of 2‐propanol viscosity). Application of a magnetic field (MF) of 0.2 T leads to deceleration of both stages of BH^● decay in EB by approximately 20%. Decay kinetics of both transients were observed in SEBS. There was no MF effect on BH^● decay in SEBS. We only observed ³B^* in PS. Decay kinetics of ³B^* in this case were described as polychromatic dispersive first‐order kinetics. We discuss the effects of polymer structure on transient kinetics and the MF effect.     

Free radicals in photolysis of mixed phosphonium–iodonium ylides and in their reactions with acetylenes

UV irradiation of mixed phosphonium–iodonium ylide in CH₂Cl₂ leads to formation of free radicals with lifetimes of a few minutes detected by EPR. In mixtures of ylides with acetylenes, the structure of radicals changes, and their concentration and stability increase. In the presence of acetylenes, the radicals contain ylide and acetylene residues, and their EPR spectra have hyperfine coupling constants typical for ³¹P nuclei in C-radicals and for ¹H nuclei, depending on the acetylene structure. It has been demonstrated that the observed radical products are formed from short-lived primary radicals.     

Possibility of Studying Neutron–Neutron Correlations in Halo Nuclei

An experimental investigation of the reaction of core pickup from ⁶He and ¹¹Li two-neutronhalo nuclei is proposed. In such experiments, neutron–neutron correlations in a halo nucleus will be assessed on the basis of the energy of a neutron–neutron quasibound state. A detailed kinematical simulation of the reaction ⁶He + ²H → ⁶Li + (nn) →⁶ Li + n + n is performed. It is shown that the energy of the quasibound state in question can determined from the shape of the energy spectrum of neutrons originating from the breakup of this state. In the proposed exclusive experiment, a beam of ⁶He (¹¹Li) nuclei with an energy of about 5 to 10 MeV per nucleon interacts with a deuterated-polyethylene target. This will permit detecting charged particles (⁶Li and ¹¹Be) and a neutron. On the basis of determining the energy of the neutron–neutron quasibound state, it will become possible to estimate the effective attraction between the valence neutrons in the field of the third particle (core).     

Mathematical simulation of few-nucleon experiments with three or more particles in the final state

A computer program for the kinematic simulation of nuclear reactions with three or more particles in the final state is described. Simulation results for the d + ²H → (nn)^S + (pp)^S → p + p + n + n reaction, which proceeds through the formation of a singlet dineutron and diproton in the intermediate state, are presented as an example. It is shown that the kinematic parameters of breakup particles depend on the parameters of the intermediate state. The results from simulating secondary neutron spectra are compared to an experimental neutron time-of-flight spectrum obtained in a kinematically complete experiment at a deuteron energy of 15 MeV. As a result of the comparison, the energy of the quasi-bound singlet ¹S₀ state of the ²n-system is determined.