We report on time resolved optical pump-probe investigations of the electron-cation recombination in alkane liquids. The alkanes were excited via two-photon excitation, using intense sub-picosecond pulses at 266 nm. Adding an electron scavenger to the liquid and probing the transient absorption at three different wavelengths (800 nm, 1500 nm, and 2250 nm) made it possible to distinguish the transient absorption due to excess electrons from that due to other absorbing species. The experimentally observed charge-recombination kinetics in n-hexane and n-octane could be fairly well reproduced by computer simulations in which the initial electron thermalization distance distribution was taken to be f(r)r2dr=(1/b)exp(−r/b)dr. Other distributions, such as a Gaussian, gave unsatisfactory results. The average electron thermalization distance in n-hexane was found to be 35±5 Å and in n-octane it was found to be 70±10 Å. The results for isooctane could be described either by the distribution (1/b)exp(−r/b)dr with an average thermalization distance of 25±5 Å or by a Gaussian distribution with an average thermalization distance 50±10 Å. The transient absorption in n-hexane and isooctane was observed to exhibit a red shift during the first 2 ps after the onset of the absorption. This spectral relaxation could be due to the slowing down of the ejected electron to thermal energies.
The anisotropic mobility of excess charge carriers in pure and mixed crystals of two polydiacetylene derivatives was determined. The charge carriers were produced by pulsed irradiation and detected by time-resolved measurement of the microwave conductivity. The charge-carrier mobility was measured as a function of the orientation of the polymer backbone in the crystal with respect to the probing electric microwave field. A lower limit in the intrinsic anisotropy in the charge-carrier mobility was found to be 15 in favor of charge transport in the direction of the polymer backbone as compared to the transverse direction for polydiacetylene-(bis p-fluorobenzene sulfonate) (pFBS), while a value of 90 was found for polydiacetylene-(bis p-toluene sulfonate) (pTS) and the 50:50 pTS/FBS copolymer. A lower limit of the charge-carrier mobility along the backbone of 3 cm2/V s was found for both pTS and pFBS. The charge-carrier mobility in the copolymer was found to be one order of magnitude lower than in the pure polymers.
Poly(di-n-hexylsilylene) (PDHS) was highly oriented by concurrent gel crystallization with UHMW-PE and subsequent ultradrawing. The orientation of the polysilylene molecules in the ultradrawn blends was demonstrated with polarized UV spectroscopy. The radiation-induced conductivity was measured, using the pulse radiolysis time-resolved microwave conductivity (TRMC) technique. Charge transport was studied both parallel and perpendicular to the polysilylene backbones. Mobilities of the charge carriers in the direction of these backbones are at least an order of magnitude higher than those perpendicular to them. The sum of the mobilities of the mobile charge carriers in the solid phase of PDHS at room temperature is at least 3 X 10~® m2 V-1 s-1 in the direction of the polysilylene backbone. The magnitude of the radiationinduced conductivity in the solid phase is related to the crystallinity of the sample and decreases at the solid to mesophase transition in all systems studied. The PE matrix appears to restrict the crystallization of the PDHS backbone in the all-trans conformation in the solid phase.
The problem of the diffusion controlled recombination of ions for the case where initially two or more pairs of oppositely charged ions are found in each other’s Coulomb field, as occurs in the track of a high-energy electron in a nonpolar liquid, is treated by means of computer simulation. Results are presented on the probability of survival as a function of time for ion pairs in clusters containing up to four ion pairs initially. The effect of an external electric field on the probability of escape from recombination is investigated. The multipair results are compared with the calculations based on the single-pair theory. The consequences of the consideration of multipair effects in addition to the single ion-pair contribution in the nonhomogeneous kinetics of the track of high-energy electron in nonpolar liquids are discussed briefly.
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