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Is it possible to design a supramolecular cage that would "solvate" the excess electron in the same fashion in which several solvent molecules do that co-operatively in polar liquids? Two general strategies are outlined for "electron encapsulation," viz. electron localization using polar groups arranged on the (i) inside of the cage or (ii) outside of the cage. The second approach is limited to polynitriles. We demonstrate that the latter faces a formidable problem: the electron attaches to the nitrile groups forming molecular anions with bent C-C-N fragments. Since the energy cost of this bending is very high, for dinitrile anions in n-hexane, the binding energies for the electron are very low and for mononitriles, these binding energies are lower still, and the entropy of electron attachment is anomalously small. Density functional theory modeling of electron trapping by mononitriles in n-hexane suggests that the mononitrile molecules substitute for the solvent molecules at the electron cavity, "solvating" the electron by their methyl groups. Such "solvated electrons" resemble multimer radical anions in which the electron density is shared (mainly) between C 2p orbitals in the solute/solvent molecules, instead of existing as cavity electrons. The way in which the excess electron density is shared by such molecules is similar to the way in which this sharing occurs in large di- and poly- nitrile anions, such as 1,2,4,5,7,8,10,11-octacyano-cyclododecane anion. The work thus reveals limitations of the concept of "solvated electron" for organic liquids. It also demostrates the feasibility of "electron encapsulation.", Comment: 44 pages & 12 fugures (low quality in some figures) + supplement w 3 tables, 10 fig, & Appendix; submitted to J Phys Chem A

The picosecond dynamics of hydroxyl radicals generated in 200 nm photoinduced dissociation of aqueous hydrogen peroxide have been observed through their transient absorbance at 266 nm. It is shown that these kinetics are nearly exponential, with a decay time of ca. 30 ps. The prompt quantum yield for the decomposition of H2O2 is 0.56, and the fraction of hydroxyl radicals escaping from the solvent cage to the water bulk is 64-68%. These recombination kinetics suggest strong caging of the geminate hydroxyl radicals by water. Phenomenologically, these kinetics may be rationalized in terms of the diffusion of hydroxide radicals out of a shallow potential well (a solvent cage) with an Onsager radius of 0.24 nm., Comment: 14 pages, 1 figure

Monomers and small clusters of such molecules can reversibly trap conduction band electrons in dilute alkane solutions. The dynamics and energetics of this trapping have been studied using pulse radiolysis - transient absorption spectroscopy and time-resolved photoconductivity. Binding energies, thermal detrapping rates, and absorption spectra of excess electrons attached to monomer and multimer solute traps are obtained and possible structures for these species are discussed. "Dipole coagulation" (stepwise growth of the solute cluster around the cavity electron) predicted by Mozumder in 1972 is observed. Acetonitrile monomer is shown to solvate the electron by its methyl group, just like the alkane solvent does. The electron is dipole-bound to the CN group; the latter points away from the cavity. The resulting negatively charged species has a binding energy of 0.4 eV and absorbs in the infrared. Molecules of straight-chain aliphatic alcohols solvate the excess electron by their OH groups; at equilibrium, the predominant electron trap is a trimer or a tetramer; the binding energy of this solute trap is ca. 0.8 eV. Trapping by smaller clusters is opposed by the entropy which drives the equilibrium towards the electron in a solvent trap. For alcohol monomers, the trapping does not occur; a slow proton transfer reaction occurs instead. For acetonitrile monomer, the trapping is favored energetically but the thermal detachment is rapid (ca. 1 ns)., Comment: will shortly be submitted to J Phys Chem A; 53 pages w 12 figures; has a Supplement of 12 pages & 22 more figures

In common nonpolar liquids, such as saturated hydrocarbons, a dynamic equilibrium between trapped (localized) and quasifree (extended) states has been postulated for the excess electron (the two-state model). Using time-resolved dc conductivity, the effect of 1064 nm laser photoexcitation of trapped electrons on the charge transport has been observed in liquid n-hexane and methylcyclohexane. The light promotes the electron from the trap into the conduction band of the liquid, instantaneously increasing the conductivity by orders of magnitude. From the analysis of the two-pulse, two-color photoconductivity data, the residence time of the electrons in traps has been estimated as ca. 8.4 ps for n-hexane and ca. 13 ps for methylcyclohexane (at 295 K). The rate of detrapping decreases at lower temperature with an activation energy of ca. 200 meV (280-320 K); the lifetime-mobility product for quasifree electrons scales linearly with the temperature. We suggest that the properties of trapped electrons in hydrocarbon liquids can be well accounted for using the simple electron bubble (Wigner-Seiz spherical well) model. The estimated localization time of the quasifree electron is 20-50 fs; both time estimates are in good agreement with the "quasiballistic" model. This localization time is significantly lower than the value of ca. 300 fs obtained using time-domain terahertz (THz) spectroscopy for the same system [E. Knoesel et al., J. Chem. Phys. 121, 394 (2004)]. We suggest that the THz signal originates from the oscillations of electron bubbles rather than the free-electron plasma; vibrations of these bubbles may be responsible for the deviations from the Drude behavior observed below 0.4 THz. Various implications of these results are discussed., Comment: 37 page, 5 figures; w Supplement of 13 pages and 5 figures; accepted by J. Chem. Phys

It is shown that 532 nm and 1064 nm laser photoexcitation of trapped electrons generated by 355 nm photolysis of aqueous titania (TiO2) nanoparticles causes rapid photobleaching of their absorbance band in the visible and near IR. This photobleaching occurs within the duration of the laser pulse (3 ns FWHM); it is caused by photoinduced electron detrapping followed by rapid recombination of the resulting free electron and a trapped hole. The quantum yield for the electron photobleaching is ca. 0.28 for 532 nm and ca. 0.024 for 1064 nm photoexcitation. Complete separation of the spectral contributions from trapped electron and hole is demonstrated using glycerol as a selective hole scavenger. When glycerol is added to the solution, some light-absorbing holes are scavenged promptly within the duration of the 355 nm photoexcitation pulse, some are scavenged at a slower rate over the first 200 ns after the 355 nm pulse, and the rest are not scavenged, even at high concentration of the scavenger (> 10 vol %). A reaction with chemi- and physi- sorbed glycerol would account for the prompt and the slow hole decay, respectively. The implications of these results are discussed., Comment: 43 pages, 11 figures + supplement: 2 pages, 7 figures; to be submitted to J. Phys. Chem. B

In this review (124 refs), several problems in radiolysis of saturated hydrocarbons are examined. Special attention is paid to the chemistry of radical cations, high-mobility holes, excited state and spur dynamics, magnetic field and spin effects, and optically detected magnetic resonance spectroscopy., Comment: modified review published in Radiation Chemsitry of Organic Liquids: Saturated Hydrocarbons, in Radiation Chemistry: Present Status and Future Prospects, in Radiation Chemistry: Present Status and Future Trends, edited by C. D. Jonah and B. S. M. Rao, Elsevier, Amsterdam, 2001; p. 175

In concentrated solutions of NaClO4 and Na2SO4, the quantum yield for free electron generated by detachment from photoexcited anions (such as I-, OH-, ClO^4-, and [SO3]^2-) linearly decreases by 6-12% per 1 M ionic strength. In 9 M sodium perchlorate solution, this quantum yield decreases by roughly an order of magnitude. Ultrafast kinetic studies of 200 nm photon induced electron detachment from Br-, HO- and [SO3]^2- suggest that the prompt yield of thermalized electron does not change in these solutions; rather, the ionic strength effect originates in more efficient recombination of geminate pairs. Within the framework of the recently proposed mean force potential (MFP) model of charge separation dynamics in such photosystems, the observed changes are interpreted as an increase in the short-range attractive potential between the geminate partners. Association of sodium cation(s) with the electron and the parent anion is suggested as the most likely cause for the observed modification of the MFP. Electron thermalization kinetics suggest that the cation associated with the parent anion (by ion pairing and/or ionic atmosphere interaction) is passed to the detached electron in the course of the photoreaction. The precise atomic-level mechanism for the ionic strength effect is presently unclear; any further advance is likely to require the development of an adequate quantum molecular dynamics model., Comment: 40 pages, 10 figures + supplement 2 pages, 9 figures; will be submitted, in a modified form, to J. Phys. Chem A

While ionization of some polyatomic molecular liquids (such as water and aliphatic alcohols) yields so-called "solvated electrons" in which the excess electron density is localized in the interstices between the solvent molecules, most organic and inorganic liquids yield radical anions and cations in which the electron and spin densities reside on the solvent molecule or, more commonly, a group of such molecules. The resulting multimer ions have many unusual properties, such as high rates of diffusive hopping. The "solvated electron" can be regarded as a variant of a multimer radical anion in which the charge, while perturbing the solvent molecules, mainly resides in the space between these molecules. We give several examples of less known modes for electron localization in liquids that yield multimer radical anions (such as C6F6, benzene, acetonitrile, carbon disulfide and dioxide, etc.) and holes localization in liquids that yield multimer radical cations (such as cycloalkanes). Current understanding of the reaction properties for these high-mobility solvent radical anions and cations is discussed., Comment: truncated chapter "Radical Ions in Liquids", to appear in "Charged Particle and Photon Interactions with Matter. Chemical, Physicochemical, and Biological Consequences with Applications", eds. Y. Hatano and A. Mozumder, Marcel Dekker, NY, 2004

Time-resolved transient absorption spectroscopy has been used to study nanosecond and sub-microsecond electron dynamics in aqueous anatase nanoparticles in the presence of hole scavengers: chemisorbed polyols and carbohydrates. These polyhydroxylated compounds are rapidly oxidized by the holes; 50-60% of these holes are scavenged within the duration of 355 nm excitation laser pulse. The scavenging efficiency rapidly increases with the number of anchoring hydroxyl groups and varies considerably as a function of the carbohydrate structure. A specific binding site for the polyols and carbohydrates is suggested that involves an octahedral Ti atom chelated by the poly-OH ligand. This mode of binding accounts for the depletion of undercoordinated Ti atoms observed in the XANES spectra of polyol coated nanoparticles. We suggest that these binding sites trap a substantial fraction of holes before the latter descend to surface traps and/or recombine with free electrons. The resulting oxygen hole center rapidly loses a CH proton to the environment, yielding a metastable C-centered radical., Comment: 22 pages, 4 figures; to appear in Chem. Phys. Lett

Research in systems, guidance and control, space sciences, engineering, telecommunications and propulsion for space exploration programs

The optimum-thrust equations for both variable and constant thrust are presented. These thrust programs are used to generate rendezvous trajectories from the Earth to Mars for various flight times and launch dates during the years 1968-71. The manner in which the propulsion requirements vary with flight time and launch date are considered, and a comparison of vehicle performance using the variable- and constant-thrust programs is presented. The optimization of the pro- pulsion system parameters is discussed, and the existence of optimum launch dates is interpreted in terms of certain transversality conditions derivable from the calculus of variations. A brief comparison of the advanced propulsion vehicle and the ballistic vehicle propulsion requirements is made for Earth-Mars rendezvous trajectories. An appendix considering the analytical basis for this work is included.