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Charge cycling cross sections of lithium and helium ions are measured for atomic metal films of titanium, zirconium and copper. The material dependence of the measured charge state fractions is investigated and compared with the material density, electron density, mean ionization potential and Fermi level. The experimental data quantitatively agrees with previously documented charge exchange cross sections for thin films and shows the expected increase in the fraction of higher charge states with increasing velocity, over the ion velocity range considered. There is an observable material dependence of the charge state fraction. The measured charge state distributions are expressed as a mean charge state and compared with various mean and effective charge formalisms. The effective charge models are demonstrated to be inadequate representations of the measured mean charge states for the ions and films probed in this study.

The phenol(+)...Ar(2) complex has been characterized in a supersonic jet by mass analyzed threshold ionization (MATI) spectroscopy via different intermediate intermolecular vibrational states of the first electronically excited state (S(1)). From the spectra recorded via the S(1)0(0) origin and the S(1)β(x) intermolecular vibrational state, the ionization energy (IE) has been determined as 68,288 ± 5 cm(-1), displaying a red shift of 340 cm(-1) from the IE of the phenol(+) monomer. Well-resolved, nearly harmonic vibrational progressions with a fundamental frequency of 10 cm(-1) have been observed in the ion ground state (D(0)) and assigned to the symmetric van der Waals (vdW) bending mode, β(x), along the x axis containing the C-O bond. MATI spectra recorded via the S(1) state involving other higher-lying intermolecular vibrational states (σ(s)(1), β(x)(3), σ(s)(1)β(x)(1), σ(s)(1)β(x)(2)) are characterized by unresolved broad structures.

Experimental measurements coupled with Monte Carlo track simulations have been used to examine the yields of hydrated electrons in the radiolysis of water with protons, helium ions, and carbon ions. Glycylglycine, in concentrations ranging from 10(-4) to 1 M, was employed as a scavenger and the production of the ammonium cation used as a probe of hydrated electron yields from about 2 ns to 20 mus. Monte Carlo track simulations employing diffusion-kinetic calculations of product yields are found to reproduce experimental observations satisfactorily. Model details are used to elucidate the heavy ion track physics and chemistry. Comparison of the heavy ion results with those found in gamma radiolysis shows intratrack reactions are significant on the nanosecond to microsecond time scale as the ion track relaxes, and that a constant (escape) yield is never attained on this time scale. Numerical interpolation techniques are used to obtain both track average and track segment yields for use in practical applications or comparison with other models. The model results give the first hints that initial ( approximately 5 ps) hydrated electron yields, and possibly other water decomposition products, are dependent on the type and energy of the incident radiation.

The chemistry of the Fricke dosimeter induced by low and high linear energy transfer (LET) radiation has been modeled using the stochastic IRT method, incorporating simulated ion tracks. Comparison of the results for track segments with experimentally determined differential yields shows excellent agreement for energetic electrons, and for nonrelativistic ions, including 1H, 4He, 12C, and 20Ne. There is a significant effect of particle type and energy on the response of the Fricke dosimeter, which reflects the competition between intra-track reaction of the radiation-induced radicals, diffusion, and scavenging. This competition is modified by changes in the ion track structure. Monte Carlo track structure simulations show that the radial energy loss profiles are similar for ions with the same velocity/charge ratio and that ∼40% of the energy is initially deposited within a water diameter of the track axis. The LET of the ionizing radiation is shown to be a poor parameter for characterizing the Fricke dosi...

Inelastic collision cross-sections, mean free paths, stopping powers, energy loss distributions, mean energy losses and csda ranges are evaluated for non-relativistic electrons and positrons in liquid water using a formalism in which the response of the medium is expressed employing the experimental dipole oscillator strength distribution. Monte Carlo track structure simulations employing the calculated inelastic collision cross-sections, and elastic cross-sections evaluated using partial wave methods, are used to determine the energy dependence of positron and electron path-lengths, penetrations, and non-homogeneous energy deposition distributions. The calculated data are discussed, and compared and contrasted. The energy loss properties of electrons and positrons that depend only on the differential inelastic collision cross-section are similar for particle energies ≳1 keV, but there are apparent differences for lower energies. The energy loss properties depending on the size of the energy transfer events and on the differential inelastic collision cross-section differ for all particle energies. The differences found are the consequence of electron indistinguishability on the inelastic cross-section for the electron. Non-homogeneous energy deposition distributions of positrons with energy less than 1 keV are significantly more forward directed than those of electrons with the same initial energy. The differences are due to the larger inelastic collision cross-section of a positron compared to an electron, and its effect on the relative numbers of inelastic and elastic collisions.

Positron track structures were simulated by stochastic modeling of the collision-by-collision slowing down of positrons in n-hexane. The details of the inelastic scattering of positrons and of electrons produced in ionizing collisions were taken into account until the particle energies had degraded to less than 25 eV. Further slowing down to thermal energy was assumed to result in a spherically symmetric thermalization distribution around the position where the particle energy becomes less than 25 eV. The dynamics of positronium formation in competition with electron−cation recombination and positron annihilation was followed by a simulation of the diffusive motion of the charged species in each other's Coulomb field. Positronium formation was found to involve reaction of the thermalized positron with electrons produced along the last few hundred nm of its track-end, corresponding to several keV of energy attenuation. Positronium formation occurs mainly during the first few tens of picoseconds after posit...

In conventional simulations of electron track structures the medium is assumed to be continuous, so events (and therefore chemical species) can be generated unphysically close together. The problem is particularly severe for low-energy electrons, which are responsible for most of the observed chemistry. Two modified simulation methods are proposed. The first modifies the local density of the medium using the radial distribution function. The second simulates the track using configurations generated by molecular dynamics. When gas-phase cross-sections are used the methods have large, but opposite effects on electron range and inter-event distances. With condensed phase cross-sections the effects are much smaller.

The energy loss and transport of electrons and positrons in gaseous cyclohexane, and solid cyclohexane, benzene, polyethylene, and polystyrene are determined and discussed. A formalism for the ener...

Computer simulations are described of the evolution of radiolytic tracks in liquid alkanes following muon implantation. The programs predict time-dependent cation and muonium yields due to electron capture as well as the amplitudes of the muon and muonium precession signals. The variation of these quantities with muon stopping distance and applied electric field is investigated. Simulations for hexane, iso-octane and neopentane, covering 3 orders of magnitude in electron mobility, are presented. Particular attention is paid to the extent to which the delay to muonium formation can explain the reported missing fractions and to predicting the electric-field effects which might be tested in new experiments.

Muonium formation in liquid hexane is examined by computer simulation. In track-end competition between muonium formation and cation–electron recombination, the muon is found to react with electrons from a significant part of the track end, corresponding to an energy attenuation of several tens of keV and a length of several microns. This muonium formation extends to microseconds following muon implantation. Delayed muonium formation leads to a much smaller amplitude of the muonium asymmetry than for prompt muonium formation during slowing down of the muon, and in this way may account for the missing polarization in transverse magnetic field experiments. If reaction of muons with electrons from their radiolysis tracks contributes to the experimentally observed muonium yield, the muon must thermalize between 60 and 150 nm from the last ionization of the track to reproduce the amplitudes of the muon and muonium asymmetries. For the smallest distance, 60 nm, the experimentally observed muonium asymmetry resu...

Monte Carlo simulation methods have been used to evaluate the effects of elastic collisions on the transport of 0.1, 1, 10, and 100 keV energy electrons in water. The simulations show that transport-related phenomena in the direction axial (parallel) to the initial direction of travel are more dependent on elastic processes than those in the radial (perpendicular) direction. A decrease in the total elastic cross section to 10% of its gas phase value increases the mean axial penetration depths and the maximum of the axial energy deposition gradients by as much as a factor of 2. The most pronounced effects are observed for electrons of 100 eV energy where even the mean radial penetrations are doubled. Except for these low energies, the mean radial penetrations and the radial energy deposition gradients are relatively unaffected by elastic processes, implying that these parameters are mainly determined by inelastic collisions.

A stochastic simulation method employing suitable experimentally based cross sections has been developed for probing the spatial distribution of energy loss and the trajectory of 100 eV to 1 MeV electrons in gaseous and liquid water. Elastic collisions and large-energy-loss inelastic collisions strongly influence the passage of electrons such that the separation between the initial and final (E = 25 eV) position is considerably smaller than the path length. At all energies, the mean axial and the mean radial penetrations are somewhat similar, however, the former is strongly influenced by inelastic collisions and the latter by elastic collisions. The effect of phase on the density-normalized range is small for energetic electrons, but differences are apparant at the lowest energies

The dipole oscillator strength distributions for solid and for gas phase cyclohexane, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, and benzene were constructed from experimentally derived optical constants and from atomic X-ray absorption cross sections. Monte Carlo simulations of the energy loss by electrons of initial energy from 10 keV to 1 MeV in these media were performed using cumulative inelastic cross sections obtained from a formulation incorporating the constructed dipole oscillator strength distributions. In the solid phase, the energy loss distributions, the most probable energy losses, and the mean energy losses for electrons show little effect due to the conjugation of $pi bonds. However, there are large differences between the gases, and there is a considerable effect due to condensation. The most probable and the mean energy losses for 1 MeV incident electrons in solid cyclohexane, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, and benzene are in the ranges 22-24 and 47-48 eV, respectively. Comparison with data for water suggests that the values for the solid phases of hydrocarbons are acceptable approximations for the liquid phase. Density normalized stopping powers, inelastic mean free paths, and ranges for electrons in the various hydrocarbons are also presented. 36 refs., 4 figs., 7 tabs.

Positronium formation in low-density polyethylene was studied using computer simulation and by 1-detector Doppler broadening measurement. The experimental Ps fraction is 35%, and is reproduced by simulation if the positron thermalization distance is 150 nm. Additional simulations were performed to investigate the effect of the mobilities of the positron-induced charged species, and of the electron and positron thermalization distances, on Ps formation.

An analytic treatment for the short-term recombination following the photolysis of water is presented. The technique is used to look at the way that the spatial distribution of the reactants resulting from the photoionization influences the escape probability and the time-dependent decay kinetics of the solvated electron. The predictions of the analytic method are compared with the experimentally observed decay of the solvated electron. Two different distributions are used to describe the initial separation of the hydrated electron from the hydroxyl radical and the hydrated proton with which it may react. To match model to experiment requires either a Gaussian of standard deviation 0.6 nm or an exponential with a characteristic width of 0.3 nm. The two distributions have the same root-mean-square separation, 1.0 nm, once the initial overlap of reactants is taken into account.

A procedure for the calculation of entity-specific ionization and excitation probabilities for water radiolysis at low linear energy transfer (LET) has been developed. The technique pays due attention to the effects of the ionization threshold and the energy dependence of the ionization efficiency. The numbers of primary ionizations and excitations are not directly proportional to the spur energy. At a given spur energy, ionization follows a binomial distribution subject to an energetically possible maximum. The excitation distribution for a spur of given energy and with a given number of ionizations is given by a geometric series. The occurrence probabilities depend upon the cross sections of ionization, excitation, and other inferior processes. Following the low-LET radiolysis of liquid water the most probable spurs contain one ionization, two ionizations, or one ionization and one excitation, while in water vapor they contain either one ionization or one excitation. In liquid water the most probable outcomes for spurs corresponding to the most probable energy loss (22 eV) and to the mean energy loss (38 eV) are one ionization and one excitation, and two ionizations and one excitation, respectively. In the vapor, the most probable energy loss is 14 eV which results in one ionizationmore » or one excitation and the mean energy loss is 34 eV for which the spur of maximum probability contains one ionization and two excitations. The total calculated primary yields for low-LET radiolysis are in approximate agreement with experiment in both phases.« less

The stopping powers, the inelastic mean free paths, the ranges, and the distributions of energy loss events for electrons in some gaseous alkanes have been calculated from experimentally based dipole oscillator strength distributions. The results show very little difference between the members of the homologous series of straight-chain hydrocarbons, C{sub n}H{sub 2n+2} where n = 2, 4, 6, and 8. For instance, the integrated path length of a 1-MeV electron in ethane of unit density is 0.38 cm and increases to 0.40 cm in octane of the same density. The stopping power of a 1-MeV electron in unit density hexane is 0.2 eV/nm, and the inelastic mean free path is 220 nm. The corresponding values for the other alkanes differ by less than 10%. The distribution of energy loss events along the track of a high-energy electron in these gaseous hydrocarbons is not significantly affected by the value of n, and it is insensitive to the incident electron energy from 10 keV to 1 MeV. The most probable energy loss along the track of a 1-MeV electron is 14 eV in ethane, 15 eV in butane and hexane, and 16 eV in octane. The mean energy loss is 25 eVmore » in butane, hexane, and octane and is 1 eV less in ethane.« less

The importance of studies with synchrotron radiation for providing dipole oscillator strength distributions in the spectral region between 10 and 100 eV is discussed. Dipole oscillator strengths compiled from photo-absorption and high energy electron inelastic scattering experiments are compared with recent synchrotron measurements for gaseous C2H2OH and i-C4H8 in the v-u.v. region. The compiled dipole oscillator strength distributions for these gaseous media and for water vapor are used to calculate the inelastic mean free path, mean excitation potential and the range distribution of low-energy electrons. The density normalized ranges are found to decrease in the order H2O > C2H5OH >i-C4H8. The actual range of a 1 keV electron in H2O is about 3 or 4 times that in C2H5OH and i-C4H8, respectively. This difference can have a pronounced effect on track structure.

The distributions of energy deposition events for electrons in gaseous and liquid water and in amorphous ice have been calculated. These calculations make use of an inelastic cross section that takes into account binding energy, exchange, and relativistic effects and that is based on experimentally determined dipole oscillator strength distributions. The most probable energy loss in a single event by a 1-MeV electron is 22.5 eV in the condensed phases and 13.5 eV in the gaseous phase, while the average energy loss is 34, 38, and 40 eV/event for gaseous and liquid water and amorphous ice, respectively. Very little dependence of the distributions on incident electron energy is found. At 1 MeV the differential or single event distribution is found to be the same as the integral or cumulative distribution. The fractions of the total energy lost in events that are less than 100 eV are 0.76, 0.75, and 0.69 for gaseous and liquid water and amorphous ice, respectively.

The dependence of the radiation chemistry of water on electron energy is probed using Monte Carlo track-structure simulation and stochastic modeling of diffusion kinetics. Decreasing the initial electron energy from 1 MeV to 100 keV has little effect on the decay kinetics of ${\rm e}{}_{{\rm aq}}{}^{-}$ and ${\rm OH}^{\bullet}$ or the formation of H2 and H2 O2. In the energy range 100 keV to 1 keV, the initial electron energy has a considerable effect on the chemistry; decreasing the electron energy increases the amount of intratrack reaction, lowering the long-time yields of ${\rm e}{}_{{\rm aq}}{}^{-}$ and of ${\rm OH}^{\bullet}$ and raising the yields of the molecular products. For electrons of initial energy lower than 1 keV, these trends in the kinetics are reversed; the amount of reaction decreases and there is more ${\rm e}{}_{{\rm aq}}{}^{-}$ and ${\rm OH}^{\bullet}$ surviv...

Experimentally derived optical constants and X-ray attenuation cross sections were used to construct the complete dipole oscillator strength distribution for solid, dry DNA. Monte Carlo simulations of the energy loss by electrons of initial energy 5 keV to 1 MeV in DNA were performed using cumulative inelastic cross sections obtained from a formulation incorporating the constructed dipole oscillator strength distribution. The energy-loss distribution, the most probable energy loss and the mean energy loss for electrons in DNA are compared to those for liquid water, gaseous water and gaseous hexane. For the most part, the calculations show that electron energy loss in DNA is very similar to that in liquid water; however, it is quite different from both gaseous water and gaseous hexane. The mean energy losses for a 1 MeV incident electron in DNA, liquid water, gaseous water and gaseous hexane are 57.9, 56.8, 50.9 and 38.4 eV, respectively. The large differences found between the predictions for DNA and for ...

Random-flights Monte Carlo simulation and stochastic master equation techniques are presented for modelling the influence of an applied electric field on the ion recombination kinetics in multiple-ion-pair spurs in low-dielectric-permittivity solvents. The calculated free-ion yield in two-, three- and four-ion-pair spurs with realistic initial spatial distributions increases linearly with the applied field strength for weak fields. The slope-to-intercept ratio predicted for the variation is numerically similar to that obtained analytically for a single ion pair as the escaping fraction of ion pairs is small and as the Fano factor is close to unity for low-dielectric-permittivity solvents. Consequently, for weak electric fields, it is predicted that experimental slope-to-intercept ratios for hydrocarbons will not provide any significant information about the spur-size distribution.

Recently we presented [Chem. Phys. Lett. 142, 385 (1987)] a method for calculating the diffusion‐controlled geminate escape probability of an ion pair in an anisotropic medium. The escape probability depends on both the length and the direction of the initial interion vector, and is reducible to a function of two dimensionless variables describing the overall anisotropy of the medium. In this paper the treatment is extended to include an initial distribution of interion distances and an external field. Inclusion of a field increases the number of independently variable parameters to five: two to describe the anisotropy of the medium and three to describe the intensity and direction of the applied field. It is shown that the effects of including anisotropy are not negligible for the typical case of anthracene and that the slope‐to‐intercept ratio of the field‐dependent escape probability for a spherically averaged distribution depends on the direction of the applied field.