The interaction of slow positrons with organic moecules above and below positronium formation thresholds

A review is given of the ionization of organic moecules by monoenergetic positrons having energies in the range of 0.5–15 eV. Two mechanisms, unique to positrons, are described. If the kinetic energy of the positron is above the positronium formation threshold, such that electrons can be removed from the molecules to form free positronium atoms, the ionization/fragmentation behavior can be explained qualitatively by a modification of the Ore gap theory. To explain how positrons can ionize and fragment molecules when their kinetic energies are below the positronium formation threshold, it is necessary to assume that energy is transferred to the molecule by the annihilation process. Ionization cross sections for positrons having kinetic energies below the positronium formation threshold are sensitive to molecular size, structure and bond types. Continuing work involves a search for positronium compound formation and measurements of the kinetic energy distributions of ions.     

Dissociative Excitation Processes upon Collisions of Slow Electrons with MgBr2 Molecules

Dissociative excitation of MgI, MgII, and MgBr upon e–MgBr₂ collisions was experimentally studied. The excitation cross sections of the spectral lines of MgI and MgII, as well as two sets of bands of the MgBr molecule were measured at an incident electron energy of 100 eV. In the electron energy range from the excitation threshold to 100 eV, ten optical excitation functions with a complex structure were recorded. The principal pathways of dissociative excitation of MgI, MgII, and MgBr by slow electrons are discussed.     

Spin asymmetry in near threshold (e, 2e) process for lithium and hydrogen

The singlet and triplet cross sections and the asymmetry parameter for e-H and e-Li ionization have been calculated in a distorted-wave Born approximation for incident energies in the range 0.5 to 20 eV above the ionization threshold. The two final-state continuum electrons are assumed to share equal energy and to come out 180° apart. The variation with respect to the angle of scattering is found to be qualitatively very different in the two cases. It is found that, in the case of lithium, the ¹D and ³F components in the scattering amplitude are quite significant even at very low energies.     

Vacuum-UV and Low-Energy Electron-Induced DNA Strand Breaks – Influence of the DNA Sequence and Substrate

DNA is effectively damaged by radiation, which can on the one hand lead to cancer and is on the other hand directly exploited in the treatment of tumor tissue. DNA strand breaks are already induced by photons having an energy below the ionization energy of DNA. At high photon energies, most of the DNA strand breaks are induced by low-energy secondary electrons. In the present study we quantified photon and electron induced DNA strand breaks in four different 12mer oligonucleotides. They are irradiated directly with 8.44 eV vacuum ultraviolet (VUV) photons and 8.8 eV low energy electrons (LEE). By using Si instead of VUV transparent CaF₂ as a substrate the VUV exposure leads to an additional release of LEEs, which have a maximum energy of 3.6 eV and can significantly enhance strand break cross sections. Atomic force microscopy is used to visualize strand breaks on DNA origami platforms and to determine absolute values for the strand break cross sections. Upon irradiation with 8.44 eV photons all the investigated sequences show very similar strand break cross sections in the range of 1.7–2.3×10⁻¹⁶ cm². The strand break cross sections for LEE irradiation at 8.8 eV are one to two orders of magnitude larger than the ones for VUV photons, and a slight sequence dependence is observed. The sequence dependence is even more pronounced for LEEs with energies <3.6 eV. The present results help to assess DNA damage by photons and electrons close to the ionization threshold.     

Electroreflectance studies on the excitons in PTS and DCHD single crystals

In polydiacetylene single crystals of PTS [poly-2,4-hexadiyne-1,6-diol-bis(p-toluene sulfonate)] and of DCHD [poly-1,6-di(N-carbazolyl)-2, 4-hexadiyne] electric field modulated reflectance spectra in the range of the π-π* excitations of the polymer indicate considerable delocalization of the electrons along the chain. Shape and size of the excitonic electroreflectance spectra are determined by a large charge transfer component of the excitons. Evaluation of the spectra yields about 20% and 40% charge transfer with excitation to neighbouring π bonds in PTS and DCHD, respectively. Large differences occur in electroreflectance in the range of vibronic excitons pointing to pronounced coupling of the carbazole side group in DCHD to the π electrons of the chain. At higher energy, ?0.48 eV above the lowest exciton transition in both polymers a large electroreflectance signal is observed which is attributed to a band transition.     

Angle-resolved photoelectron spectroscopy of cyclopropane

The angular distribution parameter, β, determined for the valence orbitals (IP < 18 eV) of cyclopropane in the 10–30 eV photon energy range using dispersed polarized synchrotron radiation. The energy dependence of β for photoelectron energies between, 2 and 10 eV above threshold was found to be similar to those found previously for other σ orbitals. The effects of Jahn-Teller splitting on β for the 3e′ orbital were found to be small but definitely present. The overall shape and magnitude of the β(hv) curve are, however, sufficiently for the different Jahn-Teller components that, for purposes of orbital assignments using β(hv) curves the shape and magnitude of the curves can be considered associated only with the initial state. Resonance photoionization features at a photon ener of ≈ 18 eV were observed in the 3e′ and 3a′₁ orbitals and tentatively assigned to autoionization.     

Cationic Rydberg states observed in resonantly enhanced electron spectra of CS2

The inner valence electron spectrum of the CS₂ molecule has been investigated in the binding energy range between 18.6 and 26.3 eV using synchrotron radiation for ionisation. Photon energies in the range from 67 to about 167 eV have been used, with particular focus on 166.70, 166.89 and 167.09 eV for which S2p electrons are resonantly transferred into Rydberg orbitals close to the ionisation threshold. From there, autoionisation takes the molecule into various cationic states characterized by two valence holes and a Rydberg spectator electron. Many new bands are observed which contain vibrational progressions with spacings around 120 meV in most cases. These are assigned as excitations of the totally symmetric stretching ν₁ mode in the cationic state. The new bands reflect states in the cation that are close to the electronic states of the dication and assignments are made by comparison to double ionisation electron spectra.     

Molecular size effect in NCO and NCS dianion resonances

Cross sections for electron-impact detachment and electron-impact dissociation of NCO- and NCS- were measured from about 3 to about 40 eV. The former are found to follow a classical prediction with a threshold energy of 9.1 +/- 0.1 eV for NCO- and 8.9 +/- 0.2 eV for NCS-. When the incoming electron binds to the monoanion, a short-lived dianion complex is formed, which is revealed as a resonance in the cross section. For NCO- a resonance is evident at 9.3 +/- 0.2 eV, which implies that the dianion lies above the monoanion by this amount of energy. In the case of NCS- two resonances are evident at 8.4 +/- 0.2 and 19.0 +/- 0.5 eV, respectively. The low-energy NCS dianion is less unstable than the dianion of NCO, which in turn is less unstable than the CN dianion (10-eV resonance). Thus the resonance shifts down in energy with the increasing size of the anion, a fact which is attributed to a decrease in Coulomb energy between the spatially separated electrons.     

Dissociative electron attachment to furan, tetrahydrofuran, and fructose

We study dissociative electron attachment to furan (FN) (C(4)H(4)O), tetrahydrofuran (THF) (C(4)H(8)O), and fructose (FRU) (C(6)H(12)O(6)) using crossed electron/molecular beams experiments with mass spectrometric detection of the anions. We find that FN and THF are weak electron scavengers and subjected to dissociative electron attachment essentially in the energy range above 5.5 eV via core excited resonances. In striking contrast to that, FRU is very sensitive towards low energy electrons generating a variety of fragment ions via a pronounced low energy feature close to 0 eV. These reactions are associated with the degradation of the ring structure and demonstrate that THF cannot be used as surrogate to model deoxyribose in DNA with respect to the attack of electrons at subexcitation energies (<3 eV). The results support the picture that in DNA the sugar moiety itself is an active part in the initial molecular processes leading to single strand breaks.     

Near threshold absolute TDCS: first results

A new method, and first results for an impact energy 2 eV above the threshold of ionisation of helium, are presented for the measurement of absolute triple differential cross sections (TDCS) in a crossed beam experiment. The method is based upon measurement of beam/target overlap densities using known absolute total ionisation cross sections and of detection efficiencies using known absolute double differential cross sections (DDCS). For the present work the necessary absolute DDCS for 1 eV electrons had also to be measured. Results are presented for several different coplanar kinematics and are compared with recent DWBA calculations.     

Single,double, and multiple double strand breaks induced in DNA by 3-100 eV electrons

Nonthermal secondary electrons with initial kinetic energies below 100 eV are an abundant transient species created in irradiated cells and thermalize within picoseconds through successive multiple energy loss events. Here we show that below 15 eV such low-energy electrons induce single (SSB) and double (DSB) strand breaks in plasmid DNA exclusively via formation and decay of molecular resonances involving DNA components (base, sugar, hydration water, etc.). Furthermore, the strand break quantum yields (per incident electron) due to resonances occur with intensities similar to those that appear between 25 and 100 eV electron energy, where nonresonant mechanisms related to excitation/ionizations/dissociations are shown to dominate the yields, although with some contribution from multiple scattering electron energy loss events. We also present the first measurements of the electron energy dependence of multiple double strand breaks (MDSB) induced in DNA by electrons with energies below 100 eV. Unlike the SSB and DSB yields, which remain relatively constant above 25 eV, the MDSB yields show a strong monotonic increase above 30 eV, however with intensities at least 1 order of magnitude smaller than the combined SSB and DSB yields. The observation of MDSB above 30 eV is attributed to strand break clusters (nano-tracks) involving multiple successive interactions of one single electron at sites that are distant in primary sequence along the DNA double strand, but are in close contact; such regions exist in supercoiled DNA (as well as cellular DNA) where the double helix crosses itself or is in close proximity to another part of the same DNA molecule.     

Laser-induced low energy electron diffraction in aligned molecules

We measured the photoelectron spectra and angular distributions of partially aligned N(2), O(2), and CO(2) in the rescattering plateau of above threshold ionization (ATI). The measured ATI electrons have relatively low collision energies (<15 eV). The photoelectron angular distributions (PAD) show clearly species and energy dependence. A simple two-center interference model was not able to consistently retrieve structural properties. We conclude that due to the interplay between the electrons and rescattering potential, the molecular structural information is obscured and cannot be extracted conveniently. However, the sensitivity of the PAD to the scattering potential in laser-induced electron diffraction promises a practical tool for studying electron-ion scattering dynamics.     

Alignment of Ar+ (2p −1 2 P 3/2) ions after electron impact ionisation in the range 1 keV to 268 eV

We have measured the alignmentA ₂₀ of Ar⁺(2p ^{?1 2} P _3/2) ions after electron impact ionization in the range of primary electron energyE ₀=1000...268 eV (range of excess energyE ₁=750...19.5 eV) via the anisotropic angular distribution ofL ₃?M _2,3 M _2,3(¹ S ₀) Auger electrons. On decomposing the Auger spectra into their components special care was taken by including the effect of the postcollision interaction on the shape of Auger lines. The present alignment values forE ₀≧350 eV agree well with previously existing experimental values of DuBois and Rodbro and with theoretical DWBA results of Berezkho and Kabachnik, but forE ₀<350 eV they deviate systematically from the DWBA values. For the lowest impact energyE ₀, which is only 19.5 eV above threshold, we obtainedA ₂₀=+0.09(16). This value clearly indicates that in the ionisation process near threshold the two low-energy electrons escape not only with a two-electron partial waveL=0, according to Wannier's original assumption, but also with partial wavesL>0.     

Differential track structure of electrons in liquid water

The differential spectrum of energy loss events produced in liquid water by electrons has been calculated using a differential cross-section incorporating exchange and binding. The average energy loss in a single event is found to vary from about 28 to about 68 eV for 100 eV for 100 eV to 1 MeV electrons, respectively. All energy loss events above a certain level (set either to 1 or to 5 keV) are assumed to produce branch tracks, which are further degraded using a Monte Carlo technique. The total track averaged energy loss per event for a 1 MeV electron is found to be about 57 eV.     

Chemical effects of low energy electron impact on hydrocarbons in the gas phase: III. Cyclopropane

The simulated radiolysis of cyclopropane with low energy electrons (3.5 to 15.0 eV) was investigated. The setup used for the irradiations has been described previously. Appearance curves of the various products formed under electron impact were determined. The features observed on these curves yield various indications.(1) Some products arise from the dissociation of excited molecules. Contributing states are the following ones: a triplet state at 7.4 eV, singlet states at 6.7 and/or 7.7 eV, at 8.55 eV, at 9.4 and/or 9.95 eV and superexcited states lying around 10.2 eV. As in other hydrocarbons studied, the electron impact excitation cross section shows a steep increase at the ionization potential. (2) Other products result from ion fragmentation and ion—molecule reactions.A reaction scheme was proposed to account for the chemical effects associated with excited states and the yields of excited molecules in dissociating states were derived from experimental data. The observations relative to excited molecule fragmentation are in conformity with photolysis data. Additional information on the decomposition processes of molecules excited in the triplet state at 7.4 eV, in the singlet states at 6.7 and/or 7.7 eV and in the superexcited states were obtained.Owing to the complexity of ionic mechanisms it was not possible to distinguish between the contributions of ionization and excitation. Only the radiation chemical yield of products, G(products), was evaluated. The values found for G(products) just above the ionization potential are close to the data obtained in conventional radiolysis which could indicate that secondary electrons having such energies play an important role in radiation chemistry.     

Spin polarization and cross sections of electrons elastically scattered from heavy alkaline-earth atoms

Semi-relativistic approach is employed to compute the differential and integrated cross sections, spin polarizationP and the spin polarization parametersT andU for the scattering of electrons from barium and strontium atoms in the energy range from 2.0–300 eV. The projectile-target interaction is represented both by real and complex optical potential in the solution of Dirac equation for the scattered electrons. The real optical potential includes the static, a parameter free correlation polarization and a modified semi classical exchange potentials. The complex optical potential is constructed by adding a model absorption potential as its imaginary part to the real optical potential.     

Spontaneous and infrared induced electron detachment from negatively charged helium nanodroplets

A beam of helium nanodroplets, with sizes ranging up to N = 10⁷ or more atoms, is produced by fragmentation of a low entropy supersonic expansion. It subsequently is excited by electron impact, producing various charged and metastable droplet states depending on the electron energy. We will describe experiments with negatively charged cluster ions, which are observed for low energy impacts when N >2×10⁵. In these experiments, after a flight time in high vacuum of several milliseconds the droplets pass through a weak transverse field above an electron multiplier. A signal from spontaneously detached electrons is observed, which suggests that the ion, while long lived, is inherently metastable. Furthermore, when the beam is crossed with an infrared light beam above the detector, the detachment rate is significantly increased. The wavelength dependence of this light induced signal has a broad peak near 1.5μm. By deflection measurements it is found that the spontaneous detachment signal comes preferentially from smaller clusters, while the light induced signal comes predominantly from larger ones. By stopping potential measurements one can conclude that both kinds of detached electrons have energies below 1eV, with photo detached electrons the more energetic.     

On the vibrational dependence of electron impact ionization of diatomic molecules

The dependence of the Na₂ electron impact ionization rate is measured as a function of vibrational excitation in a crossed molecule-electron beamm arrangement at collision energiesE _coll ≤ 3 eV above the ionization threshold. Specific vibrational distributions in theX ¹∑ _g ⁺ state with average vibrational energies of 0.17 eV, 0.276 eV, and 0.349 eV, are prepared via Franck-Condon pumping using a narrow-band cw laser. Enhancement of the ionization rate is observed only at impact energies near the ionization threshold where the ionization rate increases linearly as a function of vibrational excitation. Analysis of the experimental data is based on three model calculations. The first of these calculations equates vibrational energy with kinetic energy and agrees well with the experimental data. A second, more refined model allows for differences in state-to-state ionization rates and uses Franck-Condon factors to estimate transition probabilities, but leads to a less favorable agreement. The third one employs a semi-classical formulation of the Franck-Condon principle. It provides the best agreement with the experimental data. In contrast with an earlier study of electron impact ionization of diatomic molecules [20], we find no evidence of dynamical modification of the ionization rate, due to vibrational motion of the nuclei, at the present level of accuracy of our data and analysis.     

Vibrational and translational enhancement of endothermic reactions: K + HCl(υ = 0,1) and K + HF(υ = 0,1)

The relative integral cross section for the two endothermic reactions K + HCl(υ = 0 and 1) → KCl + H and K + HF(υ = 0 and 1) → KF + H has been measured as a function of the collision energy E using the crossed molecular beam technique. The vibrationally excited state (υ = 1) has been populated thermally by heating the beam source to temperatures around 2000 K. The variation of the collision energy from thermal up to around 2.1 eV was achieved by seeding the K-beam with various carrier gases. The molecular reaction product was detected by surface ionization in connection with a time-of-flight method. The total energy threshold of the reactions has been found to be equal to or only slightly above the corresponding endothermicities. This suggests a vanishing or very low barrier crest on the potential energy hypersurfaces which is contradictory to recent theoretical results. The inclusion of tunneling in case of K + HF leads to a negligible rise of the barrier heights. The efficacy of translational and vibrational energy in promoting the reactive process has been directly compared over a wide range of collision energies. For K + HCl the vibrational enhancement of the reactivity descends with increasing E from approximately a factor of 10 at E = 0.08 eV to around unity for E ? 0.5 eV. The good agreement of this experimental result with phase space calculations suggests that the marked enhancements are predominantly caused by the long-range attraction between reagents in connection with an “early” barrier on the potential energy surface. In case of K + HF vibrational energy is by a factor of up to 380 more favourable in promoting the reaction than the same amount of translational energy. Again, with rising collision energy its efficacy decreases but promotes the reaction still by a factor of 70 at E = 1.7 eV. Since phase space theory fails here the reaction is certainly non-statistical and we conclude that the observed large efficacy of vibrational energy is due to a “late” barrier. The proposed barrier positions for the two systems are in accordance with theoretical results.     

Shape effect and quantum size effect on small metal particles

We examine the small-energy cross section behaviour for the processe+H^?→H*+2e. The excitation-detachment function appears approximately constant above the classical threshold, with a weak undulatory component superimposed. The energy distribution function for the outgoing electrons is found almost uniform within our classical model.