Ionisation dynamics at intermediate momentum transfer: an (e, 2e) investigation on argon

Relative triple differential cross section for the coplanar asymmetric (e, 2e) reaction in argon have been measured at 1.5 KeV incident energy and 40 eV ejected electron energy in several kinematics. Depending on the scattering angle, ? _a , the chosen kinematics select either ionising collisions belonging to the Bethe ridge (? _a =9.2°) or processes in the intermediate region between the pure dipolar and binary regimes. The more relevant finding is the presence of a minimum in the recoil lobe, almost opposite to the direction of the momentum transfer. This feature is qualitatively explained by a first Born model, which describes the ejected electron by a Coulomb wave-function. This result suggests that in the investigated kinematics the interaction of the slow ejected electron with the residual ion is the dominant effect beyond the first order electron-electron interactions.     

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.     

The yields of1 P and1 D resonances in the He(e, 2e)He+ reaction

In the first Born approximation the dependence of the yields of the¹ P and¹ D resonances in the He(e, 2e)He⁺ reaction on the momentum transfer in the recoil peak region at incident energiesE ₀=1000 eV is studied. It is shown that in a certain range of the ejection angle and for the large momentum transfer the yield of the¹ D resonance dominates over the¹ P resonance one.     

Relativistic (e, 2e) collisions on atomic inner shells in symmetric geometry

We report here on an (e, 2e) experiment at relativistic electron energies (E ₀=300 keV and 500 keV) in coplanar symmetric geometry. Absolute triple differential cross section measurements forK-shell ionisation of gold, silver and copper are compared with a number of simple first order approximations. Appreciable discrepancies between theory and experiment are found, which reduce with decreasingZ and increasing primary energy. The theoretical calculations show that spin flip effects are important in symmetric geometry, in earlier works these had been neglected.     

Some considerations on the microwave electrodeless discharge: Plasma diagnostics in argon,helium or nitrogen atmospheres

Plasma diagnostics of several microwave plasmas are determined by making electrical (with double floating probes) and optical measurements in pure Ar, He or N₂ plasmas, and also in Ar plasmas containing various metals, i.e. Cs, Tl or Zn; plasma parameters, such as, electric field (E), electron (j_e) and ion (j_i) current densities, electron density (n_e), electron temperature (T_e) electron conductivity (σ_e), ion density (n_i), electron mean free path (λ_e) electron (μ_e) and ion (μ_i) mobilities and electron [(v_e)_drift] and ion [(v_i)_drift] volocities are either directly measured or calculated. The reversal temperature (T_r) of excited (0.96 eV lower level) thallium atoms is measured, and the steady-state conditions of the plasma are analyzed by the energy balance equation. The experimental measurements indicate that the electric field strength E decreases as the space charge decreases (ionization extent) increases. Although the plasma appears to be under steady-state conditions, it is not under local thermodynamic equilibrium conditions, i.e. T_e >T_r. In addition, the measurements indicate that there is a deficiency of electrons in the plasma (n_e < n_i), probably due to electron affinity processes; and the plasma has a small positive space charge.     

Optical response of bimetallic Li x Na8−x (0≦x≦8) and of doped Na8Zn clusters

Total (elastic + inelastic) cross-sections for electron scattering from C, N, O atoms and their simple molecules are studied theoretically. Thee ^–-C, N, O atomic calculations are done in the complex optical potential approach. To study the electron scattering from O₂, N₂, CO, NO, CN, C₂ as well as CO₂, N₂O, NO₂ O₃ targets, we have adopted an additivity rule, wherein the molecular cross-section is an incoherent sum of the cross-sections of the constituent atoms. The cross-sections of C, N & O atoms are presented at incident energiesE _i =10–1000 eV, the molecular cross-sections are presented atE _i =100–1000 eV. The reliability of the additivity rule is discussed against the background of experimental data.     

Electron impact and single photon ionization cross sections of neutral silver clusters

Neutral silver atoms and small clusters Ag _n (n=1...4) were generated by sputtering, i.e. by bombarding a polycrystalline silver surface with Ar⁺ ions of 5 keV. The sputtered particles were ionized by a crossed electron beam and subsequently detected by a quadrupole mass spectrometer. In alternative to the electron impact ionization, the same neutral species were also ionized by single photon absorption from a pulsed VUV laser (photon energy 7.9 eV), and the photoionization cross sections were evaluated from the laser intensity dependence of the measured signals. By in situ combining both ionization mechanisms, absolute values of the ratio σ _e (Ag _n )/σ _e (Ag) between the electron impact ionization cross sections of silver clusters and atoms could be determined for a fixed electron energy of 46 eV. These values can then be used to calibrate previously measured relative ionization functions. By calibrating the results using literature data measured for silver atoms, we present absolute cross sections for electron impact ionization of neutral Ag₂, Ag₃ and Ag₄ as a function of the electron energy between threshold and 125 eV.     

Influence of the surface structure on the initiation of nuclear-chemical processes under laser ablation of metals in aqueous media

It is shown that the efficiency of nucleochemical transformations under conditions of laser ablation of metals in aqueous media under the influence of picosecond laser pulses with peak intensity I _E ～ 10¹⁰–10¹³ W/cm² is largely determined by features of the metal’s surface relief in the region of high spatial frequencies (nanometer range) formed under these conditions. This is found through an atomic force microscopy study of the relief features of such surfaces formed with different laser ablation modes on specially prepared model samples. Analysis of the obtained images by means of flicker-noise spectroscopy with key 3D surface parameters in the nanometer range allow us to associate the rates of nuclear processes initiated upon laser ablation with sharpness factor as a measure of the chaotic constituent of the relief profile of a forming surface at the highest spatial frequencies. It is found that it is in the neighborhood of the greatest high-frequency irregularities of the surface that electric fields with the highest voltage that lowers the energy barrier to electron injection from the metal (the Frenkel effect) are located and the elevated values of mechanical tensile stresses that also contribute to reducing the work of an electron escaping from the metal (the Zhurkov effect) are found. It is concluded that the sharpness factor must play the key role in raising kinetic energy of electrons E _e to ～5–10 eV in the subsurface regions of low-temperature plasma formed upon laser ablation in the metal subsurface region in which the above nucleochemical transformations can occur.     

Kinetics of the radiative relaxation of the highly excited C60 +* ion

The radiation of fullerene molecules from the intersection area of the C₆₀ beam with an electron beam with an energy of 27≤E _e /eV≤100 was studied experimentally under conditions of a single collision. It was found that ionized C₆₀ ^+* molecules make the main contribution to the radiation. The radiation intensity and the temperature of C₆₀ ^+* as functions of the energyE _e were measured. The kinetics of the radiation cooling of C₆₀ ^+* was studied and the rate of the radiation loss of the ion energy (5.5·10⁵ eV s⁻¹) was determined at a temperature of 3150 K. For the heat model of radiation at the wavelength λ=540 nm, this corresponds to the emissivity ε=1.1·10⁻². Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 273–276, February, 2000.     

Electron spectroscopy of hydrogen fluoride resonances

Transmission electron spectroscopy has been applied to determine the energies of resonances in HF. In addition to a sharp resonance at 10.05 eV, a resonance series exhibiting both vibrational and rotational structure is resolved in the energy range between 12 eV and 13 eV and the following molecular constants are obtained: B = 20.4 cm^?1, r_e, = 0.93 Å, ω_e 0.132 eV, ω_ex_e = 0.006 eV and D_e = 0.73 eV. The resonance spectrum is analysed with reference to an electron energy loss spectrum and approximate potential energy curves are deduced. Serious discrepancies are found between the present results and the data reported by Spence and Noguchi.     

Photoexcited C60: fragmentation and delayed ionization

We investigate two reaction channels of C₆₀, excited by a pulsed laser at 355 nm: i) The relative and absolute yields of even-sized fragment ions are measured over a wide range of laser fluences. At low fluence we find a dramatic dependence of the abundance ratio of C _n?2 ⁺ versus C _n ⁺ . This result argues against the notion that unimolecular emission of C₄ or larger fragments contributes significantly to the production of even-sized fragment ions. ii) We count the number of delayed electrons emitted from C₆₀, or one of its products, after photo-excitation. For high laser fluences, this number reaches a value of (2.6 ± 1.1)% per photoexcited C₆₀. This sets a lower bound to the ratio of effective rate constants, k_e(E^* )/∑k_j(E^*), where k_e refers to electron emission, and the sum in the denominator extends over all reaction channels.     

Kinetics of photo-induced dissociation of Na clusters deposited on Mica

Na clusters bound to mica surfaces have been irradiated with pulsed and cw visible laser light. Kinetic energy and angular distributions of the Na atoms desorbing from the clusters have been determined using cw two-photon laser-induced fluorescence detection. In addition the dependence of the desorption rate on laser power, wavelength and polarization has been measured. The most probable kinetic energyE _kin of the photodesorbed atoms at the surface temperatureT _S =300 K was found to beE _kin=18±5 meV, independent of laser irradiance (3 µJ/cm²...20 mJ/cm²) and wavelength (450 nm, 505 nm, 658 nm). With increasing orientation angle between detection axis and surface normal (0°≦Θ≦90°)E _kin was observed to decrease slightly, while it was nearly independent of surface temperature betweenT _S =30 K andT _S =300 K. Also, with increasing radius of the Na clusters the desorbing Na atoms slowed down. The angular distribution of the Na atoms was of cos²-type with respect to the surface normal. These observations suggest that laser-induced desorption of Na from Na clusters bound to mica surfaces involves an initial rate-limiting step of direct surface plasmon excitation followed by a final step of delayed thermal desorption.     

Dissociation of ethane by electron impact

The absolute total dissociation cross section for ethane is reported for electron energies between 10 and 600 eV. A maximum value of 7.6 × 10^?16 cm² occurs at 80 eV while the apparent threshold is ≈ 10 eV. Dissociative ionization is more probable than dissociation into neutral fragments at all energies except in the threshold region. The data indicates that fragmentation involving methane elimination (c^? + C₂ H₆ → e^? + CH₄ + CH₂) occurs in less than 2% of the dissociative events for 50 < E < 600 eV. Arguments are presented which suggest that some of the lower excited states of ethane are stable against dissociation.     

Copolymerization of methyl 2-acetamidoacrylate and styrene in the presence of stannic chloride

Continuous variation method in UV revealed that methyl N-acetylaminoacrylate (MNA) and SnCl₄ formed the 1:1 complex. The copolymerization of MNA with styrene in tetrahydrofuran was carried out at 50 °C in the presence of SnCl₄. The resulting monomer reactivity ratios decreased with an increasing concentration of SnCl₄ added. This finding suggests that SnCl₄ participates in the propagation step of the copolymerization. Therefore, the copolymerization was analyzed by assuming terpolymerization of free MNA (M₁), complexed MNA (M₂), and styrene (M₃). The absolute copolymerization parameters were obtained as follows: k₁₁/k₁₂=0.165, k₁₁/k₁₃=3.04, k₂₂/k₂₁=0.32, k₂₂/k₂₃=0.103, k₃₃/k₃₁=0.058, k₃₃/k₃₂=0.001, Q₁=6.03, e₁=0.52, Q₂=88.57, and e₂=2.23. The complexed MNA is more reactive to polymer radicals with free MNA and styrene as the terminal unit than the free MNA. Very small values of k₂₂/k₂₃ and k₃₃/k₃₂ suggests that the copolymerization of the complexed MNA and styrene proceeds alternatingly.     

Vibrational analysis of the electronic spectrum of ethylene based onab initio SCF-CI calculations

Ab initio calculations for CH₂ twisting and CC stretching vibrational wavefunctions and energy levels are reported for various electronic states of ethylene C₂H₄. Electronic transition moments between these states are also obtained to allow a calculation of the oscillator strengths for vibrational transitions involved in various electronic band systems; from this study it is concluded that thevertical electronic energy differenceΔE _e may differ significantly from the energy of the absorption maximumΔE _max with which it is often equated. In particular it is found in the case of theπ→π ^* singlet-singlet excitation of ethylene that theΔE _e value overestimates the most probable vibrational transition energy (7.89 eV) by some 0.4 eV, thereby offering an explanation for the fact that previous attempts to predict the location of theV-N Franck-Condon absorption maximum have consistently obtained substantially higher results than the 7.66 eV value actually observed. Similar calculations for various Rydberg species and for theN-T transition are also found to obtain a quite consistent representation of the electronic spectrum of this system.     

Laser-assisted electron-impact excitation of hydrogen and helium atoms

The excitation of H(1s–2s) and He(1¹ S–2¹ S) by electron impact in the presence of a nonresonant laser field is studied in the framework of the perturbation theory. The wavelength variation of the total cross section is presented at incident electron energies of 100 eV and 200 eV for hydrogen and 200 eV for helium. The use of pseudostates as intermediate states in the study of excitation of hydrogen is also investigated.     

Computed current density in the case of slow charge-transfer coupled with a second order chemical reaction,under potentiostatic conditions (SnIV/SnII system)

Current-time curves at constant potential on plane and spherical electrode have been computed by numerical integration of the transport equations in the case of a slow charge-transfer coupled with a chemical reaction leading to a non-reactive product E₃, following the scheme:E₁+e→E₂E₁+E→E₂?E₃Results indicate that this mechanism is characterized by lower current densities than those of simple diffusion-transfer control. The current density is not proportional to depolarizer E₁ concentration. The main characteristic of these systems is that, when polarograms are analysed in terms of log k versus potential E, different straight lines are obtained depending on the concentration of the depolarizer; consequently, the apparent transfer coefficient α varies with concentration. Moreover all lines intersect at a same point lying at a potential near the equilibrium value, where the charge-transfer rate constant k is sufficiently small to be rate determining.     

Dynamical (e, 2e) studies using tetrahydrofuran as a DNA analog

Triple differential cross sections for the electron-impact ionization of the outer valence orbital of tetrahydrofuran have been measured using the (e, 2e) technique. The measurements have been performed with coplanar asymmetric kinematics, at an incident electron energy of 250 eV and at an ejected electron energy of 10 eV, over a range of momentum transfers. The experimental results are compared with theoretical calculations carried out using the molecular three-body distorted wave model. The results obtained are important for gaining an understanding of electron driven processes at a molecular level and for modeling energy deposition in living tissue.     

High resolution electron momentum spectroscopy of the valence orbitals of water

The development of a third-generation electron momentum spectrometer with significantly improved energy and momentum resolutions at Tsinghua University (ΔE = 0.45–0.68 eV, Δθ = ±0.53° and Δ? = ±0.84°) has enabled a reinvestigation of the valence orbital electron momentum distributions of H₂O with improved statistical accuracy. The measurements have been conducted at impact energies of 1200 eV and 2400 eV in order to check the validity of the plane wave impulse approximation. The obtained ionization spectra and electron momentum distributions have been compared with the results of computations carried out with Hartree Fock [HF] theory, density functional theory in conjunction with the standard B3LYP functional, one-particle Green’s function [1p-GF] theory along with the third-order algebraic diagrammatic construction scheme [ADC(3)], symmetry adapted cluster configuration interaction [SAC-CI] theory, and a variety of multi-reference [MR-SDCI, MR-RSPT2, MR-RSPT3] theories. The influence of the basis set on the computed momentum distributions has been investigated further, using a variety of basis sets ranging from 6-31G to the almost complete d-aug-cc-pV6Z basis set. A main issue in the present work pertains to a shake-up band of very weak intensity at 27.1 eV, of which the related momentum distribution was analyzed for the first time. The experimental evidences and the most thorough theoretical calculations demonstrate that this band borrows its ionization intensity from the 2a₁ orbital.     

On the validity of using the TKR sum rule to place zero angle electron impact spectra on an absolute scale

The use of the Thomas—Kuhn—Reiche (TKR) sum rule to place electron impact spectra on an absolute scale is only rigorously correct if the observed intensities can be extrapolated to their first Born zero momentum transfer limit. This note investigates the error involved in using the TKR sum rule at zero scattering angle instead of zero momentum transfer. By considering an expansion of the generalized oscillator strength at zero angle it is shown that the first order correction to the TKR sum rule can be written as ($\text{1}\text{3}\text{k}{}^2$)[|E_o|?($\text{4}\text{5}$)$\text{V}$_ee] for target systems randomly oriented in space where k² is the incident electron energy and |E_o| and $\text{V}$_ee are the magnitude of the total electronic energy and the electron—electron repulsive energy of the ground state of the target system respectively. Estimates based on Hartree—Fock calculations indicate a 0.27% error in the use of the TKR sum rule for F and 0.79% for Ar at an incident electron energy of 25 keV.