Ionisation of small molecules by state-selected Ne* (3P0, 3P2) metastable atoms in the 0.06 < E < 6 eV energy range

The velocity dependence and absolute values of the total ionisation cross section for the molecules H₂, N₂, O₂, NO, CO, N₂O, CO₂, and CH₄ by metastable Ne* (³P₀) and Ne* (³P₂) atoms at collision energies ranging from 0.06 to 6.0 eV have been measured in a crossed beam experiment. State selection of the two metastable states of Ne* was obtained by optical pumping with a cw dye laser. We observe a strongly different velocity dependence at collision energies below about 1 eV for the ionisation cross section of the systems Ne*H₂, N₂, CO, and CH₄, and the systems Ne*O₂, NO, CO₂, and N₂O, respectively. The first group shows an increasing cross section in this energy range, similar to the Ne*Ar system, while the second group shows a very flat behaviour. This behaviour correlates with the difference in character (π or σ^b) of the orbital of the electron that is removed from the target molecule. For the molecules H₂, N₂, CO, and CH₄ an electron from a σ^b orbital is removed from the molecule, whereas for O₂, NO, N₂O, and CO₂ an outer π-ortibal electron is involved. For the systems Ne* (³P₀, ³P₂)H₂ we have derived the imaginary part of the optical potential by assuming a real potential similar to the theoretically calculated ground state NaH₂ potential of Botschwina et al. The resonance width Γ(r) as a function of the internuclear distance r shows a saturation at small r (r < 2.8 Å) for both the Ne*(³P₀)H₂ and the Ne*(³P₂)H₂ interaction. This supports previous conclusions of Verheijen et al. and Kroon et al. Reliable values for the absolute value of the total ionisation cross section have been obtained by performing a careful calibration of the density—length product of the supersonic secondary beam. The results are in good agreement with the values of West et al. for experiments without state selection. The total ionisation cross sections for molecules with π-type ionisation orbitals, with their larger spatial extent, in general are larger than those for molecules with σ^b-type ionisation orbitals.     

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.     

Laser-assisted (e, 2e) collisions in helium

The influence of a strong laser field on the dynamics of fast (e, 2e) collisions in helium is analyzed in the asymmetric, coplanar geometry. The interaction of the laser field with the incident, scattered and ejected electrons is treated in a non-perturbative way, while the remaining interactions are treated by using first order perturbation theory. Detailed calculations are performed for an incident electron energyE _{k i}=600 eV, an ejected electron energyE _{k B}=5 eV and a scattering angle θ _A =4°. The influence of the laser parameters (photon energy, intensity and direction of polarization) on the angular distribution of the ejected electron is analyzed. We find that in general the triple differential cross sections are strongly dependent on the dressing of the projectile and the target by the laser field.     

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.     

Improved study of the ionization of hydrogen atoms in thermal energy collisions with metastable He(23 S) atoms

The first electron spectrometric study of the ionizing reaction of metastable He(2³ S ₁) atoms with ground state hydrogen atoms has been carried out with sufficiently high resolution to partially resolve the rotational structure due to formation of rovibrationally excited HeH⁺ (v, J) ions at two different beam source temperatures (300 K and 90 K). The electron energy spectrum has been reproduced in model quantum calculations, using a new large scale ab initio calculation of the He(2³ S)+H(1² S)²Σ-potential. The imaginary part has been adjusted to yield a satisfactory fit to the measured spectrum. The collision energy dependence of the associative ionization electron spectra and of the total and partial ionization cross sections is discussed in some detail. No significant signs for limitations of the used local complex potential method, indicated by results of an earlier study of the He(2³ S)+H(1² S) system, have been found in the present work, in which the calculations were carried out with an improved and corrected program.     

Triple differential ionisation cross-section of hydrogen atom bye ±-impact including exchange

The Triple Differential Cross-Sections for ionisation of hydrogen atom bye ^± impact are calculated including exchange effect. A correlated three-body continuum wave function is used here for the final state. Attention is paid to equal energy sharing at the final channel where scattered and ejected energies are each 200 ev., and it is found that the exchange effect is remarkably high in this case. Present electron impact results are compared with the recent work of McCarthy et al. [1].     

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.     

Photoionization of the 2s, 2p and 3s subshells of Mg+

We present calculations of total and partial photoionisation cross sections for the 2s, 2p and 3s subshells of Mg⁺ from their ionisation thresholds up to a photon energy of 220 eV. Photoelectron angular distribution parameters are also calculated for the 2p subshell. Oscillator strengths are given for discrete structure, and calculated energy levels compared with experimental values. The significance of electron correlations and two electron excitation for this atomic ion is discussed.     

Total cross-sections for electron scattering from iron at 10–5000 eV using a model optical potential

We report calculations on the total (elastic plus inelastic) electron-scattering cross sections in the energy range 10–5000 eV. A model complex optical potential, composed of static, exchange, polarisation and absorption terms, is employed to describe the collision system at each electron energy. The Iron atom is described by Dirac-Hartree-Fock-Slater self-consistent charge density. The complex phase shifts are computed in a variable phase approach. The absorption cross sections are compared with the experimental results. The experimental absorption cross sections are obtained by adding the experimental ionisation cross sections and available experimental excitation cross sections for electron impact of the allowed transitions a⁵ D → (x,y,z)⁵ D ⁰, (w,y,z)⁵ P ⁰. We have good qualitative agreement between our results and the experimental results available below 200 eV. The Born-Bethe parameters are also calculated. Elastic differential cross-sections with and without absorption are also reported at a few selected energies.     

Dissociative ionisation of CS2 and the formation of S2+

Photoelectron–photoion coincidence spectroscopy has been used to examine dissociative ionisation of CS₂ from electronic states of CS₂⁺ up to 27 eV, including the satellite states 3, 4, 6 and 10 whose decay has not been studied before. Branching ratios to the ions S⁺, CS⁺, S₂⁺ and C⁺ have been determined throughout the range and kinetic energy release distributions have been deduced from peak shapes, allowing inferences on the states of the fragments. The choice of product channel is not strongly dependent on initial parent ion state identity. The products are formed in many different final states, but kinetic energy releases less than 3 eV are favoured, corresponding to formation of highly excited states of the products. In confirmation, optical emission has been found in coincidence with photoelectrons from formation of several inner valence states of the ions. Formation of S₂⁺ occurs from several initial states of the parent ion and possible mechanisms are considered. It is concluded that a “quasi-statistical” model may best describe the dissociation of CS₂⁺ from the inner valence states.     

HeIα photoionisation cross-section determination of Br atoms

An extended electron modulation spectroscopy method is described which allows the accurate determination of photoionisation cross sections of transient species relative to those of precursor compounds. In this paper cross section at 584 Å for atomic and molecular bromine transitions from neutral ground to lowest ionic states have been measured relative to that of the HBr⁺ (X²Π_1/2,3/2)←HBr(X¹Σ⁺) ionisation. Using the cross section of this HBr transition as an absolute standard and with relative cross-section data for ionisations leading to the accessible excited ionic states of Br⁺ and Br⁺₂, absolute total angle-integrated cross sections for the valence shell ionisation process in Br⁺ and Br⁺₂ are presented.     

Total ionization and electron attachment cross sections of CCl2F2 by electron impact

The absolute total ionization cross sections from threshold to 250 eV and dissociative attachment cross sections from zero to 10 eV have been measured for the CCl₂F₂ (dichloro-difluoro-methane) molecule by using a parallel plate condenser type ionization chamber. The maximum of the ionization cross-section curve was found to be at an energy of about 90 eV with a cross section of 1.44 × 10^?19 m². The attachment cross-section curve shows three peaks, the most intense being at zero electron energy with a cross-section value of 1.80 × 10^?20 m², and the other two at energies of 0.6 eV and 3.5 eV, respectively. The maximal relative error in cross-section values is 0.08, for electron energies larger than 0.4 eV.     

Differential cross section for CN− formation from dissociative electron attachment to the cyanogen molecule C2N2

The differential cross section for CN^? formaition from dissoiiativc attachment on C₂N₂ has been obtained in a crossed-beam experiment. Below 10 eV incident electron energy thc CN^? cross section shows two broad overlapping peaks with maxima at 5.4 and 7.3 eV corresponding to the formation of CN^? in its ground electronic state ¹σ⁺ plus the CN radical in the first excited state²π and the ground stale ²Σ+ respcctively     

Translational energy distribution of the excited hydrogen atom produced by controlled electron impact on HCl

The translational energy distribution of an atom can be calculated by differentiating the Doppler line shape of its emission line taken at a high optical resolution. The Balmer-β line of the excited hydrogen atom (n = 4) produced by electron impact on HCl has been measured at a high resolution (0.033Å) and at two angles (55° and 90°) with respect to the electron beam. The translation energy distribution depends on the electron energies and has almost two groups of components: ≈ 5 eV (fast) and ≈ eV (slow). Anisotropy is imporant for the slow component. The excitation function shows the corresponding structures. It is concluded that Rydberg states converging to the ²Π state of HCl⁺ produce the fast component and Rydberg states converging to the repulsive HCl⁺ states which cross the ²Σ⁺ state produce the slow component.     

Translational spectroscopy of N+ fragments from 5 keV collision-induced dissociation of N2+ ions by helium

Collision-induced dissociation of a 5–10 keV N₂⁺ beam impinging on a helium target has been reinvestigated by translational spectroscopy. The laboratory kinetic energy distribution of N⁺ fragments exhibits height structures on a continuum. They correspond to N⁺ and N fragments ejected in the c.m. frame with kinetic energies W of 4.75, 6.4, 6.8 and 8.1 eV and they are interpreted as transitions into excited states of N₂⁺ lying at more than 30 eV above the ground state of N₂. The experimental W distribution extending over 12 eV is compared to distributions calculated using the model of vertical Franck—Condon electronic excitation with different assumptions for the initial and final states.     

Experimental and theoretical investigation of the triple differential cross section for electron impact ionization of pyrimidine molecules

Cross-section data for electron impact induced ionization of bio-molecules are important for modelling the deposition of energy within a biological medium and for gaining knowledge of electron driven processes at the molecular level. Triply differential cross sections have been measured for the electron impact ionization of the outer valence 7b(2) and 10a(1) orbitals of pyrimidine, using the (e, 2e) technique. The measurements have been performed with coplanar asymmetric kinematics, at an incident electron energy of 250 eV and ejected electron energy of 20 eV, for scattered electron angles of -5°, -10°, and -15°. The ejected electron angular range encompasses both the binary and recoil peaks in the triple differential cross section. Corresponding theoretical calculations have been performed using the molecular 3-body distorted wave model and are in reasonably good agreement with the present experiment.     

Experimental and theoretical studies of the Bi-excited collision systems He*(23 S) + He*(23 S, 21 S) at thermal and subthermal kinetic energies

We have carried out a comprehensive experimental and theoretical investigation of the autoionizing collision systems He*(2³ S, 2¹ S) + He*(2³ S). We present high resolution electron energy spectra, obtained with a single He* beam (average relative collision energy 〈E _rel〉=1.6 meV) and with crossed He* beams (〈E _rel〉> =61 meV). The spectra show substantial structure, and under single beam conditions fast oscillations due to the interference of incoming and outgoing heavy particle waves in the entrance channels are observed. Accurate ab initio potential curves for the seven lowest He*—He*(Σ) molecular states have been obtained from a Feshbach projection scheme, and width functions for He*(2³ S) + He*(2³ S) have been derived by Stieltjes imaging. Based on these ab initio data, detailed quantum mechanical calculations of the electron spectra have been carried out and provide a thorough understanding of the experimentally observed spectral features. Good overall agreement of the calculated spectra with the experimental data is observed. The close coincidence in the positions of the experimental and theoretical peaks, especially for He*(2³ S) + He*(2³ S), underlines the reliability of the ab initio potentials. In the He*(2¹ S) + He*(2³ S) electron spectrum, the dominant peak is traced to be due to autoionization from the 2³Σ⁺ _g molecular state accessed via an avoided crossing. We also present a detailed discussion of the total ionization cross sections σ_tot and of the fraction σ_AI/σ_tot for associative ionization together with a critical comparison with previous work. The ionization probabilities for close collisions in entrance channels, from which autoionization is spin-allowed, are near unity, and therefore the absolute values and the collision energy dependence of the total cross sections simply reflect the long-range behaviour of the excited state potentials.     

Excitation energy transfer in the Li-Cs collision: Li*(2P)+Cs(6S)→Li(2S)+Cs*(5D)

The measurement of the collisional cross section for the process Li*(2P)+Cs(6S)→Li(2S)+Cs*(5D) are reported. The technique of resonant Doppler-limited two-photon laser excitation with thermionic detection is applied. The population density of the Cs*5D state is probed by photoionization, and the signals of the Cs(6S)→Cs*(5D) and the Li(2S)→Li*(2P) transitions are compared. The value for cross section of 30 Ų is measured, with an accuracy of 45%.     

Absolute emission cross sections of fragments produced by 0–100 eV electron impact on NH3

The absolute emission cross sections (ECSs) of excited fragments produced by electron impact on NH₃ were investigated in an incident electron energy range of 0–100 eV. The ECS of the NH(c¹II-a¹Δ) transition was 20.4 × 10⁻¹⁹ cm² and that of the NH(A³II-X³Σ) transition was 28.1 × 10⁻¹⁹ cm² at an incident electron energy of 100 eV. The estimated uncertainties of these values were 16 and 20%, respectively. The ECSs of Balmer lines were also determined. The effective rotational temperature of the NH(c¹II, v = 0) state at the incident electron energy of 100 eV was obtained to be (1.24 ± 0.34) × 10³ K by observing the intensity destribution of the Q branch of the NH(c¹II, v = 0-a¹Σ, v″ = 0) emission band.     

Two-photon absorption study of Zn(II)tetraphenylporphin: Role of a higher excited singlet state of gerade symmetry

The S₂ state fluorescence of Zn(II)tetraphenylporphin has been studied by using two-photon absorption and optical—optical double-resonance techniques. The main process to populate the S₂ state was found to be a stepwise two-photon absorption to the S_nstate through the S₁ state. The large absorption cross section of the S_n ← S₁ transition (6.8 × 10^?16 cm² molecule^?1) at 540 nm suggests that there exists a higher excited singlet state of gerade parity.