Two-dimensional Penning ionization electron spectroscopic study on outer characteristics of molecules

Collision-energy/electron-energy-resolved two-dimensional Penning ionization electron spectroscopy (2D-PIES) has been used to study outer characteristics of molecules. Anisotropic potential energy surfaces for collisional ionization of molecules with a metastable helium atom He*(2³S) have been determined starting from ab initio model potentials via optimization based on trajectory calculations. Since ionization widths in the theoretical model of Penning ionization are functionals of target molecular orbitals, outer electron distributions of molecular orbitals can be determined via optimization procedures of calculated ionization cross-sections by trajectory calculations with respect to observed 2D-PIES data.     

Collision-energy-resolved Penning ionization electron spectroscopy of toluene and chlorotoluenes

Stereodynamics in ionization of toluene and o-, p-chlorotoluenes by collision with He^*(2³S) metastable atoms were investigated by two-dimensional collision-energy/electron-energy-resolved Penning ionization electron spectroscopy. Anisotropic interactions around the molecule were studied by the collision energy dependence of partial ionization cross-sections (CEDPICS) as well as model potential calculations, and shielding effect by the methyl group was observed in CEDPICS for ionization from Cl lone-pair orbitals of o- chlorotoluene. Attractive interaction with He^*(2³S) around the π orbital region was found to be larger for toluene rather than o-, p- chlorotoluenes. Exterior electron density (EED) calculation of partial ionization cross-sections in Penning ionization and negative CEDPICS for ionization band observed in ca. 4 eV in electron energy indicated that π^-2π⁺¹ shake-up state was observed in Penning ionization electron spectroscopy of toluene.     

Electron spectroscopy using excited atoms and photons VI. Penning ionization of HCN and some related compounds

The He*(2¹S, 2³S) Penning ionization electron spectra of HCN, (CN)₂, CH₃CN, BrCN and ICN are compared to the corresponding 584 Å photoelectron spectra. Large differences are observed for the relative populations of the ionic states of the various molecules when Penning ionization is compared to photoionization. For these molecules, the ratio of the normalized relative populations of states corresponding to removal of π bonding and nitrogen lone pair electrons is significantly greater for 584 Å photoionization than for He*(2³S) Penning ionization. Some unidentified chemi-ionization processes are observed for (CN)₂, BrCN and ICN.     

A Clear Atomic Example for the Surface Sensitivity of Penning Ionization: He*(23S) + Yb

Using a crossed-beam apparatus with a double hemispherical electron spectrometer, we have studied the spectrum of electrons released in thermal energy ionizing collisions of metastable He*(2³S) atoms with ground state Yb(4f¹⁴ 6s^{2 1}S₀) atoms, thereby providing the first Penning electron spectrum of an atomic target with 4f-electrons. In contrast to the Hel (58.4 nm) and NeI (73.6/74.4 nm) photoelectron spectra of Yb, which show mainly 4f- and 6s-electron emission in about a 5:1 ratio, the He*(2³S) Penning electron spectrum is dominated by 6s-ionization, accompanied by some correlation-induced 6p-emission (8% Yb⁺(4f¹⁴ 6p ²P) formation) and very little 4f-ionization (?2.5%). This astounding result is attributed to the electron exchange mechanism for He*(2³S) ionization and reflects the poor overlap of the target 4f-electron wavefunction with the 1s-hole of He*(2³S), as discussed on the basis of Dirac-Fock wave functions for the Yb orbitals and through calculations of the partial ionization cross sections involving semiempirical complex potentials. The presented case may be regarded as the clearest atomic example for the surface sensitivity of He*(2³S) Penning ionization observed so far.     

Electron spectroscopy using excited atoms and photons IX. Penning ionization of CO2, CS2, COS,N2O,H2S,SO2, and NO2

The electron spectra resulting from thermal collisions of He* (predominantly 2³S) metastable atoms with the seven triatomic molecules, CO₂, COS, CS², N₂O H₂S, SO₂ and NO₂, are compared with their respective 584-Å photoelectron spectra using a transmission-corrected electron spectrometer. The normalised relative electronic-state transition probabilities for production of ionic states in Penning ionization and photoionization are reported together with energy shifts (ΔE values) for He*(2³S) Penning ionization. The cross-section for Penning ionization to lower states of NO₂⁺ is extremely low as has been observed in other open shell molecules such as NO and O₂.     

Classical trajectory calculations for state-resolved Penning ionisation reactions of polycyclic aromatic hydrocarbon C10H8 in collision with He*(23S)

ABSTRACT

The reaction dynamics of Penning ionisation of a polycyclic aromatic hydrocarbon (PAH), naphthalene C₁₀H₈, in collision with the metastable He*(2³S) atom is studied by classical trajectory calculations using an approximate interaction potential energy surface between He* and the molecule, which is constructed based on ab initio calculations for the isovalent Li?+?C₁₀H₈ system. The ionisation width (rate) around the molecular surface are obtained from overlap integrals of the He 1s orbital and the molecular orbital. The calculated collision energy dependences of partial Penning ionisation cross sections (CEDPICS) in the range 50–500?meV at 300?K have reproduced the experimental results semi-quantitatively. The opacity functions, which represent the reaction probability with respect to the impact parameter b, are discussed in connection with collision energy, interaction with He* and the exterior electron density of molecular orbitals. They indicate that the collisional ionisations of C₁₀H₈ can be classified into three types: π electron ionisations with negative collision energy dependences which are predominantly determined by attractive interaction with He*; σ orbitals ionisations of the hardcore type; σ orbital ionisations which reflect interaction potentials around CH bonds. The critical impact parameters b_c become larger with increasing collision energy due to the centrifugal barrier.     

Electron spectroscopy using excited atoms and photons VII. Penning ionization of some carbonyl-containing compounds

The electron spectra resulting from thermal collisions of He*(2¹S, 2³S) metastable atoms with HCHO, CH₃CHO, CH₃COCH₃, HCOOH and CH₃COOH, are compared to the respective 584-Å photoelectron spectra. An analysis of the Penning electron band shape and the position of the peak maximum for the ground-state ions suggest there are significant differences in the Franck-Condon factors when compared with photoelectron spectra. This may be ascribed to modifications of the target potential-energy surfaces. It is also observed that the relative populations of the ionic states differ appreciably for Penning ionization and photoionization.     

Electron spectroscopy using excited atoms and photons IV. Penning ionization of ethylene

Using helium metastable atoms (2¹S, 2³S), the Penning ionization of C₂H₄ has been studied using a high resolution electrostatic electron analyzer. The Franck—Condon envelope for the ground state of C₂H₄⁺ (X²B_3u) is found to be the same for He* (2³S) Penning ionization and 584 Å photoionization. The ΔE shift values and the relative electronic transition probabilities are reported for four ionic states. Unusually large differences have been found for the relative electronic transition probabilities for Penning ionization and photoionization.     

Comparative study of He(23S)-penning ionization and He(I) photoionization of CF4, CCl4, and the chlorofluoromethanes by electron-ion coincidence spectroscopy

We present electron—ion coincidence spectra of the chlorofluoromethanes obtained after He(I)-photoionization and Penning ionization by He(2³S).Remarkable differences between both modes of ionization exist for CF₃Cl. Our tentative interpretation suggests the existence of a strongly bound interaction potential of ionic character between He(2³S) and CF₃Cl, in addition to the essentially flat covalent potential.     

Cold atomic collisions: coherent control of penning and associative ionization

Coherent control techniques are computationally applied to cold (1 mK相似文献   

Reactions of excited atoms and molecules with atoms and molecules

Penning electron distributions arising from the ionization of Na and K by He (1s 2s ^1,3 S)-metastables in thermal collisions, as well as the absolute cross section for Penning ionization of Na by He (2³ S) and relative cross sections for ionization of Na and K by He(2¹ S) and He(2³ S) are measured. It is shown that under fairly general conditions the well depth ε* of the interaction potential between the metastable and the target particle can be obtained directly from the measured electron distributions. ε*-values are reported for the moleules He(1s 2s ^1,3 S)-Na(² S), K(² S) (^2,2Ω), and for He(1s 2s ^1,3 S)-Hg(¹ S)(^1,3Ω). These latter values are obtained from previously published measurements and are to be considered preliminary. Further, additional evidence is given, that Penning ionization with metastables is an electron exchange process.     

Electron spectroscopy using excited atoms and photons: II. Penning ionization of diatomic molecules

A high resolution electrostatic electron analyser has been used to study Penning ionization electron spectra of H₂, HD, D₂, N₂, CO, NO and O₂ using helium metastable atoms (2¹S, 2³S). Results for H₂, N₂ and CO are in good agreement with other work. New data are presented for HD, D₂, NO and O₂. The Penning electron spectra are also compared to the 584 Å photoelectron spectra obtained in the same apparatus. The relative vibrational intensifies for the given electronic bands indicate that in most cases Franck—Condon factors for Penning ionization and photoionization are very similar. However for the O₂⁺(X²Π_g) band, the (2³S) Penning electron and photoelectron spectra show significant differences in the Franck—Condon envelopes This perturbation of the envelope for the Penning ionization may be explained by a competing autoionization process. The relative electronic transition probabilities are in many cases found to be different for Penning ionization and photoionization.     

Subject index

We have investigated the Penning ionization of NO₂(²A₁) by He(2³S) by means of electron—ion coincidence measurements. It is possible to identify two entrance channels. The quartet state is essentially repulsive and gives rise to an electron energy spectrum similar to that found in photoionization. The doublet entrance channel is strongly attractive with a well depth D_e of 4.8 {_?0.3^+0.1 eV. Ionization out of this channel leads to very broad features in the electron energy spectrum.     

The temperature dependence of penning ionization electron energy spectra: He(23S)—ar,N2, NO,O2, N2O,CO2

Using two intense thermal energy He(2³S) sources of different temperatures (≈400 and ≈ 1000 K, resp.) and a transmission-calibrated electron energy analyzer with about 30meV resolution, the dependence of He(2³S) Penning ionization electron spectra on collision energy for the target species Ar, N₂, NO, O₂, N₂O and CO₂ have been studied. The energy shifts of the Penning electron energy distributions and the branching ratios for the population of different electronic states in the molecular ions are determined quantitatively and compared for the two different collision temperatures. These results and the shapes of the observed Penning electron energy distributions are discussed in the light of current models for the Penning procen; the observed temperature dependences are correlated with the nature of the ionized orbitals in cases of only one entrance channel (closed shell targets) and, in addition, to the existence of qualitatively different entrance channels (open shell targets).     

Study of surface-molecule interactions using nuclear resonance photon scattering

Nuclear resonance photon scattering from the 6324-keV level in ¹⁵N was used for studying the surface-molecule interaction of nitrogen (N₂) on graphite. Other N-containing gaseous molecules (such as NO and N₂O) and other surfaces such that of activated carbon fiber (ACF) were also used. In particular, the average zero point energies of the out-of-plane vibrational and librational potentials of the molecules with respect to the adsorbing surfaces were determined. In addition, the out-of-plane orientation of the molecules versus T and as a function of the coverage were measured. In the nitrogen-graphite system, the results were used for testing the calculated in-plane and out of plane phonon spectra and for testing calculations of molecular dynamic simulations.     

Energy dependence of the Penning ionization electron spectrum of Ne* (3P2,0)+Kr

A crossed beam experiment is carried out to measure the energy of electrons emitted in Penning ionization processes by Ne^*(³P_2,0)–Kr collisions. The electron energy spectra have been measured at four different collision energies: 0.050, 0.140, 0.190, 0.460 eV. The analysis of the results allows the separation of spin orbit contributions both in the entrance and in the exit channels providing the related cross-section ratios. Some theoretical considerations have been made to clarify nature and role of interatomic potentials driving the collisions and some general features about the role of atomic fine structure in the Penning ionization processes.     

Reactions of excited atoms and molecules with atoms and molecules

From a He-beam excited by electron impact we eliminated the He(2¹ S) component to better than 0.5% by irradiating light from a He discharge. The quenching process ishv(2¹ P→2¹ S)+He(2¹ S)→He(2¹ P)→He(1¹ S +hv) (2¹ P 1)¹ S. By measuring the ions produced in collisions of the He-metastables with various target gases in a mass spectrometer, singlet to triplet Penning-cross section ratios were obtained. These ratios are without exception close to one, which is taken as evidence for the previously proposed electron exchange mechanism of the Penning ionization. In the case that more ions are produced in the collision of He (2¹ S) and He(2³ S) with a target gas, separate relative production cross sections are obtained for the two metastables. For the rare gases the measurements are performed at two temperatures of the He-beam, 320 and 90 °K. It is found that the cross section ratio of associative — to Penning ionization increases considerably as the temperature is decreased for both, He(2¹ S) and He(2³ S), the effect being much more pronounced for He(2¹ S). The results of this work are found to confirm conclusions drawn from measured energy distributions of the electrons ejected in the Penning process.     

The structure and proton affinity of N‐benzyl‐N‐(allenyl)trifluoromethanesulfonamide: FT‐IR,DFT and ab initio study,NBO analysis

The first N‐allenyl derivative of trifluoromethanesulfonamide, N‐benzyl‐N‐(allenyl)trifluoromethanesulfonamide ( 1 ), was studied experimentally by the FT‐IR spectroscopy and theoretically at the DFT and MP2 levels of theory. The intramolecular interaction of the nitrogen atom with the triflyl and the allenyl group was studied in comparison with the analogously substituted vinyl derivatives. Compound 1 in heptane solution at 295–183 K exists as an equilibrium mixture of conformational isomers. Protonation at different basic sites in a series of reference molecules is studied theoretically. The central C² atom of the allenyl group in 1 has the highest proton affinity, which is 16 kcal/mol higher than in the N‐vinyl analogues. The relative ability of the allenyl and vinyl groups to conjugation with an electron‐rich and electron‐deficient nitrogen atom lone electron pair is discussed. From the NBO analysis, the conjugation of the nitrogen lone electron pair with the allenyl group is much stronger than with the vinyl group. Copyright © 2013 John Wiley & Sons, Ltd.     

Photoionization cross-section measurements from the 2p, 3d and 3s excited states of lithium

The photoionization cross-sections from the 2p²P_{1/2, 3/2}, 3d²D_{3/2, 5/2} and 3s²S_1/2 excited states of lithium have been measured at different ionizing laser wavelengths, above the first ionization threshold. The experiments are performed by using a thermionic diode working in the space charge limited mode and the cross-sections are measured by employing the saturation technique. By changing the ionization photon energy, a smooth frequency dependence of the cross-sections has been observed for the 2p and 3d states. The cross-section from the 3s excited state has been measured at a single photon energy. The measured values of the photoionization cross are compared with the available data.     

Penning ionization of 1-bromoadamantane and bromocyclohexane by collision with He*(23S) metastable atoms: spin–orbit coupling effect and anisotropic interaction around bromine atom

The He I ultraviolet photoelectron spectra and Penning ionization electron spectra for 1-bromoadamantane and bromocyclohexane are measured and assigned on the basis of the outer valence Green’s function calculations. Spin–orbit splitting is observed clearly for the lone pair orbitals localized on the Br atoms. Slope parameters of collision energy dependence of partial ionization cross sections for two split branches in the bromocyclohexane spectra are nearly equal. Two possible explanations are discussed by a strong spin–orbit coupling effect and the coexistence of two isomers. The former breaks down the spatial electron distribution picture of molecular orbital in electron energy spectra; the latter is supported by the calculated results of anisotropic interaction around the Br atom using the unrestricted second-order Møller–Plesset perturbation theory with the 6-31+G(d,p) basis set. By comparison of the anisotropic interactions around the Br and Cl atoms, a shielding effect by the hydrocarbon group which leads to decreasing attractive interactions around the substituted Br atom is found to be relatively weak even for the large bromohydrocarbons.