Collision-energy-resolved Penning ionization electron spectroscopy of styrene, 2-vinylpyridine, and 4-vinylpyridine with He*(23S) metastable atoms

Collisional ionization of styrene (phenylethylene), 2-vinylpyridine, and 4-vinylpyridine with metastable He*(2³S) atoms were studied by means of collision-energy/electron-energy resolved two-dimensional Penning ionization electron spectroscopy. Collision energy dependence of partial ionization cross-sections, which reflects the anisotropic interactions between a He*(2³S) atom and the target molecules, indicates that attractive interaction for the out-of-plane access of a He*(2³S) atom to phenyl group is stronger than that for the out-of-plane access to vinyl group. Moreover, it was found for vinylpyridines that the attractive interaction around π electrons became weaker than that for styrene, and that the attractive interaction for the in-plane access to the nitrogen atom is stronger than that for out-of-plane π-directions. However, in 2-vinylpyridine, the hydrogen atom of vinyl group prevents a He*(2³S) atom from approaching to the nitrogen atom along in-plane directions, and thus the attractive interactions around the nitrogen atom were shielded by the vinyl group. The experimentally observed anisotropic interactions were qualitatively supported with ab initio model interaction potential calculations between a Li (He*(2³S)) atom and the target molecule. Concerning with electronic structures of investigated molecules, the assignment of Penning ionization electron spectrum for 4-vinylpyridine was discussed on the basis of different behavior of collision-energy dependence of partial ionization cross-sections, and the satellite ionization band in Penning ionization electron spectra was also reported for styrene.     

Penning ionization electron spectroscopy of molecular adsorbates on Pd and Cu surfaces

The electronic properties of CO, PF₃, NH₃, C₂H₂ and C₆H₆ adsorbed on Pd(111), Pd(110), and Cu(110) surfaces were studied by Auger deexcitation (AD) of metastable noble gas atoms (Penning ionization) and by ultraviolet photoelectron spectroscopy (UPS). Electron emission via AD is restricted to the outmost levels localized at the adsorbed particles. The AD process competes with reasonance ionization of the metastable atoms, and its probability depends on the geometry of the adsorbed species and on its adsorption sites as well as on coverage. The differences in kinetic energy of electrons emitted by AD or by photons reflect modifications of the electron affinities of the adsorbed molecules. The extreme surface sensitivity of AD spectroscopy allows in particular to probe the local density of states at the adsorbate in the energy range close to the Fermi level which arises from π “back donation” as well as σ antibonding contributions.     

Penning ionization electron spectroscopy and photo-electron spectroscopy of molecular solids. II. Ammonia and water

The Penning ionization electron spectra (PIES) and ultraviolet photoelectron spectra (UPS) of ammonia and water molecules condensed on a cold metal substrate have been measured using thermal He*(2³S, 2¹S) metastable atoms and He(I) (58.4 nm) photons. The shifts of the observed positions of the PIES peaks relative to those of the UPS peaks in the condensed phase are roughly equal to the corresponding shifts in the gas phase. The relative intensities of the 3a₁ and 1e orbital peaks are reversed in the PIES and UPS for both gaseous and condensed ammonia; the origin of this reversal is interpreted as the difference between the interactions with metastable atoms and photons. On the other hand, the relative intensities of the 3a₁ orbital peak in the PIES and UPS for condensed water decrease as compared to the gas-phase spectra. This implies participation of the 3a₁ orbital of water in the hydrogen bonding.     

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.     

Photoelectron and Penning ionization electron spectrometry with a differential retarding field analyzer

A differential retarding field analyzer of the type described by Lindau et al.² has been tested in detail by photoelectron spectrometry in the 0–10-eV range. A resolution of 17 meV was obtained together with a satisfactory line-shape and sensitivity. The important potential parameters concerning resolution and line-shape, and the energy-dependence of the transmission of the analyzer have been determined experimentally. As a first application, Penning ionization electron spectra for the systems He(2³S)-N₂ O₂, and HCl have been obtained with the transmission-calibrated analyzer.     

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.     

Electon spectroscopy using exeted atoms and photons of Penning ionization of the rare gases

A study has been made of the Penning ionization electron spectra resulting from collisions between rare gas atoms and helium metastable atoms. The results are compared and contrasted with photoelectron spectra at 584 Å. Additional bands appear in the electron spectra and are explained on the basis of fast neutral-neutral collisions involving curve crossing mechanisms. Electron spectra from mixed rare gas systems at higher pressures are examined.     

Application of Penning ionization electron spectroscopy to the study of chemical reactions on the solid surface; Photooxidation of naphthacene and rubrene

Photooxidation reactions on the solid films of naphthacene and rubrene were studied by Penning ionization electron spectroscopy (PIES) and UV photoelectron spectroscopy (UPS). PIES was found to reveal the processes of the surface reaction within the first layer selectively. On the other hand, the UPS result showed that the photooxidation reaction proceeded into the inner layers as oxygen molecules diffused into the solid. The present study indicates that the reaction process from the surface to the bulk can be effectively probed by a combination of PIES and UPS.     

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.     

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 V. Penning ionization of water,alcohols and ethers

The Penning electron spectra of water, methanol, ethanol, isopropyl alcohol, tertiary butyl alcohol, dimethyl ether, ethylene oxide, diethyl ether, tetrahydrofuran and 1,4 dioxane, obtained using helium metastable atoms (2¹S, 2³S), are compared to their respective photoelectron spectra at 584 Å. An analysis of the Penning electron band shape and the position of the peak maximum for the ground state ions suggest significant changes in the Franck—Condon factors in comparison with photoelectron spectra. This may be ascribed to modifications of the target potential energy curves. 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 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.     

Velocity dependence of total Penning ionization cross sections

The velocity dependence of the total Penning ionization cross sections,σ(v), is measured in the thermal relative velocity region, using a time of flight method.σ(v) curves are reported for the collision systems He(2¹ S)/Ar, Kr, Xe, N₂, Hg, He(2³ S)/Ar, Kr, Xe, N₂, Hg, Ne(³ P _{2, 0})/Kr, Hg, and Ar(³ P _{2, 0})/Hg. In a qualitative discussion it is shown that all features of the measuredσ(v) curves may be explained within the frame of the theory of Penning ionization, allowing to extract information on the physical quantities governing the process: on the interaction potentialV(R) and on the transition probabilityW(R). A theoretical calculation for the He(2³ S)/Ar system shows good agreement with our experimentalσ(v) curve. On the basis of the present results earlier data onσ(v), and on absolute cross sections and rate constants obtained at certain relative velocity distributions are discussed.     

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.     

Method for determining the ionization potential and electron affinity of atoms and molecules using electron spectroscopy

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 56, No. 4, pp. 570–574, April, 1992.     

Penning ionization electron spectroscopy of H2O,D2O,H2S and SO2

Electron energy peak shifts and peak shapes were determined in the ionization of H₂O, D₂O, H₂S and SO₂ by Ne(³P₂) and He(2¹S, 2³S) metastable atoms. The shifts are large, especially in ionization of H₂O and D₂O into the ionic ground state and are probably mostly due to chemical interaction during the collision.In a previous paper the electron energy distribution curves for ionization of CO, HCl, HBr, N₂O, NO₂, CO₂, COS and CS₂ by helium, neon and argon metastables and the characteristics of this ionization were described¹. In this paper the series of triatomic molecules was extended to the molecules H₂O, D₂O, H₂S and SO₂. Because all these molecules have considerable dipole moments it could be expected that the peak shifts might be enhanced as compared with other triatomic molecules.