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.     

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.     

Electron spectroscopy using excited atoms and photons; VIII penning ionization of the hydrogen halides

Electron spectra resulting from collisions of thermal 2³S metastable helium atoms with HF, HCl, HBr and HI are compared with the respective 584     

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.     

Electron spectroscopy with excited atoms and photons.: X. Halogen-substituted methanes and methyl amines

Electron binding energy spectra have been measured for CH₃Cl, CH₃Br, CH₃I, CFCl₃, CF₂Cl₂, CF₃Cl, CF₄, CH₃NH₂, (CH₃)₂ NH and (CH₃)₃N. Measurements have been made using 584 Å (21.22 eV) photons as well as with 2³S(19.82 eV) and 2¹S(20.62 eV) metastable helium atoms. Relative spectral intensities are compared for photoionization and Penning ionization.     

Electron impact ionization of diatomic molecules

Cross sections for the ionization of N₂, CO and O₂ diatomic molecules by electron impact are calculated. The applied distorted wave model is based on our previous studies for positron impact, the molecular orbitals being described by Gaussian wavefunctions. Our study emphasizes the importance of electron exchange and of using correct distorted waves for the ejected electron.     

Ratiometric comparison of intense field ionization of atoms and diatomic molecules

Intense-laser ionization rates for rare gas atoms and diatomic molecules have been precisely compared by making simultaneous measurements of ionization yield vs laser intensity for mixed atomic and molecular targets. At a given laser intensity, the N (2) and F (2) ionization yields are slightly greater than that of Ar. Conversely, comparison of O (2) and S (2) with Xe indicates significant ionization suppression in these molecules. Recent molecular ionization models that successfully describe ionization suppression in O (2) and its absence in N (2) fail to explain our observations in F (2) and S (2).     

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.     

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.     

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.     

Reactions of excited atoms and molecules with atoms and molecules

Ionizing collisions of long lived excited particles with atoms and molecules are studied by a cross beam technique. For the first time reactions of atoms in high Rydberg states are included in the investigation. In this paper we report relative cross sections for the production of the ions RH⁺, RH ₂ ⁺ , and H ₂ ⁺ by collisions of excited rare gas atoms R^* with H₂. With HD as the target molecule the isotope effect for the production of RD⁺ and RH⁺ has been determined. In the case of argon and krypton, ions are produced only by the high Rydberg states, whereas in the case of helium and neon only the metastable states contribute to a measurable extent. The data indicate, that the reaction mechanism is different in principle for metastable and highly excited atoms. Simple models are proposed to explain the experimental results.     

Ionization of long-lived highly excited atoms by collision with molecules. II

A theoretical treatment of the collisional ionization of a highly excited atom with a molecule is improved by use of the exact form factor of the hydrogen atom in highly excited states. The present treatment is still based on the previously proposed mechanism in which the Rydberg electron is ionized by gain of energy from rotational de-excitation of the molecule in a rotationally excited state. Closer comparison of experiment with theory is made for symmetric- as well as asymmetric-top molecules.     

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.     

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 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.