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 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 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.     

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 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.     

Threshold photoelectron spectrum of CO by electron attachment (TPSA)

The threshold photoelectron spectrum of CO is reported in the photon energy range from 14.0 eV (photoionization threshold) to 21 eV. The technique used is threshold photoelectron spectroscopy by electron attachment (TPSA). Vibrational levels of the X²Σ⁺ state are observed up to high quantum numbers (ν′? 12) and their population is found to be enhanced by resonant autoionization. Levels in the A²Π and B²Σ⁺ states are also observed. Here, the measured vibrational populations more closely follow Franck—Condon intensities, so that for these states direct photoionization is more important. Qualitative molecular-orbital arguments are given to make plausible the enhancements observed in CO⁺ and in previous measurements on N₂⁺.     

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).     

Comparison of methods for calculating Franck–Condon factors beyond the harmonic approximation: how important are Duschinsky rotations?

Three different approaches for calculating Franck–Condon factors beyond the harmonic approximation are compared and discussed in detail. Duschinsky effects are accounted for either by a rotation of the initial or final wavefunctions – which are obtained from state-specific configuration-selective vibrational configuration interaction calculations – or by a rotation of the underlying multi-dimensional potential energy surfaces being determined from explicitly correlated coupled-cluster approaches. An analysis of the Duschinsky effects in dependence on the rotational angles and the anisotropy of the wavefunction is provided. Benchmark calculations for the photoelectron spectra of ClO₂, HS^?₂ and ZnOH^? are presented. An application of the favoured approach for calculating Franck–Condon factors to the oxidation of Zn(H₂O)⁺ and Zn₂(H₂O)⁺ demonstrates its applicability to systems with more than three atoms.     

Calibration of electron-energy spectrometers

Relative photoionization cross-sections of strong peaks in the He(I) photoelectron spectra of N₂, CO, CO₂, and O₂, are tabulated. These data have been measured with an electron energy analyzer whose relative luminosity has been calibrated to an accuracy of ± 5 ‰ Thus, the tables will be useful for calibrating the transmission of other analyzers for electron energies below 9 eV. Correction for angular distribution effects is discussed.     

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.     

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.     

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.     

Ab initio calculation of the dipole moment of cyanamide

The He(I) photoelectron spectra of the isoelectronic series Fe(CO)₂(NO)₂, Co(CO)₃NO and Ni(CO)₄ are reported and interpreted by means of ab initio SCF-MO calculations. For the nitrosyl complexes it is found that ionization potentials calculated assuming Koopmans' theorem are seriously in error due to the considerably greater orbital relaxation accompanying ionization from metal than from NO valence orbitals. When such allowance is made for orbital relaxation by performing restricted Hartree-Fock (RHF) calculations on the ionic states, the experimental spectra are accurately reproduced and the observed similarity of the spectra of all three molecules is explained.     

Spectre de photoélectrons et calcul des facteurs de franck-condon pour H2O,D2O,HDO

The first band of the photoelectron spectrum of HDO has been recorded. In agreement with the selection rules of the group theory, the fundamental terms of the three symmetric vibrations of HDO (C_s symmetry) have been observed. Taking the geometry of the ion as parameters, the Franck-Condon factors for the ionization of H₂O, D₂O and HDO have been calculated. The geometry of the H₂O⁺, D₂O⁺, HDO⁺ ions (ground state) have been determined accurately by comparison of the calculated results with the corresponding photoelectron spectra. This geometry is approximately the same for the three ions: r_OH  1,00 Å and < HOH  110°.     

Satellite peaks in the high energy photoelectron spectra of some small first row molecules

The gas phase high energy photoelectron spectra of CH₄, NH₃, H₂O, N₂, O₂, CO and CO₂ have been recorded, and in all cases weak satellite peaks to high binding energy of the main ionization peak are observed. These peaks are assigned to transitions to ionic states in which valence electron excitation as well as core ionization has occurred. The intensity and position of these peaks, relative to the main ionization peak have been estimated from ab initio UHF calculations on the core hole states, which in general allow assignment of the satellite peaks in terms of orbital transitions of the core hole ion.     

Spektren und Winkelverteilungen der Photoelektronen von Atomen und Molekülen

The photoelectron energy spectra produced by the impact of 584 Å photons on the gases Kr, H₂, N₂, O₂, CO, NO and several alkanes are reported. The angular distribution of photoelectrons corresponding to the simultaneous formation of specific electronic states of the ion have been measured in the range 30°—130°. A preference for electron ejection in the direction of light propagation is shown in the formation of the electronic ground states of NO⁺ and O ₂ ⁺ . Both involve the ejection of an electron from aπ _g orbital. The onset energies for the various electronic states have been obtained with a resolution of ca. 40 meV. The relative transition probabilities for formation of various electronic states in a given molecular ion, as well as the Franck-Condon factors within specific states have been obtained. Similar experiments with the alkanes (methane, ethane, propane, and n-butane) reveal directly the existence of widely separated electron states (or grouping of states) in the ion, as well as the probability of forming these states. This energy distribution function, of vital importance to the study of the unimolecular decay of ions, could only be inferred previously by use of a questionable assumption.     

Penning ionization of NO and O2(1Δg) by argon in the 3P1 state

Electronically excited argon in the ³ P ₁ state was generated in a flow system by resonance absorption of the λ=106·7 nm emission line. This photochemical source is particularly convenient for kinetic studies since Ar(³ P ₁) alone is produced in the primary exciting step. Energy transfer from excited argon may lead to Penning ionization:

This process has been studied for M=NO and M=O₂(¹Δ _g ). Relative concentrations of Ar* were estimated from the Penning ion current, and it is shown that Ar* is probably in the initially excited ³ P ₁ state and not in the metastable ³ P ₂ state. Kinetic studies showed that Penning ionization of NO occurs for about one in every five gas kinetic collisions with Ar(³ P ₁). The Penning process can only compete effectively with radiative loss of Ar(³ P ₁) as a result of extensive imprisonment of resonance fluorescence. Penning ionization of O₂(¹Δ _g ) is shown to proceed at nearly the same rate as ionization of NO. The decrease in Penning ion current on addition of a quenching gas, such as N₂ or O₂, is used to derive rate constants for the quenching process relative to the rate of quenching by NO. Estimated absolute rate constants are presented in a table.     

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.     

Synchrotron radiation VUV double photoionization of some small molecules

The VUV double photoionizations of small molecules (NO,CO,CO2 ,CS2 ,OSC and NH3 ) were investigated with photoionization mass spectroscopy using synchrotron radiation. The double ionization energies of molecules were determined with photoionization efficiency spectroscopy. The total energies of these molecules and their parent dications (NO2+ ,CO2+ ,CO2+2 ,CS2+2 ,OSC2+ and NH2+3 ) were calculated using the Gaussian 03 program and Gaussian 2 calculations. Then,the adiabatic double ionization energies of the molecules were predicated by using high accuracy energy mode. The experimental double ionization energies of these small molecules were all in reasonable agreement with their respective calculated adiabatic double ionization energies. The mechanisms of double photoionization of these molecules were discussed based on a comparison of our experimental results with those predicted theoretically. The equilibrium geometries and harmonic vibrational frequencies of molecules and their parent dications were calculated by using the MP2 (full) method. The differences in configurations between these molecules and their parent dications were also discussed on the basis of theoretical calculations.     

Dipole oscillator strengths for the photoabsorption,photoionization and fragmentation of molecular oxygen

The photoabsorption, photoionization and fragmentation of O₂ have been studied using electron impact coincidence methods to obtain branching ratios and dipole oscillator strengths (cross-sections). The photoabsorption measurements cover the energy range 5–300 eV while the formation of electronic states of O₂⁺ (photoelectron spectroscopy) and the resulting ionic fragmentation (photoionization mass spectrometry) are both measured from close to threshold up to photon energies of 75 eV. The binding energy spectra of O₂ show peaks at 33, 47 and 57 eV in addition to those reported elsewhere in the literature. These peaks are assigned to multiple final ion states arising from photoionization of the inner valence orbitals. Structure in the O₂⁺ electronic state partial oscillator strength curves is in good agreement with recent theoretical work which predicts the existence of several shape resonances. A quantitative picture of the dipole-induced breakdown of O₂ is obtained for the energy range 12–75 eV. The photoionization efficiency is found to be constant above 20 eV.