Electron emission from crystal surfaces of condensed aromatics under the impact of metastable rare gas atoms

Energy distributions of electrons ejected from polycrystalline surfaces of naphthacene, perylene and coronene by the impact of metastable He ^*, Ne ^* and Ar ^* atoms have been measured. Two types of peaks, which are similar to the “non-moving” and “moving” structures in photoelectron spectra, are observed in each spectrum. The non-moving structures (ca. 1 eV) for perylene and coronene are similar to those in the photoelectron spectra, whereas the relative intensities of the two non-moving structures for naphthacene (0.6 and 1.7 eV) are remarkably different from the corresponding structures in the photoelectron spectrum. The peak positions (but not necessarily their intensities and widths) for the moving structures for all the samples (> 2 eV) agree with those of the corresponding photoelectron spectra. The origin of these moving structures is ascribed to Penning ionization on the solid surface.     

Photoelectron spectroscopy from CO adsorbed on a Cu(111) surface

He II-ultraviolet photoelectron spectra (hv = 40.8 eV) from a carbon monoxide layer adsorbed on a Cu(111) surface exhibit two peaks at 8.5 and 11.6 eV below the Fermi level and a weaker maximum centered at about 13.5 eV. The emission from the Cu d-band is markedly suppressed after adsorption. The results are discussed in terms of the recent models for assigning the UPS peaks observed after adsorption of CO on transtion metals     

The He(I) photoelectron spectra of amine N-oxides

The He(I) photoelectron spectrum of β-hydroxyethyldimethylamine N-oxide shows the degenerate pair of N-oxide ionic states to be shifted by 0.49 eV to higher ionization potential, relative to the non-hydrogen bonded system. The two bands remain nearly degenerate. The ionization of the hydroxy lone pair is shifted by > 0.6 eV to lower ionization potential. INDO calculations for the corresponding ionic configurations of a model system (NH₃O + H₂O) are compared with the experimental observations.     

Ab initio potential energy curves for the CO+ ion and a study of the photoelectron spectra of CO

The potential energy curves for the low-lying electronic states of CO⁺ are calculated by using the ab initio full valence configuration interaction method with minimal STOs. Calculated spectroscopic parameters are given for twenty five bound states, and the compared with the experimental values for four observed bound states. The band systems, observed in the 22 eV and 27 eV region of He II photoelectron spectra of CO, are analysed by using calculated potential energy curves and spectral intensities. New assignment for the fourth band system at 22 eV ～ 23 eV is proposed.     

Valence and C 1s core level photoelectron spectra of camphor

The valence photoelectron spectrum of camphor has been recorded with 95 eV synchrotron radiation, with better definition than previous He I spectra. The spectrum is interpreted by comparison with these He I results and with the aid of an outer-valence Green’s Function calculation of the orbital ionization energies. These calculations closely reproduce the observed vertical ionization energies in the outer valence region. A core level spectrum of the C 1s region (hν=357.9 eV) is also presented and reveals a marked shift of the carbonyl carbon relative to all others in the molecule.     

The photoelectron spectra of perfluoro-2-butyne and perfluorohexa-2,4-diyne. The perfluorination effect

The He(I) photoelectron spectra of perfluoro-2-butyne and perfluorohexa-2,4-diyne are reported and discussed in comparison with the photoelectron spectra of 2-butyne and hexa-2,4-diyne. Large π shifts (ca. 2 eV) towards higher ionization energies are observed as a consequence of complete substitution of hydrogen by fluorine. EXTHUC and CNDO/S calculations are found to be in good agreement with the experimental data. A new vibrational analysis of the first band of the perfluoro-2-butyne is proposed.     

Sum rule consideration on valence orbital ionization energies in methyl amines

A sum rule treatment using Hall's equivalent orbitals has been applied to the valence orbital ionization energies of mono-, di- and trimethylamine up to about 40 eV assuming Koopman's theorem. Excellent agreement is obtained between theory and experiment in partial and total energy sums. A similar treatment using p-type localized orbitals previously proposed by the authors is also applied to ionization energies below about 18 eV for the methyl amines. In these sum rule treatments we use both the vertical ionization energies obtained from He (I) photoelectron spectra measured here and the higher ionization energies deduced from the He (II) photoelectron spectra published by Potts et al. The complete sets of valence orbital ionization energies of the methyl amines have thus been identified by sum rule considerations in photoelectron spectroscopy. We show that the character of the corresponding MO's may be inferred from the sum rule. The orbital orderings and characters of the methyl amines have also been studied by CNDO/2 and MINDO/2 methods.     

Observation of two-photon above-threshold ionization of rare gases by xuv harmonic photons

We have successfully observed two-photon above-threshold ionization in rare gas atoms (Ar, Xe, and He) by the fifth harmonic (25 eV photon energy) of a KrF laser. Use of the energy-resolved photoelectron counting system together with our laser, providing strong 25 eV radiation at 40-100 Hz, enabled us to detect the very weak single-color two-photon above-threshold ionization signals. Experimental data are in good agreement with our theoretical calculations newly developed along the line of multichannel quantum defect theory.     

Photoelectron and electron impact spectrum of HCN

The electronic structures of HCN and DCN have been determined by examining high resolution He(I) photelectron spectra of HCN and DCN, He(II) photoelectron spectrum of HCN, and the electron impact energy loss spectra of HCN and DCN. The present investigation supports an earlier assignment of the orbital sequence in HCN. New vibrational data are presented and the Rydberg series and valence transitions are reinvestigated. The adiabatic ionization energies for the 1π and 5σ orbitals in HCN are found to be 13.607 ± 0.002 eV and 14.011 ± 0.003 eV respectively.As mentioned above the investigation of the Rydberg series indicated that the first IP at 13.607 eV is the 1π ionization and the second IP at 14.011 eV is the 5σ ionization. A comparison of the experimental and theoretical intensity ratio between the two first PES progressions also supports this assignment. It is further supported by the fact that in the second IP the ν₃ vibration frequency is not changed as much as it is in the first IP, which is in agreement with the PES of N₂ and CO. The analysis of the bending vibrations also supports this ordering of the orbitals.The same orbital assignment has recently been proposed by Frost et al.⁵, using a comparison with the HCP photoelectron spectrum. The present paper supports their assignment of orbitals and (00⁰0)-(00⁰0) transitions. There are, however, some disagreements concerning the vibrational analysis. This is probably due to the fact that the HCN spectrum of Frost et al.⁵ revealed less structure than ours. As indicated by Figure 5 there is possibly still more structure to be revealed.     

Ionisation of cyanate ions in solution

The He(I) photoelectron spectra of the ions OCN^? and SCN^? in adiponitrile solution have sharp bands peaking at 8.3 and 7.1 eV. In acetonitrile solution, the N-methylpyridazinium salts possess UV/visible absorption bands at 4.16 (OCN^?), 3.64 (SCN^?) and 3.37 eV (SeCN^?); these are assigned as electron transfers from anion to cation. The conclusion is drawn that the near-UV absorption bands of the anions are caused by electron transfers to the solvent.     

Vacuum ultraviolet resonance line radiation source from rare gas atoms and ions for UHV photoelectron spectroscopy

A source and differential pumping system for producing high intensity resonance line radiation from rare gas atoms and ions for ultrahigh vacuum (UHV) photoelectron spectroscopy has been developed. Photoelectron count rates from a gold sample, as measured with a double-pass cylindrical mirror analyzer at pass energy 15 eV and 0.10 eV resolution, are ～ 300,000 c s^?1 for the He(I) (21.22 eV) line and ～30000 c s^?1 for the He(II) (40.81 eV) line. The source design is based on the principle of the electrostatic charged particle oscillator and is capable of sustaining discharges over the pressure range 1 to ～ 10^?6 torr. The discharge segment consists of a cylindrical cold cathode surrounding two tungsten rod anodes which are held at high positive potential. Three stages of differential pumping are employed in order that the vacuum in the main spectrometer chamber can be maintained at 2 × 10^?10 torr during operation. The calculated helium flow reaching the main chamber under these conditions is < 101 s^?1. Details of the construction and operating characteristics of the source are presented.     

The valence band of some light rare-earth metals studied by photoemission spectroscopy

The valence bands of La, Ce, Pr, and Nd have been studied by ultraviolet photoelectron spectroscopy (UPS) (He II, hv=40.8 eV) and compared to results from X-ray photoelectron spectroscopy (XPS)(Mg Kα_{1, 2}). The energy positions of the $\text{4?}$ levels are in good agreement with earlier XPS results and calculations based on estimates from optical and thermodynamic data. The widths of the sd bands and the $\text{4?}$ levels and the relative emission intensities are discussed.     

Electronic states of neutral and ionized tetrahydrofuran studied by VUV spectroscopy and ab initio calculations

The electronic spectroscopy of isolated tetrahydrofuran (THF) in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 5.8–10.6 eV with absolute cross-section measurements derived. In addition, an electron energy loss spectrum was recorded at 100 eV and 10° over the 5–11.4 eV range. The He(I) photoelectron spectrum was also collected to quantify ionisation energies in the 9–16.1 eV spectral region. These experiments are supported by the first high-level ab initio calculations performed on the excited states of the neutral molecule and on the ground state of the positive ion. The excellent agreement between the theoretical results and the measurements allows us to solve several discrepancies concerning the electronic state spectroscopy of THF. The present work reconsiders the question of the lowest energy conformers of the molecule and its population distribution at room temperature. Electronic supplementary material  Supplementary Online Material     

The photoelectron spectra of some iron tricarbonyl complexes of 4π-electron donor ligands

The photoelectron spectra of eighteen compounds which are dienes or diene—iron tricarbonyl complexes have been investigated. A comparison of the photoelectron spectra of the dienes and corresponding iron carbonyl complexes has yielded the values of the perturbation energies for the two π orbitals of the diene moiety caused by interaction with Fe(CO)₃. These perturbation energies are relatively constant (Δπ₁ = 0.89 ± 0.07 eV, Δπ₂ = 0.22 ± 0.06 eV) throughout the series. They have been employed to estimate the π ionization energies of the organic transient species cyclobutadiene (8.29 and 11.95 eV) and trimethylenemethane (8.36 and 11.79 eV), two novel molecules which have not been studied successfully by photoelectron spectroscopy to date.     

Electronic structure of amino acids and ureas

The outer electronic levels of glycine, alanine, glycine ethyl ester, urea, and thiourea have been investigated by means of photoelectron spectroscopy using He I (584 Å) and He II (304 Å) radiation and CNDO/2 molecular orbital calculations. In the amino acids the molecular orbital (MO) ordering ha been found to be [σ core and carbonyl π > a″(O) a′(O) > a″(N)] with a first ionization potential of ～8.8 eV. Glycine and alanine are foun to exist as the undissociated amino carboxylic acids rather than zwitterions in the high temperature vapor. In urea the three lowest energy molecular o are near-degenerate [σ(4b₁) ～ π(1a₂) ～ π(2b₂)] while in thiourea only the two lowest energy MO's are near-degenerate [π(1a₂ σ(4b₁) ～ π(2b₂)]. The first ionization potentials of urea and thiourea are 9.7 and 7.9 eV respectively.     

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.     

Photoelectron spectroscopy of methyl,ethyl and isopropyl alcohol adsorbed on the (110) surface of Cu

Methyl, ethyl and isopropyl alcohol were allowed to adsorb on a clean (110) face of a copper crystal at room temperature, and photoelectron spectra were obtained using He(I) (21.2 eV) radiation. The spectra of the adsorbed species differed sufficiently from those of the free molecules to indicate a chemical change on adsorption. In accordance with studies in the literature using flash-desorption and energy-loss spectroscopy, it is suggested that the observed spectra are those of the corresponding alkoxy radicals formed as a result of dehydrogenation. In the case of the methoxy radical, calculations were carried out for a cluster of nine copper atoms and the results shown to be consistent with the experimental data. Results were also obtained on the rates of adsorption and desorption and on the dependence of the spectra on the polar angle between the normal to the surface and the photoelectron direction. Finally, results were obtained on the extent of adsorption by means of X-ray photoelectron spectroscopy.     

Enhanced nonlinear double excitation of He in intense extreme ultraviolet laser fields

Nonlinear, three-photon double excitation of He in intense extreme ultraviolet free-electron laser fields (～24.1 eV, ～5 TW/cm2) is presented. Resonances to the doubly excited states converging to the He+ N=3 level are revealed by the shot-by-shot photoelectron spectroscopy and identified by theoretical calculations based on the time-dependent Schr?dinger equation for the two-electron atom under a laser field. It is shown that the three-photon double excitation is enhanced by intermediate Rydberg states below the first ionization threshold, giving a greater contribution to the photoionization yields than the two-photon process by more than 1 order of magnitude.     

Photoelektronenspektren organischer verbindungen: III. Photoelektronenspektren acetylensubstituierter kleiner ringe

The high resolution He I photoelectron spectra of vinylacetylene, cyclopropylacetylene, cyclobutylacetylene and cyclohexylacetylene are reported and ionization potentials in the low-binding region are assigned by correlating the spectra with spectra of reference compounds, by using the information supplied by vibrational analysis and by comparing the assignment with the results of MINDO/2 calculations. The resonance integrals β_RS between the π type orbitals of the acetylene moiety (R) and the π and Walsh type orbitals of the substituents (S) are determined by means of a LCMO model, parametrized with spectra of known assignment. The interaction terms H_Rs of cyclopropylacetylene, which are 1.3 eV in the bisected conformation and 0.7 eV in the perpendicular conformation, can be considered as experimental verification of the correlation diagram for vinylcyclopropane given by Hoffmann et al. and provide an explanation of the stability of the gauche conformation of vinylcyclopropane. In the case of cyclobutylacetylene, only estimated values of the interaction term H_RS can be obtained from the spectrum.     

Reaction of oxygen with gallium surfaces

The adsorption of oxygen molecules on evaporated gallium films has been studied by UV photoelectron spectroscopy between 10 and 300 K. In addition to the oxygen levels, the chemical shift of the Ga 3d core level has been investigated using monochromatized light from a He discharge lamp at ?ω = 40.8 eV. Four different states of oxygen have been found depending on temperature. At 10 K the molecules of the first layer are physisorbed onto which several additional layers can be condensed. The rigid relaxation shifts to smaller binding energies are 2.7 eV for physisorbed and 1.3 eV for condensed oxygen. During warming-up the oxygen reacts with the gallium surface. Between 70 and 130 K an oxygen species develops which is interpreted as chemisorbed molecular oxygen. This is concluded from the valence band UP spectra, the chemical shift of the Ga 3d level, and the work function change. At 300 K oxygen is dissociatively bound and the bulk oxide grows.