Relative intensities in X-ray photoelectron spectra.: Part VIII

The photoionization cross-sections for the 3s and 3p shells of atomic Si, P, S, and Cl and the S²⁺ ion, and for the 2s and 2p shells of atomic F have been calculated using the random-phase approximation with exchange (RPAE) for the average-configuration term. Using the theoretical atomic cross-section values, the partial cross-sections for photoionization of the SF₆ molecule have been calculated for hv ? 54 eV and the photoelectron spectra have been interpreted. The calculation of relative intensities in the photoelectron spectra of H₂S is presented. The influence of the effective charge of an atom on the photoionization cross-section value for a molecular level is shown.     

Relative intensities in x-ray photoelectron spectra. Part II

Experimental data on the relative intensities of X-ray photoelectron lines of some elements with 22 ? Z ? 56, and the calculations for the photoionization cross-sections for inner levels of some elements with 21 ? Z ? 63 are reported. The relationship between photoionization relative cross-sections and line relative intensities is examined. Theoretical values of the photoionization cross-sections were used in the calculations of relative intensities of molecular valence levels for AO₄^x? (A = Cl, S, P, Se, As), AF₆^x? (A = S, Si, Al), COS, CS₂ and H₂S.     

X-ray photoionization cross sections for quantitative analysis

The relative photoemission intensities from subshells of 51 elements and simple compounds have been measured using X-ray photoelectron spectroscopy. The chemical composition of each specimen surface was monitored by Auger electron spectroscopy and contamination was minimized by Ar⁺ ion bombardment. Experimental photoelectron cross sections derived for MgKα (1254 eV) radiation were compared with theoretical Hartree-Slater subshell photoionization cross sections calculated by Scofield. Good correlation (r = 0.96) between theory and experiment was observed over two orders of magnitude for most of the elements studied. Relative cross sections for subshells of a given element are also consistent with theoretical values, with several notable exceptions. The results indicate that quantitative analysis of ESCA spectra is possible using the Scofield cross sections.     

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.     

Study of the x-ray photoelectron spectrum of tungsten—tungsten oxide as a function of thickness of the surface oxide layer

The photoelectron spectrum of tungsten metal using Al K_α X-rays has been studied as a function of a tungsten oxide layer on the surface. The photoelectron lines arising from the 4f shell of tungsten metal are clearly separated in energy from those coming from WO₃. The ratio of the intensities of these two sets of lines were measured for a series of metal samples which were anodized to a determined level of tungsten oxide. The data were shown to be consistent with a uniform deposition of oxide film. The escape depth, or thickness from which half the photoelectron intensity is derived, was found for a 1450 eV photoelectron to be 8.9 Å and 18.3 Å for W and WO₃, respectively.     

Pulsed field ionisation—ZEKE photoelectron spectrum of o-, m- and p-tolunitrile

Pulsed field ionisation—ZEKE photoelectron spectroscopy has been applied to o-, m- and p-tolunitrile in a supersonic jet. The PFI-ZEKE photoelectron spectra of m- and p-tolunitrile show well-resolved anharmonic structures in the low-frequency region, which are assigned to bands due to internal rotational motion of the methyl group in the cation. Level energies and relative transition intensities are reproduced well by a one-dimensional rotor model with a three-fold axis potential. Potential curves for the internal rotation have been determined. For o-tolunitrile, no band due to internal rotation was found in PFI-ZEKE spectrum. It is suggested that the o-tolunitrile cation has the high barrier for internal rotation and the stable conformation that is the same as that in S₁ and S₀. The barrier height and the conformation are compared with other toluene derivatives, and the relation between the electronic character of –CN and the internal rotational motion has been discussed.     

Measurements of relative line strengths in N I multiplets for transition arrays 3s-np (n=3, 4, 5)

Applying the emission method, intensities of 60 spectral lines of neutral nitrogen (N I) belonging to 15 multiplets originating from 3s-np (n=3, 4, 5) transition arrays have been measured. A wall-stabilized arc, operated at atmospheric pressure in helium with some admixture of nitrogen was applied as the excitation source. From measured line intensities, relative line strengths within multiplets have been evaluated. For transitions with Δn=1 and 2, significantly larger discrepancies from LS coupling results are found if compared to transitions 3s-3p (Δn=0). The measured relative line strengths within 3s-3p multiplets are compared with older measurements, recent calculations and with the new data recommended by NIST.     

“Photoelectron” spectroscopy by electron impact coincidence measurements of scattered and ejected electrons in CO

The relative intensities for ionization to the lowest three electronic states of CO⁺ have been obtained by detecting forward scattered 3.5 keV electrons in coincidence with electrons ejected at 90°. Data are reported for energy losses of the projectile electron in the range of 18–50 eV. A simple relation is derived between our data, photoelectron intensities and the angular anisotropy parameter β. This relation appears to be fulfilled satisfactorily for a (photon) energy of 21.2 eV, for which photoelectron intensities and β are available.     

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.     

Gas phase high temperature photoelectron spectroscopy: an investigation of the transition metals scandium and vanadium

The He(I) photoelectron spectra of atomic scandium and vanadium have been recorded in the gas-phase. For scandium two bands associated with a (4s)^?1 ionization and one band associated with a (3d)^?1 ionization of the neutral atom have been observed. Measurement of their relative intensities allows the σ_3d:σ_4s photoionization cross-section ratio in atomic scandium to be estimated as 57.1 ± 9.0. In the atomic vanadium spectrum, six bands were seen. Four of these correspond to (3d)^?1 and (4s)^?1 ionizations of the ground state of the neutral atom, the V a⁴F state, whereas two bands correspond to (3d)^?1 ionizations of an excited state, the V a₆D state, which is approximately 2100 cm^?1 above the ground state. Measurement of the intensities of bands arising from the V a₄F state allows σ_3d:σ_4s to be estimated as 29.8 ± 2.5 for vanadium. Spectra of vanadium have been recorded with both single- and multi-detector photoelectron spectrometers. Comparison of the data acquisition rates obtained with both spectrometers demonstrates the advantage of using a multidetector instrument in high temperature photoelectron spectroscopy.     

Relation among x-ray photoelectron intensities,cross sections and kinetic energies in insulating solids

The fundamental relation among X-ray (MgK_α) photoelectron intensities, theoretical orbital ionization cross sections, and photoelectron kinetic energies has been investigated through precisely measured relative intensities of orbital electrons from sublimed-condensed samples of KF, NaF, CaF₂ and KG, and other ionic solids. Although the results confirm within approximately 15% the relation and establish a linear dependence of the intensities on analyzer transmission, they suggest that either a slight deviation from the relation exists or a cancellation of errors occurs. Empirically it is found that the relative intensities are proportional to the square of the ratio of kinetic energies, and with this relation, the intensities agree with the theoretical cross sections within a few per cent. This relation suggests that the dependence on the angular parameter is inconsequential and that the mean free path is proportional to the kinetic energy.     

UV Subshell photoionisation cross-section of atomic Zn,Cd, and Hg: Experiment and theory

Partial photoionisation cross-sections have been measured at photon wavelengths of 584, 304, 256 and 243 Å for the valence d and s shells of zinc, cadmium and mercury atoms. These cross-sections have also been calculated using the GIPM, a simple one-electron method which obtains the final-state potential by an inversion of the Schrödinger equation for the initial orbital of the photoelectron in the Hartree-Fock wave function. Comparison shows good qualitative agreement between theory and experiment. The quantitative agreement is typically within a factor of 2. Angular asymmetry parameters have also been calculated and the branching ratio for the splitting of the d-subshell cross-section into ²D_{$\text{5}\text{2}$} and ²D_{$\text{3}\text{2}$} contributions has been measured.     

Gas phase high temperature photoelectron spectroscopy

The HeI photoelectron spectrum of atomic gallium has been recorded. Bands associated with ionization from both the J = 1/2 ground state and J = 3/2 excited state arising from the 4s ² 4p ¹ configuration have been observed. The NeI spectrum of the (4p)^-1 ionization is used to estimate the excitation temperature of gallium as 1100 ± 150 K and evidence is presented to show that the HeI spectrum is perturbed by an auto-ionization process from the J = 3/2 excited state. A simple intermediate coupling model is developed and shown to be useful in the prediction of relative band intensities.     

Resonant excitation of a new valence state in N2 by electron impact

A new valence state which lies below the first Rydberg state has been observed in N₂, in the 11.3 to 12.1 eV energy range, using an electron impact technique. This state is only excited from 1.3 to ～3 eV above its threshold through a short lived resonance. Some levels are not clearly resolved but their presence is deduced from the resonant profiles of their differential cross sections. A perturbation in the relative intensities of the C³Π_u vibrational levels is observed when this new state is excited.     

Relative intensities in x-ray photoelectron spectra

An experimental determination of the relative intensities of X-ray photoelectron lines corresponding to the inner levels of elements with Z ? 20, and calculations of the total photo-ionization cross-sections for all shells of these elements with the Hartree—Fock—Slater potential are reported. The experimental and theoretical values agree well for the 1s levels while marked discrepancies are revealed for the 2p levels. The theoretical values of the cross sections for the atomic valence levels are used to calculate the relative intensities of molecular levels in CF₄, BF₄^?, BeF₄^2?, LiF, NO₃^?, CO₃^2?, CO, N₂, CO₂, H₂O and C₄H₅N. The results of the calculations agree satisfactorily with experiment.     

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.     

Relative intensities in X-ray photoelectron spectra

An experimental determination of the relative intensities of X-ray photoelectron lines corresponding to the inner levels of elements with Z ? 20, and calculations of the total photo-ionization cross-sections for all shells of these elements with the Hartree—Fock—Slater potential are reported. The experimental and theoretical values agree well for the 1s levels while marked discrepancies are revealed for the 2p levels. The theoretical values of the cross sections for the atomic valence levels are used to calculate the relative intensities of molecular levels in CF₄, BF₄^?, BeF₄^2?, LiF, NO₃^?, CO₃^2?, CO, N₂, CO₂, H₂O and C₄H₅N. The results of the calculations agree satisfactorily with experiment.     

Photoionization cross sections: Interpretation of band intensities in He I and He II photoelectron spectra

Relative photoionization cross sections of molecules consisting of atoms from the first three rows of the Periodic Table are computed by a theoretical method developed previously which involves the plane-wave approximation for the photoelectron and the use of semiempirical LCAO-SCF-MO's of the CNDO- or MINDO- type for the initial orbital. The calculated values are compared with experimental photoelectron band intensities obtained by integrating the band areas in He I and He II photoelectron spectra. The observed relative intensity changes of bands in the spectrum in going from He I to He II excitation are attributed to variations in the one- and/or two-centre contributions to the cross sections from the electron density at the atoms and in the bonds, respectively. The analysis of the relative intensities of bands in the He I and He II spectra thus leads to conclusions about the electron density distribution in the initial orbital and about the assignment of the bands.In a qualitative sense, the experimental He I/He II intensity changes are usually correctly predicted by our theoretical method, but there are often considerable quantitative discrepancies between the measured and calculated values which probably arise from the inherent simplifications of our approach.     

New infrared integrated band intensities for HC3N and extensive line list for the ν5 and ν6 bending modes

The infrared spectrum of cyanoacetylene (also called propynenitrile) has been investigated from 400 to 4000 cm⁻¹ at a resolution of 0.5 cm⁻¹. Integrated intensities of the main bands and a number of weaker bands have been obtained with an uncertainty better than 5%. Inaccurate values in previous studies have been identified in particular concerning the intensity of the strong ν₅ stretching band at 663.2 cm⁻¹. Former results on the temperature dependence of integrated intensities have also been revisited.Synthetic spectra calculation has been performed for the ν₅ and ν₆ bands on the basis of the best available high resolution data. It has been shown that the GEISA line parameters for HC₃N are not sufficient to reproduce the band intensities and some hot band features observed in our experimental spectra at room temperature. As a first step, the model spectra has been improved by including a number of missing hot subbands and by calculating accurately the hot band relative intensities. Finally, a perfect agreement between calculated and observed spectra was achieved on the basis of a global analysis of HC₃N levels up to 2000 cm⁻¹ combined with the new integrated intensity measurements. A new extensive line list for the ν₅ and ν₆ bending modes of HC₃N has been compiled.