1s Threshold Research Articles

Angle-resolved Auger spectra of the molecule

Auger decay spectra of the ethyne (acetylene) molecule have been measured at photon energies below as well as above the C 1s ionization threshold. Angular anisotropy has been observed at the resonance and in the region of a continuum feature previously assigned to a resonance, but disappears far above threshold. Because of strong configuration interaction in the final state only high kinetic energy Auger transitions can be assigned to individual electronic states. In the case of resonant Auger decay, this mixing of states also blurs the division of the spectrum into participator and spectator features. Decay spectra resonances assist in the assignment of Rydberg resonances. The decay of the double excitations above the C 1s threshold has also been studied.

Near-threshold structures in inner-shell photoabsorption processes of N2and CO

The photoabsorption spectra near the 1s threshold of N-2 and CO are calculated by the multiscattering self-consistent-field method. Our calculated near-threshold spectra are in good agreement with the experimental data. All the excited states are treated in a unified manner in the framework of the quantum defect theory. The Z+1 model potential with the relaxed core hole is adopted in our calculation.

Decay of and OCS after sulphur 1s photoexcitation: I. Total ionic charge spectra and electronic processes

Deep core excitation of a small molecule leads to ejection of several electrons and dissociation into numerous alternative sets of fragment ions. We describe an experiment which identifies complete such sets arising from decay of triatomic molecules (and measures their total charge) via triply coincident detection in a time-of-flight mass spectrometer. Careful data correction furthermore enables estimation of contributions from channels comprising one neutral fragment. and OCS molecules were photoexcited by monochromatic x-rays tuned through the entire region around the sulphur 1s threshold (2478 eV). For the first time energy dependent total photoion charge spectra were obtained for deep core excited molecules. They closely resemble those of argon at its K edge, demonstrating the similarity of electronic de-excitation. Electronic processes reflected in these spectra are ejection of excited valence or Rydberg electrons via shake-off and/or Auger decay, photoelectron recapture by post-collision interaction with a fast Auger electron, and pre-edge 1s ionization which extends to tens of eV below threshold. Results on the dissociative aspects are given in the following paper.

Decay of and OCS after sulphur 1s photoexcitation: II. Dissociation channels and kinematics

and OCS molecules were excited and fragmented by x-rays of variable energy around the sulphur 1s threshold (2478 eV). Coincident detection of three ionic fragments by a time-of-flight mass spectrometer enabled the determination of branching ratios for 18 dissociation channels with total ionic charges between 2 and 7. While the electronic de-excitation is similar in both molecules (see the preceding paper), the dissociation shows differences, notably a more asymmetric distribution of charge onto the fragments of OCS. Branching among channels of the same total charge is independent of photon energy although the dominant de-excitation cascades are not. This is interpreted as an indication of strong interchannel mixing. A simple, parameter-free Coulomb explosion model to simulate the dissociative kinematics is described. Detailed results based on this model give, contrary to opinions to be found in the literature, very good agreement with observed momentum correlations whereas maximum kinetic energies are somewhat overestimated for channels of higher total charge.

Photoion spectra of C60 molecules at resonance excitation and ionization energies near the C 1s edge

Photoionization dynamics of gas-phase C60 molecules after selective excitation with synchrotron radiation has been studied with time-of-flight mass spectrometry. The energy level scheme deduced from previous photoelectron and Auger electron spectra is used to analyze the photoion data. At the vicinity of π* pre-edge resonances the fragmentation of multiply-charged ions varies strongly as a function of photon energy. The C60+ ion yield decreases slowly after 1s threshold indicating that the slow photoelectrons can get recaptured. Somewhat further above the threshold the enhanced relative fragmentation of the parent C60+ ions suggests excitation of the giant plasmon resonance due to 20 eV photoelectrons.

Photodetachment of the1s2s2pt4P state ofHe−sfrom threshold to 100 eV

Calculations of cross sections and photoelectron angular distributions from the photodetachment of the 1s2s2p${\mathrm{}}^{4}$ P metastable state of ${\mathrm{He}}^{\mathrm{\ensuremath{-}}}$ have been performed from threshold to 100 eV using multiconfiguration Hartree-Fock wave functions for discrete states and coupling continuum channels with closed channels to include resonances. Ejection of each electron was considered and ejection plus excitation was included in connection with 1s photoabsorption. The ${1\mathrm{s}2\mathrm{p}}^{2}$ and ${2\mathrm{s}2\mathrm{p}}^{2}$ ${\mathrm{}}^{4}$ P resonances were found in good agreement with recent experiments and calculations; the reason that the former lies above the 2s threshold while the latter lies below the 1s threshold is discussed. The nonresonant 2p cross section is in excellent agreement with experiments and other calculations. The 2s results, in which Cooper minima are predicted, are not in agreement with a similar calculation, particularly for the \ensuremath{\mathrm{B}} parameter. The 1s cross section shows a large maximum just above threshold, much larger than the 1s cross section in neutral He, and at somewhat higher energy the cross section is dominated by 1s detachment plus excitation to the (2s3p\ifmmode\pm\else\textpm\fi{}3s2p)${\mathrm{}}^{3}$ P and 3s3p${\mathrm{}}^{3}$ P autoionizing states of He.

Resonant photoemission of titanium

Abstract The partial photoionization cross section of the titanium 3d band has been measured using synchrotron radiation in a wide photon energy range (21–110 eV). Our prime interest has been concentrated on the resonant behaviour of the valence band in the vicinity of the 3p and 3s thresholds in which the influence of autoionization appears as drastic fluctuations of a resonant profile. The resonance close to the 3s excitation edge has been interpreted to be primarily caused by the p-character of the unoccupied band and the Cooper minimum of the 3p core level. We have also considered in details the effects which may distort a measured profile. In addition, we have examined the subthreshold Auger emission that should already occur below the edge energy. However, our study does not support the existence of the subthreshold Auger phenomenon in titanium.

Electronic state-lifetime interference observed at Ne K inter-resonance excitation

Electronic decay spectra of Ne, excited at the 1s threshold have been measured. Final states of the 2p −2 n′p configuration are reached via 1s −1 np intermediate states. Appreciable intensity from spectator shake transitions ( n ≠ n′) is found and the predictions of a simple intensity model are evaluated. The model reproduces the experimental results at on-resonance excitation. At inter-resonance excitation an apparent excitation energy dependence of the shake probability is accounted for by electronic state-lifetime interference theory.

The C 1s Auger decay spectrum of the molecule: the effects of vibrational fine structure, double excitations and shake-up transitions

Auger electron spectra of the methane molecule have been measured at various photon energies below and above the C 1s threshold. At energies corresponding to the higher Rydberg resonances the resonant Auger spectrum approaches the normal Auger spectrum both in form and peak position. Deviations from this trend are observed, however, when the core-to-Rydberg transition is accompanied by the excitation of vibrational modes. Auger spectra measured at the continuum resonance around 303 eV excitation energy display new decay features, which allows this resonance to be assigned to core-hole double excitations. Furthermore, above the C 1s threshold the satellite Auger features arising from the lowest shake-up initial states are identified.

Synchrotron radiation photoemission and photostimulated desorption of deuterated methanol on Si(111)7 × 7 and Si(100)2 × 1

Deuterated methanol adsorption on Si(111)7 × 7 and Si(100)2 × 1 surfaces has been investigated, at room temperature, by synchrotron radiation photoemission and photostimulated desorption. Photoemission experiments as a function of methanol coverage show that deuterated methanol adsorption is dissociative on both surfaces and occurs via б O-D bond breakage and б SiO bond formation. Site selectivity is displayed by the methoxy species on Si(111)7 × 7 surface, since t S 2 surface state related to the rest atoms is quickly quenched upon low coverage adsorption. The S 1 surface state is quenched only subsequently at much higher coverages. The photostimulated desorption pattern is similar for the two surfaces and strongly depends on the investigated energy range. D + is always the most abundant ion produced, but higher mass fragments are present too in the energy range including the C 1s threshold, where multiple desorption processes are possibly taking place.

Resonant photoemission in titanium metal

The photoionization cross section of the titanium 3d band, including the contributions from direct photoemission and autoionization, has been measured using synchrotron radiation on a wide photon energy range (21 – 110 eV). Particularly, we have concentrated on the resonant behavior of the valence band near the 3p and 3s threshold in which the influence of autoionization appears as drastic fluctuations of a resonant profile. In addition, we have examined subthreshold Auger emission that should already occur below the threshold energy.

Resonant photoemission in titanium metal

The photoionization cross section of the titanium 3d band, including the contributions from direct photoemission and autoionization, has been measured using synchrotron radiation on a wide photon energy range (21 – 110 eV). Particularly, we have concentrated on the resonant behavior of the valence band near the 3p and 3s threshold in which the influence of autoionization appears as drastic fluctuations of a resonant profile. In addition, we have examined subthreshold Auger emission that should already occur below the threshold energy.

Photodetachment of Li- from the Li 3s threshold to the Li 6s threshold.

Eigenchannel R-matrix calculations (including effects of long-range multipole interactions beyond the reaction volume) for ${\mathrm{Li}}^{\mathrm{\ensuremath{-}}}$ photodetachment partial cross sections from the vicinity of the Li 3s threshold to the Li 6s threshold (3.8 eV \ensuremath{\le}\ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\le} 5.65 eV) are presented. Excellent agreement with the relative total cross section measurements of U. Berzinsh et al. [Phys. Rev. Lett. 74, 4795 (1995)] in the vicinity of the Li 3s and Li 3p thresholds is found. The calculated resonance structures are analyzed in detail. In particular, the energy region between the Li 4s and Li 5p thresholds (for which there are as yet no experimental measurements) is shown to have types of doubly excited resonances which are prominent only because of the nonhydrogenic ${\mathrm{Li}}^{+}$ core; such types are weak or absent in higher-energy regions as well as in ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ photodetachment spectra. \textcopyright{} 1996 The American Physical Society.

Conjugate shake-up-enhanced Auger transitions in N2.

A substantial enhancement of intensity in the high-energy part of the ${\mathrm{N}}_{2}$ KLL Auger spectrum has been observed at photon energies a few tens of eV above the N 1s threshold. By monitoring the corresponding 1s photoelectron spectrum, we show that the effect is due to the emergence of strong conjugate shake-up satellites. As shown in previous e-2e studies for CO, such Auger spectra are useful in determining the intensity behavior of shake-up satellites in the immediate threshold regions.

Si 1s x-ray absorption spectra of epitaxial Si–Ge atomic layer superlattice and alloy films

The Si 1s (K-shell) x-ray absorption spectra of several [(Si)m(Ge)n]p atomic layer superlattices (ALS) and a range of SixGe1−x alloy thin films grown epitaxially on both Si(100) and Ge(100) have been investigated using plane-polarized synchrotron radiation. The near-edge spectral features of ALS and alloy samples with similar (average) chemical composition are remarkably similar. The spectra of both the strained ALS and alloy samples contain features at the Si 1s threshold which exhibit a small but characteristic polarization dependence. The polarization dependence is reduced or absent in strain-relaxed materials such as annealed ALS, annealed alloys, or thick alloy films. The polarization-dependent components of the signal are attributed to anisotropic states associated with strain-induced tetragonal distortions. The sense of the polarization is inverted between samples grown on Si and those grown on Ge, consistent with the expected inversion in the spatial orientation of the strain field. An explanation is proposed for the dependence of the magnitudes of the Si 1s polarization effect on the composition of alloys and ALS.

Fusion in flight from the molecular continuum of dtμ

Fusion reaction rates for the muonic molecular system dtμ are calculated in the scattering continuum above the first break-up threshold (tμ) 1s+d, and for a few of the resonant states there embedded. In order to analyze the fusion in flight reaction (tμ) + d, the molecular wave function is obtained within (a) semiclassical, (b) adiabatic, and (c) nonadiabatic descriptions, in the energy range 0.002–10 keV. The semiclassical rates are higher than the corresponding adiabatic fusion rates, but converge towards the latter for high collision energies. In the nonadiabatic treatment, the qualitatively new structure has been obtained. The fusion rate shows an oscillatory behaviour, associated with the presence of a series of virtual states and variations of the overbarrier transmission in the tμ+d potential scattering. Furthermore, the nonadiabatic fusion rates are substantially enhanced at resonant energies near the tμ 2s threshold.

Resonance structure in the 2p photoionization of atomic sodium close to the 2s threshold.

We calculated 2p inner-shell photoionization cross sections of atomic sodium in the ground state in the photon-energy region close to the 2s threshold using the R-matrix method. We included five residual ${\mathrm{Na}}^{+}$ (2${\mathit{p}}^{6}$ $^{1}$S, 2${\mathit{p}}^{5}$3s $^{3}$${\mathit{P}}^{\mathit{o}}$, $^{1}$${\mathit{P}}^{\mathit{o}}$, 2s2${\mathit{p}}^{6}$3s $^{3}$S${,}^{1}$S) states in the R-matrix expansion. The initial Na ${(}^{2}$S) bound state, the final ${\mathrm{Na}}^{+}$ plus photoelectron ${(}^{2}$${\mathit{P}}^{\mathit{o}}$) state, and the residual ionic states are represented by fairly extensive configuration-interaction wave functions. The prominent features in the cross sections are two autoionization Rydberg series converging to ${\mathrm{Na}}^{+}$ (2s2${\mathit{p}}^{6}$3s $^{3}$S${,}^{1}$S) excited-state thresholds. The present results are compared with the available experiments.

On the mechanism of the smooth increase of the mean ion charge in Ar photoionization near the K-edge

It is demonstrated, that the Auger decay of the virtually created 1s vacancy can explain semi-quantitatively the experimentally observed (by Doppelfeld et al. [J. Phys. B 26 (1993) 445]) increase of the mean charge of the ions, produced in the photoionization of Ar close but below the 1s threshold.

Window resonance in photodetachment of the negative silicon ion: strong interaction of the 3p continuum with the 3s to 3p shape resonance

Photodetachment cross section measurements have been performed on the Si-(3s23p34S) ion using a pulsed dye laser and non-linear optical techniques in the photon energy region close to the 3s threshold. A deep window resonance observed at a photon energy of 5.2 eV is a clear manifestation of electron correlation resulting from the interaction of the 3p to epsilon s, epsilon d continua with the 3s23p3 to 3s3p4 autodetaching resonance. The experimental resonance parameters are compared with theoretical predictions.

Core-hole decay studies of O/Ni(100) c(2 x 2).

A study of the core-hole decay spectra of O/Ni(100) c(2\ifmmode\times\else\texttimes\fi{}2) is presented. Spectra were recorded at photon energies corresponding to the O 1s x-ray-absorption (XA) resonance, as well as high above the O 1s threshold. Additional spectra were recorded at photon energies corresponding to characteristic features in the XA spectrum just above the O 1s threshold. These spectra were found to provide important information regarding the electronic structure of atomic oxygen on nickel. Broadenings in the decay spectra due to population of Rydberg states are observed. A shoulder on the high-kinetic-energy side of the spectra is interpreted as a result of Auger final states made up by independent valence holes. It was found that the spectroscopic behavior of the O/Ni(100) system is in many respects similar to that of pure Ni.