Conjugate Shake-up Research Articles

Cationic double K-hole pre-edge states of CS2 and SF6

Recent advances in X-ray instrumentation have made it possible to measure the spectra of an essentially unexplored class of electronic states associated with double inner-shell vacancies. Using the technique of single electron spectroscopy, spectra of states in CS2 and SF6 with a double hole in the K-shell and one electron exited to a normally unoccupied orbital have been obtained. The spectra are interpreted with the aid of a high-level theoretical model giving excellent agreement with the experiment. The results shed new light on the important distinction between direct and conjugate shake-up in a molecular context. In particular, systematic similarities and differences between pre-edge states near single core holes investigated in X-ray absorption spectra and the corresponding states near double core holes studied here are brought out.

Single photon simultaneous K-shell ionization and K-shell excitation. I. Theoretical model applied to the interpretation of experimental results on H2O.

We present in detail a theoretical model that provides absolute cross sections for simultaneous core-ionization core-excitation (K(-2)V) and compare its predictions with experimental results obtained on the water molecule after photoionization by synchrotron radiation. Two resonances of different symmetries are assigned in the main K(-2)V peak and comparable contributions from monopolar (direct shake-up) and dipolar (conjugate shake-up) core-valence excitations are identified. The main peak is observed with a much greater width than the total experimental resolution. This broadening is the signature of nuclear dynamics.

Near-Edge X-Ray Absorption Fine Structures Revealed in Core Ionization Photoelectron Spectroscopy

Simultaneous core ionization and core excitation have been observed in the C(2)H(2n) (n=1, 2, 3) molecular series using synchrotron radiation and a magnetic bottle time-of-flight electron spectrometer. Rich satellite patterns corresponding to (K(-2)V) core excited states of the K(-1) molecular ions have been identified by detecting in coincidence the photoelectron with the two Auger electrons resulting from the double core hole relaxation. A theoretical model is proposed providing absolute photoionization cross sections and revealing clear signatures of direct (monopolar) and conjugate (dipolar near-edge x-ray absorption fine structure) shakeup lines of comparable magnitude.

Magnetic Dichroism inK-Shell Photoemission from Laser Excited Li Atoms

Magnetic dichroism in the angular distribution has been demonstrated for single-electron photoemission from inner ns(2) subshells of gaseous atomic targets using the example of K-shell photoionization of polarized Li atoms laser prepared in the 1s(2)2p (2)P(3/2) excited state. The effect is pronounced for the conjugate shakeup and conjugate shakedown photoelectron lines, and less important, though observable, for the main and direct shakeup lines. The phenomenon is caused by configuration interaction in the final continuum state and is quantitatively described by the close-coupling R-matrix calculations.

Carbon K-shell photoionization of CO: Molecular frame angular distributions of normal and conjugate shakeup satellites

We have measured the molecular frame angular distributions of photoelectrons emitted from the Carbon K-shell of fixed-in-space CO molecules for the case of simultaneous excitation of the remaining molecular ion. Normal and conjugate shakeup states are observed. Photoelectrons belonging to normal Σ -satellite lines show an angular distribution resembling that observed for the main photoline at the same electron energy. Surprisingly a similar shape is found for conjugate shakeup states with Π -symmetry. In our data we identify shake rather than electron scattering (PEVE) as the mechanism producing the conjugate lines. The angular distributions clearly show the presence of a Σ shape resonance for all of the satellite lines.

Angle-resolved photoion yield and resonant Auger spectroscopy for the doubly excited Rydberg states above the C1sthreshold of CO

Doubly excited core-hole states of carbon monoxide in the photon energy region of $300--305\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, i.e., directly above the C $1s$ ionization threshold, have been studied using both angle-resolved ion-yield and high-resolution resonant Auger spectroscopies. The leading configurations of the most prominent doubly excited Rydberg states are assigned by careful analysis of the ion-yield spectra and the final-state spectra to C $1{s}^{\ensuremath{-}1}$ $(5{\ensuremath{\sigma}}^{\ensuremath{-}1}2{\ensuremath{\pi}}^{1}S=1)$ $3s\ensuremath{\sigma}$ $({v}^{\ensuremath{'}}=0,1,2)$, C $1{s}^{\ensuremath{-}1}$ $(5{\ensuremath{\sigma}}^{\ensuremath{-}1}2{\ensuremath{\pi}}^{1}S=0)$ $3s\ensuremath{\sigma}$ $({v}^{\ensuremath{'}}=0,1,2)$, and C $1{s}^{\ensuremath{-}1}$ $(5{\ensuremath{\sigma}}^{\ensuremath{-}1}2{\ensuremath{\pi}}^{1}S=1)$ $4s\ensuremath{\sigma}$ $({v}^{\ensuremath{'}}=0,1)$, which can only be populated via a conjugate shake-up process. Analysis of the resonant Auger spectra provides an assignment of several two-hole--one-electron $(2h\text{\ensuremath{-}}1e)$ final states.

Intensity Inversion between Main and Satellite Lines in Atomic Photoionization

The 1s photoionization of atomic Li was studied by photoelectron spectroscopy in the photon energy region between 85 and 140 eV for the ground state and the three lowest excited configurations Li(*) 1s(2)nl, nl=2p, 3s, 3p. The importance of electron correlations was investigated by comparing the multielectron transitions, so-called shake-up and conjugate shake-up satellites, and the direct process, so-called main lines. The relative intensity of the satellites increases with the level of initial excitation of the Li atom. The shake-up process dominates for states with an n=3 valence electron and the satellites become stronger than the main lines. This spectacular effect can be explained by the spatial overlap of the initial and final state wave functions. Surprisingly, the spatial overlap affects shake-up and conjugate shake-up lines in the same way.

5pphotoemission from laser-excited cesium atoms

Fine-structure resolved $5p$ photoemission spectra of Cs in the ground state and after laser excitation into the $[\mathrm{Xe}]6p\phantom{\rule{0.2em}{0ex}}^{2}P_{1∕2}$ and $6p^{2}P_{3∕2}$ states have been studied. The $5{p}^{5}6p$ final states have been reached by the $5{p}^{6}6s\ensuremath{\rightarrow}5{p}^{5}6p$ conjugate shakeup process from ground-state atoms as well as by direct photoemission from laser-excited atoms. The laser-excited spectra can be well described with calculations based on the $jK$-coupling model. Calculations based on a multiconfiguration Dirac-Fock approach have been performed to investigate the deviations created by intermediate coupling from the pure $jK$-coupling scheme.

Photoabsorption and resonant photoemission in the region of Ne 1s double excitations

The absolute photoabsorption cross-sections of Ne in the regions 865–873 (1s Rydberg states) and 900–940 eV (double excitations) have been measured. Several new double excitation states were detected, including a very weak member of the series converging to the 1s2p 53s conjugate shake-up ion state. States were also observed above the 1s2p 5 double ionisation potentials, and are assigned to the 1s2snsn′p series. At the first 1s2p 53p 2 double excitation state, the partial ionisation cross-section for the 1s −1 ion state resonates. The behaviour of the cross-section and Fano profile are analysed by a theoretical calculation using the time-independent scattering formalism.

Angle-resolved photoelectron spectroscopy of the satellite bands accompanying the O 1s mainline of CO 2

The lowest-energy satellite bands accompanying the O 1s mainline of CO 2 molecule have been investigated, using angle-resolved photoelectron spectroscopy. The measurements were carried out at beamline 27 at SPring-8 in Japan with a high-resolution electron spectroscopy apparatus consisting of a hemispherical energy analyzer (SES-2002) combined with a gas cell. We found that the internal inelastic scattering is significant while the conjugate shake-up plays its role as well.

Excitation mechanism of the lowest-energy satellite bands in F 1s photoemission from SF 6

The lowest-energy satellite bands accompanying the F 1s mainline of the SF 6 molecule have been investigated using angle-resolved photoelectron spectroscopy. We found that a conjugate shake-up is dominant while the internal inelastic scattering also plays a significant role.

The excitation mechanism of satellite bands in F 1s photoemission of SiF 4

We have investigated the lowest-energy satellite bands accompanying the F 1s mainline of SiF 4 molecule using angle-resolved photoelectron spectroscopy and separated the contributions from the direct shake-up and the conjugate shake-up. The direct shake-up satellites are resonantly enhanced by the shape resonance at the same kinetic energy as that of the mainline: the anisotropy parameter β of the direct shake-up satellites drops in the vicinity of the shape resonance as for the mainline. The intensity of the conjugate shake-up satellites decreases monotonically with the increase in the photon energy: their β value also varies monotonically as a function of the energy.

A high-resolution N 1s photoionization study of the molecule in the near-threshold region

The angle-resolved N 1s photoelectron spectrum of has been measured with high resolution in the threshold region. An analysis of the vibrational fine structure yields a vibrational energy of and a lifetime broadening of for the core-ionized molecule. As in the case of C 1s ionization of CO, the vibrational fine structure changes considerably with photon energy in the region of both the shape resonance and the double excitations. Both the vibrationally resolved partial photoionization cross sections and asymmetry parameters have been determined in this photon energy range. The calculated cross sections in the literature are in reasonable agreement with experiment. In addition, we have measured the cross sections and asymmetry parameters of the most intense direct and conjugate shake-up satellites. The behaviour of the satellites is found to differ significantly from that of the corresponding C 1s satellites of CO.

High-resolution C 1s photoelectron spectra of methane.

The C 1s partial photoionization cross section and photoelectron angular distribution of methane (${\mathrm{CH}}_{4}$) have been measured with high-energy resolution between threshold and 385 eV photon energy. From the analysis of the vibrational fine structure on the C 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$ photoelectron line a vibrational energy of 396\ifmmode\pm\else\textpm\fi{}2 meV and an equilibrium bond length of 1.039(\ifmmode\pm\else\textpm\fi{}0.001) \AA{} for the ${\mathrm{CH}}_{4}^{+}$ ion have been determined. The lifetime broadening was found to be 83(\ifmmode\pm\else\textpm\fi{}10) meV. The weak feature in the photoabsorption cross section just above threshold does not influence the vibrational fine structure in a way typical for a shape resonance. We therefore suggest that it is due to doubly excited states of the type C (1s${)}^{\mathrm{\ensuremath{-}}1}$(Val${)}^{\mathrm{\ensuremath{-}}1}$(Ryd${)}_{\mathit{a}}^{1}$(Ryd${)}_{\mathit{b}}^{1}$, an assignment which is supported by recent Auger decay studies. Measurements of the shakeup structure revealed six satellite lines, one of which increases strongly in intensity at threshold, thus pointing to the existence of a conjugate shakeup process. \textcopyright{} 1996 The American Physical Society.

Shape resonances and multielectron effects in the core-level photoionization of CO2.

The partial photoionization cross sections and angular distributions of the C 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$ and O 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$ single-hole states of ${\mathrm{CO}}_{2}$ have been measured with high precision near threshold. The 4${\mathrm{\ensuremath{\sigma}}}_{\mathit{u}}^{\mathrm{*}}$ shape resonance is well reproduced in the 1s single-hole cross sections, but strong site-specific effects are observed with respect to both its energy and intensity. A striking feature of the present data is the presence of fine structure in the 1s single-hole cross sections ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{SH}}$ due to multielectron excitations: virtually all spectral details in the total 1s cross sections ${\mathrm{\ensuremath{\sigma}}}_{1\mathit{s}}$ are also present in ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{SH}}$ implying that multielectron excitations couple strongly to the underlying 1s continuum. The decay dynamics of the double-excitation feature at 303.5 eV in the C 1s cross section of ${\mathrm{CO}}_{2}$ have been characterized by Auger spectroscopy. Partial cross sections and angular distributions have also been measured for the first group of strong shake-up satellites in the C 1s photoelectron spectrum. Similar to the C 1s ionization of CO, a strong enhancement of intensity near threshold is observed for some satellite channels as well as the occurrence of features that are not present in the sudden limit. The latter are attributed to conjugate shake-up.

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.

Measurement of Ne 1s and 1snl satellite photoelectron spectra near threshold.

Near-threshold inner-shell photoionization of neon has been studied by measuring threshold photoelectron spectra in the energy regions of the 1${\mathit{s}}^{\mathrm{\ensuremath{-}}1}$ main line and the 1s2${\mathit{p}}^{5}$nl satellite states. The measured linewidth (0.22\ifmmode\pm\else\textpm\fi{}0.03 eV) of the 1s\ensuremath{\rightarrow}3p transition is more consistent with the observation of the 1s hole state linewidth than previous determinations. Features, assigned as doubly excited states and conjugate shakeup transitions, have been discerned in the 1s2${\mathit{p}}^{5}$nl satellite spectrum.

Near-threshold behavior of the K-shell satellites in CO.

The study of photoelectron satellites in the near-threshold region is important for differentiating among various electron correlation mechanisms. For CO, which is a prominent test case for theories describing electron correlation effects, the threshold and near-threshold branching ratios (BR's) of the [ital K]-shell satellites were measured. Eight different C 1[ital s] satellite states were identified, showing very complex threshold behavior. The resonancelike behavior of the (2[sigma])[sup [minus]1](1[pi])[sup [minus]1](2[pi][sup *])[sup 1]([ital S][prime]=1) satellite BR agrees with recent theoretical calculations. The observed energy dependence of the singlet (2[sigma])[sup [minus]1](1[pi])[sup [minus]1](2[pi][sup *])[sup 1] satellite compares less well with the calculations. Three [sup 2][Sigma][sup +] satellites with a (2[sigma])[sup [minus]1](5[sigma])[sup [minus]1]([ital n][sigma])[sup 1] configuration have a constant satellite BR over the energy range studied. Three 2[sigma] satellites, which have been assigned as pure conjugate shake-up states, have intensity only near threshold. The threshold spectrum above the O 1[ital s] ionization threshold consists of two distinct groups of satellites. Conjugate shake-up transitions contribute to the intense feature centered at 551-eV binding energy, which is not observed above its threshold. Two [sup 2][Sigma][sup +] satellites, which have been described by configuration interaction of (1[sigma])[sup [minus]1](5[sigma])[sup [minus]1]([ital n][sigma])[sup 1] states with a singlet [pi][pi][sup *] state, showmore » indications of shape-resonance-like behavior.« less

Variation of the C (KVV) Auger angular distribution in the [formula omitted]-resonance of CO

We report on angle-resolved high-resolution measurements of C (KVV) Auger spectra of CO in the vicinity of the C (1 s) σ ∗ -shape resonance above threshold. These spectra show clear evidence for the theoretically predicted anisotropic K-shell Auger emission in molecules. Complementary results from angle-resolved photoion-spectroscopy show that the small size of the observed effect is, besides the varying intrinsic anisotropy of the Auger decay, also due to a smaller anisotropy in the primary absorption process than originally predicted but in good agreement with more recent calculations. Contrary to this, some satellite Auger transitions show large anisotropies outside the shape resonance, an effect attributed to a conjugate shakeup transition in the primary photoionization process. All Auger anisotropies show a clear and distinct variation across the shape resonance which enables a distinction between different types of Auger transitions concerning the primary photoionization process.

Photon energy dependence of the high resolution C 1s photoelectron spectrum of CO in the threshold region.

The C 1s photoelectron spectrum of CO has been measured with high resolution in the threshold region. The data exhibit hitherto unresolved vibrational structure on the C 1s main photoelectron line as well as new features in the satellite region. The vibrational energy of the ${\mathrm{CO}}^{+}$ ion is 301\ifmmode\pm\else\textpm\fi{}3 meV. The vibrational structure changes dramatically with photon energy due to the influence of the ${\mathrm{\ensuremath{\sigma}}}^{\mathrm{*}}$ shape resonance. The new satellite lines which are not present at high photon energies are attributed to conjugate shakeup processes.