Resonant Photoelectron Spectra Research Articles

Facile preparation of Ti3+ self-doped TiO2 microspheres with lichi-like surface through selective etching

A novel composite Al(acac)3/Al-TiO2 is synthesized via facile flame thermal method and Ti3+self-doped TiO2 microspheres with lichi-like surface is prepared by selective etching the composite with HCl. The samples are investigated by scanning electron microscopy (SEM), inductively coupled plasma-atomic emission spectrometry (ICP), X-ray diffraction (XRD), electron paramagnetic resonance (EPR) and X-ray photoelectron spectra (XPS), respectively. SEM shows that microspherical TiO2 with lichi-like surface is obtained and the rough surface is resulted by etching Al(acac)3 in the composite, which is further confirmed by ICP and iodoform reaction from the characterizations to the etching solution. EPR and XPS indicate the presence of Ti3+on the surface of the microspheres after HCl etching. The XPS results also imply that Ti3+may be formed by removing the partly doped Al in the resulted samples. No reducer or reducing condition are needed through the whole synthesis procedures and it would be a new strategy for the preparation of nanostructured TiO2.

Electronic structure of octahedrally coordinated Cr in CrxTiX2 (X = Se, Te) and TixCr1−xSe2

Influence of the covalence, octahedral distortion and the translation symmetry of the chromium ions in octahedral coordination on the electronic structure for compounds under investigation was clarified. Cr L2,3 and Ti L2.3 X-ray absorption spectra (XAS) along with atomic multiplet calculations and resonant Ti 2p-3d and Cr 2p-3d photoelectron spectra of valence band (RXPS) in the intercalated titanium dichalcogenides CrxTiX2 (X=Se, Te) and in the substitution compound TixCr1−xSe2 show the charge transfer between chromium and titanium sublattices.

Ti(3+) Aqueous Solution: Hybridization and Electronic Relaxation Probed by State-Dependent Electron Spectroscopy.

The electronic structure of a Ti(3+) aqueous solution is studied by liquid-jet soft X-ray photoelectron (PE) spectroscopy. Measured valence and Ti 2p core-level binding energies, together with the Ti 2p resonant photoelectron (RPE) spectra and the derived partial electron-yield L-edge X-ray absorption (PEY-XA) spectra, reveal mixing between metal 3d and water orbitals. Specifically, ligand states with metal character are identified through the enhancement of signal intensities in the RPE spectra. An observed satellite 3d peak structure is assigned to several different metal-ligand states. Experimental energies and the delocalized nature of the respective orbitals are supported by ground-state electronic structure calculations. We also show that by choice of the detected Auger-electron-decay channel, from which different PEY-XA spectra are obtained, the experimental sensitivity to the interactions of the metal 3d electrons with the solvent can be varied. The effect of such a state-dependent electronic relaxation on the shape of the PEY-XA spectra is discussed in terms of different degrees of electron delocalization.

Photoelectron Spectroscopic Study on Decay Processes of Core-Excited States of NaNO2

The absorption and resonant photoelectron spectra of NaNO2 films have been measured at Na-L, N-K, and O-K core levels. The complete understanding of the electronic structures of NaNO2 in a wide energy region is proposed in a band scheme. Resonant photoelectron spectra indicate that the lowest excited states of N-K and O-K excitations (NK-LES and OK-LES) decay predominantly through participator and spectator Auger processes, whereas the normal Auger decay process is negligibly small. The decay probabilities of the NK-LES and OK-LES through the participator Auger process are estimated to be about 47 and 54%, respectively. The delocalization lifetimes of the NK-LES and OK-LES are also estimated to be more than 40 and 28 fs, respectively.

Vibrational state-selective autodetachment photoelectron spectroscopy from dipole-bound states of cold 2-hydroxyphenoxide: o-HO(C6H4)O(-).

We report a photodetachment and high-resolution photoelectron imaging study of cold 2-hydroxyphenoxide anion, o - HO(C6H4)O(-), cooled in a cryogenic ion trap. Photodetachment spectroscopy revealed a dipole-bound state (DBS) of the anion, 25 ± 5 cm(-1), below the detachment threshold of 18 784 ± 5 cm−1 (2.3289 ± 0.0006 eV), i.e., the electron affinity of the 2-hydroxyphenoxy radical o - HO(C6H4)O(⋅). Twenty-two vibrational levels of the DBS are observed as resonances in the photodetachment spectrum. By tuning the detachment laser to these DBS vibrational levels, we obtain 22 high-resolution resonant photoelectron spectra, which are highly non-Franck-Condon due to mode-selective autodetachment and the Δv = - 1 propensity rule. Numerous Franck-Condon inactive vibrational modes are observed in the resonant photoelectron spectra, significantly expanding the vibrational information that is available in traditional high-resolution photoelectron spectroscopy. A total of 15 fundamental vibrational frequencies are obtained for the o - HO(C6H4)O(⋅) radical from both the photodetachment spectrum and the resonant photoelectron spectra, including six symmetry-forbidden out-of-plane modes as a result of resonant enhancement.

Communication: Observation of dipole-bound state and high-resolution photoelectron imaging of cold acetate anions.

We report the observation of a dipole-bound state and a high-resolution photoelectron imaging study of cryogenically cooled acetate anions (CH3COO(-)). Both high-resolution non-resonant and resonant photoelectron spectra via the dipole-bound state of CH3COO(-) are obtained. The binding energy of the dipole-bound state relative to the detachment threshold is determined to be 53 ± 8 cm(-1). The electron affinity of the CH3COO· neutral radical is measured accurately as 26 236 ± 8 cm(-1) (3.2528 ± 0.0010 eV) using high-resolution photoelectron imaging. This accurate electron affinity is validated by observation of autodetachment from two vibrational levels of the dipole-bound state of CH3COO(-). Excitation spectra to the dipole-bound states yield rotational profiles, allowing the rotational temperature of the trapped CH3COO(-) anions to be evaluated.

Vibrational State-Selective Resonant Two-Photon Photoelectron Spectroscopy of AuS(-) via a Spin-Forbidden Excited State.

Vibrational state-selective resonant two-photon photoelectron spectra have been obtained via a triplet intermediate state ((3)Σ(-)) of AuS(-) near its detachment threshold using high-resolution photoelectron imaging of cryogenically cooled AuS(-) anions. Four vibrational levels of the (3)Σ(-) excited state are observed to be below the detachment threshold. Resonant two-photon absorptions through these levels yield vibrational state-selective photoelectron spectra to the (2)Σ final state of neutral AuS with broad and drastically different Franck-Condon distributions, reflecting the symmetries of the vibrational wave functions of the (3)Σ(-) intermediate state. The (3)Σ(-) excited state is spin-forbidden from the (1)Σ(+) ground state of AuS(-) and is accessed due to strong relativistic effects. The nature of the (3)Σ(-) excited state is confirmed by angular distributions of the photoelectron images and quantum calculations.

Resonant photoelectron spectroscopy of Au2− via a Feshbach state using high-resolution photoelectron imaging

Photodetachment cross sections are measured across the detachment threshold of Au2(-) between 1.90 and 2.02 eV using a tunable laser. In addition to obtaining a more accurate electron affinity for Au2 (1.9393 ± 0.0003 eV), we observe eight resonances above the detachment threshold, corresponding to excitations from the vibrational levels of the Au2(-) ground state (X(2)Σu(+)) to those of a metastable excited state of Au2(-) (or Feshbach resonances) at an excitation energy of 1.9717 ± 0.0003 eV and a vibrational frequency of 129.1 ± 1.5 cm(-1). High-resolution photoelectron spectra of Au2(-) are obtained using photoelectron imaging to follow the autodetachment processes by tuning the detachment laser to all the eight Feshbach resonances. We observe significant non-Franck-Condon behaviors in the resonant photoelectron spectra due to autodetachment from a given vibrational level of the Feshbach state to selective vibrational levels of the neutral final state. Using the spectroscopic data for the ground states of Au2(-) (X(2)Σu(+)) and Au2(X(1)Σg(+)), we estimate an equilibrium bond distance of 2.53 ± 0.02 Å for the Feshbach state of Au2(-) by simulating the Franck-Condon factors for the resonant excitation and autodetachment processes.

Study of the electronic structure of the quasicrystalline Ti—Zr—Ni system

Photoelectron spectra and optical absorption spectra for clean surfaces of quasicrystalline samples of the Ti—Zr—Ni system are measured. The resonant photoelectron spectra are also measured in the range of photon energies corresponding to Ni 2p and Zr 3d absorption thresholds. The change in the intensities of the spectrum of the valence band near the Zr 3d threshold is insignificant. The emission of Auger electrons increases near the Ni 2p threshold in a resonance way. Our studies make it possible to reveal certain specific features of the electronic structure of quasicrystals, which can be used to construct models of actual electronic structures of quasi-crystalline materials.

Vibronic Analysis of the S1−S0 Transition of Phenylacetylene Using Photoelectron Imaging and Spectral Intensities Derived from Electronic Structure Calculations

The vibrational structure of the S(1)-S(0) electronic band of phenylacetylene has been recorded by 1 + 1 resonance-enhanced multiphoton ionization, accompanied by slow electron velocity map imaging photoelectron spectroscopy at each resonant vibrational band. Assignments of the S(1) vibrations (up to 2000 cm(-1) above the band origin) are based upon the relative intensities of the vibronic bands calculated by complete second-order vibronic coupling, vibration-rotation (Coriolis and Birss) coupling calculations, and the vibrational structure of the S(1) resonant photoelectron spectra. Although this is an allowed electronic transition, the relative intensities of the a(1) bands are often largely determined by vibronic coupling rather than simple Franck-Condon factors, and second-order coupling is substantial. Nonsymmetric vibrations have intensities obtained through either vibronic or Coriolis coupling, and the calculations have been instrumental in discriminating between alternate possibilities in the assignments. Strong vibronic effects are expected to be present in the spectra of most monosubstituted benzenes, and the calculations presented here show that theoretical treatments based upon electronic structure calculations will generally be useful in the analysis of their spectra.

Grafting of a hydroxylated poly(ether ether ketone) to the surface of single-walled carbon nanotubes

A hydroxylated poly(ether ether ketone) (HPEEK) derivative has been covalently grafted onto the surface of acid-treated single-walled carbon nanotubes (SWCNTs) following two different esterification approaches. The hydroxylation degree of the HPEEK and the extent of the grafting reactions were determined by thermogravimetric analysis. Microscopic observations revealed an increase in the bundle diameter of the SWCNTs and the heterogeneous composition of the synthesized samples. Infrared, nuclear magnetic resonance and X-ray photoelectron spectra corroborated the grafting success, showing the appearance of signals associated with the ester group. The polymer-grafted SWCNTs display higher decomposition temperatures and a wider range of thermal degradation than the HPEEK. The esterification decreases the crystallization and melting temperature as well as the crystallinity of the HPEEK. The semicrystalline nature of the grafted samples was confirmed by X-ray diffraction analysis, which reveals a diminution in the crystal size of the polymer. Dynamic mechanical studies show an exceptional increase in the storage modulus and glass transition temperature of the polymer by the attachment to the SWCNTs. Slightly better thermal and mechanical properties are observed for the sample with the higher esterification degree. The HPEEK-grafted SWCNTs can be used as fillers to prepare PEEK nanocomposites with enhanced performance.

Study of the electronic structure of the Al-Cu-Fe quasicrystalline system

The photoelectron spectra, photon absorption spectra, and electron energy loss spectra have been measured in reflection geometry for the clean surface of quasicrystalline samples and ω-phase samples of identical chemical composition that belong to the Al-Cu-Fe system. The investigations performed made it possible to reveal some specific features of the electronic structure of quasicrystals; these features can be successfully used in construction of the model of the actual electronic structure of quasicrystalline materials. The resonant photoelectron spectra have been measured in the range of photon energies corresponding to the Fe2p and Cu2p absorption edges. A sharp increase (by a factor of 3–4) in the spectral intensity is found in the valence band near the Fe2p edge. Near the Cu2p edge, the Auger electron emission resonantly increased, while the emission from the valence band almost did not change.

Preferential oxidation of CO in rich H2 over CuO/CeO2: Details of selectivity and deactivation under the reactant stream

A CuO/CeO2 catalyst is examined with respect to its performance for preferential oxidation of CO in H2-rich streams. Catalytic activity results are explained on the basis of characterization by operando-DRIFTS and complemented with the analysis of redox properties by electron paramagnetic resonance (EPR) and X-ray photoelectron spectra (XPS). General catalytic activity features are accounted for by comparative analysis of the activities for individual CO and H2 oxidation, for which similar CuO and CeO2 interfacial active sites appear to be involved. An interesting particularity is related to observation of a low temperature hydrogen oxidation process in which CO apparently acts as gaseous promoter. A deactivation process taking place rapidly under the reactant stream is evidenced and attributed to accumulation of hydroxyls on the interfacial active sites and/or to copper sintering in the course of the run.

Profile of resonant photoelectron spectra versus the spectral function width and photon frequency detuning

The outermost, singly ionized valence state of N-2, the X (2)Sigma(g)(+) state, is investigated in detail as a function of the photon frequency bandwidth for core excitation to the N 1s-->pi(*) resonance, where the photon frequency is tuned in between the first two vibrational levels of this bound intermediate electronic state. We find a strong, nontrivial dependence of the resulting resonant photoemission spectral profile on the monochromator function width and the frequency of its peak position. For narrow bandwidth excitation we observe a well resolved vibrational fine structure in the final electron spectrum, which for somewhat broader bandwidths gets smeared out into a continuous structure. For even broader monochromator bandwidths, it converts again into a well resolved vibrational progression. In addition, spectral features appearing below the adiabatic transition energy of the ground state of N-2(+) are observed for broadband excitation. A model taking into account the interplay of the partial scattering cross section with the spectral function is presented and applied to the X (2)Sigma(g)(+) final state of N-2(+).

In situ photoelectron spectroscopy of LaMnO 3 and La 0.6Sr 0.4MnO 3 thin films grown by laser molecular beam expitaxy

We have constructed a high-resolution photoelectron spectroscopy system combined with a laser molecular beam epitaxy (laser-MBE) chamber and have characterized composition-controlled La 1− x Sr x MnO 3 (LSMO) thin films. The importance of atomically flat surfaces by in situ photoelectron spectroscopy for revealing the intrinsic electronic structures has been demonstrated by comparing O1s, O2s and valence band spectra from the laser-MBE-grown LaMnO 3 and LSMO films with those from the scraped samples. Even for the laser-MBE-grown LSMO films, core levels and band structure exhibit strong dependence on surface morphology. For atomically flat LSMO films, we have also elucidated the hole-doping features into Mn3d e g band by substituting La with Sr by resonant photoelectron spectra.

Potential barrier effects in Cs 3d resonance photoemission of CsF

We present and discuss the Cs 3d X-ray absorption spectra and resonant photoelectron emission spectra of CsF. The resonant photoelectron spectra excited at the Cs 3d absorption maxima, exhibit a strong resonance behavior. The different behaviour of participator and spectator channels and interference effects of the 4d partial photoionisation cross-section at the 3d resonances are discussed. The experimental evidence of the phenomenon of spin-orbit activated interchannel coupling in the 3d photoionization of Cs is presented.

Role of electronic localization and charge-vibrational coupling in resonant photoelectron spectra of polymers: Application to poly(para-phenylenevinylene)

A combination of x-ray absorption and resonant photoemission (RPE) spectroscopy has been used to study the electronic structure of the one-dimensional conjugated polymer poly (para-phenylenevinylene) in nonordered (as prepared) thin films. The dispersion of RPE features for the decay to localized and delocalized bands are qualitatively different. A theory for band dispersion of RPE in polymers is given, showing the important roles of electronic state localization and vibrational (phonon) excitations for the character of the dispersion.

Generalization of the duration-time concept for interpreting high-resolution resonant photoemission spectra

The duration-time concept, vastly successful for interpreting the frequency dependence of resonant radiative and nonradiative x-ray scattering spectra, is tested for fine-scale features that can be obtained with state of the art high-resolution spectroscopy. For that purpose resonant photoelectron (RPE) spectra of the first three outermost singly ionized valence states X 2Σg+, A 2Πu, and B 2Σu+, are measured for selective excitations to different vibrational levels (up to n=13) of the N 1s→π* photoabsorption resonance in N2 and for negative photon frequency detuning relative to the adiabatic 0-0 transition of this resonance. It is found that different parts of the RPE spectrum converge to the spectral profile of direct photoionization (fast scattering) for different detunings, and that the RPE profiles are asymmetrical as a function of frequency detuning. The observed asymmetry contradicts the picture based on the simplified notation of a common scattering duration time, but is shown to agree with the here elaborated concept of partial and mean duration times. Results of the measurements and the simulations show that the duration time of the scattering process varies for different final electronic and different final vibrational states. This owes to two physical reasons: one is the competition between the fast “vertical” and the slow “resonant” scattering channels and the other is the slowing down of the scattering process near the zeros of the real part of the scattering amplitude. (Less)

Role of stray light in the formation of high-resolution resonant photoelectron spectra: an experimental and theoretical study of N 2

We show that the undular stray light, diffusely scattered by the optical system of a synchrotron beamline, can play an important role in the formation of high-resolution resonant photoelectron (RPE) spectra. The influence of the stray light is mediated through the Stokes doubling effect, with the Lorentzian tail of the spectral function being replaced by a more complicated form. This effect is shown to appear in the high-resolution resonant photoelectron spectrum of the N 2 molecule in which the spectral shape of the “non-Raman (NR) bands” differs qualitatively for the A 2Π u and X 2Σ + g final states. A particularly large enhancement of the non-Raman Stokes line is observed for the A-state while the picture is inverted for the X-state where the non-Raman band is suppressed. It is shown that the resonant photoemission profile is affected by two qualitatively different detunings, the detuning of the monochromatized line relative to the photoabsorption line and the detuning of the undulator harmonic relative to the same reference line. The experimental data show that the relative intensity of the non-Raman line strongly depends on the tuning of the undulator harmonic with respect to the selected monochromator bandpass, leading to a strong decrease of the Stokes line intensity for certain undulator detunings. A clear red-shift asymmetry for the decrease in the Stokes line intensity is observed when the monochromator line is detuned towards negative photon frequencies, whereas the picture is reverted for the situation of a positively detuned monochromator line. The results show the necessity to control the stray light and to investigate both the Raman and non-Raman contributions to the spectral profiles in order to avoid misinterpretation and in order to make full use of the information available in resonant photoemission spectra of molecules.

A study of O2(a 1Δg) with photoelectron spectroscopy using synchrotron radiation

The atmospherically important species O2(a 1Δg) has been studied by photoelectron spectroscopy using vacuum ultraviolet radiation from a synchrotron as the photon source. Constant-ionic-state (CIS) spectra, recorded for vibrational levels of O2+(X 2Πg) v+=0,1,2,3 accessed from O2(a 1Δg) v″=0, exhibit intense signals in the photon energy region 14.0–15.5 eV which are shown to arise from autoionization from a Rydberg state with an O2+(C 2Φu) core. On the basis of the results obtained and earlier evidence derived from vacuum ultraviolet absorption spectroscopy, this state is assigned as a (C 2Φu,3sσg) 1Φu Rydberg state. Photoelectron spectra recorded for O2(a 1Δg) at positions of strong resonances have allowed extended vibrational structure to be obtained in the first photoelectron band. The relative vibrational component intensities in the resonant photoelectron spectra are in good agreement with computed relative intensities obtained via Franck–Condon calculations, confirming the vibrational numbering of the resonances in the 1Φu state. Competition between autoionization and predissociation in the 1Φu Rydberg state is discussed on the basis of the results obtained. Weaker structure is observed in CIS spectra recorded in the photon energy regions 12.5–13.5 and 15.0–20.0 eV. Suggestions are made for the nature of the highly excited states of O2 associated with this structure, based on available ionization energies and spectroscopic constants of known ionic states accessible from O2(a 1Δg). For example, two broad bands centered at ≈16.4 and ≈17.75 eV are assigned to excitation to Rydberg states arising from the configurations (D 2Δg,3pπu) and (D 2Δg,4pπu), respectively.