1s Photoabsorption Research Articles

C–C bond unsaturation degree in monosubstituted ferrocenes for molecular electronics investigated by a combined near-edge x-ray absorption fine structure, x-ray photoemission spectroscopy, and density functional theory approach

We present the results of an experimental and theoretical investigation of monosubstituted ethyl-, vinyl-, and ethynyl-ferrocene (EtFC, VFC, and EFC) free molecules, obtained by means of synchrotron-radiation based C 1s photoabsorption (NEXAFS) and photoemission (C 1s XPS) spectroscopies, and density functional theory (DFT) calculations. Such a combined study is aimed at elucidating the role played by the C-C bond unsaturation degree of the substituent on the electronic structure of the ferrocene derivatives. Such substituents are required for molecular chemical anchoring onto relevant surfaces when ferrocenes are used for molecular electronics hybrid devices. The high resolution C 1s NEXAFS spectra exhibit distinctive features that depend on the degree of unsaturation of the hydrocarbon substituent. The theoretical approach to consider the NEXAFS spectrum made of three parts allowed to disentangle the specific contribution of the substituent group to the experimental spectrum as a function of its unsaturation degree. C 1s IEs were derived from the experimental data analysis based on the DFT calculated IE values for the different carbon atoms of the substituent and cyclopentadienyl (Cp) rings. Distinctive trends of chemical shifts were observed for the substituent carbon atoms and the substituted atom of the Cp ring along the series of ferrocenes. The calculated IE pattern was rationalized in terms of initial and final state effects influencing the IE value, with special regard to the different mechanism of electron conjugation between the Cp ring and the substituent, namely the σ/π hyperconjugation in EtFC and the π-conjugation in VFC and EFC.

Resonant inelastic x-ray scattering and UV–VUV luminescence at the Be 1s edge inBeO

We carried out a combined study of UV–VUV luminescence and resonant x-ray emissionfrom BeO single crystals with incident photon energies in the vicinity of the Be 1sabsorption edge. The x-ray emission spectra show that at the Be 1s photoabsorption edgethe lattice relaxation processes in the excitation site take place already on the timescale ofthe radiative decay of the core excitation. Comparison of the x-ray emission andthe luminescence spectra indicates that the maximum energy loss of the processof lattice relaxation during the decay of inner-shell holes is similar to the lossthat occurs in the self-trapping process of valence excitons. The possible decaychannels of core excitations have been discussed and the mechanism for the creationof 5.2 eV luminescence at the photoabsorption resonances has been suggested.

Resonant inelastic X-ray scattering at the Be 1s edge in BeO

We present the resonant inelastic X-ray scattering (RIXS) spectra of BeO at the Be 1s photoabsorption edge. The RIXS spectra excited in the vicinity of the core exciton resonance show two principal features: the scattering on a valence exciton (which at higher excitation energies verges into the characteristic K α emission), and a remarkably strong energy loss side-band to the elastic scattering peak, which has a maximum width of 6 eV and has intensity, at resonance, comparable to the valence scattering. The energy loss side-band appears to result from rapid lattice relaxation at the absorption site on the X-ray scattering time-span.

The 1s photoabsorption transitions in Br I and Br II

Relativistic configuration interaction calculations have been performed for the 1s →np (n = 4–8) photoabsorption transitions in Br I and Br II, and transition energies and transition probabilities have been evaluated. These data are necessary to interpret the experiment photoabsorption spectra. The K edge energies of Br I and Br II have also been computed and compared to the existing values when available.

Dichroic O 1s photoabsorption and resonant X-ray scattering in haematite (Fe 2O 3)

X-ray photoabsorption and resonant X-ray emission at the oxygen K edge of haematite (Fe 2O 3) single crystals have been measured with linearly polarized light. The X-ray absorption is strongly dichroic, in agreement with previous results from thin films: there are substantial differences between spectra with the electric vector of the incident light parallel and perpendicular to the c axis. This is explained in terms of published electronic structure calculations. The X-ray emission under resonant Raman conditions however is only weakly dichroic, and this is ascribed to delocalization of the excited electron in the conduction band.

Resonant inelastic X-ray scattering at the K edge of oxygen and fluorine in insulators

Here we present the results of a study of the electronic structure using the resonant inelastic scattering process at the F 1s photoabsorption edge in LiF and O 1s edge of MgO crystals. In case of excitations in the sub-threshold region, the Raman-type linear dispersion of X-ray fluorescence peak was observed for the studied compounds. In LiF, the narrowing of the X-ray fluorescence peak is observed at pre-threshold excitation, which can be related to the creation of a core exciton, but no core excitons were identified in MgO.

Valence in the Rydberg/continuum region in molecular inner-shell spectroscopy

The core-to-valence, especially core-to-σ*, excitation is sometimes obscure in molecular soft X-ray absorption spectra. The potential energy curve (PEC) of the core-to-σ* state is repulsive for the bond elongation; on the other hand, the PECs of the core ionized state and the core-to-π* state are bound and the core-to-Rydberg states are almost parallel to the PEC of the core ionized state. Thus, the σ* state can have interactions with the Rydberg and continuum states, and even with the π* state in some cases, around the Franck–Condon region. Using schematic PECs, we have discussed, where the σ* state is located in the 1s photoabsorption spectra of N 2, O 2, CH 3F, and C 2H 2, and how we know evidence for the σ* state in the 1s photoabsorption spectra of CH 4.

Na K PHOTOABSORPTION AND RESONANT KLL AUGER SPECTRA IN NaF AND NaCl

The nature of Na 1s photoabsorption in NaF and NaCl is investigated using the resonant Auger decay of Na 1s core excitations. The appearance of new peaks in Auger spectra, when the energy of excitation coincides with the photoabsorption thresholds, shows that the core excitons are created in the threshold region of the Na 1s photoabsorption spectrum. The core excitons around 1074.2 eV originate from the dipole-forbidden 1s -1 3s final state, the population of which is made possible by the symmetry disruption at the photoabsorption site. Another core exciton, excited at photon energies of 1076.8 eV in NaF and 1076.3 eV in NaCl, originates from 1s -1 3p states similar to atoms of Ne and Na. Resonant Auger spectra show also that in NaCl the first sharp photoabsorption maximum at 1076.6 eV has only partially excitonic character.

High-resolution inner-shell photoabsorption of the OH and OD free radicals

The O 1s photoabsorption (ion-yield) spectra of OH (2Πi,v″=0) and OD (2Πi,v″=0) radicals have been studied in the gas phase for the first time. Spectroscopic parameters for the O 1s→π excited Σ+2 state of OH and OD, observed at ∼525.8 eV, have been obtained from a Franck–Condon analysis comparing the spectra of both molecules. The lifetime width of this state has been measured as Γ=147±11 meV and Γ=156±7 meV for OH and OD, respectively. Excellent quantitative agreement is found between the experimental spectroscopic parameters re, ωe, and T0, measured in this work for the Σ+2 state and recent theoretical results obtained for OH by high-level ab initio calculations. The spectroscopic parameters predicted by the equivalent-core model are close to the experimental values within a few percent. This model therefore provides an accurate description of the lowest-lying excitation process in both OH and OD molecules.

Central atom 1s photoabsorption spectra of nitrogen and phosphorus AX 3 (A=N, P and X=F, H) molecules

The newly commissioned SGM beamline [1] at CSRF (the Canadian Synchrotron Radiation Facility at the Synchrotron Radiation Center in Madison WI) offers excellent performance at the nitrogen 1s absorption edge. We have therefore expanded our ongoing X-ray absorption spectroscopy study of electronic structure and chemical bonding of small molecules to include measurements of the N(1s) absorption edges for comparison to the edges of heavier elements in the same group. Here we present the central atom (N, P) 1s spectra of NF 3, NH 3, PF 3 and PH 3 measured at high resolution with synchrotron radiation. The phosphorus edges were measured with the Canadian DCM beamline at CSRF. The observed features were assigned with the aid of MS-Xα and ab initio calculations. Spectral differences are explained in terms of differences in the electronic structure of these four related molecules. All molecules show a strong central atom (A) core (1s) to 1e * transition and additional weaker features which arise from transitions to other virtual molecular orbitals. Chemical shifts resulting from the molecular field of the substituents are observed. Our NH 3 spectrum agrees with that previously published [2].

A comparison of the oxygen 1s photoabsorption spectra of SO 2 and NO 2

Oxygen 1s photoabsorption spectra of the bent triatomic molecules SO 2 and NO 2 have been measured at high resolution using synchrotron radiation. Spectra were assigned with the aid of Xα and gaussian 94 calculations. Analysis of the spectra reveals similarities and differences in the electronic structure between SO 2, which is classified as a hypervalent molecule and NO 2, which is normally not classified as such. The spectral differences observed can be explained in terms of differences in the valence shell electron population and molecular geometry. There is no indication that the sulphur 3d orbitals play a significant role in the molecular bonding of SO 2. The results support the interpretation that partial ionic bonding plays a greater role in the formation of hypervalent molecules than the promotion of electrons into the unoccupied d orbitals.

Sodium 1s photoabsorption spectra of Na and NaF clusters deposited in rare gas matrices

Abstract Na and NaF clusters were investigated by Na 1s edge excitation using synchrotron radiation provided by the Super Aco storage ring in Orsay. The Na cluster spectra show a close resemblance to the solid Na spectrum which consists essentially in an absorption jump. Only for very small clusters discrete structures appearing below the absorption spectrum are visible with small oscillator strength. This contrasts with the spectrum of Na vapour which show intensive lines attributed to Rydberg states, indicating that even in small Na clusters the valence electrons are delocalised over the whole aggregates. The NaF cluster spectra present a strong decrease in the excitonic structures with decreasing size, approaching a metallic threshold for the smallest ones. This, together with mass spectroscopy observations, allow us to infer the presence of non-stoichiometric clusters for the smallest aggregates which undergo a ‘metallisation’. The discrete excitations in NaF clusters are described in terms of a model which considers perturbation of the ion-like Rydberg states by the environment of the absorbing cation and this allows simulations in satisfactory agreement with the experimental data.

Chemical shifts in P-1s photoabsorption spectra of gaseous phosphorus compounds

Abstract Phosphorus 1s photoabsorption spectra of a series of gaseous trivalent and pentavalent phosphorus compounds scanning coordination numbers 3, 4 and 5 were measured using synchrotron radiation. Spectra were assigned with the aid of Xα and Gaussian 94 (CIS) calculations. The main pre-edge peak shifts with chemical substitution and edge differences (relative to PH3) were analysed in terms of simple potential and charge models. Comparing these shifts with those of corresponding photoelectron and Auger shifts indicates that similar structural and electronic effects operate.

Electron-ion coincidence measurements on CO after C 1s and O 1s photoabsorption

Electron-ion coincidence measurements of carbon monoxide following 1sC2p (h = 287.4 eV) and 1sO2p (h = 534.2 eV) resonant photoabsorption and following 1sC photoionization (h = 350 eV) by synchrotron radiation are presented. Coincidence time-of-flight mass spectra were obtained for several pronounced electron emission channels. Fragmentation is observed and released kinetic energies are estimated. For resonant excitation 1sC2p branched decay into C++O and C+O+ is shown to take place in coincidence with electrons of 255.0 eV. In coincidence with electrons of 249.6 eV after 1sC2p photoabsorption, fragmentation into C++O+ is detected which can only be explained by double ionization in a two-step process.

Theoretical study of the Cl 1s and 2p near edge photoabsorption spectra of HCl by accurate ab-initio configuration interaction and density functional approaches

Calculations of the discrete Cl 1s and 2p excitation spectra of HCl are performed employing accurate ab-initio Configuration Interaction (CI) and Density Functional (DF) approaches. The core excited spectra above edge are described with a new DF method based on a one-centre approximation. The highly correlated quasi-degenerate perturbation scheme gives results in excellent accord with experimental data and allows a definite assignment of the Rydberg excited states below edge. The simpler 1h–1p CI scheme shows some deficiencies essentially attributed to the use of inadequate orbitals, although it appears adequate for a qualitative description of the spectra. The DF approach provides dependable results for the lower-energy states of the discrete spectra while some limitations affect the present description of the higher Rydberg states. Good agreement with experimental data is obtained for the calculated cross-section profiles relative to the Cl 1s and 2p photoionization. Also the theoretical asymmetry parameter profiles are analysed.

Electron–ion coincidence measurements on N 2 after 1s photoabsorption

Results of coincidence measurements between energy selected electrons and ions after 1s photoabsorption of N 2 are presented. A nitrogen 1s electron was either resonantly excited (1s → 1 π g ) or ionized with synchrotron radiation of hν=400.9 and 415.0 eV, respectively. The decay of the resulting 1s −1 state was traced for several strong electron emission channels by taking in coincidence time of flight mass spectra. For some decay channels fragmentation of the excited nitrogen was observed. Kinetic energies of the fragments were determined and, where possible, the molecular ionic states involved identified. A double electron emission process after resonant excitation (1s → 1 π g ) was observed.

Photoabsorption spectrum of ozone in the K-edge region

The K-shell (1s) photoabsorption spectrum of ozone has been studied for the first time by using synchrotron radiation as a light source. The total photoabsorption cross section exhibits two distinct peaks at 529.1 and 535.4 eV attributed to the π ∗ ← O(1 s) transitions of the terminal and the central oxygen atoms in ozone, respectively. The σ ∗ ← O(1 s) transition bands were also observed.

Core Excitation Spectroscopy of Phenyl- and Methyl-Substituted Silanol, Disiloxane, and Disilane Compounds: Evidence for π-Delocalization across the Si−CphenylBond

The Si 1s and 2p solid state photoabsorption (total electron yield) spectra of triphenylsilanol, hexaphenyldisiloxane, and hexaphenyldisilane and the Si 1s spectra (total ion yield) of gaseous trimethylsilanol, hexamethyldisiloxane, hexamethyldisilane, and trimethylmethoxysilane have been recorded using synchrotron radiation. These spectra are compared to inner shell electron energy loss spectra of gaseous triphenylsilanol, hexaphenyldisilane, trimethylmethoxysilane, hexamethyldisiloxane, and hexamethyldisilane in the Si 2p and C 1s regions, measured under scattering conditions where electric dipole transitions dominate (2.5 keV residual energy, θ ≤ 2°). Comparison of the Si 1s and Si 2p spectra of the Ph3Si−X and Me3Si−X species shows there are low-lying transitions at Si which occur exclusively in the Ph3Si−X species. These transitions are attributed to (Si 1s-1,π*Si-Ph) and (Si 2p-1,π*Si-Ph) states in which the core excited electron is delocalized across the Si−C(phenyl) bond into the π* levels of the ...

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.

C 1s and F 1s photoabsorption and subsequent electronic decay of CH 4, CH 3F, CH 2F 2, CHF 3, and CF 4

High-resolution electron yield spectra of CH 4, CH 3F, CH 2F 2, CHF 3, and CF 4 are observed in the vicinity of C 1 s and F 1 s thresholds and interpreted with the help of ab initio SCF calculations. Resonance spectator and participant Auger spectra are observed following the C 1 s and F 1 s excitations of these molecules and discussed in terms of the characters of the orbitals of the electrons involved in the electronic decay. The electronic decay from the fragments is also observed for CH 2F 2, CHF 3, and CF 4 at the C 1s → σ ∗ CF antibonding excitation.