1s Threshold Research Articles

Electronic structure of fluorinated multiwalled carbon nanotubes studied using x-ray absorption and photoelectron spectroscopy

This paper presents the results of combined investigation of the chemical bond formation in fluorinated multiwalled carbon nanotubes (MWCNTs) with different fluorine contents (10-55 wt %) and reference compounds (highly oriented pyrolytic graphite crystals and white graphite fluoride) using x-ray absorption and photoelectron spectroscopy at C 1s and F 1s thresholds. Measurements were performed at BESSY II (Berlin, Germany) and MAX-laboratory (Lund, Sweden). The analysis of the soft x-ray absorption and photoelectron spectra points to the formation of covalent chemical bonding between fluorine and carbon atoms in the fluorinated nanotubes. It was established that within the probing depth (similar to 15 nm) of carbon nanotubes, the process of fluorination runs uniformly and does not depend on the fluorine concentration. In this case, fluorine atoms interact with MWCNTs through the covalent attachment of fluorine atoms to graphene layers of the graphite skeleton (phase 1) and this bonding is accompanied by a change in the hybridization of the 2s and 2p valence electron states of the carbon atom from the trigonal (sp(2)) to tetrahedral (sp(3)) hybridization and by a large electron transfer between carbon an fluorine atoms. In the MWCNT near-surface region the second fluorine-carbon phase with weak electron transfer is formed; it is located mainly within two or three upper graphene monolayers, and its contribution becomes much poorer as the probing depth of fluorinated multiwalled carbon nanotubes (F-MWCNTs) increases. The defluorination process of F-MWCNTs on thermal annealing has been investigated. The conclusion has been made that F-MWCNT defluorination without destruction of graphene layers is possible. (Less)

Dielectronic recombination into Mg-like ions

Dielectronic recombination (DR) rate coefficients for Na-like ions forming Mg-like ions of Fe, Zn, Kr, and Mo are calculated. Energy levels, radiative transition probabilities, and autoionization rates for 1s22s22p63l'nl, 1s22s22p64l'nl, and 1s22s22p53l"3l'nl states are calculated by using the Hatree-Fock-relativistic method (Cowan code). Total and state-selective DR rate coefficients are derived as a function of electron temperature. Configuration mixing plays an important role for DR rates of 1s22s22p63snl states. Energy level structure near the first 3s threshold is important for DR rate coefficients behaviour at low temperature.

Characterization of fluorinated multiwalled carbon nanotubes with X-ray absorption, photoelectron and emission spectroscopies

The combined investigation of the chemical bond formation in fluorinated multiwalled carbon nanotubes with 15 wt.% fluorine concentrations (MWCNTs + F 15 wt.%) using X-ray absorption, emission and photoelectron spectroscopy at C 1s and F 1s thresholds is presented. All measurements were performed at BESSY II. The analysis of the soft X-ray and photoelectron spectra point to the formation of covalent chemical bonding between fluorine and carbon atoms in the fluorinated nanotubes. Based on results of this combined study the depth dependent effects are discussed.

Observation of site-specific decay processes in Ar–Kr mixed clusters following K-shell excitation of Kr

Multiple ion coincidence momentum imaging measurements have been performed for an Ar–Kr mixed rare-gas cluster with an average cluster size 〈 N〉 = 100 in the 1s threshold region of Kr. The signals resulting due to the photoabsorption by Kr have been extracted on the basis of the difference in the spectra between 14.08 and 14.50 keV, i.e., on both sides of the 1s threshold for Kr. The time-of-flight spectra for the clusters reveal that multiply charged cluster ions produced as a result of inner-core excitation emit many singly charged ions such as Kr +, Ar +, ArKr +, and Ar 2 +. The abundance of daughter ions suggests the transfer of positive charges from the X-ray absorbing atom to the surrounding atoms before the Coulomb explosion is suggested from. The momentum of daughter ions is deduced from the time-of-flight and position data, and the momentum distribution shows the characteristic size dependence of the ions on the number of coincidence signals N coin, i.e., the average momentum of the Ar + ions increases with N coin, while that of the Kr + ions decreases. The N coin dependence of momentum is interpreted according to the site-dependent decay processes.

Bound states and resonance states of the plasma-embedded tdμ and ddμ molecular ions

We have investigated the bound states and resonance states of plasma-embedded tdμ and ddμ molecular ions using accurate correlated basis functions. The plasma effect has been taken care of by considering the Debye shielding approach of plasma modeling which admits a variety of plasma conditions. The density of resonance states are calculated using the stabilization method. The ground and excited states energies, and the S-wave resonance energies of tdμ and ddμ molecular ions immersed in plasmas are reported for various shielding parameters, along with the 1S and 2S threshold energies of the tμ and dμ atoms.

Effect of surface treatments on the electronic properties of ultra-nanocrystalline diamond films

We present the soft x-ray spectroscopic study of the ultra-nanocrystalline diamond (UNCD) films with different surface treatments. The samples were prepared by means of microwave plasma enhanced chemical vapor deposition (MPECVD) and the different surface treatments are applied to alter their field emission properties. The electronic properties were subsequently examined by the soft x-ray absorption and x-ray emission spectroscopy at carbon 1s threshold. From the experimental results, there is no significant variation in electronic structure of oxygen- and hydrogen-plasma treated UNCD films. On the other hand, the biased treated UNCD film shows more remarkable change on the sp 2 and sp 3 states. The formation of sp 2 bonding and the reduction of sp 3 bonding are the consequence of the improved electron field emission properties.

X-ray Absorption Microscopy of Bacterial Surface Protein Layers: X-ray Damage

The electronic structure of individual sheets of the bacterial surface protein layer (S layer) of Bacillus sphaericus NCTC 9602 was studied using a photoemission electron microscope (PEEM) operating in near-edge X-ray absorption fine structure spectroscopy mode. The laterally resolved measurements performed at the C 1s, N 1s, and O 1s thresholds on fresh samples revealed characteristic differences compared to the laterally integrated data, where substrate contributions were taken along with the protein signals. During the PEEM experiments an irradiation-induced increase of the C-C bond density at the cost of the densities of the C-O and C-N bonds related to a rearrangement of the contributing atoms of the S layer deposited onto a Si substrate was observed. The critical irradiation doses for the C-O and C-N bond breaking and formation of the new C-C bonds were derived.

Characteristics of photoionization in the XUV domain for the excited 1s22s2p1,3Po states of the Be-like C2+ ion

Resonance structures and their characteristics in the photoionization of the first excited p-states of Be-like C2+ ion which interact with the even-parity continua have been studied up to the C3+ 3d threshold limit. Using the variational R-matrix method, calculations have been performed in the initial 20 and final 22 state close-coupling approximations in the photon wavelength range 15–35 nm. The detailed photoionization processes of included multichannels are displayed in the partial photoionization cross sections, which contain a number of autoionizing Rydberg series of resonances converging to the C3+ 2p, 3s, 3p and 3d thresholds. In particular, the lower members of the Rydberg series showing irregular patterns of resonances, which lie above the C3+ 2p threshold, are identified and resonance positions Er, effective quantum numbers n*, and widths Γ are presented. Excellent agreement between length and velocity gauge results is found, along with overall good agreement with previous work, except for the discrepancies with the threshold behaviour of the OP (opacity project) result.

A high-resolution study of resonant Auger decay processes in N2O after core electron excitation from terminal nitrogen, central nitrogen and oxygen atoms to the 3π LUMO

Decay spectra of N2O following excitation to the N terminal (Nt) → π*, N central (Nc) → π* and O 1s → π* intermediate states are reported. The final states reached after participator decay show resonant enhancement consistent with a local-density-of-states analysis based on the Mulliken population of the valence molecular orbitals. In particular, the X-state is resonantly enhanced mostly after excitation from the Nt 1s and the O 1s core levels to the π*, while the B-state is mostly enhanced following the excitation of the Nc 1s → π* intermediate state. Below the Nt 1s threshold, the lowest lying peak related to spectator decay falls at lower binding energy than the highest lying participator peak. This can be attributed to a particularly strong screening effect exerted by the excited electron in the LUMO.

Synchrotron radiation study of chloromethane clusters: Effects of polarizability and dipole moment on core level chemical shifts

Free neutral chloromethane clusters have been produced by adiabatic expansion and investigated by means of photoelectron spectroscopy. By studying the differences between the cluster binding energy shifts at the Cl 2p and C 1s thresholds we have shown that antiparallel packing is the dominant local structure of the clusters. This geometry is induced by the polar character of the constituent molecules, and it accords with the structures of chloromethane solids and dimers. Furthermore, results obtained from the analysis of the outer valence levels of the clusters support the suggested local structure. The roles of the polarizability and of the dipole moment of the constituent molecules in the cluster binding energy shifts are discussed in comparison with a recent investigation of bromomethane clusters.

Resonant processes and Coulomb interactions in (C59N)2

We have determined the on-site molecular Coulomb interaction energy U of a (C59N)2 bulk film and find values ranging from 1.10+/-0.10 eV for the highest occupied molecular orbital to 1.35+/-0.10 eV for the deeper lying orbitals, comparable to values found in C60. The on-site Coulomb interaction between a carbon core hole and valence electrons, Uc, is, however, substantially lower than in C60 at 1.35+/-0.07 eV. Resonant photoemission (RESPES) results show a weakened participator decay channel, especially around the N 1s threshold, where resonance of the highest occupied molecular orbital shoulder is absent. Near-edge x-ray absorption fine structure and constant initial state measurements, taken in parallel with the RESPES data, indicate, however, that matrix element effects cannot be ruled out.

The ground state and doubly-excited 1,3P° states of hot-dense plasma-embedded Li+ ions

We have investigated the ground state and the doubly excited 1,3P○ resonance states of plasma-embedded Li+ ion. The plasma effect is taken care of by using a screened Coulomb potential obtained from the Debye model. A correlated wave function has been used to represent the correlation effect between the charged particles. The ground state of Li+ in plasmas for different screening parameters has been estimated in the framework of Rayleigh-Ritz variational principle. In addition, a total of 18 resonances (9 each for 1P○ and 3P○ states) below the n=2 Li+ thresholds has been estimated by calculating the density of states using the stabilization method. For each spin state, this includes four members in the 2snp+ (2≤n ≤5) series, three members in the 2snp- (3≤n ≤5) series, and two members in the 2pnd (n=3, 4) series. The resonance energies and widths for various Debye parameters ranging from infinity to a small value for these 1,3P○ resonance states along with the ground state energies of Li+ and the Li2+ (1S), Li2+ (2S) threshold energies are reported. Furthermore, the wavelengths for the photo-absorption of lithium ion from its ground state to such 1P○ resonance states for different Debye lengths are also reported.

Lyman and Balmer emission following core excitations in methane and ammonia molecules

The intensities of hydrogen Lyman-α and Balmer series emission lines as a result of photoexcitation of free CH4 and NH3 molecules at the C 1s and N 1s edges have been measured. For methane, the total fluorescence yield in the visible region (300–650 nm) was also recorded. Excitation functions of the Balmer lines show relative intensity enhancement of Balmer-β emission in comparison with Balmer-α at higher core-to-Rydberg excitations. The Lyman-α emission intensity, in general, follows the relations observed in the corresponding total ion yield measurements. Additionally, the Balmer-γ and -δ yields were measured for ammonia molecules and they show intensity maxima at photon energies shifted closer to the N 1s threshold than Balmer-α and -β. A quantum defect analysis is performed to find out which core excitations are responsible for this enhanced intensity. Dissociation pathways leading to the emission in H atoms are discussed.

Doubly excited 1,3Pe meta-stable bound states and resonance states of helium in weakly coupled plasmas

The doubly excited 1,3Pe meta-stable bound states and resonance states of helium embedded in weakly coupled plasmas are calculated using Slater orbital basis functions. The plasma effects are taken care of by using the screened Coulomb potential obtained from the Debye model. We have calculated the 2pnp 1Pe (n = 3, 4) and 2pnp 3Pe (n = 2, 3, 4) meta-stable bound state energies of He in plasmas for various Debye lengths. In addition, a total of ten resonances (five each for 1Pe and 3Pe states) below the N = 3 He+ thresholds has also been obtained by calculating the density of states using a stabilization method. For the 1Pe states, this includes three members from the 3pnp (n = 4, 5, 6) series, and two members from the 3dnd (n = 4, 5) series. For the 3Pe state, this includes three members from the 3pnp (n = 3, 4, 5) series, and two members from the 3dnd (n = 3, 4) series. The resonance energies and widths for the 1,3Pe resonances for different screening parameters below the He+(3S) threshold are reported along with the eigen-energies of the 1,3Pe meta-stable bound states lying below the He+(2S) threshold.

Simultaneous measurements of photoionization cross-sections of lithium isotopes from 3p 2P1/2, 3/2

We present the first measurements of the photoionization cross-section of the 3p 2P1/2, 3/2 excited states for both the lithium isotopes Li6 and Li7 in the region close to the Li+ 1s2 1S threshold (from 0 to 3 eV) using the two-step photoionization and the saturation technique. Narrow linewidth dye lasers have been used in combination with a locally developed atomic beam-TOF mass spectrometer to measure the photoion signals as a function of energy density of the ionizing laser. It is observed that the cross-section is maximum near the ionization threshold and decreases monotonically as the excess photon energy of the ionizing laser is increased. The deduced number densities of the isotopes are Li6 (1.2 × 108 atoms cm−3) and Li7 (1.2 × 109 atoms cm−3), which are found to be close to their natural abundance ratio within the experimental error. Our experimental results of the measurement of the cross-section are also in good agreement with earlier theoretical work.

Experimental and Theoretical Investigation of the Electronic Structure of 5-Fluorouracil Compounds

We present a comparison between experimental and theoretical X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) of 5-fluorouracil compounds, with an emphasis on the effects of the inclusion of nickel in the structure. By focusing on the 1s thresholds of carbon, nitrogen, oxygen, and fluorine it was possible to provide a complete picture of the occupied and unoccupied partial density of states of the 5-fluorouracil systems. Spectra calculated using density functional theory are compared to experimental results. Most experimental results agree well with our theoretical calculations for the XAS and XES of the compounds. All spectral features are assigned. Our results reveal that the nickel in the compound is coordinated with the nitrogen sites of the 5-fluorouracil ligands.

Three-body muon-transfer collisions from hydrogen isotope to He2+ and Li3+ ions

The muon transfer rates from hydrogen isotopes (p,d) to 3,4He2+ and 6,7Li3+ ions are calculated in the hyperspherical close coupling method. Well converged results are obtained. The present rates are comparable to those of existing calculations for He2+, but they are much larger for Li3+. The resonance parameters are also calculated for resonances near the (Hμ)1s threshold.

MCHF studies of partial photoionization cross section of atomic fluorine

We present results of a theoretical investigation of the partial and total photoionization cross sections between the 1D and 1S thresholds of atomic fluorine using the multiconfiguration Hartree–Fock method of bound and continuum wavefunctions. The 2p4(1S)ns, md series observed by an experiment through their decay into the allowed 2p4(3P)kl and 2p4(1D)kl ionization channels are carefully identified. The 2s2p6 2S resonance is seen to interact strongly with the nearby 2p4(1S)4s 2S resonance. The results of the partial cross sections are first compared with the experimental observations. The total cross sections are compared with the available experimental and theoretical data. The energy positions of the resonance series 2p4(1S)ns, md as well as 2s2p6 2S excitation are found to be in very good agreement with the experimental observations.

Numerical calculations of energy, nucleus size and coulomb decay rate for ddμ* resonance states in the variational approach using new wavefunctions

This paper provides a theoretical complement to the experimental measurement of the population of excited dμ(2s) and dμ(1s) atoms in a deuterium. The population of these atoms plays an important role in a muon catalyzed fusion cycle. Symmetric and non-symmetric muonic molecular ions have been predicted to form in excited states in collisions between excited muonic atoms and hydrogen molecules. One example is the ddμ*, which is a muonic deuterium–deuterium symmetric ion in excited state and is initially produced in the interaction of dμ(2s) atoms with deuterium nuclei. Our calculations interpret the experimental findings in terms of the so-called side-path model. This model essentially deals with the interaction mentioned above in which the ddμ* ion undergoes Coulomb de-excitation where the excitation energy is shared between a dμ(1s) atom and one deuterium. The structure of ddμ* is studied here using the numerical, variational method and the given wavefunctions. Few resonance energies for ddμ* molecular states are calculated below the 2s threshold. For more precise assessment of the reliability of the given wavefunctions, the nucleus sizes and Coulomb decay rates for the zeroth, first and second vibrational meta-stable states of the mentioned ion are also calculated. The obtained results are close to those previously reported. The advantage of the given method over previous methods is that the used wavefunction has only two terms, which simplifies the calculations with the same results as those from the complicated coupled rearrangement channel method with a Gaussian basis set. These energies are the base data required for size, formation and decay rate calculations of the ddμ* ion.

Efficient production of metastable fragments around the 1s ionization threshold in N2

Metastable formation has been investigated in the inner-shell region of N2. Enhanced N* formation has been observed around the 1s threshold, and is shown to result from dissociation of high N2+* Rydberg states. These N2+* states are populated by spectator Auger decay from core-excited states, as well as by the recapture of slow photoelectrons into the Rydberg orbitals. The present measurement demonstrates that the metastable observation is a new and sensitive tool to study the decay dynamics of core-excited states and the photoelectron recapture due to post-collision interaction.