Cross Sections For Transitions Research Articles

Adsorption of acetonitrile(CH3CN)onSi(111)−7×7at room temperature studied by synchrotron radiation core-level spectroscopies and excited-state density functional theory calculations

The room temperature adsorption of acetonitrile $({\mathrm{CH}}_{3}--\mathrm{C}\mathrm{N})$ on $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ is examined by synchrotron radiation N $1s$ x-ray photoemission and x-ray absorption spectroscopies. The experimental spectroscopic data point to multiple adsorption geometries. Candidate structures are optimized using density functional theory (DFT), the surface being simulated by silicon clusters encompassing one (adjacent) adatom--rest atom pair. This is followed by the DFT calculation of electron transition energies and cross sections. The comparison of theoretical spectra with experimental ones indicates that the molecule is adsorbed on the surface under two forms, a nondissociated geometry (an $s{p}^{2}$-hybridized CN) and a dissociated one (leading to a pendent sp-hybridized CN). In the nondissociative mode, the molecule bridges an adatom--rest atom pair. For bridge-type models, the discussion of the core-excited state calculations is focussed on the so-called silicon-molecule mixed-state transitions that strongly depend on the breaking or not of the adatom backbonds and on the attachment of the nitrogen end either to the adatom or to the rest atom. Concerning the dissociated state, the CH bond cleavage leads to a cyanomethyl $(\mathrm{Si}\text{\ensuremath{-}}{\mathrm{CH}}_{2}\text{\ensuremath{-}}\mathrm{CN})$ plus a silicon monohydride, which accounts for the spectroscopic evidence of a free $\mathrm{C}\mathrm{N}$ group (we do not find at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ any spectroscopic evidence for a $\mathrm{C}\mathrm{N}$ group datively bonded to a silicon atom via its nitrogen lone pair). Therefore the reaction products of acetonitrile on $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ are similar to those detected on the $\mathrm{Si}(001)\text{\ensuremath{-}}2\ifmmode\times\else\texttimes\fi{}1$ surface at the same temperature, despite the marked differences in the reconstruction of those two surfaces, especially the distance between adjacent silicon broken bonds. In that respect, we discuss how adatom backbond breaking in the course of adsorption may explain why both surface orientations react the same way with acetonitrile.

Benchmarking two-photon absorption with CC3 quadratic response theory, and comparison with density-functional response theory

We present a detailed study of the effects of electron correlation on two-photon absorption calculated by coupled cluster quadratic response theory. The hierarchy of coupled cluster models CCS, CC2, CCSD, and CC3 has been used to investigate the effects of electron correlation on the two-photon absorption cross sections of formaldehyde (CH2O), diacetylene (C4H2), and water (H2O). In particular, the effects of triple excitations on two-photon transition cross sections are determined for the first time. In addition, we present a detailed comparison of the coupled cluster results with those obtained from Hartree-Fock and density-functional response theories. We have investigated the local-density approximation, the pure Becke-Lee-Yang-Parr (BLYP) functional, the hybrid Becke-3-parameter-Lee-Yang-Parr (B3LYP), and the Coulomb-attenuated B3LYP (CAM-B3LYP) functionals. Our results show that the CAM-B3LYP functional, when used in conjuction with a one-particle basis-set containing diffuse functions, has much promise; however, care must still be exercised for diffuse Rydberg-type states.

The Differential Interference Angle of Λ-Related Quantum Interference of <sup>1</sup>Π-state Diatom

Collisional quantum interference (CQI) on rotational energy transfer was observed by Sha et al. (J. Chem. Phys., 1995, 102: 2772) in static cell, and the integral interference angle which determines the magnitudes of the transition cross sections was measured. To obtain more precise information, the experiment in the molecular beam should be taken, as the integral interference angle is the average effect of the differential interference angle. The differential interference angle of Λ-Related Quantum Interference of 1Π-state diatom was presented to measure the degree of coherence, by using the first order Born approximation of time-dependent perturbation theory. The anisotropic Lennard-Jones interaction potential is employed to simulate their interference angles. The relationships of the differential interference angle versus the factors of the experiment in the molecular beam, including experimental temperature, partners, the distance of the interaction and rotational quantum number, are obtained. And the various factors that influence the interference angles are discussed. This theoretical model is important to understand or perform in the experiment of the molecular beam.

Electron excitation cross sections for spectral transitions in SrII out of atomic metastable states

Electron excitation cross sections for spectral transitions in SrII out of atomic metastable states

Spectroscopic properties of Er3+ transitions in SrWO4 crystal

Er 3 + : Sr W O 4 crystal of good optical quality was grown by Czochralski technique method. Polarized absorption and emission spectra of Er3+ ions in SrWO4 crystal have been investigated. The Judd-Ofelt (JO) theory was applied to the polarized absorption spectra of Er3+:SrWO4 to obtain the three JO parameters: Ω2=7.41×10−20cm2, Ω4=0.25×10−20cm2, and Ω6=1.71×10−20cm2. The radiative probabilities, radiative lifetimes, and branch ratios of Er3+:SrWO4 were calculated. The stimulated emission cross sections of the potential laser transitions at 1535nm were determined using the reciprocity method and the Fuchtbauer-Ladenberg formula, and the potential laser gain versus wavelength for the I13∕24→I15∕24 transition has been estimated and discussed.

Model study of the Landau–Zener approximation

We reexamine the accuracy of the Landau–Zener approximation, whose analytic simplicity has made it often cited and applied over the past 70years in describing nonadiabatic transitions. We find that this approximation may be in substantial disagreement with exact numerical results for a model physical system and a wide range of coupling strengths and relative velocities. We conclude that the Landau–Zener approximation should be used with caution for quantitative estimates of nonadiabatic transition probabilities or cross sections.

Ionization excitation of helium by the(e,2e)reaction

The $(e,2e)$ cross sections for transitions to the $n=1$, 2, and 3 final state of ${\mathrm{He}}^{+}$ have been measured at impact energies of 1000 and $1600\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ by using a newly developed energy and momentum dispersive spectrometer. Binding energy spectra in the range of $8\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}86\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and momentum profiles for the transitions to the ground and excited ion states of ${\mathrm{He}}^{+}$ are reported at different impact energies, and the experimental results are compared with plane wave impulse approximation calculations. The impact energy dependence of cross section ratios for the ${\mathrm{He}}^{+}\phantom{\rule{0.3em}{0ex}}n=1$, 2, and 3 final states are obtained. Some discrepancies are observed between experimental data and theoretical calculations.

Close-coupling study of rotational energy transfer of CO (υ=2) by collisions with He atoms

Quantum close-coupling scattering calculations of rotational energy transfer in the vibrationally excited CO due to collisions with He atom are presented for collision energies between 10(-5) and approximately 1000 cm-1 with CO being initially in the vibrational level upsilon=2 and rotational levels j=0,1,4, and 6. The He-CO interaction potential of Heijmen et al. [J. Chem. Phys. 107, 9921 (1997)] was adopted for the calculations. Cross sections for rovibrational transitions and state-to-state rotational energy transfer from selected initial rotational levels were computed and compared with recent measurements of Carty et al. [J. Chem. Phys. 121, 4671 (2004)] and available theoretical results. Comparison in all cases is found to be excellent, providing a stringent test for the scattering calculations as well as the reliability of the He-CO interaction potential by Heijmen et al.

Two-centre dielectronic transitions in fast highly charged ion–atom collisions

We consider two-centre dielectronic transitions occurring in collisions of fast highly charged hydrogen-like projectiles with neutral atomic targets. In such collisions, in addition to the reaction channel due to the two-centre electron–electron interaction, the reaction channel induced by the two-centre electron–nucleus interactions becomes important. We show that the account of distortions of initial and final states of the target caused by the field of the projectile nucleus (which can also be viewed as imposing the Coulomb boundary conditions on these states) leads to an approach in which both these reaction channels are combined in a natural and relatively simple way. As a result, this approach enables one to extend calculations of different cross sections for two-centre dielectronic transitions from the high-velocity to the intermediate-to-high collision velocity regime.

(e,2e)and(e,3−1e)studies on double processes of He at large momentum transfer

The double processes of He in electron-impact ionization, single ionization with simultaneous excitation and double ionization, have been studied at large momentum transfer using an energy- and momentum-dispersive binary $(e,2e)$ spectrometer. The experiment has been performed at an impact energy of $2080\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in the symmetric noncoplanar geometry. In this way we have achieved a large momentum transfer of $9\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.}$, a value that has never been realized so far for the study on double ionization. The measured $(e,2e)$ and $(e,3\ensuremath{-}1e)$ cross sections for transitions to the $n=2$ excited state of ${\mathrm{He}}^{+}$ and to doubly ionized ${\mathrm{He}}^{2+}$ are presented as normalized intensities relative to that to the $n=1$ ground state of ${\mathrm{He}}^{+}$. The results are compared with first-order plane-wave impulse approximation (PWIA) calculations using various He ground-state wave functions. It is shown that shapes of the momentum-dependent $(e,2e)$ and $(e,3\ensuremath{-}1e)$ cross sections are well reproduced by the PWIA calculations only when highly correlated wave functions are employed. However, noticeable discrepancies between experiment and theory remain in magnitude for both the double processes, suggesting the importance of higher-order effects under the experimental conditions examined as well as of acquiring more complete knowledge of electron correlation in the target.

Inner-shell excitation of acetylene by electron impact

The distorted-wave approximation (DWA) is applied to study $K$-shell excitation in ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$ by electron impact. More specifically, calculated differential and integral cross sections for the $X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{g}^{+}\ensuremath{\rightarrow}^{1,3}\ensuremath{\Pi}_{g}(1s{\ensuremath{\sigma}}_{g}\ensuremath{\rightarrow}1p{\ensuremath{\pi}}_{g})$ and $X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{g}^{+}\ensuremath{\rightarrow}^{1,3}\ensuremath{\Pi}_{u}(1s{\ensuremath{\sigma}}_{u}\ensuremath{\rightarrow}1p{\ensuremath{\pi}}_{g})$ transitions in this target in the $300--800\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ incident energy range are reported. The triplet-to-singlet ratios of respective integral cross sections, namely, RI(3:1), calculated by dividing the integral cross sections for transitions leading to the triplet core-excited states by those leading to the corresponding singlet states, are also reported as a function of incident energies. In general, our calculated sums of the generalized oscillator strength for transitions leading to the $^{1}\ensuremath{\Pi}_{g}$ and $^{1}\ensuremath{\Pi}_{u}$ excited states are in good agreement with the available experimental data. On the other hand, the present calculated integral cross sections and the corresponding data for its isoelectronic species CO are significantly different. Possible physical origins for this difference are discussed.

Excitation of PtI Transitions Terminating on Singlet Levels

The method of extended crossing beams is used to investigate the excitation of platinum atom transitions terminating on singlet levels. Thirty eight excitation cross sections are measured at the electron energy of 30 eV; nine optical excitation functions are recorded in the electron energy range from zero to 200 eV. Also measured are eight excitation cross sections of transitions which have even upper levels. Possible excitation channels are discussed, which define the absolute values of cross sections and the form of optical excitation functions.

Excitation of Giant Resonances in Deuteron Stripping Reactions on 10B far below the Coulomb Barrier

The reaction threshold for deuteron induced reactions on 10 B amounts to 25.2 MeV, which enables an excitation of the giant resonances (GR) in the compound nucleus 12 C even at very low projectile energies. In our recent experimental and theoretical investigations of the 10 B(d,p) 11 B reaction, we have found a strong reaction contribution arising from the giant dipole resonance (GDR) excitation. This kind of transition is only possible due to a significant isospin impurity of the GDR in 12 C. In the present work, we study an influence of the GDR as well as of the giant quadrupole (GQR) excitation on the 10 B(d,p) 11 B reaction cross section for transitions to the ground as well as the first three excited states at astrophysically relevant deuteron energies below 3 MeV. Based on our new measurement of angular distributions and absolute cross sections as well as on experimental results of other authors we have determined the total astrophysical S-factor of the 10 B(d,p) 11 B reaction down to deuteron energies close to zero. Additionally, the importance of the interference between resonant and direct reaction amplitudes is discussed.

Inelastic scattering matrix elements, cross sections and rate constants for transition B( 2P 1/2) + H 2( j = 0) ↔ B( 2P 3/2) + H 2( j = 0)

The non-adiabatic wave packet collisions of B( 2P 1/2) + H 2( j = 0) ↔ B( 2P 3/2) + H 2( j = 0) were calculated using the time dependent Channel Packet Method to compute transition probabilities, cross sections and rate constants. While the H 2 angular momentum j was fixed to 0, the total angular momentum of the system J, was varied from 1/2 to 153/2. The feature of the Stückelberg oscillation was shown in transition probabilities. The transition from B( 2P 3/2) to B( 2P 1/2) state was shown to be favored over the reverse process. The ratio of the computed rate constants was well compared with that of the analytic result obtained from the Boltzmann factor and the detailed balance.

Absorption intensities and emission cross sections of principal intermanifold and inter-Stark transitions of Er3+(4f11) in polycrystalline ceramic garnet Y3Al5O12

A comparative spectroscopic study is performed on Er3+(4f11) ions doped in polycrystalline ceramic garnet Y3Al5O12 (YAG) and single-crystal laser rod, both containing nominal 50 at. % of Er3+. The standard Judd–Ofelt (JO) model is applied to the room-temperature absorption intensities of Er3+(4f11) transitions in both hosts to obtain the phenomenological intensity parameters. These parameters are subsequently used to determine the radiative decay rates, radiative lifetimes, and branching ratios of the Er3+ transitions from the upper multiplet manifolds to the corresponding lower-lying multiplet manifolds LJ2S+1 of Er3+(4f11) in these garnet hosts. The emission cross sections of the intermanifold Er3+I13∕24→I15∕24 (1.5 μm) transition as well as the principal inter-Stark transition Y1→Z4 (1550 nm) within the corresponding multiplet manifolds have been determined. The room-temperature fluorescence lifetimes of the I13∕24→I15∕24 (1.5 μm) transition in both polycrystalline ceramic and single-crystal YAG samples were measured. From the radiative lifetimes determined from the JO model and the measured fluorescence lifetimes, the quantum efficiencies for both samples were determined. The comparative study of Er3+(4f11) ions performed suggests that polycrystalline ceramic YAG is an excellent alternative to single-crystal YAG rod for certain applications.

Absence of the enhanced intra-4f transition cross section at 1.5μm of Er3+ in Si-rich SiO2

We present measurements of the optical absorption cross section of the I15∕24→I13∕24 transition at 1.5μm of Er3+ ions embedded in SiO2 and Si-rich oxide, using cavity ringdown spectroscopy on thin films. The peak absorption cross section for Er3+ embedded in Si-rich oxide (10at.% excess Si) was found to be (8±2)×10−21cm2 at 1536nm, similar to typical values for Er embedded in SiO2. The data imply that the silicon nanoclusters incorporated in Si-rich oxide do not enhance the peak cross section of the Er3+ I15∕24−I13∕24 transition by 1-2 orders of magnitude, contrary to what has been reported in earlier work.

Observation of a Molecular Frame(e,2e)Cross Section: An(e,2e+M)Triple Coincidence Study onH2

We report the first experimental results showing transition-specific anisotropy of molecular frame (e, 2e) cross sections. Vector correlations between the two outgoing electrons and the fragment ion have been measured for specific ionization-excitation processes of H2. The results enable us to obtain molecular frame (e, 2e) cross sections for transitions to the 2ssigma(g) and 2psigma(u) excited states of H(2)(+), thereby making stereodynamics of the electron-molecule collisions directly visible.

Ultrasonic testing of railway axles with the phased array technique ? experience during operation

The increasing use of high-speed trains, resulting in higher stress to the material and serious accidents with human injuries in the past, lead to a demand for shorter inspection intervals, extended scope of inspection and more stringent test requirements to be applied to the critical components. One component, which is inspected in regular intervals, is the axle of the railway wheel set. Critical areas for cracks are mainly the wheel and brake disc seats and the cross section transition areas. The manual ultrasonic inspection of used axles by experienced personal is time-consuming (since several sound beam angles have to be used to cover the complete test area), expensive and, of course, subjective. It represents a bottleneck in the inspection line. One way to overcome this bottleneck is the automation of the test process and the use of phased array probes. GE Inspection Technologies, in co-operation with the German railways (DB) and the 'Bundesanstalt fur Materialprufung'(BAM), developed a system to automate this inspection. This system applies the phased array technique to detect transversal cracks in the most critical areas of the axle. The different angles of several conventional probes are replaced by steering the beam angle of one phased array probe accordingly. Mechanical movement is substituted by software control. Two of these automated systems have been delivered to the German Railways. When taking these systems into operation, several problems arose, caused by the variety of the axles, coupling surface conditions and differences in the sound attenuation even within the axle. This paper describes the technical concept and reports some practical experience, problems and solutions in the commissioning phase of the automatic inspection lines.

Charm production in antiproton–nucleus collisions at the [formula omitted] and the [formula omitted] thresholds

We discuss the production of charmonium states in antiproton–nucleus collisions at the ψ′ threshold. It is explained that measurements in p¯A collisions will allow to get new information about the strengths of the inelastic J/ψN and ψ′N interaction, on the production of Λc and D¯ in charmonium–nucleon interactions and for the first time about the nondiagonal transitions ψ′N→J/ψN. The inelastic J/ψ-nucleon cross section is extracted from the comparison of hadron–nucleus collisions with hadron–nucleon collisions. We extract the total J/ψ-nucleon cross section from photon–nucleon collisions by accounting for the color transparency phenomenon within the frame of the GVDM (generalized vector meson dominance model). We evaluate within the GVDM the inelastic ψ′-nucleon cross section as well as the cross section for the nondiagonal transitions. Predictions for the ratio of J/ψ to ψ′ yields in antiproton–nucleus scatterings close to the threshold of ψ′ production for different nuclear targets are presented.

Experimental and theoretical NEXAFS/XPS study of the room-temperature adsorption of acetonitrile onSi(001)−2×1

Combining synchrotron radiation N $1s$ x-ray photoemission and x-ray absorption spectroscopy to density functional theory calculations of electron transition energies and cross sections performed on silicon clusters, we have re-examined the issue of acetonitrile adsorption on single- and two-domain $\mathrm{Si}(001)\text{\ensuremath{-}}2\ifmmode\times\else\texttimes\fi{}1$ surfaces, taking into consideration the various adsorption models proposed in the recent theoretical works. It is shown that at 300 K and saturation coverage, the molecule is chemisorbed both under nondissociated and dissociated forms. The nondissociative adsorption mode results from a $\mathrm{di}\text{\ensuremath{-}}\ensuremath{\sigma}$ bonding involving the cyano group. The use of a vicinal surface allows us to show that the side-on (a cycloadditionlike product on the Si dimer) is the majority species of $\mathrm{di}\text{\ensuremath{-}}\ensuremath{\sigma}$ type. The datively bonded molecule (end-on) is not observed at 300 K. The molecule also dissociates on the surface, under the form of a cyanomethyl plus a silicon monohydride, a model which has not been so far proposed by theoretical works.