Experimental Energy Range Research Articles

Temporary Anions of Benzoxazole in Charge-Transfer Cluster Photodetachment.

The photoelectron spectra of cluster anions of superoxide (O2-) solvated by one molecule of benzoxazole (BzOx) reveal two competing photodetachment mechanisms: a direct photoemission from the solvated cluster core and an indirect pathway involving temporary anion states of benzoxazole accessed via the O2-·BzOx → O2·BzOx- charge-transfer transitions. Benzoxazole is a bicyclic unsaturated organic molecule that does not form permanent anions. However, its low-lying vacant π* orbitals permit a resonant capture of the electron emitted from the O2- cluster core. The non-Hermitian theory using a complex absorbing potential predicts the existence of two BzOx- π* resonances within the experimental energy range: resonance A (π1*) at 0.891 eV and resonance B (π2*) at 1.76 eV, relative to the onset of the BzOx + e- continuum at the ground-state geometry of neutral BzOx. Within the clusters, the O2·BzOx- charge-transfer states are partially stabilized relative to the free-electron limit by interactions with the O2 molecule. These interactions depend on the electronic states of both species. The theory predicts that at the O2-·BzOx cluster geometry, the O2(X3Σg-)·BzOx-(A) and O2(a1Δg)·BzOx-(A) states lie at 0.56 and 0.47 eV (vertically) above the respective neutral states. The O2(3Σg-)·BzOx-(B) resonance is found 1.43 eV (vertically) above O2(X3Σg-)·BzOx. Intense signatures of both BzOx- resonances and the three above-mentioned charge-transfer cluster states, O2(X3Σg-)·BzOx-(A), O2(a1Δg)·BzOx-(A), and O2(3Σg-)·BzOx-(B) are observed in the 355 nm (3.495 eV) and 532 nm (2.330 eV) photoelectron spectra of the O2-·BzOx cluster anion.

Transmission through graphene of electrons in the 30 – 900 eV range

Here, we report on accurate transmission measurements of electrons below 1 keV through suspended monolayer graphene. Monolayer graphene was grown via chemical vapor deposition and transferred onto transmission electron microscopy (TEM) grids. A monochromatic electron gun has been employed to perform the measurements in the 30 – 900 eV range in ultra-high vacuum. The graphene transparency is obtained from the absolute measurement of the direct beam current and the transmitted one, by means of a Faraday cup. We observed a transmission going from ∼20 to ∼80% for monolayer graphene within the experimental electron energy range. The high quality and the grid coverage of the suspended graphene has been proved via micro-Raman, X-ray photoemission, electron energy loss spectroscopies and field-emission scanning electron microscopy. After a 550 °C in-vacuum annealing of the samples, the main contribution to the C 1s spectrum is due to the sp2 component and the evidence of suspended monolayer graphene has been observed through the π-plasmon excitation.

Estimation of production yield of 93mMo and other residues from a 7Li-induced reaction

The crux of the present work is to analyze the yield of the residues produced in the 7Li-induced reaction on 89Y target in the low energy range. The experimental yields of 93mMo, which is a significant product in the energy range considered, and other coproduced radionuclides, such as 92mNb, 89Zr, 91mY, and 90mY have been measured within 19–40 MeV energy, and are compared with the compound and pre-compound models in the framework of the statistical reaction model code EMPIRE. The maximum measured yield for 93mMo is 374 MBq/C at 31.3 MeV in a 3 mg/cm2 thick 89Y target corresponding to a cross-section of 396.8 ± 38.6 mb from the 89Y(7Li,3n)93mMo reaction. The model estimations well reproduce the experimental yields of 93mMo. The possible production of stable isotopes has also been estimated theoretically from the model over the experimental energy range. The reaction could be an alternative route for the production of no-carrier-added 93mMo radionuclide provided that the radionuclides are chemically separated from the bulk 89Y matrix.

Prominent role of multielectron processes inK-shell double and triple photodetachment of oxygen anions

The photon-ion merged-beams technique was used at a synchrotron light source for measuring absolute cross sections of double and triple photodetachment of O$^{-}$ ions. The experimental photon energy range of 524-543 eV comprised the threshold for K-shell ionization. Using resolving powers of up to 13,000, the position, strength and width of the below-threshold 1s 2s2 2p6 2S resonance as well as the positions of the 1s 2s2 2p5 3P and 1s 2s2 2p5 1P thresholds for K-shell ionization were determined with high precision. In addition, systematically enlarged multi-configuration Dirac-Fock calculations have been performed for the resonant detachment cross sections. Results from these ab-initio computations agree very well with the measurements for the widths and branching fractions for double and triple detachment, if double shake-up (and -down) of the valence electrons and the rearrangement of the electron density is taken into account. For the absolute cross sections, however, a previously found discrepancy between measurements and theory is confirmed.

Comparative stereodynamics in molecule-atom and molecule-molecule rotational energy transfer: NO(A(2)Σ(+)) + He and D2.

We present a crossed molecular beam scattering study, using velocity-map ion-imaging detection, of state-to-state rotational energy transfer for NO(A(2)Σ(+)) in collisions with the kinematically identical colliders He and D2. We report differential cross sections and angle-resolved rotational angular momentum polarization moments for transfer of NO(A, v = 0, N = 0, j = 0.5) to NO(A, v = 0, N' = 3, 5-12) in collisions with He and D2 at respective average collision energies of 670 cm(-1) and 663 cm(-1). Quantum scattering calculations on a literature ab initio potential energy surface for NO(A)-He [J. Kłos et al., J. Chem. Phys. 129, 244303 (2008)] yield near-quantitative agreement with the experimental differential scattering cross sections and good agreement with the rotational polarization moments. This confirms that the Kłos et al. potential is accurate within the experimental collisional energy range. Comparison of the experimental results for NO(A) + D2 and He collisions provides information on the hitherto unknown NO(A)-D2 potential energy surface. The similarities in the measured scattering dynamics of NO(A) imply that the general form of the NO(A)-D2 potential must be similar to that calculated for NO(A)-He. A consistent trend for the rotational rainbow maximum in the differential cross sections for NO(A) + D2 to peak at more forward angles than those for NO(A) + He is consistent with the NO(A)-D2 potential being more anisotropic with respect to NO(A) orientation. No evidence is found in the experimental measurements for coincident rotational excitation of the D2, consistent with the potential having low anisotropy with respect to D2. The NO(A) + He polarization moments deviate systematically from the predictions of a hard-shell, kinematic-apse scattering model, with larger deviations as N' increases, which we attribute to the shallow gradient of the anisotropic repulsive NO(A)-He potential energy surface.

Shielding behaviors of some polymer and plastic materials for gamma-rays

Gamma-rays interaction parameters with six polymer and plastic materials have been computed and investigated for their shielding behaviours in the experimental energy range 10–1400keV. Low mass-density and ability to transform into thin-sheets of H-containing plastic and polymer materials generate the curiosity for present study. Gamma Ray Interaction Coefficients (GRIC)-toolkit: a computer program designed in MS-Excel, 2007 for the investigation. The toolkit has the capability to compute different parameters linked to shielding properties of the sample-material such that; mass attenuation coefficients (μ/ρ), mass energy absorption coefficients (μen/ρ), equivalent atomic number (Zeq), KERMA (kinetic energy released per unit mass). Polyvinyl Chloride (PVC) seems to be the best shield for gamma radiations in energy range 10–110keV. The interaction parameters for selected samples have been computed and provided in the extended energy range 0.015–15MeV.

An analysis of elastic scattering reactions with a Fermi–Dirac pomeron opaqueness in impact parameter space

In the Bourrely–Soffer–Wu model we introduce for the pomeron a new opaqueness in impact parameter space in terms of different quark contributions described by a Fermi–Dirac distribution. In order to check the validity of this assumption we consider \(p~p\), \(\bar{p}~p\), and \(\pi ^{\pm }~p\) elastic scattering. We emphasize the role of the gluon above the diffraction peak in the differential cross sections. Once these contributions are determined we extend the model to light nuclei elastic reactions like \(p~d\), \(p~^4\hbox {He}\) and \(\pi ^{\pm }~^4\hbox {He}\). The results obtained show a good description of all these elastic processes over the available experimental energy range and moderate momentum transfer.

Internal energy selection in vacuum ultraviolet photoionization of ethanol and ethanol dimers

Internal energy selected ethanol monomer and ethanol dimer ions were prepared by threshold photoionization of a supersonic molecular beam seeded with ethanol. The dissociative photoionization processes of the monomer, the lowest-energy CH3-loss channel of the dimer, and the fragmentation of larger clusters were found to be disjunct from the ionization onset to about 12 eV, which made it possible to determine the 0 K appearance energy of C-C bond breaking in the H-donor unit of the ethanol dimer cation as 9.719 ± 0.004 eV. This reaction energy is used together with ab initio calculations in a thermochemical cycle to determine the binding energy change from the neutral ethanol dimer to a protonated ethanol-formaldehyde adduct. The cycle also shows general agreement between experiment, theory, and previously published enthalpies of formation. The role of the initial ionization site, or rather the initial photoion state, is also discussed based on the dimer breakdown diagram and excited state calculations. There is no evidence for isolated state behavior, and the ethanol dimer dissociative photoionization processes appear to be governed by statistical theory and the ground electronic state of the ion. In the monomer breakdown diagram, the smoothly changing branching ratio between H and CH3 loss is at odds with rate theory predictions, and shows that none of the currently employed few-parameter rate models, appropriate for experimental rate curve fitting, yields a correct description for this process in the experimental energy range.

A Phase Space Theory for Roaming Reactions

We describe a new, simple theory for predicting the branching fraction of products in roaming reactions, compared to the analogous barrierless bond dissociation products. The theory uses a phase space theory (PST) formalism to divide reactive states in the bond dissociation channel into states with enough translational energy to dissociate and states that may roam. Two parameters are required, ΔEroam, the energy difference between the bond dissociation threshold and the roaming threshold, and the roaming probability, Proam, the probability that states that may roam do roam rather than recombine to form reactants. The PST-roaming theory is tested against experimental and theoretical data on the dissociation dynamics of H2CO, NO3, and CH3CHO. The theory accurately models the relative roaming to bond dissociation branching fraction over the experimental or theoretical energy range available in the literature for each species. For H2CO, fixing ΔEroam = 146 cm(-1), the midpoint of the experimental bounds for the roaming threshold, we obtain Proam = 1. The best-fit value, ΔEroam = 161 cm(-1), is also consistent with the experimental bounds. Using this value, the relative roaming to dissociation branching ratios are predicted to be similar in D2CO and H2CO, consistent with experimental observation. For NO3, we fix ΔEroam = 258.6 cm(-1), the experimental threshold for NO + O2 production, and we model low-temperature experimental branching fractions using the experimental rotational and vibrational temperatures of Trot = 0 K and Tvib = 300 K. The best fit to the experimental data is obtained for Proam = 0.0075, with this very small Proam being consistent with the known geometric constraints to formation of NO + O2. Using Proam = 0.0075, our PST-roaming theory also accurately predicts the low-temperature NO yield spectrum and quantum yield data for room-temperature NO3 photolysis. For CH3CHO, we fix ΔEroam = 385 cm(-1), based on theoretical calculations, and obtain a best-fit value of Proam = 0.21, fitting to reduced dimensional trajectory calculations. These values of ΔEroam and Proam yield PST-roaming theory results that are also consistent with two experimental room-temperature data points. The combination of other kinetic theories and the PST-roaming theory will provide rate coefficients for roaming reactions.

Experimental absolute cross section for photoionization of Xe7+

The cross section for photoionization of Xe7+ has been measured on an absolute scale with the photon-ion merged beams technique at a synchrotron light source. The experimental photon energy range of 95 145 eV comprises the ground state ionization threshold at about 106 eV and the prominent 4d9 5s 5f2P resonance at 122.14 o 0.2 eV. The strength of this resonance is found to account for 15% of the 4d absorption oscillator strength. With an experimental photon energy spread of 50 meV the 4d9 5s 5f2P resonance width could be determined with 8% uncertainty to be 72 meV.

Evaluation of w values for carbon beams in air, using a graphite calorimeter

Despite recent progress in carbon therapy, accurate values for physical data such as the w value in air or stopping power ratios for ionization chamber dosimetry have not been obtained. The absorbed dose to graphite obtained with the graphite calorimeter was compared with that obtained using the ionization chambers following the IAEA protocol in order to evaluate the w values in air for mono-energetic carbon beams of 135, 290, 400 and 430 MeV/n. Two cylindrical chambers (PTW type 30001 and PTW type 30011, Farmer) and two plane-parallel chambers (PTW type 23343, Markus and PTW type 34001, Roos) calibrated by the absorbed dose to graphite and exposure to the 60Co photon beam were used. The comparisons to our calorimeter measurements revealed that, using the ionization chambers, the absorbed dose to graphite comes out low by 2–6% in this experimental energy range and with these chamber types and calibration methods. In the therapeutic energy range, the w values in air for carbon beams indicated a slight energy dependence; we, however, assumed these values to be constant for practical use because of the large uncertainty and unknown perturbation factors of the ionization chambers. The w values in air of the carbon beams were evaluated to be 35.72 J C−1 ± 1.5% in the energy range used in this study. This value is 3.5% larger than that recommended by the IAEA TRS 398 for heavy-ion beams. Using this evaluated result, the absorbed dose to water in the carbon beams would be increased by the same amount.

Ab initio potential energy surfaces and nonadiabatic interactions in the H++NO collision system

Ab initio calculations on the H(+)+NO system have been carried out in Jacobi coordinates at the multireference configuration interaction level employing Dunning's correlation-consistent polarized valence triple zeta basis set to analyze the role of low-lying electronic excited states in influencing the collision dynamics relevant to the experimental collision energy range of 9.5-30 eV. The lowest two adiabatic potential energy surfaces, asymptotically correlating to H(+)+NO(X (2)Pi) and H((2)S)+NO(+)(X (1)Sigma(+)), have been obtained. Using ab initio procedures, the (radial) nonadiabatic couplings and the mixing angle between the lowest two electronic states (1 (2)A' and 2 (2)A') have been obtained to yield the corresponding quasidiabatic potential energy matrix. The strengths of the computed vibrational coupling matrix elements reflect a similar trend, as has been observed experimentally in the magnitudes of the state-to-state transition probability for the inelastic vibrational excitations [J. Krutein and F. Linder, J. Chem. Phys. 71, 559 (1979); F. A. Gianturco et al., J. Phys. B 14, 667 (1981)].

Similarities and differences between electron and positron scattering from molecules

In this paper a comparative study is carried out between total cross sections (TCS) for electron impact with those for positron impact using both experimental and theoretical procedures. The method used for the measurements is a retarding potential time-of-flight (RP-TOF) while the theoretical procedure is the Continuum Multiple Scattering (CMS) model. The experimental energy range is 0.2–1000 eV while that for the CMS is 1.5–100 eV. For polar molecules, the increase in TCS observed below 2 eV is expected due to the dipole induced long range interaction that has been observed only for some molecules for electron impact. In the energy range 3–60 eV electron TCS are characteristically larger than positron TCS due to resonances in the former. Some peculiar cases have been studied whereby positron TCS become unexpectedly greater than electron TCS at energies below 10 eV. The first Born approximation has been observed to hold for the impact energy region above a few hundred of eV for all molecules studied in this laboratory, albeit with different threshold energies.

Strangeness and the Discovery of Quark-Gluon Plasma

Strangeness flavor yield s and the entropy yield Sare the observables of the deconfined quark-gluon state of matter which can be studied in the entire available experimental energy range at AGS, SPS, RHIC, and, in near future, at the LHC energy range. We present here a comprehensive analysis of strange, soft hadron production as function of energy and reaction volume. We discuss the physical properties of the final state and argue how evidence about the primordial QGP emerges.

Dissociation of the OCS+ ion in low-lying electronic states studied using multiconfiguration second-order perturbation theory

Complete active space self-consistent-field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) calculations with atomic natural orbital basis sets were performed to investigate the S-loss direct dissociation of the 1 2Pi(X 2Pi), 2 2Pi(A 2Pi), 1 2Sigma+(B 2Sigma+), 1 4Sigma-, 1 2Sigma-, and 1 2Delta states of the OCS+ ion and the predissociations of the 1 2Pi, 2 2Pi, and 1 2Sigma+ states. Our calculations indicate that the S-loss dissociation products of the OCS(+) ion in the six states are the ground-state CO molecule plus the S+ ion in different electronic states. The CASPT2//CASSCF potential energy curves were calculated for the S-loss dissociation from the six states. The calculations indicate that the dissociation of the 1 4Sigma- state leads to the CO + S+ (4Su) products representing the first dissociation limit; the dissociations of the 1 2Pi, 1 2Sigma-, and 1 2Delta states lead to the CO + S+(2Du) products representing the second dissociation limit; and the dissociations of the 2 2Pi and 1 2Sigma+ states lead to the CO + S+(2Pu) products representing the third dissociation limit. Seams of the 1 2Pi-1 4Sigma-, 2 2Pi-1 4Sigma-, 2 2Pi-1 2Sigma-, 2 2Pi-1 2Delta, and 1 2Sigma(+)-1 4Sigma- potential energy surface intersections were calculated at the CASPT2 level, and the minima along the seams were located. The calculations indicate that within the experimental energy range (15.07-16.0 eV) the 2 2Pi(A 2Pi) state can be predissociated by 1 4Sigma- forming the S+(4Su) ion and can undergo internal conversion to 1 2Pi followed by the direct dissociation of 1 2Pi forming S+(2Du) and that within the experimental energy range (16.04-16.54 eV) the 1 2Sigma+(B 2Sigma+) state can be predissociated by 1 4Sigma- forming the S+(4Su) ion and can undergo internal conversion to 2 2Pi followed by the predissociation of 2 2Pi by 1 2Sigma- and 1 2Delta forming the S+(2Du) ion. These indications are in line with the experimental fact that both the 4Su and 2Du states of the S+ ion can be formed from the 2 2Pi and 1 2Sigma+ states of the OCS+ ion.

Positron scattering from carbon dioxide

We report total cross section measurements for positron scattering from carbon dioxide $({\mathrm{CO}}_{2})$. The energy range of the present measurements is $0.1--20.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The present study is undertaken to both try and resolve a discrepancy in the literature between the earlier low-energy works of Hoffman et al. [Phys. Rev. A 25, 1393 (1982)] and Kimura et al. [J. Chem. Phys. 107, 6616 (1997)], and to extend the available data to lower energies. We find generally good agreement with the data of Hoffman et al. over the common experimental energy range. A comparison of the present data with available calculations is also made, as is a comparison with corresponding electron total cross section data.

Coincidental emission of molecular ions from keV carbon cluster impacts

We report here experimental data for the emission of one and two phenylalanine (Ph) molecular ions per impact from 10 to 21 keV impacts of coronene (C 24H 12), C 60, and gramicidin S (C 60N 12O 10H 92) projectiles as a function of projectile mass, energy, and geometry. Secondary ion mass spectrometry (SIMS) experiments were conducted using event-by-event bombardment and detection. With this method, individual projectile impacts and the resulting secondary ions are recognized as singular events resolved in time and space. The target surface was an equa-molar mixture of phenylalanine (Ph H, C 9H 11NO 2) and deuterated phenylalanine (Ph D, C 9H 3D 8NO 2). This allowed for the detection of two co-emitted [M-H] − phenylalanine ions in a dual time-of-flight instrument. In contrast to the linear dependence yield of Ph versus energy for single Ph ion emission, the yield for two Ph ion emission versus energy is nonlinear within the experimental energy range. For the co-emission of two Ph ions, C 60 was found to be more efficient than coronene at equal velocities.

Photoionization ofSc2+ions by synchrotron radiation: Measurements and absolute cross sections in the photon energy range 23–68 eV

Cross sections for the photoionization (PI) of ${\mathrm{Sc}}^{2+}$ ions with $[\mathrm{Ar}]3d$ ground-state configuration have been measured by employing the merged ion-photon beams method. The ${\mathrm{Sc}}^{2+}$ ions were produced from metallic vapor in an electron cyclotron resonance ion source, and the photon beam was generated by an undulator in the electron-synchrotron storage ring of the advanced light source of the Lawrence Berkeley National Laboratory. The experimental photon energy range 23--68 eV encompasses the direct $3d$ and $3p$ photoionization thresholds. The experimental photoion spectrum is dominated by autoionizing resonances due to $3p$ excitations predominantly decaying via Coster-Kronig and super-Coster-Kronig transitions. Individual resonances located around $E\ensuremath{\approx}40.2\mathrm{eV}$ have been measured with an instrumental energy spread $\ensuremath{\Delta}E$ as low as 1.2 meV, corresponding to a resolving power of $E/(\ensuremath{\Delta}E)$ around 33 500. The fractions of metastable ions in the ${\mathrm{Sc}}^{2+}$ ion beam are obtained by comparing the photoionization cross section with the recently measured [Schippers et al., Phys. Rev. A 65, 042723 (2002)] cross section for the time-reversed process of photorecombination of ${\mathrm{Sc}}^{3+}$ ions. Absolute strengths of several ${3p}^{5}{3d}^{2}$ and ${3p}^{5}3d4s$ PI resonances have been determined. They are the same as the corresponding resonance strengths for isoelectronic ${\mathrm{Ca}}^{+}$ ions.

Measurement of the field-induced dielectronic-recombination-rate enhancement ofO5+ions differential in the Rydberg quantum numbern

The influence of electromagnetic fields on the dielectronic recombination of ${\mathrm{O}}^{5+}$ ions has been measured at the Stockholm heavy ion storage ring CRYRING. The experimental energy range covered all $2\stackrel{\ensuremath{\rightarrow}}{s}2p$ $\ensuremath{\Delta}n=0$ resonances. Electric fields up to 600 V/cm were applied and a noticeable enhancement of the dielectronic recombination cross section as a function of electric-field strength was observed. Field ionization of highly exited Rydberg states was used to investigate the enhancement of the recombination rate for two narrow bands of n states ranging from $n\ensuremath{\approx}19\char21{}22$ and from $n\ensuremath{\approx}23\char21{}25.$ The experimental data are compared to theory. After applying a field-ionization model to the calculations, the field-free spectra show good agreement with the experimental data. Although experiment and theory show the same trend of increasing strength of dielectronic recombination with increasing external electric field and with increasing n, quantitative discrepancies remain.

Tensor low-energy electron diffraction analysis of the surface structure of NaCl(100) thin films grown on Pd(100) and Pt(111)

Thin films of NaCl were molecularly deposited and subsequently ordered on Pd(100) and Pt(111). These films exhibited the same adsorbate–substrate interaction on Pd(100) and Pt(111) as evidenced by the lone multilayer desorption peak observed in the temperature programmed desorption spectra of NaCl on these two substrates. The ordered structures of the films were produced by exposing a heated substrate to a flux of NaCl emanating from a Knudsen cell and investigated through the use of low-energy electron diffraction (LEED). On Pd(100), the adsorbed multilayer yielded a (1×1) NaCl(100) LEED pattern. The (1×1) pattern was seen as the result of the film's thickness being greater than the sampling depth of the elastically scattered electrons used in the LEED experiment, thus removing the contribution of the substrate's lattice to the observed pattern. For the (1×1) pattern of NaCl(100) on Pd(100), it was found that the NaCl film grew along the [010]-type surface directions of Pd(100) to take advantage of a near 1:1 lattice match. On Pt(111), the ordered NaCl multilayer gave a (1×1) NaCl(100) LEED pattern also, but, unlike Pd(100), one of the overlayer's unit cell vectors lies parallel to that of the substrate's with the other rotated by 90° from the previous one to preserve the geometry of the NaCl(100) lattice. Multiple intensity versus electron energy ( IV) data sets were gathered for the NaCl(100) multilayer film on both Pd(100) and Pt(111). An analysis of the I– V curves of the multilayer NaCl(100) pattern on both substrates showed that the multilayer has the same structure on both substrates to a depth sampled by the electrons in the experimental energy range. Data from both systems were used in a fully dynamical LEED calculation. In the final structure, which refined down to a Pendry R-factor ( R P) of 0.16, the largest deviation from the ideally terminated NaCl(100) structure was the movement of the surface Na + towards the bulk, thereby causing a 0.12±0.03 Å corrugation of the surface layer. This shift of the surface cations was not followed by a corresponding displacement of the underlying anions, so the first interlayer spacing, d(Cl 2–Na 1), was reduced to 2.72±0.03 Å from its bulk value of 2.82 Å. Deviations from the bulk were smaller in subsequent layers, e.g. the second interlayer spacing, d(Na 3–Cl 2), was 2.77±0.03 Å, and there was essentially no buckling of the second layer.