Photo-dissociation of naphthalene dimer cations stored in a compact electrostatic ion storage ring.

Naphthalene dimer cations [C10H8]2 + have been produced by using an electron cyclotron resonance plasma ion source and stored in a compact electrostatic ion storage ring. We show that the radiative cooling of these cations is much slower than the isolated monomer naphthalene cations. We also report on photo-dissociation studies in the gas phase of naphthalene dimer cations at high internal energy. The dissociation energy is estimated to 0.5 eV in close agreement with previous measurements but a factor of 2 smaller than recent (density functional theory (DFT) and ab initio) theoretical studies. As uncertainties on theory as well as on the experiment cannot be as large as this difference, we conclude that this discrepancy may be due to temperature effects with possible isomerization. As an interpretation of the photo-dissociation spectrum of naphthalene dimer cations, we propose a tentative simple analytical model based on effective Morse potentials. These effective potentials are expected to "average" temperature effects that would apparently result in a smaller energy difference between the fundamental and dissociation states due to the twisting vibration modes of the naphthalene dimer cations.

Fragmentation dynamics of meso-tetraphenyl iron (III) porphyrin chloride dication under energy control.

Meso-tetraphenyl iron (III) porphyrin chloride dications (FeTPPCl2+)* were prepared in collisions with F+ and H+ at 3 keV. The dominant fragmentation channels were observed to involve the loss of the Cl atom and the successive loss of neutral phenyl groups for both collisional systems. The mass spectra in correlation with the deposited excitation energy distributions of the parent ions for the main fragmentation channels were measured by using the collision induced dissociation under energy control method. The global excitation energy distribution was found to be shifted to lower energies in collisions with H+ compared to collisions with F+ showing a noteworthy change of the excitation energy window using different projectile ions. Partial excitation energy distributions of the parent ions FeTPPCl2+ were obtained for each fragmentation group. In a theoretical work, we have calculated the dissociation energies for the loss of one and two phenyl groups, including phenyl and (phenyl ± H). The energy barrier for the hydrogen atom transfer during the loss of (phenyl-H) has been also calculated. The measured energy difference for the successive loss of two phenyl groups was compared with the theoretical values.

IRMPD Spectroscopy Sheds New (Infrared) Light on the Sulfate Pattern of Carbohydrates.

IR spectroscopy of gas-phase ions is proposed to resolve positional isomers of sulfated carbohydrates. Mass spectrometric fingerprints and gas-phase vibrational spectra in the near and mid-IR regions were obtained for sulfated monosaccharides, yielding unambiguous signatures of sulfated isomers. We report the first systematic exploration of the biologically relevant but notoriously challenging deprotonated state in the near IR region. Remarkably, anions displayed very atypical vibrational profiles, which challenge the well-established DFT (Density Functionnal Theory) modeling. The proposed approach was used to elucidate the sulfate patterns in glycosaminoglycans, a ubiquitous class of mammalian carbohydrates, which is regarded as a major challenge in carbohydrate structural analysis. Isomeric glycosaminoglycan disaccharides from heparin and chondroitin sources were resolved, highlighting the potential of infrared multiple photon dissociation spectroscopy as a novel structural tool for carbohydrates.

Photodissociation of Trapped Rb_{2}

The direct photodissociation of trapped ^{85}Rb_{2}^{+} (rubidium) molecular ions by the cooling light for the ^{85}Rb magneto-optical trap (MOT) is studied, both experimentally and theoretically. Vibrationally excited Rb_{2}^{+} ions are created by photoionization of Rb_{2} molecules formed photoassociatively in the Rb MOT and are trapped in a modified spherical Paul trap. The decay rate of the trapped Rb_{2}^{+} ion signal in the presence of the MOT cooling light is measured and agreement with our calculated rates for molecular ion photodissociation is observed. The photodissociation mechanism due to the MOT light is expected to be active and therefore universal for all homonuclear diatomic alkali metal molecular ions.

First principles calculations on nitrogen reactivity on tungsten surfaces.

Using the spin-polarized gradient-corrected density functional theory we investigate the adsorption and dissociation of nitrogen molecule and hydrogen nitride radicals on two model surfaces of tungsten, W{1 0 0} and W{1 1 0}. The goal of the investigations is to predict the number and nature of the nitrogenized moieties that could be found on a tungsten surface after nitrogen-seeded hydrogen (deuterium) plasma discharge. The surfaces are considered clean or saturated in hydrogen or nitrogen atoms as they are expected to be after nitrogen seeded hydrogen plasma irradiation. We find that the radicals NH, NH2 and NH3 are dissociated by the catalytic action of the metallic surfaces if they are animated with an initial kinetic energy of less than 2.5 eV. The products of the reaction are a nitrogen atom incorporated into the metal surface and an adsorbed proton. The general conclusion is that nitrogen reduces hydrogen retention on the surface and more generally that the nitrided surfaces should be less reactive than the clean tungsten even if the two surfaces we consider behave quite differently.

Hydrogen retention in beryllium: concentration effect and nanocrystalline growth.

We herein report on the formation of BeD2 nanocrystalline domes on the surface of a beryllium sample exposed to energetic deuterium ions. A polycrystalline beryllium sample was exposed to D ions at 2 keV/atom leading to laterally averaged deuterium areal densities up to 3.5 10(17) D cm(-2), and studied using nuclear reaction analysis, Raman microscopy, atomic force microscopy, optical microscopy and quantum calculations. Incorporating D in beryllium generates a tensile stress that reaches a plateau at ≈1.5 10(17) D cm(-2). For values higher than 2.0 10(17) cm(-2), we observed the growth of ≈90 nm high dendrites, covering up to 10% of the surface in some zones of the sample when the deuterium concentration was 3 × 10(17) D cm(-2). These dendrites are composed of crystalline BeD2, as evidenced by Raman microscopy and quantum calculations. They are candidates to explain low temperature thermal desorption spectroscopy peaks observed when bombarding Be samples with D ions with fluencies higher than 1.2 10(17) D cm(-2).

Hydrogen retention and diffusion in tungsten beryllide.

Beryllide compounds are often used in various domains because they are more resilient to oxidation than pure beryllium and at the same time they keep some of the properties of this metal. Nevertheless, the data about their properties during atomic hydrogen exposure are very scarce: numerous experiments have been conducted in the past few years on solid hydride deposition under beryllium-seeded plasma action or on energetic hydrogen implantation into metallic beryllium; many others have been devoted to hydrogen retention and diffusion in tungsten. There have been fewer studies about hydrogen interaction with the alloys of these metals, although the beryllium-tungsten mixed compounds have been experimentally detected in laboratory experiments. This article reports on calculations carried out using first-principles density functional theory (DFT) on tungsten beryllide crystal (Be12W) taken as a model alloy. The formation and reactivity of atomic vacancies are investigated in the domain of temperature ranging from 0 to 500 K, together with atomic hydrogen retention and diffusivity in the bulk and in/out vacancies.

Nitrogen reactivity toward beryllium: surface reactions.

Recent experiments with nitrogen as a seeding gas in fusion plasma devices together with the option of using beryllium as an armor material in the future ITER tokamak (International Thermonuclear Experimental Reactor) have raised new interest in the interactions of beryllium surfaces with nitrogen (atomic or molecular). The strong reactivity of nitrogen implies the formation of beryllium nitrite and, in conjunction with oxygen and other possible impurities, experimentalists have to consider the probability of generating various complex moieties such as imine, amine or oxyamine, and amide radicals. This chemistry would obviously dramatically perturb the plasma, and quantum investigations can be of great predictive help. Nitrogen adsorption on beryllium basal surfaces is investigated through quantum density functional theory. Different situations are examined: molecular or atomic nitrogen reactions; nitride radical adsorption or formation on surfaces; hydrogen retention on surfaces; combined nitrogen/oxygen reactivity and hydrogen retention. A tentative comparison with experiment is also proposed.

Deuterium uptake in magnetic-fusion devices with lithium-conditioned carbon walls.

Lithium wall conditioning has lowered hydrogenic recycling and dramatically improved plasma performance in many magnetic-fusion devices. In this Letter, we report quantum-classical atomistic simulations and laboratory experiments that elucidate the roles of lithium and oxygen in the uptake of hydrogen in amorphous carbon. Surprisingly, we show that lithium creates a high oxygen concentration on a carbon surface when bombarded by deuterium. Furthermore, surface oxygen, rather than lithium, plays the key role in trapping hydrogen.

Combined electrospray ionization source with a velocity map imaging spectrometer for studying large gas phase molecular ions.

We present a new compact and versatile experimental set-up that has been designed to perform electron and ion imaging experiments on large multiply charged gas phase molecular and cluster species. It combines an electrospray ionization source, a quadrupole mass filter guiding ion optics and a velocity map imaging spectrometer. Characterization of the spectrometer has been performed on atomic ions. Results obtained on molecular species (stilbene 420 dianions) demonstrate the possibility offered by this experimental set-up.

Fragmentation of singly charged adenine induced by neutral fluorine beam impact at 3 keV.

The fragmentation scheme of singly charged adenine molecule (H(5)C(5)N(5)(+)) has been studied via neutral fluorine impact at 3 keV. By analyzing in correlation the kinetic energy loss of the scattered projectile F(-) produced in single charge transfer process and the mass of the charged fragments, the excitation energy distribution of the parent adenine molecular ions has been determined for each of the main dissociation channels. Several fragmentation pathways unrevealed in standard mass spectra or in appearance energy measurements are investigated. Regarding the well-known hydrogen cyanide (HCN) loss sequence, we demonstrate that although the loss of a HCN is the dominant decay channel for the parent H(5)C(5)N(5)(+) (m = 135), the decay of the first daughter ion H(4)C(4)N(4)(+) (m = 108) involves not only the HNC (m = 27) loss but also the symmetric breakdown into two dimers of HCN.

Ab initio and long-range investigation of the Ω((+∕-)) states of NaK dissociating adiabatically up to Na(3s 2S(1/2)) + K(3d 2D(3/2)).

A theoretical investigation of the electronic structure of the NaK molecule including spin-orbit effects has been performed for the 34 Ω((+∕-)) states dissociating adiabatically into the limits up to Na(3s(2)S(1/2)) + K(3d(2)D(3/2)) from both an ab initio approach and a long-range model. Equilibrium distances, transition energies, harmonic frequencies as well as depths of wells and heights of humps are reported for all the states. Formulas for calculating the long-range energies for all the 0(+∕-), 1, 2, and 3 states under investigation are also displayed. They are expressed in terms of the C(n) (n = 6,8, ...) long-range coefficients and exchange integrals for the (2S+1)Λ((+)) parent states, available from literature. As present data could help experimentalists we make available extensive tables of energy values versus internuclear distances in our database at the web address: http://www-lasim.univ-lyon1.fr/spip.php?rubrique99.

Study of the sticking of a hydrogen atom on a graphite surface using a mixed classical-quantum dynamics method.

The sticking of one hydrogen atom chemisorbed on the (0001) graphite surface is investigated using a mixed classical-quantum method. The phonon modes of the system in the collinear scattering approach are included in the dynamics calculations. The vibrational degrees of freedom of the surface (phonons) are treated classically, while the H-surface motion is treated using a one-dimensional quantum wave packet propagation method. The sticking probabilities are calculated and the individual contributions of the phonon bands to the collision dynamics are analyzed for surface temperatures of 10, 150, and 300 K and hydrogen kinetic energies ranging from 0.13 to 1.08 eV. An analytical form of the sticking probability as a function of the surface temperature is also proposed.

Theoretical investigation of the Omega(g,u)(+/-) states of K2 dissociating adiabatically up to K(4p 2P(3/2)) + K(4p 2P(3/2)).

A theoretical investigation of the electronic structure of the K(2) molecule, including spin-orbit effects, has been performed. Potential energies have been calculated over a large range of R up to 75a(0) for the 88 Omega(g,u)(+/-) states dissociating adiabatically into the limits up to K(4p (2)P(3/2))+K(4p (2)P(3/2)). Equilibrium distances, transition energies, harmonic frequencies, as well as depths for wells and heights for barriers are reported for all of the bound Omega(g,u)(+/-) states. Present ab initio calculations are shown to be able to reproduce quite accurately the small structures (wells and barrier) displayed at very long-range (R>50a(0)) by the (2,3)1(u) and (2)0(g)(-) purely long-range states. As the present data could help experimentalists, we make available extensive tables of energy values versus internuclear distances in our database at the web address http://www-lasim.univ-lyon1.fr/spip.php?rubrique99.

Quantum modeling (DFT) and experimental investigation of beryllium-tungsten alloy formation.

Beryllium, tungsten and carbon are planned as wall-cladding materials for the future international tokamak ITER. Be and W will be the dominant components and therefore the formation of binary Be-W alloys under plasma action is one of the most important issues in plasma-wall interaction processes at the first wall. This paper proposes a first-principles density functional theory (DFT) study of beryllium atom retention in tungsten, and a discussion of the results in relation to the available experimental data. In a first step, the beryllium adsorption energy is calculated on the W(100) and W(111) surfaces. Further, the activation barrier for the surface-subsurface diffusion step and subsequent bulk diffusion steps are considered. For each calculation, the electronic structure of the formed compound is analyzed through projected density of states (DOS) calculations.

Theoretical calculation of the low lying electronic states of the molecular ion RbH(+) with spin-orbit effects.

The potential energy has been calculated for the 42 lowest electronic states of symmetries Omega=12,32,52, for the molecular ion RbH(+). Using an ab initio method, the calculation is based on nonempirical pseudopotentials and parametrized [script-l]-dependent polarization potentials. Gaussian basis sets have been used for both atoms, and spin-orbit effects have been taken into account. The spectroscopic constants for 19 electronic states have been calculated by fitting the calculated energy values to a polynomial in terms of the internuclear distance r. The permanent dipole moment and the transition dipole moments have been calculated for the considered Omega states. Through the canonical functions approach the eigenvalue E(v), the abscissas of the corresponding turning points (r(min) and r(max)) and the rotational constants B(v) have been calculated. The comparison of the present results with those available in literature shows a very good agreement.

Quantum dynamic of sticking of a H atom on a graphite surface.

A quantum study of the sticking of a hydrogen atom chemisorbed onto graphite (0001) surface was carried out also including the phonon modes of the system in the collinear scattering approximation. A new model was developed to extract the substrate vibrational modes from density functional theory (DFT) calculation and include them in the total system dynamics. The resulting coupled-channel equations are numerically developed along time using the wave packet methods. The sticking coefficients are calculated for hydrogen atoms incident energies ranging from 0.17 and 1.3 eV for a surface temperature of 10 K and between 0.17 and 0.2 eV for a surface temperature of 150 K. The results are found to be in good agreement with the experimental work.

Quantum chemistry-based NMR spin Hamiltonian parameters of GABA for quantitation in magnetic resonance spectroscopy.

Chemical shifts delta and spin-spin coupling constants J have been calculated using quantum chemistry approaches for the gamma-amino butyric acid GABA which is a brain metabolite. Two theoretical methods HF and DFT/B3LYP, two basis sets 6-31G* and 6-311+G(2d,p) and two gauge-invariant methods CSGT and GIAO have been used. From delta and J values, NMR spectra have been obtained from the strongly coupled spin system Hamiltonian using the NMR-SCOPE package. Solvent effects have been considered within the polarisable continuum model. Comparisons between calculated and experimental NMR spectra at 300 MHz show that our best results correspond to the B3LYP/6-311+G(2d,p)-GIAO calculations. They are seen to be in good agreement with experiment. This demonstrates the usefulness of quantum chemistry methods for estimating NMR spin Hamiltonian parameters involved in specific algorithms used for quantitation of metabolites such as GABA.

Theoretical calculation of the low laying electronic states of the molecule NaCs with spin-orbit effect.

The potential energy curves have been calculated for the 59 lowest electronic states of the molecule NaCs including the spin-orbit effect within the range of 4.5a(0)-20.0a(0) of the internuclear distance R. Using an ab initio method, the calculation is based on a nonempirical pseudopotentials which take into consideration the spin-orbit effect. Gaussian basis sets have been used for both atoms, and the spin-orbit effects have been taken into consideration. The spectroscopic constants have been calculated for 56 electronic states. The components of the spin-orbit splitting have been identified for the states (1,2,4)(3)Pi. The comparison of the present results with those available in the literature shows a very good agreement.

Theoretical study of mixed silicon-lithium clusters Si(n)Li(p)(+) (n=1-6, p=1-2).

Theoretical study on the structures of neutral and singly charged Si(n)Li(p)((+)) (n=1-6, p=1-2) clusters have been carried out in the framework of the density functional theory (DFT) with the B3LYP functional. The structures of the neutral Si(n)Li(p) and cationic Si(n)Li(p)(+) clusters are found to keep the frame of the corresponding Si(n), Li species being adsorbed at the surface. The localization of the lithium cation is not the same one as that of the neutral atom. The Li(+) ion is preferentially located on a Si atom, while the Li atom is preferentially attached at a bridge site. A clear parallelism between the structures of Si(n)Na(p) and those of Si(n)Li(p) appears. The population analysis show that the electronic structure of Si(n)Li(p) can be described as Si(n)(p)(-)+pLi(+) for the small sizes considered. Vertical and adiabatic ionization potentials, adsorption energies, as well as electric dipole moments and static dipolar polarizabilities, are calculated for each considered isomer of neutral species.