Calculation of transport coefficient profiles in modulation experiments as an inverse problem.

The calculation of transport profiles from experimental measurements belongs in the category of inverse problems which are known to come with issues of ill-conditioning or singularity. A reformulation of the calculation, the matricial approach, is proposed for periodically modulated experiments, within the context of the standard advection-diffusion model where these issues are related to the vanishing of the determinant of a 2×2 matrix. This sheds light on the accuracy of calculations with transport codes, and provides a path for a more precise assessment of the profiles and of the related uncertainty.

Frequency-fluctuation model applied to Stark-Zeeman spectral line shapes in plasmas.

A very fast method for calculating line shapes in the presence of an external magnetic field accounting for charge particle dynamics is proposed. It is based on a reformulation of the frequency fluctuation model, which provides an expression of the dynamic line shape as a functional of the static distribution function of frequencies. In the presence of an external magnetic field, the distribution of intensity and polarization of the emission depends on the angle between the observation line and the magnetic field's direction.

Hydrogen-graphite interaction: Experimental evidences of an adsorption barrier.

The interaction of H atoms having relatively low average kinetic energy (∼0.025 eV) with both perfectly clean and D-covered HOPG surfaces is investigated using high resolution electron energy loss spectroscopy. From this study we confirm, in a controlled fashion, the presence of the theoretically predicted adsorption barrier since no adsorption is detected for such H atoms on HOPG.

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.

New insights into the photodynamics of acetylacetone: isomerization and fragmentation in low-temperature matrixes.

UV and IR photoreactivities of acetylacetone isolated at 4.3 K in four matrixes (N(2), Ne, Ar, Xe), pure and doped with O(2) are investigated, using either tunable UV and IR optical parametric oscillators, or a broad band mercury lamp. Samples are probed by UV and FTIR spectroscopies: electronic and vibrational transitions are assigned and irradiation kinetics are analyzed.

Evidence from numerical simulations of transport-barrier relaxations in tokamak edge plasmas in the presence of electromagnetic fluctuations.

The dynamics of transport barriers in fusion plasmas is studied in the presence of electromagnetic fluctuations. The work is based on numerical simulations using a new three-dimensional electromagnetic fluid turbulence code (EMEDGE3D). In these simulations, the transport-barrier exhibits intermittent relaxation cycles.

Experimental and theoretical investigation of HC5N adsorption on amorphous ice surface: simulation of the interstellar chemistry.

HC 5N adsorbed on amorphous water ice at 10 K presents an interaction with the ice surface and induces the restructuring of the ice amorphous bulk. Warming up the sample induces the HC 5N desorption from the H 2O ice film, between 120 and 160 K, and the associated desorption energy is 90 kJ/mol. This value is in good agreement with that calculated E d (80 kJ/mol) and gives evidence that the amorphous ice surface is essentially dynamic.

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.

Formation of neutral methylcarbamic acid (CH3NHCOOH) and methylammonium methylcarbamate [CH3NH3+][CH3NHCO2-] at low temperature.

We study low temperature reactivity of methylamine (CH3NH2) and carbon dioxide (CO2) mixed within different ratios, using FTIR spectroscopy and mass spectrometry. We report experimental evidence that the methylammonium methylcarbamate [CH3NH3(+)][C3NHCO2(-)] and methylcarbamic acid (CH3NHCOOH) are formed when the initial mixture CH3NH2:CO2 is warmed up to temperatures above 40 K. An excess of CH3NH2 favors the carbamate formation while an excess of CO2 leads to a mixture of both methylammonium methylcarbamate and methylcarbamic acid.

C(5)N(-) anion and new carbenic isomers of cyanodiacetylene: a matrix isolation IR study.

Products of the vacuum-UV photolysis of cyanodiacetylene (HC(5)N) in solid argon -- the anion C(5)N(-), imine HNC(5), and the branched carbene C(4)(H)CN -- have been identified by IR absorption spectroscopy, in addition to the already discovered isonitrile HC(4)NC. Spectral assignments were assisted by deuterium substitution experiments, by BD(T) calculations, and by the results of a recent density functional theory study.

Multiscaling dynamics of impurity transport in drift-wave turbulence.

Intermittency effects and the associated multiscaling spectrum of exponents are investigated for impurities advection in tokamak edge plasmas. The two-dimensional Hasagawa-Wakatani model of resistive drift-wave turbulence is used as a paradigm to describe edge tokamak turbulence. Impurities are considered as a passive scalar advected by the plasma turbulent flow.

When can the Fokker-Planck equation describe anomalous or chaotic transport?

The Fokker-Planck equation, applied to transport processes in fusion plasmas, can model several anomalous features, including uphill transport, scaling of confinement time with system size, and convective propagation of externally induced perturbations. It can be justified for generic particle transport provided that there is enough randomness in the Hamiltonian describing the dynamics. Then, except for 1 degree of freedom, the two transport coefficients are largely independent.

Terahertz frequency standard based on three-photon coherent population trapping.

A scheme for a terahertz frequency standard based on three-photon coherent population trapping in stored ions is proposed. Assuming the propagation directions of the three lasers obey the phase matching condition, we show that stability of few 10(-14) at 1 s can be reached with a precision limited by power broadening to 10(-11) in the less favorable case. The referenced terahertz signal can be propagated over long distances, the useful information being carried by the relative frequency of the three optical photons.

Water/cyanobutadiyne complexes: an infrared matrix isolation and theoretical study.

The structures and energies of the 1:1 HC5N:H2O complexes in solid argon matrices have been investigated using FTIR spectroscopy and ab initio calculations, at the B3LYP/6-31G** and MP2/6-31G** levels of theory. Two types of 1:1 complexes are observed. The first one corresponds to the NH structure characterized by a hydrogen bond between H2O and the nitrogen of HC5N.

Spectroscopy of cyanodiacetylene in solid argon and the photochemical generation of isocyanodiacetylene.

Following the measurements of UV and mid-IR spectra of cyanodiacetylene, H-(CC)2-CN, isolated in low temperature Ar matrices, the first photochemical study on this compound and on its 2H isotopomer was carried out with the laser light tuned to 267 nm and with far-UV discharge lamps. Evidence for the formation of isocyanodiacetylene, H-(CC)2-NC, was found in infrared absorption spectra interpreted with the aid of available theoretical predictions.

UV and IR photoisomerization of acetylacetone trapped in a nitrogen matrix.

UV- and IR-induced photoisomerization of acetylacetone trapped in a nitrogen matrix at 4.3 K have been carried out using a tunable optical parametric oscillator type laser, or a mercury vapor lamp, coupled with Fourier Transform IR and UV spectroscopies. After deposition, the main form present in the cryogenic matrix is that chelated (enol).

Interstellar ice surface site modification induced by dicyanoacetylene adsorption.

Dicyanoacetylene adsorbed on amorphous ice water at 10 K presents an interaction with the dangling H site and induces a s(4) adsorption site formation due to the restructuring of the ice bulk. Warming up the sample provokes the dicyanoacetylene desorption from the H(2)O ice film, which could be due to the beginning of the ice crystallization process. The desorption activation energy measured by temperature-programmed desorption (E(d) = 42 +/- 5 kJ x mol(-1)) is in good agreement with that calculated (E(d) = 46 kJ x mol(-1)) and gives evidence of a hydrogen-bonded adsorbed state on amorphous ice films.

Breaking of ketene bonds during HCl addition to trimethylsilylketene.

IR spectroscopy is coupled with the matrix isolation technique to study the reaction of trimethylsilylketene with HCl. From 50 K trimethylsilylketene reacts with hydrogen chloride, leading to the cleavage of the Si-C bond and the formation of trimethylsilyl chloride and acetyl chloride, through intermediate trimethylsilylacetyl chloride which was identified. A reaction profile for this result is proposed based on a theoretical study carried out at the DFT level.

Photoreactivity of cyanoacetylene trapped in water ice: an infrared, isotopic and theoretical study.

Photoreactivity of cyanoacetylene with water was successively studied in cryogenic matrixes and in the solid phase at lambda > 120 nm. These studies were performed using FTIR spectroscopy, isotopic experiments and DFT calculations at the B3LYP/6-31G level of theory. The photolysis of cyanoacetylene complexed with water in an argon cryogenic matrix led to the formation of two products.

Vacuum ultraviolet (VUV) photodecomposition of urea isolated in cryogenic matrix: first detection of isourea.

Vacuum ultraviolet (VUV) irradiation at wavelengths of lambda > 160 nm of urea-h4 (NH2CONH2) and urea-d4 (ND2COND2) has been monitored by Fourier transform infrared spectroscopy in argon and xenon matrixes. Several primary photoproducts, such as HNCO:NH3 (isocyanic acid:ammonia), CO:N2H4 (carbon monoxide:hydrazine) molecular complexes, and isourea (H2N(OH)C=NH), which is reported for the first time, were characterized. The assignment of complexes was achieved by co-depositing the pairs of respective species, whereas the isourea identification was based on the comparison between the experimental and theoretical (B3LYP) infrared spectra.

Carbodiimide production from cyanamide by UV irradiation and thermal reaction on amorphous water ice.

Cyanamide (NH(2)CN), an interstellar molecule, is a relevant molecule in prebiotic chemistry, because it can be converted into urea in liquid water. Carbodiimide (HNCNH), the most stable cyanamide isomer, is able to assemble amino acids into peptides. In this work, using FTIR spectroscopy, we show that carbodiimide can be formed from cyanamide at low temperature (10 K), by a photochemical process in argon matrix, in water matrix, or in solid film.

UV photolysis of C4N2 in water ices: new possible route of synthesis of ammonium bicarbonate and ammonium formate.

Laboratory experiments involving ultraviolet (UV) irradiation of dicyanoacetylene (C(4)N(2)) trapped in water ice at 10 K have been conducted and monitored by infrared spectroscopy (FTIR). By the support of isotopic experiments and theoretical calculations, the irradiation of a DCA/H(2)O ice mixture at lambda > 230 nm has been found to be a possible source of NH(4)(+)HCO(3)(-) (ammonium bicarbonate) and NH(4)(+)HCOO(-) (ammonium formate). These latter compounds can arise from a proton-transfer reaction between H(2)O and the CN radical, which is issued from photolyzed C(4)N(2).

Experimental and theoretical UV characterizations of acetylacetone and its isomers.

Cryogenic matrix isolation experiments have allowed the measurement of the UV absorption spectra of the high-energy non-chelated isomers of acetylacetone, these isomers being produced by UV irradiation of the stable chelated form. Their identification has been done by coupling selective UV-induced isomerization, infrared spectroscopy, and harmonic vibrational frequency calculations using density functional theory. The relative energies of the chelated and non-chelated forms of acetylacetone in the S0 state have been obtained using density functional theory and coupled-cluster methods.

Photochemical synthesis of the cyanodiacetylene HC(5)N: a cryogenic matrix experiment.

The reactivity of intermolecular complexes cyanoacetylene:acetylene and dicyanoacetylene:acetylene, trapped in solid argon matrixes at 10 K and irradiated with vacuum UV, has been studied. FTIR measurements, together with (2)H, (15)N, (13)C labeling experiments and with density functional theory (DFT) calculations (B3LYP/aug-cc-pVTZ), pointed to the formation of cyanodiacetylene HC(5)N (cyanobutadiyne). This synthetic route is potentially important for chemical models of the Titan's atmosphere.

Hydrogen adsorption on graphite (0001) surface: a combined spectroscopy-density-functional-theory study.

The adsorption of H/D atoms on the graphite (0001) surface is investigated by means of both high-resolution electron-energy loss spectroscopy (HREELS) and periodic first-principle density-functional theory. The two methods converge towards two modes of adsorption: adsorption in clusters of about four hydrogen atoms and adsorption in pairs of atoms on contiguous carbon sites. The desorption energies estimated from the calculated dissociation energies range from 8 to 185 kJ mol(-1) leading to an estimated surface coverage at saturations of 30-44 at.