Ernest Ilisca

Paris Diderot University | UP7 · Matériaux et Phénomènes Quantiques (MPQ) UMR 7162

Microporous Composite Materials, made of metal oxide ionic fragments inserted in carbonaceous substrates, might be used as intermediates in hydrogen phase transitions, allowing progressive switch between their gas and liquid states. When crossed by an hydrogen flow these composite materials combine adsorbing, magnetic, catalytic and thermal propert...

The following conference report considers hydrogen gases with odd and even rotational quantum number as two separate gases, the ortho and para varieties which do not interconvert in absence of a catalyst. The physical catalysis of hydrogen is interpreted in terms of symmetry breakings introduced by the solid to pass round the peculiar selection rul...

Most of the hydrogen storage vessels meant for vehicles to run the electric motor via a fuel cell during transport are designed for drives of only a few tenths of kilometers per day. The present report, however, describes a vessel model that is conceived to hold the hydrogen energy only for short periods during transport, such as a few hours. This...

Hydrogen molecules exist in the form of two distinct isomers that can be interconverted by physical catalysis. These ortho and para forms have different thermodynamical properties. Over the last century, the catalysts developed to convert hydrogen from one form to another, in laboratories and industries, were magnetic and the interpretations relied...

Recent NMR experiments have measured the conversion rate of hydrogen molecules dissolved in paramagnetic organic solvents. The registered proton signals display negative peaks during an initial period. The present interpretation describes a polarization effect: the transient behavior of the nuclear spins directed on the average along the magnetic e...

The interpretation of the conversion patterns of molecular hydrogen adsorbed on amorphous solid water confirms the electromagnetic nature of the process. Magnetic spin-orbit coupling and hyperfine contact introduce some electron spin-orbital excitation in the H2-ASW ground state. The molecule-solid electron repulsion evacuates the H2 rotation momen...

The ortho-para conversion of hydrogen molecules oscillating inside tetrahedral cages of silicon compounds relies on the interaction of the nuclear protons with the silicon electrons. At each collision against the cage hard walls, the electron repulsion changes the molecular rotation while projecting a valence electron in the antibonding molecular s...

The hydrogen conversion patterns on non-magnetic solids sensitively depend upon the degree of singlet/triplet mixing in the intermediates of the catalytic reaction. Three main ‘symmetry-breaking’ interactions are brought together. In a typical channel, the electron spin–orbit (SO) couplings introduce some magnetic excitations in the non-magnetic so...

Models are provided and discussed to interpret new experiments on the ortho-para conversion of hydrogen “physisorbed” on dielectric and diamagnetic surfaces. Electro-static and dynamical molecule- surface interactions complemented by hyperfine contacts are shown to be generally more effective than the magnetic ones. Coulomb repulsion induces exchan...

Non-magnetic insulating catalysts are shown to be able to convert non-equilibrium mixtures of hydrogen by an electric mechanism. The molecular electrons feel the difference of temperature between the nuclei and the thermal bath, through their “Fermi” contact interaction, and consequently transfer the rotational angular momenta to the catalyst. This...

EELS experiments of H2 absorbed at low temperature on Noble Metals have indicated that the o-p H2 conversion can be very fast on metals, even in absence of chemisorption, in contradiction with the usual belief that the catalyst must be magnetic to be efficient. We examine a large family of processes which convert the hydrogen molecules on the basis...

The theoretical expressions of the hyperfine tensors must be modified, for molecu les with identical nuclei in equivalent positions. We illustrate the procedure by considering molecules with half-integer identical nuclear spins in C3V symmetry. Application of the Pauli Principle to these molecular spin systems reduces the degeneracy of the molecula...

A theoretical model of coherent control of nuclear-spin isomers by microwave radiation has been developed. The model accounts the M degeneracy of molecular states and molecular center-of-mass motion. The model has been applied to the 13CH3F molecules. Microwave radiation excites the para state (J=11, K=1) which is mixed by the nuclear-spin-spin int...

Theoretical model of the coherent control of nuclear spin isomers by microwave radiation has been developed. Model accounts the M-degeneracy of molecular states and molecular center-of-mass motion. The model has been applied to the 13 CH 3 F molecules. Microwave radiation excites the para state (J=11,K=1) which is mixed by the nuclear spin-spin int...

Localized and traveling modes are worked out for an infinite anharmonic atomic chain by implementing a shooting algorithm and the finite difference and Newton procedures, respectively. These methods unlike the other currently used ones yield exact solutions in simulations carried out by integrating the equation of motion under the relevant initial...

A theoretical analysis of nuclear-spin conversion in ethylene ${}^{13}{\mathrm{CCH}}_{4}$ has been performed. The conversion rate was found to be $$\simeq${}3\ifmmode\times\else\texttimes\fi{}{10}^{$-${}4}$ ${\mathrm{s}}^{$-${}1}/\mathrm{Torr},$ which is in qualitative agreement with the recently obtained experimental value. It was shown that the o...

The existence of bell- and kink-shaped solitons moving at constant velocity while keeping a permanent profile is studied in infinite periodic monoatomic chains of arbitrary anharmonicity by taking advantage of the equation of motion being integrable with respect to solitons. A second-order, non-linear differential equation involving advanced and re...

All bell- and kink-shaped solitons sustained by an infinite periodic atomic chain of arbitrary anharmonicity are worked out by solving a second-order, nonlinear differential equation involving advanced and retarded terms. The asymptotic time decay behaves exponentially or as a power law according to whether the potential has a harmonic limit or not...

The ortho - para conversion rate of hydrogen molecules physisorbed on transition metal oxides is shown to be drastically enhanced for selected values of the parameters. This new electron - nucleus resonant mechanism is suggested to interpret the observed accelerations of the conversion rate by an external magnetic field. The sensitivity of the magn...

A new electron-nucleus resonant mechanism is shown to increase the ortho-para conversion rate of hydrogen molecules adsorbed on transition metal oxides. This mechanism is suggested to interpret the observed conversion rate accelerations by an external magnetic field and similar effects in a variety of chemical reactions. Since the resonances occur...

EELS experiments of H2 physisorbed on noble metals at low temperature have indicated that the ortho-para (O-P) conversion can be very fast on non-magnetic catalysts. Two processes are described in which the molecular nuclear angular momenta are dissipated by the emission of metal electron-hole pairs through a metal-molecule spin-orbit coupling. Bot...

The orbital angular momenta of paramagnetic catalytic centres are shown to induce the ortho - para hydrogen conversion process. The considered hyperfine coupling includes the dipolar interactions between the two molecular nuclear spins and the orbital angular momenta of the catalyst electrons. The resulting orbital rates reach their magnetic dipole...

The role of the intramolecular spin-spin and the spin-rotation interactions in the level-crossing spectra of the (CH3F)-C-13 nuclear spin conversion is studied theoretically. we suggest an experiment to measure the nuclear spin conversion of (CH3F)-C-13 molecules in the presence of a strong electric field (6-7 kV cm(-1)). The Stark-induced rotation...

Different channels in the ortho-para conversion of hydrogen molecules physisorbed on a transition metal oxide surface are investigated. The Wigner process, which assumes catalysts to be point dipoles, is analysed and compared to the dipolar and contact processes, which include the orbital degrees of freedom of the surface electrons. Three possible...

The nuclear spin conversion of ${\mathrm{CH}}_{3}\mathrm{F}$ molecules induced by the intramolecular spin-rotation interactions is calculated and shown to be the leading process in ${}^{12}{\mathrm{CH}}_{3}\mathrm{F}.$ For ${}^{13}{\mathrm{CH}}_{3}\mathrm{F}$ molecules, it complements the spin conversion induced by spin-spin interactions. Compariso...

We predict the influence of an external homogeneous magnetic field on nuclear spin conversion in molecules. The effect arises either from the Zeeman splittings or from the magnetic shielding of the molecular nuclei. The magnitude of the effects in CH3F molecules is estimated. The Zeeman effect can be large. The shielding effect is found to be relat...

We consider an ABO3 perovskite surface in which a transition metal ion I is substituted to the cation B. An H2 molecule is assumed to sit on the top of the metal ion at a distance d = 4.5 b. We consider the perturbation of the ionic electron by the molecular electrons and protons and calculate the resulting molecule → ion charge transfer admixtures...

The magnetic interaction of a hydrogen molecule physisorbed on a chromia / alumina catalyst is presented and the influence of the Cr3d dangling bond on the ortho-para hydrogen conversion stressed. The Wigner process, which assumes catalyst point dipoles, is analyzed and compared to the magnetic dipolar process which includes the orbital degrees of...

Recent EELS experiments on hydrogen molecules adsorbed, at low temperature, on noble metals have indicated that the o-p conversion process on non-magnetic metal catalysts might be very efficient. The Wigner magnetic dipolar mechanism, used since 1933 to interpret experimental data, is thus clearly insufficient to account for these new features. A g...

The o–p conversion rate of H2 adsorbed or condensed on a Ag (111) surface, induced by “crystal-derived” (SS) and “barrier derived” (IS) surface states, is calculated. Owing to their protuberance far outside in the vacuum region, the image states are found to be very efficient in the conversion process. An experimental measure of these IS is therefo...

Electron-energy-loss experiments of H{2} physisorbed at low temperatures on a Ag(111) surface have indicated that the o-p H{2} conversion rate is very fast , in contradiction with the usual belief that the catalyst must be magnetic to be efficient. We examine a large family of processes valid for non-magnetic and non-dissociative metals which physi...

Electron-energy-loss experiments of H2 physisorbed on noble metals at low temperature indicate that the ortho-para conversion rate is 2 orders of magnitude faster on a Ag(111) surface than on a Cu(100) one. We suggest a process in which a metal electron is virtually transferred back and forth from a surface band to the molecle antibondoing orbital,...

In physcial molecular adsorption and catalysis the building of surface-molecule complexes includes surface symmetries and opens therefore new intra-molecular catalytic channels. We illustrate this general property by investigating the o-p conversion of a hydrogen molecule adsorbed on a cold metal surface. A large family of processes is described wi...

A new ortho-para conversion mechanism is suggested as a substitute for Wigner's which yields theoretical rates often more than two orders of magnitude smaller than the experimental ones. This mechanism is illustrated for catalysts composed of transition metal magnetic impurities dispersed on a diamagnetic surface. It relies on the hyperfine contact...

We indicate that an optical study of the ortho-para hydrogen magnetic conversion is possible for molecules adsorbed on a magnetic substrate. It is shown that photons are emitted during the catalytic conversion and that this emission, although weak, might be detected. We suggest that the optical pumping of an adsorbed hydrogen layer would strengthen...

A new channel in ortho-para hydrogen conversion is suggested as a tentative interpretation of large experimental reaction rates. This process occurs in two steps, via a virtually excited electronic state. The first step involves an exchange of electrons between the molecule and a nearby magnetic impurity while the second step is due to the hyperfin...

The existence conditions of localized states in linear chains with one or two impurities, are investigated in the tight-binding model. The influence of the coupling between impurity and its neighbour is stressed, in the cases of infinite and semi-infinite chains.

The dynamical evolution of a surface spin state submitted to repeated H2 molecular scattering is investigated by means of a stochastic model. When the striking molecules librate during their surface lifetime, it is found that the surface spins become fully polarized. In particular, the surface magnetization is enhanced by a factor of 100 when a mag...

A general quantum formulation of an ortho-para hydrogen conversion reaction on dilute paramagnetic catalysts by means of tensorial algebra is developed. An efficiency operator describing the tensorial structure of the conversion rate is built, and important selection rules pertaining to the tensorial ranks are derived. Taking into account the time...

The application of an external magnetic field, during the conversion of o-H2 into p-H2, mixes the catalyst paramagnetic impurity wave functions, opening, therefore, new o-p transitions of different symmetry. The dynamical polarization of either the surface electronic spins or the H2 nuclear spins, induced by the H2 rotational system, strongly enhan...

It is a well-known result that an external magnetic field may enhance catalytic activity. A tentative interpretation of this effect in the case of ortho-para hydrogen conversion is given. When the energy of activation is of magnetic origin, the conversion rate is found to be related to the structure of the ferromagnetic domains (or to the magnetic...

A theoretical study of ortho-para hydrogen conversion on paramagnetic surfaces is given. In the absorbed layer, hydrogen molecules are assumed to behave like plane rotators, and their translation at the surface is described by means of a two-dimensional diffusion equation. The spatial dependence of the dipolar matrix elements is found to be importa...

The spin-wave spectrum of a Heisenberg ferromagnet is derived in the framework of the method of De Wames and Wolfram, with the next-nearest-neighbor coupling, the surface and bulk exchange, and the surface anisotropy taken into account. The calculations are performed for a {100} surface in a simple-cubic structure. When the next-nearest-neighbor co...

A simple theoretical semiclassical calculation of the surface ferromagnetic equilibrium is given. The equilibrium orientation of the magnetization in the layers which are near the surface is tilted from the direction of the magnetization in the bulk. This rotation depends on both surface and bulk anisotropies. Thus, some instabilities in the spectr...

The energies of the surface spin waves are calculated. By taking into account surface anisotropy, the existence the nature and the shape of the solutions obtained are discussed by means of Six parameters of surface exchange and anisotropy. The case where the orientation of the surface magnetization differs from the bulk one is also investigated as...

Surface spin-wave modes are calculated for (100) surface of simple cubic structure. An isotropic part as well as an anisotropic part are introduced in the spin Hamiltonian. The anisotropic factor is allowed to vary on the first two surface layers. Dispersion curves are derived by a Green function formalism.

A simple explanation is given of the activity of ferromagnetic catalysts and of the increase in the rate of conversion observed at low temperature by Buyanov. In contrast to preceding interpretations the conclusion is reached that ferromagnetism can enhance catalytic activity. Research sponsored in part by the Air Force Office of Scientific Researc...

Surface spin-wave modes are calculated for (100) surfaces of simple cubic structure. In contradiction to De Wames and Wolfram results, in most cases no optical branch is found in the surface spin-wave spectrum.

Evidence is presented for a new effect in ortho-para H2 conversion on magnetic solid surfaces at low temperature. This effect is a resonant exchange of energy between the spins of the magnetic impurities and H2 molecules which are absorbed in their vicinity. This process could explain the strong activities of many catalysts.

"Article principal recouvrant en partie." Thesis (doctoral)--Université de Paris VII, 1971.

Electron-energy-loss experiments of H$_{2}$ physisorbed at low temperatures on a Ag(111) surface have indicated that the o-p H$_{2}$ conversion rate is very fast , in contradiction with the usual belief that the catalyst must be magnetic to be efficient. We examine a large family of processes valid for non-magnetic and non-dissociative metals which...