Bruno Lepetit

The H3+ rovibrational spectrum revisited with a global electronic potential energy surface

In this paper, we have computed the rovibrational spectrum of the H(3) (+) molecule using a new global potential energy surface, invariant under all permutations of the nuclei, that includes the long range electrostatic interactions analytically. The energy levels are obtained by a variational calculation using hyperspherical coordinates. From the comparison with available experimental results for low lying levels, we conclude that our accuracy is of the order of 0.1 cm(-1) for states localized in the vicinity of equilateral triangular configurations of the nuclei, and changes to the order of 1 cm(-1) when the system is distorted away from equilateral configurations. Full rovibrational spectra up to the H(+)+H(2) dissociation energy limit have been computed. The statistical properties of this spectrum (nearest neighbor distribution and spectral rigidity) show the quantum signature of classical chaos and are consistent with random matrix theory. On the other hand, the correlation function, even when convoluted with a smoothing function, exhibits oscillations which are not described by random matrix theory. We discuss a possible similarity between these oscillations and the ones observed experimentally.

Quantum studies of H atom trapping on a graphite surface

The trapping and sticking of H and D atoms on the graphite (0001) surface is examined, over the energy range of 0.1-0.9 eV. For hydrogen to chemisorb onto graphite, the bonding carbon must pucker out of the surface plane by several tenths of an angstrom. A quantum approach in which both the hydrogen and the bonding carbon atoms can move is used to model the trapping, and a potential energy surface based on density functional theory calculations is employed. It is found, for energies not too far above the 0.2 eV barrier to chemisorption that a significant fraction of the incident H or D atoms can trap. The forces on the bonding carbon are large, and it can reconstruct within 50 fs or so. After about 100 fs, most of the trapped H atoms scatter back into the gas phase, but the 5%-10% that remain can have lifetimes on the order of a picosecond or more. Calculations of the resonance eigenstates and lifetimes confirm this. An additional lattice degree of freedom is included quantum mechanically and is shown to significantly increase the amount of H that remains trapped after 1 ps. Further increasing the incident energy destabilizes the trapped state, leading to less H remaining trapped at long times. We estimate that for a full dissipative bath, the sticking probabilities should be on the order of 0.1.

Ar⋯I2: A model system for complex dynamics

We review spectroscopic and photodissociation dynamical studies in the region of the B L X transition of the Ar ··· I 2 van der Waals complex, both below and above the dissociation limit of the state. This very simple system constitutes a prototype for a wide range of molecular processes: vibrational predissociation involving intramolecular vibrational relaxation, electronic predissociation, cage effect, …. Each of these processes has been or still is the subject of differing interpretations: intramolecular vibrational relaxation involved in the vibrational predissociation of this system can be in the sparse or statistical regime, vibrational and electronic predissociation are in competition and a direct, ballistic interpretation of the cage effect as well as a non-adiabatic one have been proposed. The study of the dependence of these dynamical processes on the relative orientation of the two partners of the complex (stereodynamics) is made possible by the coexistence of two stable Ar ··· I 2 ) isomers. Experimental as well as theoretical results are reviewed. Experiments range from frequency-resolved to time-dependent studies, including the determination of final state distributions. Theoretical studies involve potential energy surface calculations for several electronic states of the complex and their couplings and adiabatic as well as non-adiabatic dynamical simulations.

Proton Zemach radius from measurements of the hyperfine splitting of hydrogen and muonic hydrogen

While measurements of the hyperfine structure of hydrogen-like atoms are traditionally regarded as test of bound-state QED, we assume that theoretical QED predictions are accurate and discuss the information about the electromagnetic structure of protons that could be extracted from the experimental values of the ground state hyperfine splitting in hydrogen and muonic hydrogen.Using recent theoretical results on the proton polarizability effects and the experimentalhydrogen hyperfine splitting we obtain for the Zemach radius of the proton the value 1.040(16) fm. We compare it to the various theoretical estimatesthe uncertainty of which is shown to be larger that 0.016 fm.This point of view gives quite convincing arguments in support of projects to measure the hyperfine splitting of muonic hydrogen.

Calculation of argon trimer rovibrational spectrum

Rovibrational spectra of Ar3 are computed for total angular momenta up to J=6 using row-orthonormal hyperspherical coordinates and an expansion of the wave function on hyperspherical harmonics. The sensitivity of the spectra to the two-body potential and to the three-body corrections is analyzed. First, the best available semiempirical pair potential (HFDID1) is compared with our recent ab initio two-body potential. The ab initio vibrational energies are typically 1-2 cm-1 higher than the semiempirical ones, which is related to the slightly larger dissociation energy of the semiempirical potential. Then, the Axilrod-Teller asymptotic expansion of the three-body correction is compared with our newly developed ab initio three-body potential. The difference is found smaller than 0.3 cm-1. In addition, we define approximate quantum numbers to describe the vibration and rotation of the system. The vibration is represented by a hyper-radial mode and a two-degree-of-freedom hyperangular mode, including a vibrational angular momentum defined in an Eckart frame. The rotation is described by the total angular momentum quantum number, its projection on the axis perpendicular to the molecular plane, and a hyperangular internal momentum quantum number, related to the vibrational angular momentum by a transformation between Eckart and principal-axes-of-inertia frames. These quantum numbers provide a qualitative understanding of the spectra and, in particular, of the impact of the nuclear permutational symmetry of the system (bosonic with zero nuclear spin). Rotational constants are extracted from the spectra and are shown to be accurate only for the ground hyperangular mode.

ArI2(X)→Ar+I2(B) photodissociation: Comparison between linear and T-shaped isomers dynamics

Quantum dynamical calculations on ArI2 photodissociation have been performed using ab initio and semi-empirical potential energy surfaces, which support both linear and T-shaped isomers in the ground electronic state. Whereas the photon absorption spectra for the T-shaped isomer consist of narrow and intense bands, those for the linear isomer result from the superposition of a continuous background and peaks due to linear quasi-bound states. Vibrational distributions for the linear isomer are broader than those originating from the T-shaped one. Rotational distributions for the linear isomer are smooth and characteristic of a fast dissociation dynamics, whereas those for the T-shaped isomer are highly oscillatory. Implications of these results on the interpretation of experimental data are discussed.

R&D around a photoneutralizer-based NBI system (Siphore) in view of a DEMO Tokamak steady state fusion reactor

ince the signature of the ITER treaty in 2006, a new research programme targeting the emergence of a new generation of neutral beam (NB) system for the future fusion reactor (DEMO Tokamak) has been underway between several laboratories in Europe. The specifications required to operate a NB system on DEMO are very demanding: the system has to provide plasma heating, current drive and plasma control at a very high level of power (up to 150 MW) and energy (1 or 2 MeV), including high performances in term of wall-plug efficiency (η  >  60%), high availability and reliability. To this aim, a novel NB concept based on the photodetachment of the energetic negative ion beam is under study. The keystone of this new concept is the achievement of a photoneutralizer where a high power photon flux (~3 MW) generated within a Fabry–Perot cavity will overlap, cross and partially photodetach the intense negative ion beam accelerated at high energy (1 or 2 MeV). The aspect ratio of the beam-line (source, accelerator, etc) is specifically designed to maximize the overlap of the photon beam with the ion beam. It is shown that such a photoneutralized based NB system would have the capability to provide several tens of MW of D0 per beam line with a wall-plug efficiency higher than 60%. A feasibility study of the concept has been launched between different laboratories to address the different physics aspects, i.e. negative ion source, plasma modelling, ion accelerator simulation, photoneutralization and high voltage holding under vacuum. The paper describes the present status of the project and the main achievements of the developments in laboratories.

Ab initio study of methyl-bromide photodissociation in the A band.

We performed a theoretical study of the photodissociation dynamics of CH(3)Br in the A band using a wave packet propagation technique on coupled ab initio potential energy curves. The present model involves the (3)Q(1) and (1)Q(1) excited states which can be populated from the ground state by a perpendicular transition and which are correlated at large methyl-bromide distance to the ground bromide spin-orbit state, as well as the (3)Q(0) and 4E states which can be excited by a parallel and perpendicular transition (respectively) and both correlate to excited Br(*) spin-orbit state. The model provides absorption cross sections and branching ratios in excellent agreement with experimental results. Due to weak spin-orbit interaction, the (1)Q(1) state is the dominant contributor to the absorption cross section, except for the red wing of the band where (3)Q(0) and (3)Q(1) states have significant absorption. However, spin-orbit coupling is strong enough to induce nonadiabatic transitions between the (3)Q(1) and (1)Q(1) states during the dissociation process which should be experimentally detectable in the alignment properties of the fragments. Nonadiabatic transitions at the conical intersection between (3)Q(0) and (1)Q(1) are shown to play a minor role in this system.

Electronic and vibrational predissociation in ArI2 photodissociation dynamics

Quantum dynamical calculations on the photodissociation process: ArI2(X)+hν→Ar+I2(B) or Ar+I+I have been performed using diatomics-in-molecule semiempirical potential energy surfaces in the spectral region of the I2(B,v=15–25)←I2(X,v=0) transition. The B state responsible for vibrational predissociation producing Ar+I2(B) is coupled to four dissociative states inducing electronic predissociation to Ar+I(2P3/2)+I(2P3/2). These dissociative states correlate to the a(1g), a′(0g+), B″(1u), 1(2g) electronic states of I2. Both linear and perpendicular initial ArI2(X) isomers are considered. For the linear isomer, only the a′ state has non-negligible effect on photodissociation dynamics, although total photon absorption cross sections are not significantly modified when coupling to a′ is taken into account, partial cross sections corresponding to vibrational predissociation are smaller. For the perpendicular isomer, resonance decay rates are increased, mainly by the coupling to a′(0g+), 1(2g), and a(1g) states. ...

Sticking and desorption of hydrogen on graphite: a comparative study of different models.

We study the physisorption of atomic hydrogen on graphitic surfaces with four different quantum mechanical methods: perturbation and effective Hamiltonian theories, close coupling wavepacket, and reduced density matrix propagation methods. Corrugation is included in the modeling of the surface. Sticking is a fast process which is well described by all methods. Sticking probabilities are of the order of a few percent in the collision energy range 0-25 meV, but are enhanced for collision energies close to those of diffraction resonances. Sticking also increases with surface temperature. Desorption is a slow process which involves multiphonon processes. We show, however, how to correct the close coupling wavepacket method to account for such phenomena and obtain correct time constants for initial state decay. Desorption time constants are in the range of 20-50 ps for a surface temperature of 300 K.