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The reaction between atomic carbon and acetylene has been investigated using complementary crossed molecular beam techniques. Differential cross sections have been obtained for the reactions of both ground and excited carbon atoms, C(³P[sub J], ¹D[sub 2])+ C[sub 2]H[sub 2](X ¹Σ[sup +, sub g], in experiments conducted with continuous supersonic beams, mass spectrometric detection, and time-of-flight analysis at a relative translational energy of 29.3 kJmol[sup -1]. The reaction C(³P[sub J]) + C[sub 2]H[sub 2](X ¹Σ[sup +, sub g]) has been found to lead to C[sub 3]H+H and C[sub 3]+H[sub 2] products in comparable amounts. Both H and H[sub 2] elimination pathways are found to proceed through the formation of a C[sub 3]H[sub 2] long-lived intermediate complex whose lifetime may be comparable to its rotational period. The spin-forbidden H[sub 2] elimination channel is attributed to the occurrence of intersystem-crossing between the triplet and singlet manifolds of the C[sub 3]H[sub 2] potential-energy surfaces. The reaction C(¹D[sub 2])+C[sub 2]H[sub 2](X ¹Σ[sup +, sub g]) has been found to lead to formation of C[sub 3]H+H, with a C[sub 3]H center-of-mass angular distribution strongly forward peaked, indicating a short-lived intermediate complex. Integral cross sections have been obtained for the C(³P[sub J]j) + C[sub 2]H[sub 2] (X &sup1Σ[sup +, sub g]) → C[sub 3] H + H(²S[sub ½]) reaction in experiments conducted with pulsed, supersonic molecular beams in the range of relative translational energies E[sub T]=0.38 -25.5 kJ mol[sup -1], the H(²S[sub ½) product being detected by laser-induced fluorescence. The reaction has been found to be without a barrier, relative integral cross sections being proportional to (Er)[sup -0.80&plumsn;0.03] below E[sub T]=10 kJmol[sup -1]. These findings provide direct evidence that the C(³P[sub J]) +C[sub 2]H[sub 2] reaction can occur under... [ABSTRACT FROM AUTHOR]

A joint crossed beam and quantum mechanical investigation of the rotationally inelastic collisions of CO with ortho- and para-D2 molecules is reported. A new 4D potential energy surface (PES) averaged over the ground vibrational states of D2 and CO is used to calculate the rovibrational bound states of the ortho-D2–CO complexes. Close coupling calculations are then performed in the rigid rotor approximation for ortho- and para-D2 colliding with CO for the experimentally investigated transition of CO (j = 0 → 1) and for collision energies ranging from 0.1 to 25 cm−1. The agreement between theory and experiment is found to be very good for both the bound state energies of the ortho-D2–CO complexes and for the inelastic scattering cross-sections showing that the free rotation of two rigid rotors is a very good model of the D2–CO system in this low collision energy domain. [ABSTRACT FROM AUTHOR]

Context. Oxygen molecules (O2) are of particular interest because of their crucial role in astrochemisty. Modelling of O2 molecular emission spectra from interstellar clouds requires the calculation of rate coefficients for excitation by collisions with the most abundant species. Aims. Rotational excitation of O2(X3Σ -) by H2 is investigated theoretically and experimentally and we check the validity of the local thermodynamic equilibrium (LTE) approach for interpreting O2 observations. Methods. Using a new ab initio potential energy surface, collisional excitation of O2 is studied using a full close-coupling approach. The theoretical calculations are validated by comparison with crossed beam scattering experiments. We also performed calculations for the excitation of O2 from a large velocity gradient (LVG) radiative transfer code using the new rate coefficients. Results. State-to-state rate coefficients between the 27 lowest levels of O2 were calculated for temperatures ranging from 5 K to 150 K. The critical densities of the O2 lines are found to be at ≳104 cm-3 for temperatures higher than 50 K. This value is slightly larger than the one previously determined using previous He rate coefficients. Conclusions. The new rate coefficients will help in interpreting O2 emission lines observed where LTE conditions are not fully fulfilled and enable an accurate determination of the O2 abundance in the interstellar medium. [ABSTRACT FROM AUTHOR]

The dynamics of the C+NO→CN+O and C+N2O→CN+NO reactions are reinvestigated using a pulsed supersonic crossed molecular beam apparatus. Laser vaporization of graphite at the exit of a pulsed nozzle is used to produce the atomic carbon beam. Laser induced fluorescence spectra of the CN scattered product have been obtained for both reactions. [ABSTRACT FROM AUTHOR]

We present a combined experimental and theoretical study on the rotationally inelastic scattering of heavy water, D 2 O, with normal -H 2 . Crossed-molecular beam measurements are performed in the collision energy range between 10 and 100 cm -1 , corresponding to the near-threshold regime in which scattering resonances are most pronounced. State-to-state excitation cross-sections are obtained by probing three low-lying rotational levels of D 2 O using the REMPI technique. These measurements are complemented by quantum close-coupling scattering calculations based on a high-accuracy D 2 O-H 2 interaction potential. The agreement between experiment and theory is within the experimental error bars at 95% confidence intervals, leading to a relative difference of less than 7%: the near-threshold rise and the overall shape of the cross-sections, including small undulations due to resonances, are nicely reproduced by the calculations. Isotopic effects (D 2 O versus H 2 O) are also discussed by comparing the shape and magnitude of the respective cross-sections.

New molecular beam scattering experiments are reported for the water-hydrogen system. Integral cross sections of the first rotational excitations of para - and ortho -H 2 O by inelastic collisions with normal -H 2 were determined by crossing a beam of H 2 O seeded in He with a beam of H 2 . H 2 O and H 2 were cooled in the supersonic expansion down to their lowest rotational levels. Crossed-beam scattering experiments were performed at collision energies from 15 cm -1 (below the threshold for the excitation to the lowest excited rotational state of H 2 O: 18.6 cm -1 ) up to 105 cm -1 by varying the beam crossing angle. The measured state-to-state cross-sections were compared to the theoretical cross-sections (close-coupling quantum scattering calculations): the good agreement found further validates both the employed potential energy surface describing the H 2 O-H 2 van der Waals interaction and the state-to-state rate coefficients calculated with this potential in the very low temperature range needed for the modeling of interstellar media.

Fine-structure populations and collision-induced energy transfer in atoms are of interest for many fields, from combustion to astrophysics. In particular, neutral carbon atoms are known to play a role in interstellar media, either as probes of physical conditions (ground state 3 P j spin-orbit populations), or as cooling agent (collisional excitation followed by radiative decay). This work aims at investigating the spin-orbit excitation of atomic carbon in its ground electronic state due to collisions with molecular deuterium, an isotopic variant of H 2 , the most abundant molecule in the interstellar medium. Spin-orbit excitations of C( 3 P j ) by H 2 or D 2 are governed by non-adiabatic and spin-orbit couplings, which make the theoretical treatment challenging, since the Born-Oppenheimer approximation no longer holds. Inelastic collisional cross-sections were determined for the C( 3 P 0 ) + D 2 → C( 3 P j ) + D 2 (with j = 1 and 2) excitation process. Experimental data were acquired in a crossed beam experiment at low collision energies, down to the excitation thresholds (at 16.42 and 43.41 cm -1 , respectively). C-atoms were produced mainly in their ground spin-orbit state, 3 P 0 , by dissociation of CO in a dielectric discharge through an Even-Lavie pulsed valve. The C-atom beam was crossed with a D 2 beam from a second valve. The state-to-state cross-sections were derived from the C( 3 P j ) ( j = 1 or 2) signal measured as a function of the beam crossing angle, i.e., as a function of the collision energy. The results show different quantum behaviors for excitation to C( 3 P 1 ) or C( 3 P 2 ) when C( 3 P 0 ) collides with ortho -D 2 or normal -D 2 . These experimental results are analyzed and discussed in the light of highly accurate quantum calculations. A good agreement between experimental and theoretical results is found. The present data are compared with those obtained for the C-He and C-H 2 collisional systems to get new insights into the dynamics of collision induced spin-orbit excitation/relaxation of atomic carbon.

Nonadiabatic effects are of fundamental interest in collision dynamics. In particular, inelastic collisions between open-shell atoms and molecules, such as the collisional excitation of C( 3 P j ) by H 2 , are governed by nonadiabatic and spin-orbit couplings that are the sole responsible of collisional energy transfer. Here, we study collisions between carbon in its ground state C( 3 P j=0 ) and molecular hydrogen (H 2 ) at low collision energies that result in spin-orbit excitation to C( 3 P j=1 ) and C( 3 P j=2 ). State-to-state integral cross sections are obtained experimentally from crossed-beam experiments with a source of almost pure beam of C( 3 P j=0 ) and theoretically from highly accurate quantum calculations. We observe very good agreement between experimental and theoretical data that demonstrates our ability to model nonadiabatic dynamics. New rate coefficients at temperatures relevant to astrochemical modeling are also provided. They should lead to an increase of the abundance of atomic C( 3 P) derived from the observations of interstellar clouds and a decrease of the efficiency of the cooling of the interstellar gas due to carbon atoms.

Background: Hereditary enamel defect diseases are regrouped under the name "Amelogenesis Imperfecta" (AIH). Both dentitions are affected. Clinical expression is heterogeneous and varies between patients. Mutations responsible for this multigene disease may alter various genes and the inheritance can be either autosomal dominant or recessive, or X-linked. Until now, no therapeutic consensus has emerged for this rare disease., Case Presentation: The purpose of this article was to report treatments of AIH patients from childhood to early adulthood. Treatment of three patients of 3, 8 16 years old are described. Each therapeutic option was discussed according to patients' age and type of enamel alteration. Paediatric crowns and resin based bonding must be preferred in primary teeth. In permanent teeth, non-invasive or minimally invasive dentistry should be the first choice in order to follow a therapeutic gradient from the less invasive options to prosthodontic treatments., Conclusion: Functional and aesthetic issues require patients to be treated; this clinical care should be provided as early as possible to enable a harmonious growth of the maxillofacial complex and to prevent pain.