Large amplitude motion within acetylene-rare gas complexes hosted in helium droplets.

Near-infrared spectroscopy of the C2H2-Ar, Kr complexes was performed in the spectral region overlapping the ν3/ν2 + ν4 + ν5 Fermi-type resonance of C2H2. The experiment was conducted along the HElium NanoDroplet Isolation (HENDI) technique in order to study the coupling dynamics between a floppy molecular system (C2H2-Ar and C2H2-Kr) and a mesoscopic quantum liquid (the droplet). Calculations were performed using a spectral element based close-coupling program and state-of-the-art 2-dimensional potential energy surfaces to determine the bound states of the C2H2-Ar and C2H2-Kr complexes and simulate the observed spectra. This furnished a quantitative basis to unravel how the superfluid and non-superfluid components of the droplet affect the rotation and the deformation dynamics of the hosted complex.

Application of the spectral element method to the solution of the multichannel Schrödinger equation.

We apply the spectral element method to the determination of scattering and bound states of the multichannel Schrödinger equation. In our approach, the reaction coordinate is discretized on a grid of points whereas the internal coordinates are described by either purely diabatic or locally diabatic (diabatic-by-sector) bases. Bound levels and scattering matrix elements are determined with spectral accuracy using relatively small number of points. The scattering problem is cast as a linear system solved using state-of-the-art sparse matrix non-iterative packages. Boundary conditions can be imposed so as to compute a single column of the matrix solution. A comparison with log-derivative propagators customarily used in molecular physics is performed. The same discretization scheme can also be applied to bound levels that are computed using direct scalable sparse-matrix solvers.

S((1)D) + ortho-D2 Reaction Dynamics at Low Collision Energies: Complementary Crossed Molecular Beam Experiments and Theoretical Investigations.

The excitation function of the S((1)D) + D2 reaction was determined in a crossed molecular beam apparatus for collision energies ranging from 1817 to 47 J mol(-1) in the near-cold regime. A very good overall agreement was found between experimental data and the theoretical results obtained using the ab initio potential energy surface built by Ho and coworkers and different methods: time-independent quantum dynamics (QM), semiclassical mean potential capture theory (sc-MPCT), and quasi-classical trajectories (QCT). The general trend of the experimental excitation function is well reproduced in most of the range by a simple capture calculation with an R(-6) dispersion potential. The present results are discussed in the light of previous studies on the isotopic variants S((1)D) + H2 and HD.

Low-temperature rate coefficients for vibrational relaxation of (3)Σ(u)(+)Rb2 molecules by (3)He and (4)He atoms.

We present quantum-scattering calculations of (4)He and (3)He colliding with (87)Rb2. For both helium isotopes, the elastic and inelastic rate coefficients are strongly influenced by the J = 1 partial wave. For the lighter isotope, a strong resonance feature of the J = 1 partial wave is responsible for an extremely efficient vibrational relaxation process. We also perform bound-state calculations of the Rb2He complex for even Rb permutation symmetry and nonzero total angular momentum. The global Rb2He (3)Σu(+) potential-energy surface used supports four bound states for (4)He and a single one for (3)He. We propose an analysis of the (87)Rb2(4)He spectrum separating the contributions of Rb2 rotation and helium motion.

Dynamics of the S(1D2)+HD(j=0) reaction at collision energies approaching the cold regime: a stringent test for theory.

We report integral cross sections for the S(1D2)+HD(j=0)→DS+H and HS+D reaction channels obtained through crossed-beam experiments reaching collision energies as low as 0.46 meV and from adiabatic time-independent quantum-mechanical calculations. While good overall agreement with experiment at energies above 10 meV is observed, neither the product channel branching ratio nor the low-energy resonancelike features in the HS+D channel can be theoretically reproduced. A nonadiabatic treatment employing highly accurate singlet and triplet potential energy surfaces is clearly needed to resolve the complex nature of the reaction dynamics.

Full dimension Rb2He ground triplet potential energy surface and quantum scattering calculations.

We have developed a three-dimensional potential energy surface for the lowest triplet state of the Rb(2)He complex. A global analytic fit is provided as in the supplementary material [see supplementary material at http://dx.doi.org/10.1063/1.4709433 for the corresponding Fortran code]. This surface is used to perform quantum scattering calculations of (4)He and (3)He colliding with (87)Rb(2) in the partial wave J = 0 at low and ultralow energies. For the heavier helium isotope, the computed vibrational relaxation probabilities show a broad and strong shape resonance for a collisional energy of 0.15 K and a narrow Feshbach resonance at about 17 K for all initial Rb(2) vibrational states studied. The broad resonance corresponds to an efficient relaxation mechanism that does not occur when (3)He is the colliding partner. The Feshbach resonance observed at higher collisional energy is robust with respect to the isotopic substitution. However, its effect on the vibrational relaxation mechanism is faint for both isotopes.

Observation of partial wave structures in the integral cross section of the S((1)D2) + H2(j = 0) reaction.

The integral cross section of the S((1)D(2)) + H(2)(j = 0) → SH + H reaction has been measured for the first time at collision energies from 0.820 down to 0.078 kJ mol(-1) in a high-resolution crossed beam experiment. The excitation function obtained exhibits a non-monotonic variation with collision energy and compares well with the results of high-level quantum calculations. In particular, the structures observed in the lower energy part, where only a few partial waves contribute, can be described in terms of the sequential opening of individual channels, consistent with the theoretical calculations.

Kinetics and dynamics of the S(1D2) + H2 → SH + H reaction at very low temperatures and collision energies.

We report combined studies on the prototypical S(1D2) + H2 insertion reaction. Kinetics and crossed-beam experiments are performed in experimental conditions approaching the cold energy regime, yielding absolute rate coefficients down to 5.8 K and relative integral cross sections to collision energies as low as 0.68 meV. They are supported by quantum calculations on a potential energy surface treating long-range interactions accurately. All results are consistent and the excitation function behavior is explained in terms of the cumulative contribution of various partial waves.

On the dynamics of the H+ +D2(v=0,j=0)-->HD+D + reaction: a comparison between theory and experiment.

The H+ +D2(v=0,j=0)-->HD+D + reaction has been theoretically investigated by means of a time independent exact quantum mechanical approach, a quantum wave packet calculation within an adiabatic centrifugal sudden approximation, a statistical quantum model, and a quasiclassical trajectory calculation. Besides reaction probabilities as a function of collision energy at different values of the total angular momentum, J, special emphasis has been made at two specific collision energies, 0.1 and 0.524 eV. The occurrence of distinctive dynamical behavior at these two energies is analyzed in some detail. An extensive comparison with previous experimental measurements on the Rydberg H atom with D2 molecules has been carried out at the higher collision energy. In particular, the present theoretical results have been employed to perform simulations of the experimental kinetic energy spectra.

Interactions and dynamics in Li+Li2 ultracold collisions.

A potential energy surface for the lowest quartet electronic state ((4)A(')) of lithium trimer is developed and used to study spin-polarized Li+Li(2) collisions at ultralow kinetic energies. The potential energy surface allows barrierless atom exchange reactions. Elastic and inelastic cross sections are calculated for collisions involving a variety of rovibrational states of Li(2). Inelastic collisions are responsible for trap loss in molecule production experiments. Isotope effects and the sensitivity of the results to details of the potential energy surface are investigated. It is found that for vibrationally excited states, the cross sections are only quite weakly dependent on details of the potential energy surface.

Theoretical spectroscopy of the calcium dimer in the A 1Sigma(u)+, c3Pi(u), and a3Sigma(u)+ manifolds: an ab initio nonadiabatic treatment.

Nonadiabatic theory of molecular spectra of diatomic molecules is presented. It is shown that in the fully nonadiabatic framework, the rovibrational wave functions describing the nuclear motions in diatomic molecules can be obtained from a system of coupled differential equations. The rovibrational wave functions corresponding to various electronic states are coupled through the relativistic spin-orbit coupling interaction and through different radial and angular coupling terms, while the transition intensities can be written in terms of the ground state rovibrational wave function and bound rovibrational wave functions of all excited electronic states that are electric dipole connected with the ground state. This theory was applied in the nearly exact nonadiabatic calculations of energy levels, line positions, and intensities of the calcium dimer in the A (1)Sigma(u) (+)(1 (1)S+1 (1)D), c (3)Pi(u)(1 (3)P+1 (1)S), and a (3)Sigma(u) (+)(1 (3)P+1 (1)S) manifolds of states. The excited state potentials were computed using a combination of the linear response theory within the coupled-cluster singles and doubles framework for the core-core and core-valence electronic correlations and of the full configuration interaction for the valence-valence correlation, and corrected for the one-electron relativistic terms resulting from the first-order many-electron Breit theory. The electric transition dipole moment governing the A (1)Sigma(u) (+)<--X (1)Sigma(g) (+) transitions was obtained as the first residue of the frequency-dependent polarization propagator computed with the coupled-cluster method restricted to single and double excitations, while the spin-orbit and nonadiabatic coupling matrix elements were computed with the multireference configuration interaction wave functions restricted to single and double excitations. Our theoretical results explain semiquantitatively all the features of the observed Ca(2) spectrum in the A (1)Sigma(u) (+)(1 (1)S+1 (1)D), c (3)Pi(u)(1 (3)P+1 (1)S), and a (3)Sigma(u) (+)(1 (3)P+1 (1)S) manifolds of states.

A detailed quantum mechanical and quasiclassical trajectory study on the dynamics of the H+ + H2 --> H2 + H+ exchange reaction.

The H+ + H2 exchange reaction has been studied theoretically by means of a different variety of methods as an exact time independent quantum mechanical, approximate quantum wave packet, statistical quantum, and quasiclassical trajectory approaches. Total and state-to-state reaction probabilities in terms of the collision energy for different values of the total angular momentum obtained with these methods are compared. The dynamics of the reaction is extensively studied at the collision energy of E(coll)=0.44 eV. Integral and differential cross sections and opacity functions at this collision energy have been calculated. In particular, the fairly good description of the exact quantum results provided by the statistical quantum method suggests that the dynamics of the process is governed by an insertion mechanism with the formation of a long-lived collision complex.

Experimental and theoretical differential cross sections for the N(2D) + H2 reaction.

In this paper, we report a combined experimental and theoretical study on the dynamics of the N(2D) + H2 insertion reaction at a collision energy of 15.9 kJ mol(-1). Product angular and velocity distributions have been obtained in crossed beam experiments and simulated by using the results of quantum mechanical (QM) scattering calculations on the accurate ab initio potential energy surface (PES) of Pederson et al. (J. Chem. Phys. 1999, 110, 9091). Since the QM calculations indicate that there is a significant coupling between the product angular and translational energy distributions, such a coupling has been explicitly included in the simulation of the experimental results. The very good agreement between experiment and QM calculations sustains the accuracy of the NH2 ab initio ground state PES. We also take the opportunity to compare the accurate QM differential cross sections with those obtained by two approximate methods, namely, the widely used quasiclassical trajectory calculations and a rigorous statistical method based on the coupled-channel theory.

Global 1 1A" potential energy surface of CH2 and quantum dynamics of a sideways insertion mechanism for the C(1D) + H2 --> CH(2pi) + H reaction.

A global adiabatic potential energy surface (PES) corresponding to the second singlet state 1 1A" (1 1B1) of CH2 has been computed in a similar way as the first singlet state 1 1A' in our previous work [B. Bussery-Honvault et al., J. Chem. Phys., 2001, 115, 10 701]. This PES has a calculated well depth of 79.9 kcal mol(-1) relative to the C(1D) + H2 asymptote and correlates to CH(2pi) + H. It presents large barriers in the C(1D) + H2 arrangement for both collinear and perpendicular geometries but no barrier for geometries about 60 degrees and leads to a sideways insertion mechanism for the reaction C(1D) + H2 --> CH(2pi) + H. The ab initio calculations were carried out for 4644 geometries and the resulting energies were fitted to a many-body expansion. Accurate three-dimensional quantum mechanical scattering calculations have been performed for the C(1D) + H2(v = 0, j = 0) reaction on this ab initio 1 1A" PES in the collision energy range [0-11.5 kcal mol(-1)]. The J = 0 reaction probabilities show dense resonance structures as those obtained with the 1 1A' PES. However some different dynamical features have been found.

Ultracold collisions involving heteronuclear alkali metal dimers.

We carry out the first quantum dynamics calculations on ultracold atom-diatom collisions in isotopic mixtures. The systems studied are spin-polarized 7Li + 6Li7Li, 7Li + 6Li2, 6Li + 6Li7Li, and 6Li + 7Li2. Reactive scattering can occur for the first two systems even when the molecules are in their ground rovibrational states, but is slower than vibrational relaxation in homonuclear systems. Implications for sympathetic cooling of heteronuclear molecules are discussed.

Ultracold Li + Li2 collisions: bosonic and fermionic cases.

We have carried out quantum dynamical calculations of vibrational quenching in Li + Li(2) collisions for both bosonic (7)Li and fermionic (6)Li. These are the first ever such calculations involving fermionic atoms. We find that for the low initial vibrational states considered here (v < or = 3), the quenching rates are not suppressed for fermionic atoms. This contrasts with the situation found experimentally for molecules formed via Feshbach resonances in very high vibrational states.

Quantum dynamics of ultracold Na+ Na2 collisions.

Ultracold collisions between spin-polarized Na atoms and vibrationally excited Na2 molecules are investigated theoretically, using a reactive scattering formalism (including atom exchange). Calculations are carried out on both pairwise additive and nonadditive potential energy surfaces for the quartet electronic state. The Wigner threshold laws are followed for energies below 10(-5) K. Vibrational relaxation processes dominate elastic processes for temperatures below 10(-3)-10(-4) K. For temperatures below 10(-5) K, the rate coefficients for vibrational relaxation (v=1-->0) are 4.8x10(-11) and 5.2x10(-10) cm(3) s(-1) for the additive and nonadditive potentials, respectively. The large difference emphasizes the importance of using accurate potential energy surfaces for such calculations.

Quantum effects in the differential cross Ssctions for the insertion reaction N(2D) + H2.

The quantum (QM) scattering theory has been difficult to apply to the family of insertion reactions and the approximate quasiclassical trajectory (QCT) method or statistical calculations were mostly applied. In this Letter, we compare the experimental differential cross sections for the title insertion reaction with the results of QM and QCT calculations on an ab initio potential energy surface. The QM results reproduce well the crossed beam experiment, while a small, but significant, difference in the QCT ones points to quantum effects, possibly the occurrence of tunneling through the combined potential and centrifugal barrier.