539 research outputs found

    Cold collisions of heavy 2Σ^2\Sigma molecules with alkali-metal atoms in a magnetic field: Ab initio analysis and prospects for sympathetic cooling of SrOH(2Σ)(^2\Sigma) by Li(2^2S)

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    We use accurate ab initio and quantum scattering calculations to explore the prospects for sympathetic cooling of the heavy molecular radical SrOH(2Σ^2\Sigma) by ultracold Li atoms in a magnetic trap. A two-dimensional potential energy surface (PES) for the triplet electronic state of Li-SrOH is calculated ab initio using the partially spin-restricted coupled cluster method with single, double and perturbative triple excitations and a large correlation-consistent basis set. The highly anisotropic PES has a deep global minimum in the skewed Li-HOSr geometry with De=4932D_e=4932 cm−1^{-1} and saddle points in collinear configurations. Our quantum scattering calculations predict low spin relaxation rates in fully spin-polarized Li+SrOH collisions with the ratios of elastic to inelastic collision rates well in excess of 100 over a wide range of magnetic fields (1-1000 G) and collision energies (10−5−0.1^{-5}-0.1~K) suggesting favorable prospects for sympathetic cooling of SrOH molecules with spin-polarized Li atoms in a magnetic trap. We find that spin relaxation in Li+SrOH collisions occurs via a direct mechanism mediated by the magnetic dipole-dipole interaction between the electron spins of Li and SrOH, and that the indirect (spin-rotation) mechanism is strongly suppressed. The upper limit to the Li+SrOH reaction rate coefficient calculated for the singlet PES using adiabatic capture theory is found to decrease from 4×10−104\times 10^{-10}~cm3^3/s to a limiting value of 3.5×10−103.5\times 10^{-10} cm3^3/s with decreasing temperature from 0.1 K to 1 μ\muK

    Theoretical And Experimental Studies Of Collision-Induced Electronic Energy Transfer From v=0-3 Of The E(0g+) Ion-Pair State Of Br2: Collisions With He And Ar

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    Collisions of Br(2), prepared in the E(0(g)(+)) ion-pair (IP) electronic state, with He or Ar result in electronic energy transfer to the D, D(\u27), and beta IP states. These events have been examined in experimental and theoretical investigations. Experimentally, analysis of the wavelength resolved emission spectra reveals the distribution of population in the vibrational levels of the final electronic states and the relative efficiencies of He and Ar collisions in promoting a specific electronic energy transfer channel. Theoretically, semiempirical rare gas-Br(2) potential energy surfaces and diabatic couplings are used in quantum scattering calculations of the state-to-state rate constants for electronic energy transfer and distributions of population in the final electronic state vibrational levels. Agreement between theory and experiment is excellent. Comparison of the results with those obtained for similar processes in the IP excited I(2) molecule points to the general importance of Franck-Condon effects in determining vibrational populations, although this effect is more important for He collisions than for Ar collisions

    Collision-Induced Non-Adiabatic Transitions Between The Ion-Pair States Of Molecular Iodine: A Challenge For Experiment And Theory

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    The ion-pair states of molecular iodine provide a unique system for studying the efficiency, selectivity, and mechanisms of collision-induced non-adiabatic transitions. Non-adiabatic transitions between the first-tier ion-pair states in collisions with molecular partners and rare gases are analyzed and discussed. The qualitative features of the rate constants and product state distributions under single collision conditions are summarized and interpreted in terms of appropriate theoretical approaches. Two mechanisms for the non-adiabatic transitions are clearly identified. The first, operative for collisions involving molecular partners possessing permanent or transition electrostatic moments, is highly selective. It connects the initially prepared level in the E 0(g)(+) electronic state with the near-resonant vibronic level of the D 0(u)(+) state with a minimum change of the total angular momentum. In an extreme quasi-resonant case when the gap between initial and final rovibronic level is less than 1 cm(-1), this mechanism has a giant cross section, 40 times that of a gas kinetic collision. An electrostatic model, which includes the coupling of the giant E-D transition dipole moment with a moment of the colliding partner and the semiclassical Born approximation, provides a plausible interpretation of this mechanism. A second mechanism is shown to govern collisions with rare gas atoms. It results in population of several ion-pair states and broad distributions over rovibronic levels. This mechanism is successfully interpreted by quantum scattering calculations based on the diatomics-in-molecule diabatic potential energy surfaces and coupling matrix elements. The calculations provide good agreement with experimental measurements and reveal different mechanisms for the population of different electronic states. Unexplained features of the non-adiabatic dynamics and directions of future work are outlined

    Electronic and vibrational predissociation in Ari2 photodissociation dynamics

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    A quantum dynamical study of the ArI2 predissociation where both vibrational and electronic processes can take place was performed. A set of 5 coupled diatomics-in-molecules (DIM) electronic potentials was used. Both perpendicular and linear initial ArI2(X) isomers were considered. Only the a′ state had non-negligible effect on photodissociation dynamics for the linear isomer. Decay rates oscillated as a function of the vibrational excitation of I2(B) but the intramolecular vibrational energy was the main source of energy which occurred in vibrational predissociation.This work has been supported by DGICYT @Ministerio de Educacio´n y Ciencia ~MEC!, Spain# under Grant No. PB95-0071, INTAS under Grant No. 97-31573, and the Spanish–French PICASSO Project No. HF1999-0132. A.A.B. also thanks MEC for sabbatical fellowship.Peer Reviewe
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