Université de Bordeaux CNRS
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Cold and Ultra cold collisions in the gas phase

T. Stoecklin, P. Halvick.

Recent PHD's: Grégoire Guillon (2006-2009); Florence Turpin (2008-2011)

Open Phd position: see our offer

1. Interstellar Chemistry

Astrochemistry is a fast expanding field which benefited greatly from the progresses done in the recent years in the measurements of the rotational spectra of the molecules of the dense and diffuse interstellar clouds. These progresses were also made possible by the collaborative work performed with the national program of CNRS Physicochimie du Milieu Interstellaire and with the three successive European Networks to which we participated and were dedicated to this subject. The major question raised by the astronomers measurements is to evaluate the corresponding molecular abundances of the detected species. With typical densities of astrophysical environments, the strengths of collisional and radiative transitions are often comparable, resulting in non-Local thermodynamic equilibrium molecular level excitation. Then, the level populations have to be calculated by explicitly taking into account the different processes that regulate them, and for this, the collisional coefficients have to be known.

Figure 1. Comparison of the QCT rate coefficients for abstraction and exchange reaction with the RIOSA-NIP reaction rate coefficient, along with experimental results (open and solid circles) for abstraction reaction.

 
figure k(T)-v1

In most environments, inelastic collisions with helium and molecular hydrogen are dominant, but in some conditions (in the presence of ionizing UV or X ray photons) collisions with electrons can become important, particularly for ions. We continuously develop our own Close Coupling inelastic scattering code and adapt it to the open shell structure of the collision partners. The systems we treat and the corresponding potential energy surfaces models and scattering codes we develop are for collisions between atom and diatom; diatom and diatom as well as for electron and linear molecules. Collisional excitation rate coefficients which are calculated are made available for the astronomers on the BASECOL database (http://basecol.obspm.fr/) which is developed in collaboration with Marie Lise Dubernet in Meudon. The detailed knowledge of the reactive processes between C or H and simple molecules are also needed and we recently developed a new time independent quantum approach based on the use of Negative Imaginary Potential(NIP). We also compare our reactive results with those obtained with our home made quasi-classical trajectory code. We give an example of such comparison for the reactive collisions H + CH+ which we treated recently in Figure 1.

 

Recent Publications in this field

 

* Electron-impact rotational and hyperfine exciatation of HCN, HNC, DCN, and DNC. A. Faure, H. N. Varambhia, T. Stoecklin and J. Tennyson, Mon. Not. R. Astron. Soc. (2007) 382: 840

* Differential cross sections and product energy distributions for the C(3P)+OH(X2P) → CO(X1S+)+H(2S) reaction using a quasiclassical trajectory method. A. Zanchet, Ph. Halvick, B. Bussery-Honvault and P. Honvault, J. Chem. Phys. 128, 204301 (2008).

* Correlation-polarisation effects in electron/positron scattering from acetylene: a comparison of computational models. J. Franz, F. A. Gianturco, K. L. Baluja, J. Tennyson, R. R. Lucchese, T. L. Gibson, T. Stoecklin, Nucl.Instr..Meth. B (2008) 266: 425

* A comparative multi-property analysis of existing models for the He-N2 potential energy surface. T. Stoecklin, A. Voronin, A. K. Dham, J. Sanchez-Fortun Stoker and F. R. W. McCourt, Mol. Phys. (2008) 106: 75

* Three dimensional atom-diatom quantum reactive scattering calculations using absorbing potential: Speed up of the propagation scheme. T. Stoecklin, Phys. Chem. Chem. Phys. (2008) 10: 5045

* Rotational relaxation of HF by collision with ortho and para-H2 molecules. G. Guillon, T. Stoecklin, A. Voronin et P; Halvick, J. Chem. Phys. (2008) 129: 094397

* Scattering of electrons by gaseous CS(1S) The role of short-range forces on the near-threshold 2P resonance

F. Sebastianelli, F.A. Gianturco; T. Stoecklin; I. Baccarelli, Chem. Phys. Lett.(2009) 476: 182

* Non-threshold, threshold, and nonadiabatic behavior of the key interstellar C+C2H2 reaction. M. Costes, Ph. Halvick, K. M. Hickson, N. Daugey, C. Naulin, Astrophys. J. 703, 1179 (2009).

* O+OH → O2+H: A key reaction for interstellar chemistry. New theoretical results and comparison with experiment. F. Lique, M. Jorfi, P. Honvault, P. Halvick, S. Y. Lin, H. Guo, D. Q. Xie, P. J. Dagdigian, J. Klos, M. H. Alexander, J. Chem. Phys. 131, 221104 (2009)

* On the statistical behavior of the O+OH → H+O2 reaction: a comparison between quasi-classical trajectory, quantum scattering and statistical calculations. M. Jorfi, P. Honvault, P. Bargueno, T. Gonzalez-Lezana, P. Larrégaray, L. Bonnet and Ph. Halvick, J. Chem. Phys. 130, 184301 (2009).

* The ionic pathways of lithium chemistry in the early universe: Quantum calculations for LiH+ reacting with H. S. Bovino, T. Stoecklin and F. A. Gianturco, ApJ (2010) 708: 1560

* Rotational excitation and de-excitation of CH+ molecules by 4He atoms. F. Turpin, T. Stoecklin1 and A. Voronin, A&A 511, A28 (2010)

* Rate constant for the C(3P)+ OH(X2P) → CO(X1S+) + H(2S) reaction using various dynamical methods and potentials. Mohamed Jorfi, Béatrice Bussery-Honvault, and Pascal Honvault, Thierry Stoecklin and Pascal Larregaray, J. Phys. Chem. A, 114: 7494 (2010)

* Is H+ an efficient destroyer of LiH molecules? A quantum investigation at the Early Universe conditions. S. Bovino, M. Tacconi, and F. A. Gianturco, T. Stoecklin (ApJ (2010) in press)

 

2. Cold and ultracold chemistry

Recent experimental advances have made possible to cool gas-phase molecules to temperatures far below 1 K and trap them in well-defined internal states. Translational temperatures in the Microkelvin range have been achieved for molecules in optical traps, and even quantum degenerate gases of weakly bound molecules have been experimentally realized. These novel capabilities open the way to new technologies based on quantum control of molecular processes at very low temperatures and provide us with new ways to explore the fundamental principles that govern cold collisions. Ultracold molecules are considered for applications like implementation of qubits for scalable quantum information devices; tests of fundamental symmetries, or entanglement tests.

Figure 2: Highest positive Close Coupling Q matrix eigenvalues as a function of the magnetic field for the collision of 3He with respectively NH( a=1) and MJ=MT=-1 on the lower panel and NH( a=2) and MJ=MT=0 on the higher panel. The value of the applied electric field is indicated on each curve and is given in kV/cm.

 
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Instead of being driven by thermal processes, ultracold collisions are governed by quantum dynamics involving tunneling or scattering resonances and the long range potential at work may be strongly modified by applying combinations of moderate electric and magnetic fields. The theoretical studies which we perform are mainly concerned with the buffer gas cooling and the magnetic trapping techniques. We are currently developping codes allowing to theoretically study the best molecular candidates to be cooled and trapped using these techniques. We for example treated the case of atom + S molecule collisions submitted to a combination of electric and magnetic fields. An example of application to the 3He + NH(3S) collisions is given in Figure 2 where the lifetime of a magnetically tuned zero energy Feshbach resonances is studied as a function of the magnitude of a superimposed parallel electric field. .

 

Recent Publications in this field

 

* Effect of spin-rotation interaction in cold and ultra cold collision of N2+(2S+) with 3He and 4He. G. Guillon, T. Stoecklin and A. Voronin, Phys. Rev. A. (2007), 75:052722

* Zeeman relaxation of N2+ (2Σ+) in collisions with 3He and 4He. G. Guillon, T. Stoecklin and A. Voronin, Eur. Phys. J. D. (2008) 46: 83

*Vibrational and rotational energy transfer of CH+ in collisions with 4He and 3He. T Stoecklin and A. Voroni, Eur. Phys. J. D. (2008) 46: 259

* Spin depolarisation of N2+ (2Σ+) in collisions with 3He and 4He in a magnetic field. G. Guillon, T. Stoecklin and A. Voronin, Phys. Rev. A. (2008) 77: 042718

* Exact, Born-Oppenheimer, and quantum chemistry-like calculations in Helium clusters doped with light molecules: The He-N2 system. O. Roncero, M. P. de Lara-Castells, G. Delgado-Barrio, P. Villarreal, T. Stoecklin, A. Voronin and J. C. Rayez, J. Chem. Phys., (2008) 128: 164313

* Analytical calculation of the Smith lifetime Q matrix using a Magnus propagator: Applications to the study of resonances occurring in ultra cold inelastic collisions with and without an applied magnetic field. G. Guillon and T. Stoecklin J. Chem. Phys. (2009) 130: 144306

* Combining electric and magnetic static field for the tuning of the lifetime of zero energy Feshbach resonances: Application to 3He+NH(3S) collisions

T. Stoecklin, Phys. Rev. A. (2009) 80: 012710

* Spin depolarisation of N2+ (2Σ+) in collisions with 3He in a magnetic field: general behaviour and zero energy Feshbach resonances

G. Guillon, T. Stoecklin and A. Voronin, Phys. Scr. (2009) 80: 048118

* A comment about the tuning of the lifetime of zero energy Feshach resonances using parallel electric and magnetic fields. T. Stoecklin, Faraday Discussion (2009) 142: 226

* The interaction of MnH(X7S) with He: Ab initio potential energy surface and bound states. Florence Turpin, Philippe Halvick and Thierry Stoecklin, J. Chem. Phys. 132: 214305 (2010)

* Collisional Zeeman relaxation of MnH (X7Σ+): mechanism and comparison with experiment. F. Turpin, T. Stoecklin and Ph. Halvick, (submitted)