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Forschergruppe Laboratory Astrophysics

Forschergruppe Laboratory Astrophysics. +. +. +. +. +. +. +. +. Interstellar Molecules. Schlemmer, Cook , Saykally, Science 265 , 1686 (1994) Cook, Schlemmer, Saykally, Nature 380 , 227 (1996) Cook, Schlemmer, et al. J.Phys.Chem. A 102 , 1465 (1998).

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Forschergruppe Laboratory Astrophysics

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  1. Forschergruppe Laboratory Astrophysics

  2. + + + + + + + + Interstellar Molecules

  3. Schlemmer, Cook , Saykally, Science 265, 1686 (1994)Cook, Schlemmer, Saykally, Nature 380, 227 (1996)Cook, Schlemmer, et al. J.Phys.Chem. A102, 1465 (1998)

  4. Forschergruppe Laboratory Astrophysics

  5. Cosmic Abundance of some elements Element Abundance hydrogen (H) 1.000.000 helium 80.147 oxygen 739 carbon 445 neon 138 nitrogen 91 magnesium 40 Silcon 37 Sulfur 19

  6. Physics and Chemistry of the Interstellar Medium T = 10 K n = 104 cm-3

  7. Helsinki Winter School The Role of Gas Phase Ion – Molecule Reactions - Laboratory Studies Stephan Schlemmer WHAT? Kinetics of ion-molecule reactions radiative association (one example) WHY?Identification of Species (Column Densities) Formation and Destruction HOW? Experimental Techniques (Laboratory work) December 13, 2006

  8. Forschergruppe Laboratory Astrophysics

  9. Importance of Ion-Molecule Reactions E. Herbst, The Physics and Chemistry of Interstellar MCs (1993) Products EA Reactants Cosmic Ray Ionization Ion-Molecule Reactions Recombination Arrhenius: k(T) = <sv> = A exp (-EA/kT) Neutral-Neutral Reactions A 10-11 cm3/s EA  2000 K TMC = 10 K Ion-Molecule Reactions 10-7 cm3/s >A> 10-9 cm3/s EA  0 K

  10. - + V = - q a / r4 + - + - + p b Kinetic energy E = p2/2m Angular momentum L = b p = l h Centrifugal barrier V = + L2/(2mr2) Ion – induced dipole interaction long range forces

  11. Ion – induced dipole interaction Rate of Reaction Cross section s(E): s(E) = 2p * b db = p bmax2 Rate coefficient: k = <s v>T ion-induced dipole: kL = const. p b Kinetic energy E = p2/2m Angular momentum L = b p = l h

  12. Temperature Dependence of Ion-Molecule Reactions Langevin Rate Coefficient: M.A. Smith (1994)

  13. Rotational Energy, Zero Point Energy and Fine Structure Energy Details of Potential Energy Surface Transition State E Reactants kT Products Association

  14. Low Temperature Collisions in Flow Systems and Traps (state-of-the-art experiments)

  15. T0, p0 A B [B] T const. z, t Pulsed Uniform Supersonic Expansion A + BProducts k = (t [B])-1 k(T)

  16. CRESU: Cinétique de Réaction en Ecoulement Supersonique Uniforme B. Rowe (Rennes), I. Sims (Rennes), M. Smith (Tucson)

  17. Pulsed Uniform Supersonic Expansion ln [A] A + BProduct t d[A]/dt = - k [A] [B] t k = (t [B])-1 t  Dz/v ~ 0.1/500 s ~ 200 µs [B]  1015 cm-3 k  510-12 cm3/s

  18. 22-Pole Low Temperature Ion Trap detector primary ions rf I rf II 22-pole sapphire cold head 10 K shield 50 K

  19. Trap Experiment A + BProducts d[A]/dt = - k [A] [B] k = (t [B])-1 1 ms t  100 s 108 cm-3  [B]  1015 cm-3 k  10-17 cm3/s

  20. Method:Electrodynamical Trapping

  21. RF Ion Trap Mechanical Model Quadrupole 22-Pole

  22. Trajectories of ions in 22-pole trap f= 17MHz V0=20V + m=20u × m=2u

  23. Trajectories of ions in 22-pole trap f= 17MHz V0=20V + m=20u × m=2u

  24. Low Temperature 22 Pole Ion Trap

  25. Low Temperature 22 Pole Trap

  26. Initial reactions in dense interstellar clouds D. Smith and P. Spanel, Mass Spectrometry Reviews, 14 (1995) 255-278.

  27. Forschergruppe Laboratory Astrophysics

  28. Initial reactions in dense interstellar clouds D. Smith and P. Spanel, Mass Spectrometry Reviews, 14 (1995) 255-278.

  29. Deuteration of H3+ HD HD HD D3+ H3+ H2D+ D2H+ k1 k2 k3 [HD] = 1.8x1012 cm-3 T = 10 K

  30. Initial reactions in dense interstellar clouds D. Smith and P. Spanel, Mass Spectrometry Reviews, 14 (1995) 255-278.

  31. Example I:Negative Temperature Dependence NH3+ + H2 NH4+ + H Gas Phase Formation of Ammonia NHn+ + H2 NHn+1+ + H n = 0,1,2,3

  32. Implications of a barrier along reaction path Products EA Reactants Arrhenius: k(T) = <sv> = A exp (-EA/kT) k T / K 10 100 1000

  33. Negative Temperature Dependence NH3+ + H2 NH4+ + H Experiment Theoretical Prediction Herbst et al. J.Chem.Phys. 1991

  34. van der Waals binding Lowest energy path for NH3+ + H2 NH4+ + H kcal/mol Herbst et al. J.Chem.Phys. 1991

  35. Negative Temperature Dependence NH3+ + H2 NH4+ + H l k << kL 1 in 10000 collisions leads to reaction l Turnover at 100 K l barrier height 4.8 kcal/mol (2400 K) l Tunneling is a dominant mechanism at low temperatures

  36. TP5 LABORATORY ASTROPHYSICS Example II: Radiative Association CH3+ + H2O  CH5O+ + hn A. Luca, D. Voulot, and D. Gerlich, WDS'02 Proceedings of Contributed Papers, Part II, Safrankova (ed), Matfyzpress, 294-300 (2002)

  37. Initial reactions in dense interstellar clouds D. Smith and P. Spanel, Mass Spectrometry Reviews, 14 (1995) 255-278.

  38. Results: Formation of protonated methanol

  39. Radiative Association AB+ + C A + BC+ ABC+ ~ ~

  40. Density Dependence of Association Reaction Ternary k3 Radiative kr

  41. Radiative Association: kr Comparison to Statistical Theories

  42. Helsinki Winter School The Role of Gas Phase Ion – Molecule Reactions - Laboratory Studies Stephan Schlemmer WHAT? Kinetics of ion-molecule reactions radiative association (one example) WHY?Identification of Species (Column Densities) Formation and Destruction HOW? Experimental Techniques (Laboratory work) December 13, 2006

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