DECELERATION OF MOLECULES

Transcription

1 Seminar Ultra Cold Molecules DECELERATION OF MOLECULES Universität Hannover Matthias Pospiech 29th June 2004

2 Contents Contents: 1 Introduction Why of interest Lasercooling? 2 Deceleration Experiment Pulsed valve Deceleration Principle The Stark decelerator Measurements Results Limitations 3 Prospects 4 Summary Universität Hannover Matthias Pospiech 2 / 29

3 Introduction: Why of interest Why is deceleration/cooling of molecules of interest? What means cold for experiments air temperature: 300 K 300 m/s This limits the observation time Universität Hannover Matthias Pospiech 3 / 29

4 Introduction: Why of interest Why is deceleration/cooling of molecules of interest? What means cold for experiments air temperature: 300 K 300 m/s This limits the observation time very low velocities (1 K 5 m/s) long interaction time Universität Hannover Matthias Pospiech 3 / 29

5 Introduction: Why of interest Why is deceleration/cooling of molecules of interest? What means cold for experiments air temperature: 300 K 300 m/s This limits the observation time very low velocities (1 K 5 m/s) long interaction time fundamental studies high precision spectroscopy cold molecule molecule collisions cold chemistry molecular Bose-Einstein condensate Universität Hannover Matthias Pospiech 3 / 29

6 Introduction: Lasercooling? Deceleration is Laser Cooling applicable? Properties Laser cooling - very successful with atoms Universität Hannover Matthias Pospiech 4 / 29

7 Introduction: Lasercooling? Deceleration is Laser Cooling applicable? Properties Laser cooling - very successful with atoms Deceleration by momentum transfer absorption emission directional total momentum: p = n k statistically distributed total momentum: p t = 0 Universität Hannover Matthias Pospiech 4 / 29

8 Introduction: Lasercooling? Deceleration is Laser Cooling applicable? Properties Laser cooling - very successful with atoms Deceleration by momentum transfer Absorption-Emission Cycle necessary two-level atom absorption emission directional total momentum: p = n k statistically distributed total momentum: p t = 0 Universität Hannover Matthias Pospiech 4 / 29

9 Introduction: Lasercooling? Problems with Laser cooling Level structure of Rubidium Universität Hannover Matthias Pospiech 5 / 29

10 Introduction: Lasercooling? Problems with Laser cooling Level structure of Rubidium In case of molecules complex energy levels molecules distributed over vibrational states after spontaneous emission Universität Hannover Matthias Pospiech 5 / 29

11 Introduction: Lasercooling? Problems with Laser cooling Level structure of Rubidium In case of molecules complex energy levels molecules distributed over vibrational states after spontaneous emission how many repumper necessary? Universität Hannover Matthias Pospiech 5 / 29

12 Introduction: Lasercooling? Problems with Laser cooling Level structure of Rubidium In case of molecules complex energy levels molecules distributed over vibrational states after spontaneous emission how many repumper necessary? very difficult to find a closed two-level system in molecules Universität Hannover Matthias Pospiech 5 / 29

13 The experiment Deceleration Experiment: 1. generation by supersonic pulsed valve 2. passes through a skimmer 3. Stark decelerator 4. Time of flight measurements (various methods) Universität Hannover Matthias Pospiech 6 / 29

14 The experiment Deceleration Experiment: 1. generation by supersonic pulsed valve 2. passes through a skimmer 3. Stark decelerator 4. Time of flight measurements (various methods) Universität Hannover Matthias Pospiech 6 / 29

15 Pulsed valve Deceleration Experiment: Pulsed valve Adiabatic cooling in a supersonic pulsed gas expansion: Principle cooled gas gas expansion high pressure into vacuum multiple collisions pulsed gas expansion Universität Hannover Matthias Pospiech 7 / 29

16 Pulsed valve Deceleration Experiment: Pulsed valve Adiabatic cooling in a supersonic pulsed gas expansion: Principle cooled gas gas expansion high pressure into vacuum multiple collisions fast molecules slowed down high average velocity narrow distribution pulsed gas expansion Universität Hannover Matthias Pospiech 7 / 29

17 Deceleration Experiment: Pulsed valve Pulsed valve Properties vibrational temp.: < 50 K rotational temp.: < 5 K translational temp.: < 1 K density: < m 3 CO - Experiment: absolute velocity: 275 m/s velocity width: 0.5 K pulsed gas expansion Universität Hannover Matthias Pospiech 8 / 29

18 Experimental setup Universität Hannover Matthias Pospiech 9 / 29 The experiment Deceleration Experiment: 1. generation by supersonic pulsed valve 2. passes through a skimmer 3. Stark decelerator 4. Time of flight measurements (various methods)

19 Deceleration Experiment: Deceleration Principle Deceleration Principle momentum transfer is not possible (Lasercooling) Universität Hannover Matthias Pospiech 10 / 29

20 Deceleration Experiment: Deceleration Principle Deceleration Principle momentum transfer is not possible (Lasercooling) transfer of kinetic energy from the molecules into potential energy? Universität Hannover Matthias Pospiech 10 / 29

21 Deceleration Experiment: Deceleration Principle Deceleration Principle momentum transfer is not possible (Lasercooling) transfer of kinetic energy from the molecules into potential energy? polar molecules electric dipole moment high electric field Universität Hannover Matthias Pospiech 10 / 29

22 Deceleration Experiment: Deceleration Principle Deceleration Principle momentum transfer is not possible (Lasercooling) transfer of kinetic energy from the molecules into potential energy? polar molecules electric dipole moment high electric field Stark effect energy gain in polar molecule by electric field Universität Hannover Matthias Pospiech 10 / 29

23 Deceleration Experiment: Deceleration Principle Stark effect on polar molecules Stark Shift for ground state OH molecules in high electric fields J.R. Bochonski, University of Colorado, Boulder Universität Hannover Matthias Pospiech 11 / 29

24 Deceleration Experiment: Deceleration Principle Stark effect on polar molecules polar molecules intrisic seperation of charge dipole d e dipole alignes with an external electric field E Potential energy: H = d e E. Universität Hannover Matthias Pospiech 12 / 29

25 Deceleration Experiment: Deceleration Principle Stark effect on polar molecules polar molecules intrisic seperation of charge dipole d e dipole alignes with an external electric field E Potential energy: H = d e E. spatial varying electric field v 2 f v i, v f : initial and final velocity, M : mass. = 2 E pot M + v i 2 Universität Hannover Matthias Pospiech 12 / 29

26 Deceleration Experiment: Deceleration Principle Stark effect on polar molecules polar molecules intrisic seperation of charge dipole d e dipole alignes with an external electric field E Potential energy: H = d e E. spatial varying electric field v 2 f v i, v f : initial and final velocity, M : mass. = 2 E pot M + v i 2 gain in potential energy compensated by loss in kinetic energy Deceleration of molecules Universität Hannover Matthias Pospiech 12 / 29

27 Force on molecules Deceleration Experiment: Deceleration Principle Force in spatial varying electric field F = ( d ee) Universität Hannover Matthias Pospiech 13 / 29

28 Force on molecules Deceleration Experiment: Deceleration Principle Force in spatial varying electric field F = ( d e E) Energy low field seeking low-field seeking states Stark energy shift increases with increasing electric field molecules are moving towards the lowest electric field Electric Field Universität Hannover Matthias Pospiech 13 / 29

29 Force on molecules Deceleration Experiment: Deceleration Principle Force in spatial varying electric field F = ( d e E) Energy high-field seeking states Stark energy shift decreases with increasing electric field molecules are moving towards the highest electric field high field seeking Electric Field Universität Hannover Matthias Pospiech 13 / 29

30 Deceleration Experiment: Deceleration Principle Deceleration principle - Animation Universität Hannover Matthias Pospiech 14 / 29

31 Deceleration Experiment: Deceleration Principle Deceleration principle - Animation Universität Hannover Matthias Pospiech 14 / 29

32 Deceleration Experiment: Deceleration Principle Deceleration principle - Animation Universität Hannover Matthias Pospiech 14 / 29

33 Deceleration Experiment: Deceleration Principle Deceleration principle - Animation Universität Hannover Matthias Pospiech 14 / 29

34 Deceleration Experiment: Deceleration Principle Deceleration principle - Animation Universität Hannover Matthias Pospiech 14 / 29

35 Deceleration Experiment: Deceleration Principle Deceleration principle - Animation Universität Hannover Matthias Pospiech 14 / 29

36 Deceleration Experiment: Deceleration Principle Deceleration principle - Animation Universität Hannover Matthias Pospiech 14 / 29

37 Deceleration Experiment: Deceleration Principle Deceleration Properties : Focussing minimum electric field on the molecular beam axis Sebastian Jung, Hannover Universität Hannover Matthias Pospiech 15 / 29

38 Deceleration Experiment: Deceleration Principle Deceleration Properties : Focussing minimum electric field on the molecular beam axis low-field seeking states experience a focusing force Sebastian Jung, Hannover Universität Hannover Matthias Pospiech 15 / 29

39 Deceleration Experiment: Deceleration Principle Deceleration Properties : Focussing minimum electric field on the molecular beam axis low-field seeking states experience a focusing force no focussing in the plane parallel to the electrodes Sebastian Jung, Hannover Universität Hannover Matthias Pospiech 15 / 29

40 Deceleration Experiment: Deceleration Principle Deceleration Properties : Focussing minimum electric field on the molecular beam axis low-field seeking states experience a focusing force no focussing in the plane parallel to the electrodes alternately horizontally and vertically positioned pairs Sebastian Jung, Hannover Universität Hannover Matthias Pospiech 15 / 29

41 Deceleration Experiment: The Stark decelerator The Stark decelerator Universität Hannover Matthias Pospiech 16 / 29

42 Deceleration Experiment: The Stark decelerator The Stark decelerator Experiment from Meijer 63 equidistant electric field stages length of 35 cm maximum voltages of plus and minus 10 kv maximum electric fields of 125 kv/cm Universität Hannover Matthias Pospiech 16 / 29

43 Deceleration Experiment: The Stark decelerator Deceleration Properties Phase Definition of phase: relative position of a molecules witch its timing sequence deceleration requirements the bunch of molecules must be kept together molecules must always have the same phase switching intervals T must be gradually increased Universität Hannover Matthias Pospiech 17 / 29

44 Deceleration Experiment: The Stark decelerator Deceleration Properties Phase molecules with wrong phase Potential energy for some molecules having the same velocity but different phase. Universität Hannover Matthias Pospiech 18 / 29

45 Deceleration Experiment: The Stark decelerator Deceleration Properties Phase oscillation of phase and velocity What happens to molecules with a slightly different phase or velocity? Take: ϕ > ϕ 0, v = v 0. Molecules will oscillate with both phase and velocity around the equilibrium values. Universität Hannover Matthias Pospiech 19 / 29

46 Deceleration Experiment: The Stark decelerator Deceleration Properties Phase oscillation of phase and velocity What happens to molecules with a slightly different phase or velocity? Take: ϕ > ϕ 0, v = v 0. Molecules will oscillate with both phase and velocity around the equilibrium values. Universität Hannover Matthias Pospiech 19 / 29

47 Deceleration Experiment: The Stark decelerator Deceleration Properties Phase oscillation of phase and velocity What happens to molecules with a slightly different phase or velocity? Take: ϕ > ϕ 0, v = v 0. molecule looses more energy Molecules will oscillate with both phase and velocity around the equilibrium values. Universität Hannover Matthias Pospiech 19 / 29

48 Deceleration Experiment: The Stark decelerator Deceleration Properties Phase oscillation of phase and velocity What happens to molecules with a slightly different phase or velocity? Take: ϕ > ϕ 0, v = v 0. molecule looses more energy phase will get smaller until it lags behind. Molecules will oscillate with both phase and velocity around the equilibrium values. Universität Hannover Matthias Pospiech 19 / 29

49 Deceleration Experiment: The Stark decelerator Deceleration Properties Phase oscillation of phase and velocity What happens to molecules with a slightly different phase or velocity? Take: ϕ > ϕ 0, v = v 0. molecule looses more energy phase will get smaller until it lags behind. molecule will loose less energy Molecules will oscillate with both phase and velocity around the equilibrium values. Universität Hannover Matthias Pospiech 19 / 29

50 Deceleration Experiment: The Stark decelerator Deceleration Properties Phase oscillation of phase and velocity What happens to molecules with a slightly different phase or velocity? Take: ϕ > ϕ 0, v = v 0. molecule looses more energy phase will get smaller until it lags behind. molecule will loose less energy Molecules will oscillate with both phase and velocity around the equilibrium values. Universität Hannover Matthias Pospiech 19 / 29

51 Experimental setup Universität Hannover Matthias Pospiech 20 / 29 The experiment Deceleration Experiment: 1. generation by supersonic pulsed valve 2. passes through a skimmer 3. Stark decelerator 4. Time of flight measurements (various methods)

52 Deceleration Experiment: Measurements Time of flight (TOF) Measurement molecules impinge on a flat gold surface electron detected as a function of time since laser preparation Universität Hannover Matthias Pospiech 21 / 29

53 Deceleration Experiment: Measurements Time of flight (TOF) Results original beam: lower curve decelerated beam: upper curve additional peaks from faster and slower bunches Simulation: gray curves NH 3 Experiment Universität Hannover Matthias Pospiech 22 / 29

54 Deceleration Experiment: Results TOF results from CO-Experiment timings sequence set for a center velocity of 225 m/s velocity distribution of 4 m/s centered around this central velocity is captured molecules outside the interval are not affected only a subset of molecules on the order of 1% is decelerated Universität Hannover Matthias Pospiech 23 / 29

55 Applicable Molecules Deceleration Experiment: Results requirements sufficient large positive Stark shift ( 1 cm 1 ) in experimentally feasible electric fields ( 200 kv/cm) sufficient low initial kinetic energy sufficient low mass/stark energy Universität Hannover Matthias Pospiech 24 / 29

56 Applicable Molecules Deceleration Experiment: Results demonstrated candidates CO NH 3 (ND 3 ) (Ammonia) OH (diatomic hydroxyl radical) YbF 225 m/s 98 m/s m/s 91.8 (12.0) m/s range: 550 m/s 0 m/s Universität Hannover Matthias Pospiech 25 / 29

57 Applicable Molecules Deceleration Experiment: Results demonstrated candidates CO NH 3 (ND 3 ) (Ammonia) OH (diatomic hydroxyl radical) YbF 225 m/s 98 m/s m/s 91.8 (12.0) m/s range: 550 m/s 0 m/s high-field seeker different setup, principle demonstrated Universität Hannover Matthias Pospiech 25 / 29

58 Applicable Molecules Deceleration Experiment: Results demonstrated candidates CO NH 3 (ND 3 ) (Ammonia) OH (diatomic hydroxyl radical) YbF 225 m/s 98 m/s m/s 91.8 (12.0) m/s range: 550 m/s 0 m/s high-field seeker different setup, principle demonstrated 1. deceleration acceleration comparable to laser cooling 2. molecules can be brought to stillstand 3. velocity can be chosen precisely (OH) Universität Hannover Matthias Pospiech 25 / 29

59 Deceleration Experiment: Limitations Limitations of this cooling technique molecules polar molecules low mass/stark energy induced polarization more deceleration stages Universität Hannover Matthias Pospiech 26 / 29

60 Deceleration Experiment: Limitations Limitations of this cooling technique molecules polar molecules low mass/stark energy techical maximum obtainable electric field induced polarization more deceleration stages production problem Universität Hannover Matthias Pospiech 26 / 29

61 Deceleration Experiment: Limitations Limitations of this cooling technique molecules polar molecules low mass/stark energy techical maximum obtainable electric field induced polarization more deceleration stages production problem experimental decelerator selects absolute velocities inherit property Universität Hannover Matthias Pospiech 26 / 29

62 Prospects: What else can be done further... Better and Larger Decelerator Gerard Meijer, FHI Berlin Universität Hannover Matthias Pospiech 27 / 29

63 Prospects: What else can be done further... Trapping of Molecules Gerard Meijer, Rijnhuizen S.Y.T. van de Meerakker, Gerard Meijer, Rijnhuizen Universität Hannover Matthias Pospiech 27 / 29

64 Summary what have we discussed Summary: 1. Why are cool molecules of interest Universität Hannover Matthias Pospiech 28 / 29

65 Summary: Summary what have we discussed 1. Why are cool molecules of interest 2. Laser cooling is not applicable for molecules Universität Hannover Matthias Pospiech 28 / 29

66 Summary what have we discussed Summary: 1. Why are cool molecules of interest 2. Laser cooling is not applicable for molecules 3. Deceleration with time varying electric fields supersonic pulsed valve expansion Stark effect on polar molecules The Stark Decelerator and its properties Results Universität Hannover Matthias Pospiech 28 / 29

67 Summary what have we discussed Summary: 1. Why are cool molecules of interest 2. Laser cooling is not applicable for molecules 3. Deceleration with time varying electric fields supersonic pulsed valve expansion Stark effect on polar molecules The Stark Decelerator and its properties Results 4. Problems and prospects Universität Hannover Matthias Pospiech 28 / 29

68 Literature Literature: Hendrik L. Bethlem. Deceleration and Trapping of Polar Molecules using Time-varying Electric Fields. Hendrik L. Bethlem, Gerard Meijer. Deceleration and trapping of ammonia using time-varying electric fields. Physical Review A, 65, Gerard Meijer. Cold molecules. Physik Journal, 1(5):41 46, Gerard Meijer Hendrick L Bethlem, Giel Berden. Decelerating neutral dipolar molecules. Physical Review Letters, 83(8): , Aug J.R. Bochonski, E.R. Hudson, H.J. Lewandoski, Jun Ye. Cold free radical moleculs in the laboratory frame. Universität Hannover Matthias Pospiech 29 / 29