Experiments with Supersonic Beams as a Source of Cold Atoms

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An atom in an excited state may, in most cases, decay to any one of a number of lower level states. For laser cooling to work properly, the atom must decay back to the original lower level state, where the laser wavelength is near resonant. If it does not, the atom can fall into a dark state, where the laser does not interact with the atom, and be lost. The branching ratio governs the rates at which atoms in a particular excited state will decay into the allowed lower level states. For most atoms, the internal structure is complex enough that the branching ratio has non- zero components in several lower level states that are not resonant with the cooling laser, and most of the atoms will be lost after several thousand scattering events. This limits the number of atoms which can be laser cooled. In practice, only the alkali atoms, and a few others are amenable to laser cooling due to this limitation. Another cooling method is required for those atoms that are not laser coolable due to a more complex internal structure, which has motivated the work found in this dissertation. 1.1.2 Buffer-Gas Cooling Instead of using light to cool atoms, another approach is to use collisions with a cold background gas. This is known as Buffer Gas Cooling [4–7]. The entire experiment takes place inside a dilution refrigerator, with helium typically used as the buffer gas, to reach the cold temperatures desired. Using this method, a gas jet is allowed to expand into the cold vacuum chamber with the helium buffer gas. As the injected gas collides with the helium, it thermalizes with the background gas. This is typically done in the region of a magnetic trapping potential. When the temperature of the injected gas is below the trap depth, it will be trapped if it is in the correct state. The helium is then pumped away, leaving a cold sample in the trap. While this is a general method, there are several drawbacks that have kept buffer gas cooling from seeing widespread use. Needing to have the experiment take 3
place inside a dilution refrigerator adds significant cost and complexity to the experiment. Optical access to the trapped sample is also limited because of the cryogenic environment. This lack of access limits the experiments which can be performed, as well as the ability to effectively observe the sample. Thus, a general method which is able to trap the sample in a room temperature chamber with good optical access would decrease complexity, while enhancing the experimental utility of the apparatus. 1.2 Methods of Controlling Supersonic Beams Because supersonic beams make excellent sources of cold atoms, numerous groups have developed methods to control atoms and molecules cooled by supersonic beams. The challenge is that while the atoms or molecules have been cooled by the expansion, they are still moving fast in the lab frame. Even in this form, supersonic beams are a staple of physical chemistry [8–10]. For many experiments though, the high speed of the beams is a serious detriment, which has led to many efforts to control the motion of supersonic beams. One approach to controlling the beam is to employ mechanical methods. The velocity of the supersonic flow is relative to the nozzle, not the vacuum chamber itself, suggesting that it should be possible to produce slower beams by moving the nozzle opposite to the expansion. This is the approach taken by Gupta and Hershbach [11], who mounted their nozzle at the end of a rapidly rotating arm. Further mechanical control methods have included focusing by reflecting the beams from bent crystals [12], and stopping via collisions between atoms in crossed beams [13]. Another approach is to use external fields to control the motion of atoms or molecules in the beam. Pulsed laser fields have been used to stop atoms in a beam [14]. Pulsed electric fields are used to slow and trap molecules with permanent electric dipole moments [15–17]. The pulsed electric field approach is also used to slow and 4
Page 1 and 2: Copyright by Adam Alexander Libson
Page 3 and 4: General Methods of Controlling Atom
Page 5 and 6: Acknowledgments First and foremost,
Page 7 and 8: to problems, and I frequently went
Page 9 and 10: General Methods of Controlling Atom
Page 11 and 12: Table of Contents Acknowledgments v
Page 13 and 14: Chapter 5. Towards Trapping and Coo
Page 15 and 16: List of Figures 2.1 Comparison of e
Page 17 and 18: 5.12 Faraday Rotation Signal During
Page 19: producing a cold sample. Alternativ
Page 23 and 24: atoms or molecules are slowed using
Page 25 and 26: 2.1 Thermodynamics of Ideal Gases F
Page 27 and 28: Since there will be no heat exchang
Page 29 and 30: Using equation 2.17 to modify equat
Page 31 and 32: Normalized Effusive Beam Flux Norma
Page 33 and 34: general method for producing cold a
Page 35 and 36: The gas throughput of the nozzle ca
Page 37 and 38: Chapter 3 Slowing Supersonic Beams
Page 39 and 40: 3.1 Using Helium for Atom Optics Ex
Page 41 and 42: Figure 3.1: Calculated elastic scat
Page 43 and 44: that they can be prepared ex-situ a
Page 45 and 46: successful. Any water that remains
Page 47 and 48: Skimmer 300 l/s Turbo Pump Even-Lav
Page 49 and 50: Figure 3.5: A CAD image of the dete
Page 51 and 52: from tubular aluminum welded togeth
Page 53 and 54: Figure 3.7: A CAD image of the roto
Page 55 and 56: Figure 3.10: A CAD image of large c
Page 57 and 58: Figure 3.11: This plot shows the am
Page 59 and 60: approximations are made in this cal
Page 61 and 62: 3.3.3 Detection An SRS [61] residua
Page 63 and 64: TTL pulse to the data acquisition c
Page 65 and 66: A comparison of the time-of-flight
Page 67 and 68: eceding crystal. Each curve is the
Page 69 and 70: calculated slow beam velocity is qu
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the RGA does have an effect on the
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Chapter 4 The Atomic and Molecular
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field. In the L − S coupling regi
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3 P2 E Figure 4.2: This graph gives
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For intermediate fields, the full p
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Figure 4.4: This figure illustrates
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The same effect is also responsible
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(a) (b) (c) Time Figure 4.5: A pict
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which the particles enter). The oth
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the coil can instead be switched be
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the magnitude of the field some dis
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nozzle front surface aluminum catho
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Figure 4.9: A CAD overview of the s
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258V 1MΩ 5V 1mF DC/DC Converter TT
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Figure 4.12: An oscilloscope trace
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(a) (b) Figure 4.14: A CAD image of
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-HV PS 4 M MCP Anode 10 M 1 M Trans
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Signal [V] 2.5 2.0 1.5 1.0 0.5 refe
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Figure 4.20: An exploded view of th
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(a) (b) (c) (e) Figure 4.21: A pict
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258V 1MΩ TTL 2.2mF 50Ω TTL 5V 5V
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closed. With the new thyristor gate
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HeNe M2 M1 BB /2 L BB PC coil PC PD
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Table 4.1: Peak magnetic fields mea
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Figure 4.28: A cut-away CAD image o
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ameters. The coilgun chamber consis
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Table 4.2: Final velocities (vf), s
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MCP Signal [arb. units] 2.0 1.5 (1)
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viability for molecules essential.
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8 Ω LN2 Liquid Nitrogen Dewar Nozz
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Figure 4.33: Molecular oxygen slowi
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Chapter 5 Towards Trapping and Cool
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Energy meV 0.05 0.00 0.05 0.0 0.2 0
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10 mm 2.0 T 1.8 1.6 1.4 1.2 1.0 0.8
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250V 1MΩ 3x 2.2mF TTL 5V feedback
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Magnetic Field (T) 1.9 1.7 1.5 1.3
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5.2.2.1 Principle of Operation of t
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Figure 5.8: Photograph of the trapp
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4.1.3. In an anti-Helmholtz trap, t
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Figure 5.11: Photograph of the hydr
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Photodiode Signal (V) Time (s) Figu
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Figure 5.13: Time of flight plot of
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500 l/s Turbo Pump Supersonic Nozzl
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Ionizer 500 l/s Turbo Pump Ion Opti
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guided the design of the apparatus.
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Z − Velocity (m/s) 100 60 30 0
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Atom Number 60 50 40 30 20 10 0.6 0
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scattered photon to extract informa
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where k1 · pi = − k2 · pi, wh
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the electron g-factor which causes
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Figure 5.24: A schematic of the tel
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prevent the determination of the is
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(a) (b) time Figure 5.25: A 1D illu
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experiment, where a being is capabl
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potential position 2 x 243 nm Lα |
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Appendix 164
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y x V i Figure A.1: The geometry us
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Bibliography [1] H.J.MetcalfandP.va
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[17] Brian C. Sawyer, Benjamin L. L
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[34] R. Campargue. Progress in over
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[51] O. Carnal, M. Sigel, T. Sleato
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[70] Edvardas Narevicius, Adam Libs
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[87] Willis E. Lamb and Robert C. R
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[96] G.Gabrielse,N.S.Bowden,P.Oxley
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[110] N. Kolachevsky, J. Alnis, S.
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[125] Andreas Osterwalder, Samuel A
Page 203 and 204:
[142] C. Cohen-Tannoudji, B. Diu, a
Page 205 and 206:
[161] Paulo F. Bedaque, Aurel Bulga
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Experiments with Supersonic Beams as a Source of Cold Atoms

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