5. Simple method of augmenting the lab frame velocity of molecular beams
Precision Spectroscopy
Long-range electric dipole−dipole interaction
Cold molecular chemistry
Ultracold chemical reactions
EM Deflection & Trapping
5
8. Supersonic vs Effusive Molecular Beams
Adiabatic Expansion
Orifice >> Mean Free Path
Converts thermal energy to kinetic energy
Result: Dense, translationally &
internally cold sample with high velocity
𝛼 =
2𝑘𝑇𝑜
𝑚
𝑢 =
2𝑘𝑇𝑜
𝑚
𝛾
𝛾 − 1
1 −
𝑇∥
𝑇
8
9. 9
Initial Conditions Relevant Parameters Nozzle Beam
Pulsed Beam Skimmed Beam Stationary Source
Simple method of augmenting the lab frame velocity of molecular beams
10. Simple method of augmenting the lab frame velocity of supersonic beams
Detector
Skimmer
Nozzle
AC Motor
(below)
Rotor
Gas Feed
(above)
𝑋 = 𝑉𝑠𝑠 + 𝑉𝑟𝑜𝑡 10
11. Gupta, M. (2000). A
mechanical means of
producing cold, slow
beams of molecules.
PhD, Harvard.
11
1st Generation Rotating Source
15. Experimental Layout
15
Rotor (4)
Gas Feed System (6)
AC Motor
Eddy Current Sensor (2)
Foil Shield (3)
Skimmer (3)
Pulsed Inlet Valve (2)
Valve Control Electronics (2)
Detector (2)
Data Acquisition Card
16. Rotor 1 & 2
Improvements:
1. Length
• 𝑉𝑅 = 2𝜋𝑅 𝐹 −1
2. Nozzle
• Defines Spread
• Doesn’t Fall Off
3. Positioning Ring
16
23. 23
Background Removal
Vrot = −141 m s Vrot = −282 m s Vrot = +282 m s Vrot = +24 m s
FIG Output contains 3 signals
1) Detection Chamber background
2) Effusive Beam from Main Chamber
3) Rotor Signal
RAW Data
Background Signal
Corrected Signal
28. • Beam Shape does NOT contain any reflected peaks, shoulders, or double peaks
• Time of flight spectrum reflects appropriate shift due to rotor movement
• Velocity spreading limits lowest velocities measured
• Centrifugal enhancement of input pressure is observed
• Skimmer interference observed at the highest beam densities
• Beam narrowing restricts pulse width in speeded beams
28
Conclusions 1
34. 34
• Most peaks have shoulders or even
a double peak from skimmer edge
• Best signal to background ratio
occurs with the 3 mm skimmer
• Shield can completely remove the
beam scattered from the skimmer
• Shield protects beam from scattering
as the main chamber pressure rises
35. 35
X ρ
𝑈𝑠𝑠
𝑋 = 𝑈𝑠𝑠 + 𝑉𝑟𝑜𝑡
What dictates the choice of gas type?
1) Pure Beams
2) Seeding
SupersonicFlowVelocity(m/s)
36. 36
• Most peaks have shoulders or even
a double peak from skimmer edge
• Best signal to background ratio
occurs with the 3 mm skimmer
• Shield can completely remove the
beam scattered from the skimmer
• Shield protects beam from scattering
as the main chamber pressure rises
• Centrifugal enhancement only effects
high mass gas types
• Other gasses perform in a similar
manner to xenon
38. 38
• Most peaks have shoulders or even
a double peak from skimmer edge
• Best signal to background ratio
occurs with the 3 mm skimmer
• Shield can completely remove the
beam scattered from the skimmer
• Shield protects beam from scattering
as the main chamber pressure rises
• Centrifugal enhancement only effects
high mass gasses
• Other gasses perform in a similar
manner to xenon
• Location of density maximum
is highly pressure dependent
• Verifies the centrifugal
enhancement effect
41. 41
• Most peaks have shoulders or even
a double peak from skimmer edge
• Best signal to background ratio
occurs with the 3 mm skimmer
• Shield can completely remove the
beam scattered from the skimmer
• Shield protects beam from scattering
as the main chamber pressure rises
• Centrifugal enhancement only effects
high mass gasses
• Other gasses perform in a similar
manner to xenon
• Location of density maximum
is highly pressure dependent
• Verifies the centrifugal
enhancement effect
• Time of flight is affected by the
shield position
• Slow Frequencies effected much
more by shield position
42. 42
3rd Generation Rotating Source
Primary Topic Ex/Th Year Author
Construction Ex 1999 Gupta and Herschbach
Construction Ex 2000 Gupta and Herschbach
EM Deflection Th 2006 Timko et al.
Construction Ex 2010 Strebel et al.
Scattering Ex 2012 Strebel et al.
Merging Th 2012 Wei et al.
Construction Ex 2012 Sheffield et al.
Slowing Ex 2013 Spieler et al.
Construction Ex 2015? (This work)
2030?
1st
2nd
3rd ?
43. 43
3rd Generation Rotating Source
Primary Difference Experimental Impact
Better performing Al alloys Higher limiting frequency
Increase rotor length to 10” Ability to slow light atomic gasses
Dynamic balancing of the rotor More stability at higher frequencies
Secondary detection method Measure beam temperature
Narrow edge skimmers Limits beam heating
Improved cryopump geometry Faster pumping speeds
56. Uses of Decelerated Molecular Beams
Molecules have enhanced sensitivity (compared with atoms) to VIOLATIONS OF FUNDAMENTAL SYMMETRIES, such as
the possible existence of the electron electric dipole moment, and parity-violating nuclear moments
The internal degrees of freedom of polar molecules have been proposed as qubits for QUANTUM COMPUTERS and are ideal
for storage of quantum information.
The LONG-RANGE ELECTRIC DIPOLE−DIPOLE INTERACTION between polar molecules may give rise to novel
quantum systems.
PRECISION SPECTROSCOPY performed on vibrational or hyperfine states of cold molecules can probe the time variation of
fundamental constants, such as the electron-to-proton mass ratio and the fine structure constant.
Studies of COLD MOLECULAR CHEMISTRY in the laboratory play an important role in understanding gas-phase chemistry
of interstellar clouds, which can be as cold as 10 K.
ULTRACOLD CHEMICAL REACTIONS have been observed at a temperature of a few hundred nanokelvins, with reaction
rates controllable by external electric fields.
Molecular collisions in the few partial wave regime reveal the molecular interaction in great detail.
56
