Practical Considerations of Pulsed Molecular Beam

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Practical Considerations of Pulsed Molecular Beam
Jim Jr-Min Lin (???) Office R335, Lab L410, IAMS
?? ? ??
Reference Book Atomic and Molecular Beam
Methods edited by G. Scoles, Oxford University
Press
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General Features of a Molecular Beam
backing pressure P0 0.1 100 atm
source chamber 10-6 10-2 torr (far away from
nozzle)
Expansion (Huge pressure drop gt103)
Very Efficient Translational Cooling
Translational Temperature (Ttrans) can easily
reach 1K10K
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General Features of a Molecular Beam
S Speed Ratio
in Center-of-Mass frame
for mono-atomic gas
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General Features of a Molecular Beam
Supersonic Beam
travel speed gt information speed external
boundary exists??
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General Features of a Molecular Beam
Calculate v0 from Energy Conservation
for one mole of mono-atomic gas
Etrans
work
for one mole of di-atomic gas
Vibrational energy cant relax
Erot
Etrans
work
for mixture gas
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General Features of a Molecular Beam
Very Efficient TranslationalRotational Energy
Transfer
Very Efficient Rotational Cooling
Rotational Temperature (TROT) can easily reach
1K10K
Poor TranslationalVibrational Energy Transfer
Inefficient Vibrational Cooling
Vibrational Temperature (Tvib) can keep the
nozzle temperature or only slightly lower (e.g.,
cooled to 200K from 300K)
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General Features of a Molecular Beam
Type of processes of collisions
velocity deflection 1
translational relaxation 10
rotational relaxation 102
vibrational relaxation 106
nozzle expansion 103
Relaxation rates depend on Energy Gap, Buffer
Gas, T, etc
of collisions during expansion is scaled to P0d
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wall
Ideal Pulsed Beam Source
pulsed valve
Pumping
P1
P2
L
pump
_at_ pump inlet
_at_free jet
(steady state), Here P means number density
w pulse width, f repetition rate
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Ideal Pulsed Beam Source
V Volume S Pumping Speed S2
Pump out time
Typical example, V100 l, S1000 l/s
Rep-Rate 10Hz
PjetP1 gtgt P2Pchamber
PSS
pulsed valve
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Practical Pulsed Beam Source
Delay Time Issue
pulsed valve
not equal to not proportional to may be
anti-correlated with
we
wg
Large delay time due to Mechanical Delay
Bigger we or Higher power
Stronger Force
Shorter delay time
Possible Smaller wg
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Practical Pulsed Beam Source
Intensity Issue
Free Jet No skimmer
If nozzle is fully open,
Usually pumping speed is enough for low
repetition rate system
Opening is limited by (plunger speed) x wg
If wg can be large enough, nozzle is fully open.
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Skimmed Beam skimmer interaction
pulsed valve
Jet Density is a function of Time Space
Attenuation is also a function of Time Space
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Skimmed Beam skimmer interaction
Usually Attenuation are the major parameters for
a high intensity beam
Short wg
Delayed Attenuation
More Intense Beam after Skimmer
Opening is limited by (plunger speed) x wg
plunger speed wg is crucial to Beam Intensity
Practically, full open is rare.
Beam Intensity
P0
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Generation of O(1D) beam
O2
CO2
O(1D) O(3P)
skimmer
20 mm
hv 157 nm
Pulsed valve
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Generation of F atom beam
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Types of Available Pulsed Valves
Name Mechanism / Seal Advantage Disadvantage Comment
General Valve Solenoid / Plastic poppet (Teflon, Kel-F, PEEK, etc) cheap medium short wg 50 ms if well shimmed long wg gt300 ms follows we poppet deformed during operation shimming is skillful various shimming under various T
Even-Lavie Valve Miniature Solenoid / Kel-F gasket short wg gt10 ms stable _at_ wide T range LN2 to 250 oC high rep-rate (lt1000Hz) expensive fixed wg small nozzle 0.25 mm
Trickl Valve Piezoelectric Translator / O-ring stable medium wg high rep-rate (lt1000Hz) moderate T only
Jordan Valve Magnetic repulsion (single loop) / O-ring short wg gt10 ms stable rep-rate 10Hz 5000 Amp! moderate T only No personal experience
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Important Operation Notes
Electric Driving does not directly correlate with
Mechanical Motion
Gas Width wg Human sensors are not good for lt
1ms Average does not represent Instantaneous at
all Fast Ionization Gauge or other Diagnosis
Temperature only 20 oC except for Even-Lavie
valve ( 200 oC) Various thermal expansions
Sealing issue
Anti-Corrosive Dry is very important. e.g.,
F2, HF, Cl2, HCl are OK when dry Good gas line
vacuum
Extension channel aspect ratioL/d lt 5 or gt 5