1. 實況 與 經驗 Practical Considerations of Pulsed Molecular Beam
Jim Jr-Min Lin (林志民)
Office: R335, Lab: L410, IAMS
實況 與 經驗
Reference Book:
Atomic and Molecular Beam Methods
edited by G. Scoles, Oxford University Press

2. 10-6 ~ 10-2 torr (far away from nozzle)
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 >103)
Very Efficient Translational Cooling
Translational Temperature (Ttrans) can easily reach 1K~10K

3. in Center-of-Mass frame
General Features of a Molecular Beam
S = Speed Ratio
in Center-of-Mass frame
for mono-atomic gas

4. travel speed > information speed external boundary exists??
General Features of a Molecular Beam
Supersonic Beam
travel speed > information speed
external boundary exists??

5. Calculate v0 from Energy Conservation
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 can’t relax
Etrans
Erot
work
for mixture gas

6. Very Efficient Translational–Rotational Energy Transfer
General Features of a Molecular Beam
Very Efficient Translational–Rotational Energy Transfer
Very Efficient Rotational Cooling
Rotational Temperature (TROT) can easily reach 1K~10K
Poor Translational–Vibrational Energy Transfer
Inefficient Vibrational Cooling
Vibrational Temperature (Tvib) can keep the nozzle temperature or only slightly lower (e.g., cooled to 200K from 300K)

7. Type of processes # of collisions
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

8. wall
Ideal Pulsed Beam Source
Pumping:
pulsed valve P1 L P2
pump
@free jet
@ pump inlet
(steady state), Here P means number density
w = pulse width, f = repetition rate

9. Pjet=P1 >> P2=Pchamber
Ideal Pulsed Beam Source
V = Volume
S = Pumping Speed = S2
Pump out time
Typical example, V=100 ℓ, S=1000 ℓ/s
Rep-Rate = 10Hz
Pjet=P1 >> P2=Pchamber
PSS
pulsed valve

10. may be anti-correlated with
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

11. 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.

12. Skimmed Beam : skimmer interaction
pulsed valve
Jet Density is a function of Time & Space
Attenuation is also a function of Time & Space

13. 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

14. Generation of O(1D) beam
CO2
O(1D) + O(3P)
skimmer
20 mm hv
157 nm
Pulsed valve

15. hv F
CH4
Generation of F atom beam

16. 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 >300 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 >10 ms
wide T range
LN2 to 250 oC
high rep-rate (<1000Hz)
expensive
fixed wg
small nozzle 0.25 mm
Trickl Valve
Piezoelectric
Translator /
O-ring
stable
medium wg
moderate T only
Jordan Valve
Magnetic repulsion
(single loop) /
rep-rate ≤ 10Hz
5000 Amp!
No personal experience

17. Important Operation Notes:
Electric Driving does not directly correlate with Mechanical Motion
Gas Width wg: Human sensors are not good for < 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 ratio=L/d < 5 or > 5