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

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

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

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

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

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)

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

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

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

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

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.

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

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

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

hv F CH4 Generation of F atom beam

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 stable @ 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

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