World famous Surface chemists

1. World famous Surface chemists

2. Professor Gabor A. Somorjai Department of Chemistry, University of California, Berkeley

3. Developing low-energy electron diffraction (LEED) for surface crystallography. • Using LEED, high-resolution electron energy loss spectroscopy (HREELS), and sum frequency generation (SFG) to identify the bonding of hydrocarbons as being similar to that in organometallic clusters. • the development of molecular surface science at high pressures, pioneered the use of monolayer sensitive techniques that could be used for molecular studies at the solid-gas and solid-liquid interface using high pressure-high temperature STM and SFG.

4. Published more than 775 papers in surface sciences, heterogeneous catalysis, and solid state chemistry; • Received several honorary degrees from several international universities. • Educated a generation of leading scientists in the field, including 93 Ph.D. students and 112 postdoctoral fellows. Editor-in-chief of Catalysis Letters and serves on the editorial board of eight other journals

5. Professor Gerhard Ertl Director, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin

6. Fundamental reactivity knowledge of catalytic mechanisms gained from the modern surface science approach. • Correlate catalytic reactivity with the structure and composition of the heterogeneous catalytic surface as in ammonia synthesis. • Further applied this knowledge base to the synthesis of specific microstructures on the surface to carry out specific reactions in high selectivity.

7. Studies on carbon monoxide oxidation on specific Pt crystallographic planes revealed the dynamics of the oscillatory behavior of the chemisorbed surface species. • The development of the instrumentation that makes these observations possible is regarded as a breakthrough in surface science and a key step in developing our understanding of the very rapid and dynamic changes that many heterogeneous catalyzed reactions experience.

8. UHV system Vacuum Technology

9. Vacuum Technology How to construct a simplified Ultra-high Vacuum (UHV) system?

10. The need for Ultra-high Vacuum • Vacuum theory and pumpimg laws • Measurement of Pressure • Vacuum pumps and their characteristics • Simplified Vacuum System Design & Construction

11. Why is ultra-high vacuum (UHV) necessary? • “Monolayer time” ~ the time it takes to contaminate a surface with a single layer of molecular adsorbates • the monolayer time can be estimated as: t = 4.2 ×10-6 / P where t ~ seconds, P ~ Torr.

12. Want 1 hour to do an experiment? 1 atmosphere = 1.0133 bar 1 atmosphere = 760 torr 1 atmosphere = 1.0133×105 Pa 1 torr = 1 mm Hg 1 micron Hg = 1 milliTorr 1 millibar = 100 Pa 1 torr = 133.32 Pa 1 millibar = 0.75 Torr The pressure needed for for one hour to “monolayer time” is equal to P < 1×10-9 Torr

13. Base pressure? At least P < 2× 10-10 Torr

14. A common unit for “gas dose” • Langmuir (L) is defined as, an exposure of gas at room temperature at a pressure of P = 1× 10-6 Torr for 1 second (L = 10-6 Ts) If one monolayer is created for 1 L exposure, one should get 1 monolayer in one hour at a pressure of P = 10-6 / 3600 = 3 ×10-10 Torr.”

15. Vacuum theory and pumping laws How the vacuum is created?

16. Production of vacuum • to reduce gas density in given volume to below atmospheric pressure with pump • enclosed vessel has continuous sources which launch gas into volume and present pump with continuous gas load • vacuum achievable at steady state is result of dynamic balance between gas load and ability of pump to remove gas form volume

17. Vacuum Theory using Ideal Gas Properties • Mean velocity of a gas molecules of mass M, at absolute temperature T, is given by At T = 0 oC He ~ 1200 m/s; Ar ~ 380 m/s N2 ~ 453 m/s; H2O ~ 564 m/s

18. Mean free path, which is used to define the various regions of gas flow, is given by for air at R.T.,  (mm) = 6.6/P, P in Pa • Particle flux, or the number of particle striking a surface per unit area, or passing through an imaginary plane of unit area, is given by

19. The pressure, according to ideal gas law, is given by P = nkT • For a fixed volume containing a mixture of different non-interacting gases,

20. The Three Regions of Gas Flow • When /d << 1, the flow is vicious, where the vicious force is independent o the pressure. • When /d >> 1, it is in the free-molecular flow regime, where the vicious drag is linearly proportional to pressure. • A third regions of gas-flow, Knudsen or transition flow, is often used to describe the region between these two limits.

21. Molecular Transport and Pumping Laws Three parameters P, S, Q • P: pressure [Torr] • S: volumetric flow [liter/sec] • Q: throughput [Torr·liters/sec] Q[Torr·liters/sec] = P[Torr]S[liter/sec]

22. Complete pumping equation is Q = SP + VdP/dt • No pumping (S = 0), just a closed chamber with a constant gas load from outgassing and/or leaks. P = (Q/V)t • Negligible outgassing or other leaking sources, Q = 0, corresponding to Q << SP, P = Poe-(S/V)t

23. Pumping law in the High and Ultra-high vacuum regions • The ultimate pressure is the behavior of the gas load over time. • In the HV and UHV region, the pressure decrease with time (no leaks!), P(t2) = P(t1)(t1/t2) • The final base pressure is related to some ultimate values of Q and S, Po = Qo/So

24. Sources of Gases in Vacuum Systems • Leaks through vacuum vessel. • Virtual leaks from trapped gas volumes. • Vaporization of volatile material. • Surface outgassing from adsorbed gases on walls of vessel.

25. Volume outgassing from diffusion of dissolved gases in bulk material of vessel. • Permeation through porous material or seals of vessel. • Backstreaming of volatile fluids from pump.

26. Idealized initial pumpdown of a 100 L system, size 50×50×40 cm, with a roughing pump and UHV pump.

27. So, it drops! The UHV region can only be achieved by bakeout.

28. Measurement of pressure • Mechanical phenomena gauges: measure actual force exerted by gas (e.g. manometer). • Transport phenomena: measuring gaseous drag on moving body (e.g. spinning rotor gauge) or thermal conductivity of gas (e.g. thermocouple gauge). • Ionization phenomena gauges: ionize gas and measure total ion current (e.g. ion gauge). • Partial pressure residual gas analyzers:mass spectrometers.

29. Vacuum gauges must calibrated by • Comparison with absolute standard calibrated from its own physical properties. • Attachment to calibrated vacuum system. • Comparison with calibrated reference gauge.

30. Ultrahigh (< 10-7 Torr) High (10-3 ~ 10-7 Torr) Medium (1 ~ 10-3 Torr) Low (760 ~ 1 Torr) Manometer Manometer Thermocouple Thermocouple Spinning rotor Spinning rotor Ionization Ionization Mass spectrometer Mass spectrometer Vacuum gauges used in vacuum systems

31. Thermocouple gauge For roughing vacuum (molecular flow regime) measurements

32. Ionization gauges • Thermionic/hot cathode ionization gauges. • Energetic beam of electrons (constant I-) used to ionize gas molecules and produce ion current. I+ = p KI -, K: ion gauge sensitivity • Upper pressure limit (10-3 Torr): secondary ion ionization excitation, filament burn out. • Lower pressure limit (10-10 Torr): secondary electron current from X-ray emission.

33. 阳极 收集极 阴极 Diagram of an ion gauge for measuring UHV

34. Residual gas analyzers • More compact mass spectrometers with higher sensitivity. • Gaseous ions formed in ion source box by electron bombardment, extracted with suitable fields, separated in analyzer and then collected and measured. • Magnetic sector analyzer: masses separated by static magnetic and electric fields. • Quadrupole mass analyzer: masses separated in oscillating quadrupolar electric field.

35. The RGA 100 Residual Gas Analyzer

36. Quadrupole Mass Filter Components

37. Principles of Filter Operation

38. Residual Gas Analysis

39. Vacuum pumps and their characteristics • Gas transfer pumps: (a) Positive displacement pumps that transfer repeated volumes of gas from inlet to outlet by compression ( e.g. rotary pump). (b) Kinetic pumps that continuously transfer gas from inlet to outlet by imparting momentum to gas molecules (e.g. Diffusion pump, turbomolecularpump).

40. Entrapment/capture pumps, retain molecules by sorption or condensation on internal surfaces (e.g. sorption pump, sublimation pump, sputter ion pump, cryogenic pump).

41. Ultrahigh (< 10-7 Torr) High (10-3 ~ 10-7 Torr) Medium (1 ~ 10-3 Torr) Low (760 ~ 1 Torr) Rotary Rotary Sorption Sorption Diffusion Diffusion Turbomolecular Turbomolecular Sublimation Sublimation Sputter ion Sputter ion Cryogenic Cryogenic The different vacuum pumps

42. 1. Roughing pumps (1 atmosphere to 1-10 micron) • Rotary vane (oil) mechanical pumps low cost, durable, long life high pumping speed oil-backstreaming must be controlled • Cryosorption pumps (sorption pumps) very clean inexpensive and simple limited capacity, frequent reconditioning

43. Rotary vane mechanical pumps

45. Sorption pumps The sorption pump has no moving parts and therefore no oils or other lubricants.(5 liters of liquid nitrogen)

47. 2. Diffusion pumps (high vacuum and UHV) • Low cost per unit pumping speed, very high pumping speeds • Very well understood • Hard to destroy • Continuous operating expense (LN2) • Potential for serious vacuum accidents • “Open system”:Forbidden in certain applications

49. 3. Turbomolecular pumps (high vacuum and UHV) • Medium to high cost per unit pumping speed • Very clean, pumps rare gases • Requires periodic maintenance which can be expensive • Difficult to reach very low UHV base pressures • “Open system”:Forbidden in certain applications

50. A typical turbomolecular pump