Module 00: The Surface
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What Lives on Your Metal
A "clean" stainless steel surface is actually a layer cake of adsorbed species. Click each layer to learn what it is. Then drag the slider to watch a bake strip them away, layer by layer.
DOMINANT H₂O multilayer
PHASE Pre-bake
Click a layer
Click on any layer in the cross-section to learn what it is, why it matters, and how baking affects it.
Gas Phase Composition
H₂O
HC
CO
CO₂
H₂
Timeline of a Bakeout
A typical bake-out at 150°C progresses through distinct phases. The dominant species shifts from water to CO to H₂ as layers are stripped away.
0 – 1 hour
Multilayer water desorbs explosively. Pressure spikes. The turbo works hard.
Dominant: M18 (H₂O) • M17 (OH) • M1 (H)
1 – 6 hours
Physisorbed water leaves. CO starts appearing as the oxide layer heats up. Pressure drops fast.
Dominant: M18 declining • M28 (CO) rising • M44 (CO₂)
6 – 24 hours
Chemisorbed water and CO leaving. HC contamination becomes visible if present. FP14 monitored.
Dominant: M28 (CO) • M18 declining • HC masses 47-100 if contaminated
24 – 48 hours
Deep cleanup. Hydrocarbons should be below FP14 threshold by now. CO is the main residual.
Dominant: M28 (CO) • M2 (H₂) rising • HC should be below threshold
48+ hours
H₂ endgame. Only bulk diffusion remains. This is your ultimate pressure.
Dominant: M2 (H₂) • Trace M28 • All else at noise floor
Why Clean Assembly Matters
The difference between a 24-hour bake and a 72-hour failure often comes down to how the parts were handled before assembly.
Clean Assembly
Gloves, cleaned parts, no fingerprints. Contaminant layer nearly absent.
Contaminant layer: <0.1 monolayer
FP14 PASS in ~24 h
Contaminated Assembly
Fingerprints, residual oils, poor cleaning. Thick contaminant layer.
Contaminant layer: 10+ monolayers
72 h+ or FAIL
Contamination Crime Scene
You are the vacuum detective. Examine the RGA spectrum, click on suspicious peaks to collect evidence, then identify the culprit. Based on real SLAC investigations.
CASE #001
Solved: 0 / 5
The Leaky Flange
Click suspicious peaks to collect evidence
Evidence Notebook (0 / 3)
Click peaks in the spectrum above to investigate...
Surface Adsorption Theory
Langmuir adsorption (monolayer):
θ = K·P / (1 + K·P)
θ = fractional coverage, K = adsorption equilibrium constant, P = pressure. At UHV, θ « 1 for most species.
θ = K·P / (1 + K·P)
θ = fractional coverage, K = adsorption equilibrium constant, P = pressure. At UHV, θ « 1 for most species.
BET multilayer adsorption:
V = Vm · c·x / ((1−x)(1−x+c·x))
x = P/P0, Vm = monolayer volume, c = BET constant. Explains why water forms multilayers at atmospheric humidity.
V = Vm · c·x / ((1−x)(1−x+c·x))
x = P/P0, Vm = monolayer volume, c = BET constant. Explains why water forms multilayers at atmospheric humidity.
Monolayer formation time:
tML = 4 / (n · v̅ · s)
n = molecular density, v̅ = mean speed, s = sticking coefficient. At 10−6 Torr: ~1 s. At 10−10 Torr: ~3 hours. Work fast after bake!
tML = 4 / (n · v̅ · s)
n = molecular density, v̅ = mean speed, s = sticking coefficient. At 10−6 Torr: ~1 s. At 10−10 Torr: ~3 hours. Work fast after bake!
| Parameter | Value | Note |
|---|---|---|
| 1 monolayer (FCC metal) | ~1015 molecules/cm² | ~1.5 × 1015 for N₂ on SS |
| Sticking coeff. H₂O @ 25°C | ~1.0 | Nearly every molecule sticks |
| Sticking coeff. N₂ @ 25°C | ~0.1 | Most bounce off |
| Sticking coeff. H₂ @ 25°C | ~0.01 | Very low — must dissociate first |
| Native oxide thickness (SS) | 3–5 nm | Cr₂O₃ passivation layer |
| Fingerprint mass | ~1 µg | ≈ 1015 molecules of fatty acid |
H₂ diffusion in stainless steel:
D = 1.2×10−3 · exp(−0.57 eV / kBT) [cm²/s]
Wall thickness 1.5 mm: transit time ~hours at 150°C, ~years at 25°C. This is why H₂ is the endgame species.
D = 1.2×10−3 · exp(−0.57 eV / kBT) [cm²/s]
Wall thickness 1.5 mm: transit time ~hours at 150°C, ~years at 25°C. This is why H₂ is the endgame species.
References
[1] P.A. Redhead, J.P. Hobson, E.V. Kornelsen, The Physical Basis of Ultrahigh Vacuum, AIP, 1968.
[2] K. Jousten (ed.), Handbook of Vacuum Technology, 2nd ed., Wiley-VCH, 2016 — Ch. 5: Gas–Surface Interactions.
[3] J.F. O'Hanlon, A User's Guide to Vacuum Technology, 3rd ed., Wiley, 2003 — Ch. 2: Gas Properties, Surfaces.
[4] M. Li & H.F. Dylla, "Model for the outgassing of water from metal surfaces," JVST A 11, 1702 (1993).
[5] C. Benvenuti et al., "Influence of the surface oxide on the hydrogen outgassing from austenitic stainless steels," Vacuum 53 (1999) 317.
[2] K. Jousten (ed.), Handbook of Vacuum Technology, 2nd ed., Wiley-VCH, 2016 — Ch. 5: Gas–Surface Interactions.
[3] J.F. O'Hanlon, A User's Guide to Vacuum Technology, 3rd ed., Wiley, 2003 — Ch. 2: Gas Properties, Surfaces.
[4] M. Li & H.F. Dylla, "Model for the outgassing of water from metal surfaces," JVST A 11, 1702 (1993).
[5] C. Benvenuti et al., "Influence of the surface oxide on the hydrogen outgassing from austenitic stainless steels," Vacuum 53 (1999) 317.