1. Secondary electron yield of cryogenic surfaces as a function of physisorbed gases
Asena Kuzucan
TE-VSC-SCC

2. Experimental set-up Experimental set-up SEY measurements
Different adsorbates Kr
CO2 CO
CH4 N2
II. Different substrates
Copper
Aluminium
Electro polished copper
Physisorbtion on amorphous carbon
coatings
III. Discussion
Electron beam induced conditioning on
H2O
4. Discussion
Asena Kuzucan
TE-VSC-SCC

3. Experimental set-up
Electron gun: with a spot size/max beam current of 1mm/10μA,, beam energy of 50eV-2500eV
Residual Gas Analyzer, B&A gauge, Penning gauge: for the quantitative evaluation of adsorbates on the sample (desorption)
Cold head: which allows us to keep sample at low temperatures (designed for 4.2K, reaches 4.7K. The measurements are performed at 4,7K.)
Gas injection system
Kelvin Probe: To perform work function measurement in order to investigate the effect of the work function on SEY
Asena Kuzucan
TE-VSC-SCC

4. Kr Adsorption on copper
Results
Kr Adsorption on copper
The yield increases to very high values with increasing number of layers
Measurements with Krypton are done
in order to compare the results with
existing reference (N.Hilleret).
The results are in aggrement.
Asena Kuzucan
TE-VSC-SCC

5. CO2 Adsorption on copper
Results
CO2 Adsorption on copper
Initially SEY decreases and then increases to the bulk value of CO2.
Asena Kuzucan
TE-VSC-SCC

6. CO Adsorption on copper
Results
CO Adsorption on copper
The SEY decreases to low values with CO.
Asena Kuzucan
TE-VSC-SCC

7. CH4 Adsorption on copper
Results
CH4 Adsorption on copper
SEY decreases to low
values with increasing
amount of methane.
Asena Kuzucan
TE-VSC-SCC

8. Results N2 Adsorption on copper
Initially SEY decreases and then increases to the bulk value of N2 .
Asena Kuzucan
TE-VSC-SCC

9. Comparison of different substrates
Results
Comparison of different substrates
The yield for each substrate gets the same with increasing number of monolayers due to the reason that
the adsorbate gets the bulk and the substrate lose its importance on yield.
Asena Kuzucan
TE-VSC-SCC

10. Physisorption on amorphous carbon coating
Results
Physisorption on amorphous carbon coating
Carbon is a substrate with SEY lower than the adsorbates. Measurements with carbon monoxide as adsorbate are scheduled.
Asena Kuzucan
TE-VSC-SCC

11. Tentative interpretation of coverage dependence of SEY1 2 3
Secondary electron generation occurs in the substrate material and in the adsorbate layer. Which part has the dominant effect depends on the thickness and type of the adsorbate layer.
Thin layers (range of the primary electrons > thickness of the adsorbate): the substrate dominates
Intermediate thickness
Thick layers (range of the primary electrons < thickness of the adsorbate): the adsorbate dominates
Asena Kuzucan
TE-VSC-SCC

12. Tentative interpretation of coverage dependence of SEY1 2 3
Thin layers: a large part of the primary electrons enters the substrate material, where they produce secondary electrons. Some of the secondary electrons may experience scattering at the adsorbate/substrate interface and therefore get attenuated decrease of SEY
With increasing distance d to the metal surface, the primary electrons generate secondary electrons in the adsorbate layers as well as in the substrate metal. Therefore both, the substrate and the adsorbate contribute in secondary electron generation.
3. Once reaching thick layers, where the primary electrons cannot penetrate anymore in the metal substrate, the adsorbate layer is the only contribution to the secondary electron generation.
Asena Kuzucan
TE-VSC-SCC

13. Possible explanation of SEY of “bulk” adsorbate
Why different adsorbates have different SEY? Kr
100keV
Mean free path ∝ Valence electrons, band gap
Stopping power ∝ Number of electrons Z, density of solid ρ
Range of electrons ∝ density of solid ρ
Asena Kuzucan
TE-VSC-SCC

14. Electron beam induced conditioning on physisorbed H2O
Results
Electron beam induced conditioning on physisorbed H2O
25 monolayers of water have been physisorbed on the cold electro polished copper substrate
The sample has been irradiated several times with electron beam
Each irradiation took 100s with an electron energy of 300eV.
SEY as a function of irradiation dose
Asena Kuzucan
TE-VSC-SCC

15. Electron beam induced conditioning on physisorbed CO
Results
Electron beam induced conditioning on physisorbed CO
25 monolayers of carbon monoxide have been physisorbed on the cold electro polished copper substrate
The sample has been irradiated several times with electron beam
Each irradiation took 100s with an electron energy of 300eV.
Asena Kuzucan
TE-VSC-SCC

16. Discussion for conditioning
Water
The bombardment of an electro polished substrate with 25 monolayers of physisorbed water decrease the yield due to desorption of the adsorbed water. After desorbing all the water layers from the surface, the yield continues decreasing. With an electron dose of 1.5·10-2 C/mm2 the yield reaches its minimum of This yield is even smaller than the yield of the electro polished copper before water adsorption (usual conditioning effect). CO
The bombardment of electro polished copper with 25 monolayers of carbon monoxide increase the yield slightly for a very short moment due to the desorption of the carbon monoxide, which has a lower yield than the copper. But it decreases again with continuing conditioning.
CO Fast decrease as a function of dose
H2O Slow decrease and “saturation” at higher value
The explanation could be:
Equilibrium desorption/ adsorption
Low cross section for water desorption
Asena Kuzucan
TE-VSC-SCC

17. Thank you for your attention

18. Physisorption on amorphous carbon coating
Results
Physisorption on amorphous carbon coating
The yield of carbon is higher compared with other results, the reason might be the thickness of the carbon coating. In this case it is quite thin.
Asena Kuzucan
TE-VSC-SCC