Secondary electron yield of cryogenic surfaces as a function of physisorbed gases 06.05.2011 Asena Kuzucan TE-VSC-SCC

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 06.05.2011 Asena Kuzucan TE-VSC-SCC

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 06.05.2011 Asena Kuzucan TE-VSC-SCC

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. 06.05.2011 Asena Kuzucan TE-VSC-SCC

CO2 Adsorption on copper Results CO2 Adsorption on copper Initially SEY decreases and then increases to the bulk value of CO2. 06.05.2011 Asena Kuzucan TE-VSC-SCC

CO Adsorption on copper Results CO Adsorption on copper The SEY decreases to low values with CO. 06.05.2011 Asena Kuzucan TE-VSC-SCC

CH4 Adsorption on copper Results CH4 Adsorption on copper SEY decreases to low values with increasing amount of methane. 06.05.2011 Asena Kuzucan TE-VSC-SCC

Results N2 Adsorption on copper Initially SEY decreases and then increases to the bulk value of N2 . 06.05.2011 Asena Kuzucan TE-VSC-SCC

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. 06.05.2011 Asena Kuzucan TE-VSC-SCC

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. 06.05.2011 Asena Kuzucan TE-VSC-SCC

Tentative interpretation of coverage dependence of SEY 1 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 06.05.2011 Asena Kuzucan TE-VSC-SCC

Tentative interpretation of coverage dependence of SEY 1 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. 06.05.2011 Asena Kuzucan TE-VSC-SCC

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 06.05.2011 Asena Kuzucan TE-VSC-SCC

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 06.05.2011 Asena Kuzucan TE-VSC-SCC

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. 06.05.2011 Asena Kuzucan TE-VSC-SCC

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 1.36. 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 06.05.2011 Asena Kuzucan TE-VSC-SCC

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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. 06.05.2011 Asena Kuzucan TE-VSC-SCC