Three experimental runs were performed introducing either He, N2 or H2 gas in the UHV chamber. Before each run, the tip was annealed to form a protrusion being the emission center. Therefore, possible variations in the tip apex size influencing the electron emission voltage and the maximal contrast cannot be ruled out. The measurement started at a background gas pressure of 109 mbar and electron interferograms were recorded with a signal acquisition time of 25 s for He or N2 and 22 s for H2. Then a further small amount of gas was introduced through the nozzle. At equilibrium another interference pattern was recorded. This process was repeated stepwise with increasing pressure for the different gases. Background images for the same integration time were acquired in the experiments with He or N2 by switching off the field emission subsequently to each recording. This data was subtracted from the interferograms. For the H2 measurement no background subtraction was necessary since the ion getter pump, as the main source of background, was turned off. The recorded images were analyzed by adding all counts in the pixel-rows of the detector along the x-direction of the interference pattern and dividing the sum by the amount of pixel-columns. The distribution of the resulting average interference pattern IðyÞ versus the y-direction normal to the interference stripes was fitted by the following expression to determine the mean intensity I0, the pattern periodicity ds and the contrast K [22]
Three experimental runs were performed introducing either He, N2 or H2 gas in the UHV chamber. Before each run, the tip was annealed to form a protrusion being the emission center. Therefore, possible variations in the tip apex size influencing the electron emission voltage and the maximal contrast cannot be ruled out. The measurement started at a background gas pressure of 109 mbar and electron interferograms were recorded with a signal acquisition time of 25 s for He or N2 and 22 s for H2. Then a further small amount of gas was introduced through the nozzle. At equilibrium another interference pattern was recorded. This process was repeated stepwise with increasing pressure for the different gases. Background images for the same integration time were acquired in the experiments with He or N2 by switching off the field emission subsequently to each recording. This data was subtracted from the interferograms. For the H2 measurement no background subtraction was necessary since the ion getter pump, as the main source of background, was turned off. The recorded images were analyzed by adding all counts in the pixel-rows of the detector along the x-direction of the interference pattern and dividing the sum by the amount of pixel-columns. The distribution of the resulting average interference pattern IðyÞ versus the y-direction normal to the interference stripes was fitted by the following expression to determine the mean intensity I0, the pattern periodicity ds and the contrast K [22]
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