In Fig. 2 the resulting contrast is plotted versus the background gas pressure for He and N2. The contrast distribution is rather constant for the whole measured pressure range indicating the electrons remain coherent. At higher pressures the ion pump produced an increasing noise level of ions on the detector that leads to greater error bars and a higher dispersion of the data points. For hydrogen we were able to work without the ion getter pump and stabilized the pressure only with the cryopump. This significantly reduced the background counts resulting in a more stable signal. The pressure-dependent contrast for hydrogen is shown in Fig.3.Itisconstantaround67%.Theinsetillustratesatypical interference pattern recorded at 7:3 109 mbar.
Additionally, the mean intensity I0 of the interference pattern on the detector was determined for hydrogen with increasing pressure. It represents the center line of the cosine-function in Eq. (1). The data is plotted in Fig. 4. As expected, a significant signal drop is determined. This is presumably due to increasing collisions between electrons and H2 molecules that decrease the count rate. At a pressure of 9:3 105 mbar only a fraction of 6% of the original electron signal is left.