Why do we need such a good vacuum?

Matter-wave interference experiments with molecular beams require a good vacuum. Otherwise, collisions with gas particles would prevent the molecular beam from propagating freely from the source to the detector. At atmospheric pressure air molecules only fly for about \(60 \mathrm{nm}\) before they collide with another particle. In our experiments even larger molecules need to fly more than \(2 \mathrm{m}\) without collision. A closer inspection shows that this is only possible at pressures as low as \(10^{-8} \, \mathrm{mbar}\) which is a hundred billion times lower than atmospheric pressure.

Such an ultra-high vacuum can be achieved using a two-stage system. Backing pumps (scroll and piston pumps) reduce the pressure to a hundred-thousandth of the atmospheric pressure. For the final pressure we additionally use turbo molecular pumps.

Extra: Mathematical background
Extra: Lab techniques