# Vacuum

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# 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