Non-negligible collisions of alkali atoms with background gas in buffer-gas-free cells coated with paraffin

We measured the rate of velocity-changing collisions (VCCs) between alkali atoms and background gas in buffer-gas-free anti-relaxation-coated cells. The average VCC rate in paraffin-coated rubidium vapor cells prepared in this work was $1 \times 10^{6}$ s$^{-1}$, which corresponds to $\sim 1$ mm in the mean free path of rubidium atoms. This short mean free path indicates that the background gas is not negligible in the sense that alkali atoms do not travel freely between the cell walls. In addition, we found that a heating process known as "ripening" increases the VCC rate, and also confirmed that ripening improves the anti-relaxation performance of the coatings.