Mean Free Path

Standard Atmospheric Pressure (STP) is defined as 760, where 1 Pascal is 7.5mTorr. Generally our pressure ranges from 1mTorr (ICP) to 1Torr (plasma cleaning)

Molecules must collide with each other as they have a finite size, if they approach within 1 molecular diameter they will collide.

The distance that molecules travel between collisions is called the Mean Free Path. Smaller molecules have larger mean free paths, as well as lower pressure .

Ex: At STP $\text{ce}O2$ has a mean free path of $60nm$ and mean free path varies by 1/pressure. So for $\text{ce}O2$ at 1mT the mean free path is:

$$60nm\ast 760\ast 10^3=40mm$$