1. Estimate the degree of contamination in the grown material simply as
2. Verify that the mean free path L is much greater than the size of the apparatus. If the molecules have diameter d, they will hit all other molecules in a volume πd2x while travelling a distance x. On average they hit one other molecule when travelling one mean free path, so πd2 Ln = 1. (The same argument will be used for the mean free path of an electron in a solid in Section 8.2.) Estimate L both for the background gas and for the molecular beams, using the foregoing conditions.