Measurement of Rb-Rb van der Waals coefficient via quantum diffractive universality

Collisions between trapped atoms or trapped molecules with room-temperature particles in the surrounding vacuum induce loss of the trapped population at a rate proportional to the density of the background gas particles. The total velocity-averaged loss-rate coefficient (sigma totv)for such collisions and the variation of the loss rate with trap depth has been shown to depend only on the long-range interaction potential between the collision partners. This collision universality was previously used to realize a self-calibrating, atom-based, primary pressure standard and was validated by indirect comparison with an orifice flow standard. Here, we use collision universality to measure (sigma totv)= 6.44(11)(5) x 10-15 m3/s for Rb-Rb collisions and deduce the corresponding C6 = 4688(198)(95)Eha60, in agreement with predictions based upon ab initio calculated and previously measured C6 values.