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I am reading some materials on the scattering. There is simple model to consider atom as a rigid body but with radius replaced by the so-called scattering length. I didn't learn the scattering theory systematically but my understanding on the scattering length is the minimum separation between two atoms when the scatter or interaction becomes significant. I do some research on some typical atoms like sodium. But what confusing me is the article gives 2 scattering length labeled as ##a_{1,-1}## and ##a_{2,2}##, which both are in some unit of ##a_0##. So why there are two different scattering length? Is ##a_0## the bohr radius?

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# About scattering length

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