1. Mar 26, 2015

### KFC

Hi all,
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?

2. Mar 27, 2015

### vanhees71

As any physically relevant quantity on scattering, the scattering length is a parameter derived from the S-matrix. It characterizes low-energy (long-wavelength) scattering on a short-ranged potential. For first details, see the pretty good Wikipedia article:
http://en.wikipedia.org/wiki/Scattering_length