Consider a Ioffe-Pritchard trap geometry.

Evaporative cooling works by decreasing the trap depth to remove atoms which are more energetic than average; this leads to cooling. This is done by using RF radiation to flip atoms from trapped states ($m_F>0$) to untrapped states ($m_F\leq 0$). My question is about these transitions and the selection rules. The alkalis from which BECs are formed are quasi single electron atoms therefore we expect the single electron jump condition $\Delta l=\pm 1$ to mean that these transitions ($|F,m_F\rangle\rightarrow|F,m_F'\rangle$) are not dipole allowed. Is this the case? Are we, by saying that such transitions can occur, outside of the realm of Russel-Saunders coupling?

  • $\begingroup$ It's a very long time ago, but isn't this a magnetic transition? $\endgroup$
    – Semoi
    Dec 10, 2019 at 22:15

1 Answer 1


First of all, you should not quote the $\ell$ selection rules but the $F$ and $m_F$ ones, see below.

This is an adaptation from the wikipedia table: enter image description here

So the $\Delta m_F = \pm 1$ transitions are allowed.

What you should really be asking is how are these transitions allowed within the same principal quantum number n, so that parity is unchanged?

Electric dipole transitions of this type would be forbidden, but magnetic ones are allowed.

RF/MW evaporation use magnetic transitions.


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