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It seems that alkali atoms are often used in cold atom experiments. The first BEC was formed with alkali atoms, and many modern experiments used Alkalis. What is special about having a single electron in the outer most orbital that is useful for cold atom experiments?

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2 Answers 2

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Alkali atoms have several benefits!

  • The one outer electron makes them "hydrogen-like". Therefore, it is "easy" to calculate the energy levels which makes predictions and calculations using these elements far easier!
  • Since the energy structure is very simple, you can find closed cooling cycle. E.g. cooling rubidium requires the use of only one repumping laser beam to close the cooling cycle.
  • Why not use just hydrogen? Alkali atoms feature transition frequencies, which are easily accessible (laser technology is very advanced for visible to near infrared light). Hydrogen would require UV laser light, which is hard to produce and air is not transparent for this light.
  • Feshbach resonances are certainly nice to have, but without the points I mentioned above, you could not even cool the atoms, rendering the study of Feshbach resonances impossible.
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    $\begingroup$ > which is hard to produce, but air is not transparent for this light. shouldn't that be: which is hard to produce and air is not transparent for this light. $\endgroup$
    – mcocdawc
    Commented Feb 16 at 14:28
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    $\begingroup$ @mcocdawc sure. Changed that $\endgroup$
    – kai90
    Commented Feb 16 at 17:32
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    $\begingroup$ Apologies but I did not understand the transition frequencies point. What is it and why is it important for cooling atoms and/or making BECs? Why does H2 need UV light? $\endgroup$
    – KugelBlitz
    Commented Feb 21 at 2:28
  • $\begingroup$ No problem. You need to look at the mechanics of laser cooling. In order to cool the atoms with laser light, you need to find an electronic transition (electron gets from a ground state to an excited state and vice versa). A laser needs to address this transition with the correct wavelength. Light of the correct wavelength is nice and easy to have for alkali atoms (e.g. 780nm for Rubidium, 767nm for Potassium). But for hydrogen this wavelength is ~120nm, deep in the UV. $\endgroup$
    – kai90
    Commented Feb 21 at 9:02
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    $\begingroup$ Yes... Rb BEC was the first one. But Ketterle made a Na BEC just a few days/weeks after. I layed out the reason why they chose Rb in the first place. $\endgroup$
    – kai90
    Commented Feb 21 at 13:18
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This is probably related to the presence of a Feshbach resonance, that allows to tune the scattering length via an externally applied magnetic field. A key ingredient to have a Feshbach resonance is the hyperfine coupling between electronic and nuclear degrees of freedom, which is significant when the total electronic angular momentum is non-vanishing (it is $J=1/2$ in the ground state of alkali atoms). In the alkali atoms, the Feshbach resonance can be reached with relatively small magnetic fields, and it can be tuned in a very wide range (from large and negative, to large and positive), thus spanning the whole crossover between the BCS and the BEC regimes. Furthermore, alkali atoms are very popular because they are hydrogenoid, which means that we can describe the electronic orbitals with a very good accuracy adapting the solution to the Schrödinger equation for the hydrogen atom.

Probably there's much more, these are my two cents :)

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