Is it possible to have a diatomic molecule of sodium in gaseous state?

Already I know that hydrogen, all the halogens, nitrogen and oxygen forms diatomic molecules. But I am confused about $\ce{Na}$? So I would like to know about that.




According to molecular orbital theory, disodium should be stable in the gas phase, with a bond order of one. The molecular orbital diagram is the same for all the alkali metals since they all have one valence electron in an $s$ orbital.


![enter image description here](https://i.stack.imgur.com/H4JRk.jpg)


([image source](http://2012books.lardbucket.org/books/principles-of-general-chemistry-v1.0/s13-molecular-geometry-and-covalen.html#averill_1.0-ch09_s03_s02_f02))


NIST chemistry webbook has a small page on [disodium](http://webbook.nist.gov/cgi/cbook.cgi?ID=C25681792&Units=SI&Mask=1#Thermo-Gas) and quotes an enthalpy change of formation of $142.07~\mathrm{kJ~mol^{-1}}$ which is moderately endothermic.


[This](http://2012books.lardbucket.org/books/principles-of-general-chemistry-v1.0/s13-molecular-geometry-and-covalen.html#averill_1.0-ch09_s03_s02_f02) page has a brief discussion of alkali metal dimers and claims that they are all stable in the gas phase, in agreement with the prediction from MO theory.


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The disodium molecule $\ce{Na2}$ has first been observed by M. Polanyi and collaborators in the diluted flame of sodium vapor and chlorine $\ce{Cl2}$. When sodium metal is heated in a vacuum, it gets vaporized. If now a tiny amount of gas $\ce{Cl2}$ is sent into this vapor, the famous yellow flame of $\ce{Na}$ is produced at the point where both gases are in contact to one another. This diffusion flame is due to three consecutive reactions. First : $$\ce{Na + Cl2 -> NaCl + Cl}$$ Then the chlorine atom cannot react with atomic sodium $\ce{Na}$ since there is no third body to remove the reaction energy. So the $\ce{Cl}$ atom does react with the dimer $\ce{Na2}$ in the reaction $$\ce{Cl + Na2 -> NaCl + Na'}$$ and this reaction is exothermic enough to produce an excited $\ce{Na'}$ atom. Sorry ! this $\ce{Na'}$ should have been printed as $\ce{Na}$ with a star, but my keyboard refuses to print this symbol (\*) in index. Now this excitation energy ir reemitted as the yellow D-line of sodium ($\pu{589.89 nm}$) $$\ce{Na' -> Na + h\nu }$$ This was the first proof that the dimer $\ce{Na2}$ exists in the sodium vapor.


Ref,: 1) M. Polanyi, Atomic reactions, William and Norgate, London, 1932.
2) M. G. Evans, M. Polanyi, Transactions Faraday Soc. 35, 178, 1935


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