Re: Have you heard of an experiment involving metastable Nitrogen and Arsenic?

Metastable nitrogen is an excited state of the N2 molecule, and as such is much more reactive than ground-state nitrogen. It behaves in a similar manner to singlet oxygen, but is generally harder to make. It tends to react as a 1,2-diradical, as in the reaction with simple alkanes to form a pair of RNH radicals (Bull. Chem. Soc. Jpn., 1997 (70) 2951), but I'm afraid I couldn't find any references to the experiment you mention. It is very probable that such a high-energy species would react with arsenic, possibly explosively.

However, just because a reaction proceeds explosively doesn't automatically mean it is useful as rocket fuel. Rocket fuels need to react quickly, cleanly, and with the release of as many gaseous molecules per mole of fuel as possible. Hence the use of hydrogen/oxygen and hydrazine based fuels: both of these reactions can proceed explosively if unchecked, but the utility lies in the manner in which they can be controlled, not the impressiveness of the explosion.

As for your last point, not having found the reference you mention (do you know when this experiment was performed?), I can't really say what else the same procedure could be applied to. Helium is already the stablest species we know of -- it can't even be solidified -- so I doubt that further stabilisation would really help. Also by "single H" do you mean the monatomic H radical (which can be produced, and is plentiful in outer space), or singlet H, another excited state like metastable nitrogen? The former would be a useful thing to turn into rocket fuel, and there have been experiments going on into the possibility of stabilizing it in the form of "hydroxonium", a hypothetical metallic substance which contains H3O+ ions metallically bonded in a 'sea' of free electrons. See this article for more information.

Hope that helped... if you find out any more about the reaction, I might be able to get further.