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It is well-known that there exists the $\ce{He2^2+}$ dication, and also $\ce{HHe+}$ and $\ce{H2+}$ cations. But I could not find any definite information about the existence or inexistence of the $\ce{HHe^2+}$ cation.

It would be isoelectronic to $\ce{H2+}$, but would have a weaker bonding due to greater helium nucleus charge. It looks unlikely to be stable, but did anyone prove it by calculation or experiment?

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    $\begingroup$ There is a paper by S. K. Knudson, Semiclassical energies of low-lying states of one-electron diatomics. By skipping through the paper I found some calculations on your compound. But I didn't have time to read through yet. So perhaps it's not helpful at all to you. $\endgroup$ – Justanotherchemist Dec 18 '18 at 12:32
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    $\begingroup$ Just in case you want to look at the paper though: doi.org/10.1016/S0301-0104(00)00006-9 In my first skim I found the following quote (tl;dr): 'The only bound ground state is that for $\ce{H2+}$; the other cations have repulsive ground states. ' $\endgroup$ – Martin - マーチン♦ Dec 18 '18 at 16:57
  • $\begingroup$ @Martin-マーチン, however they might have stable excited states. Surely there must be at least one. This is also the case for the He2 dimer. $\endgroup$ – user1271772 Dec 18 '18 at 19:22
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I am not sure how to elaborate much beyond saying that yes, there are bound states of $\ce{HHe^{2+}}$. I cite three papers below which give numerical calculations of various states, ref. 1 gives calculations for the lowest 20 states of the system.

It appears to be that the 1s$\sigma$ is not a bound state in this system, but the 2p$\sigma$ state is bound. I would expect there are some other bound states as well.

References:

[1] Winter, T. G., Duncan, M. D., & Lane, N. F. (1977). Exact eigenvalues, electronic wavefunctions and their derivatives with respect to the internuclear separation for the lowest 20 states of the HeH2+ molecule. Journal of Physics B: Atomic and Molecular Physics, 10(2), 285.

[2] Laaksonen, L., Pyykkö, P., & Sundholm, D. (1983). Two‐dimensional fully numerical solutions of molecular Schrödinger equations. I. One‐electron molecules. International Journal of Quantum Chemistry, 23(1), 309-317.

[3] Buehler, E. J., Gooch, E. E., Dial, J. L., & Knudson, S. K. (2000). Semiclassical energies of low-lying states of one-electron diatomics. Chemical Physics, 253(2-3), 219-230.

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