The abstract of the new Nature paper Astrophysical detection of the helium hydride ion HeH+ tell us that infrared spectroscopy from SOFIA detected
...rotational ground-state transition of HeH+ at a wavelength of 149.1 micrometres in the planetary nebula NGC 7027.
It also says:
In this metal-free and low-density environment, neutral helium atoms formed the Universe’s first molecular bond in the helium hydride ion HeH+ through radiative association with protons. As recombination progressed, the destruction of HeH+ created a path to the formation of molecular hydrogen.