January 2016


The helium hydride ion, HeH+, is thought to be involved in some of the earliest chemistry to occur in the universe, formed through the radiative association reaction

He + H+ → HeH+ + hν

In fact, recent calculations by Bovino et al. indicate that its abundance in the early universe may have been 10 times greater than previously thought.

Since HeH+ is readily destroyed by reactions with most neutral species, it has proven difficult to detect it in the modern era. The most recent effort was a search for the J=1–0 transition toward a high redshift quasar by Zinchenko et al. in 2011. While they observed something very weak shifted slightly from the expected frequency, the weakness, narrow linewidth, and anomalous velocity cast of the feature doubt that it is due to HeH+. Previously, Liu et al. had unsuccessfully sought for HeH+ toward the planetary nebula NGC 7027.

Although isoelectronic with H2, the hydrogen molecule, HeH+ (or protonated helium) has a dipole moment, which gives it two advantages that H2 doesn't have: it can potentially be observed via its rotational spectrum, and it is easier to stabilize when formed via radiative association (stabilization of vibrationally hot species is inefficient if dipole transitions are forbidden as they are in H2). The next protonated noble gas in the series, NeH+, has not been detected to date, but ArH+ has been observed.

UPDATE (4/2019): HeH+ was detected toward NGC 7027 in work reported in April 2019 by Güsten and co-workers.

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