TL;DR

• “Helonium” is a historical/common name for the helium hydride cation, chemical formula HeH⁺ — the simplest heteronuclear ion and, by many accounts, the first molecular bond to form in the cooling early Universe.
• It was first produced in laboratory experiments in the 1920s and—after decades of searching—detected in space in 2019 using the SOFIA airborne observatory.
• HeH⁺ is an object of outsized importance for astrochemistry and fundamental molecular physics: it’s extremely reactive, impossible to store under normal conditions, yet its properties (spectra, formation and destruction chemistry, dipole moment) make it a critical piece of the story of how atoms turned into molecules after the Big Bang. (Wikipedia)

 

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1 — What is “Helonium”? Names, formula, and why it matters

“Helonium” is a non-systematic name sometimes used for the helium hydride ion, more properly written HeH⁺ or “hydridohelium(1+)”. It’s a diatomic molecular cation made of a helium nucleus bound to a protonated hydrogen (one electron removed overall), so it has two nuclei and one electron pair in total. HeH⁺ is the lightest heteronuclear ion (different nuclei) and is isoelectronic with H₂ (i.e., similar electron count), but because the two nuclei differ (He and H), the ion has a permanent dipole moment and a richer spectroscopic signature than the homonuclear H₂⁺ system. (Wikipedia)

Why does this tiny ion get so much attention? Because in the standard cosmological timeline HeH⁺ is thought to be the first molecular bond to form when the hot early Universe cooled after Big-Bang nucleosynthesis. That makes it a cornerstone of early-Universe chemistry and a gateway species in the chemical networks that ultimately led to H₂ and then star formation. The astrophysical detection of HeH⁺ confirms a key step in our understanding of primordial chemistry. (arXiv)

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2 — Short history: discovery in the lab and the long hunt in space

Laboratory production and early experiments
HeH⁺ was first produced and identified in gas-phase experiments in the 1920s (work often cited to Hogness & Lunn, 1925). Early mass-spectrometric and ion-beam experiments showed that when hydrogen and helium mixtures are ionized, the helium hydride cation can form transiently. Those early laboratory observations established HeH⁺ as a real chemical species long before astronomers knew where to look for it in space. (Physical Review Links)

Astrophysical quest and the 2019 detection
For decades astrophysicists predicted HeH⁺ should exist in certain hot, ionized astrophysical plasmas (planetary nebulae, helium-rich white dwarfs, fast shocks). But actually detecting its rotational line from Earth is hard: the strongest rotational transition of HeH⁺ lies in the far-infrared / terahertz band (~149.1 µm), a region partially blocked or contaminated by Earth’s atmosphere and by spectral lines from other molecules (e.g., CH). In April 2019 an international team using the GREAT spectrometer on board the SOFIA (Stratospheric Observatory for Infrared Astronomy) high-altitude airplane reported the first unambiguous astrophysical detection of HeH⁺ in the planetary nebula NGC 7027, confirming long-standing theory. This detection was published in Nature and announced by collaborating institutions. (Nature)

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3 — Molecular structure and fundamental physical properties

Key numbers (order-of-magnitude and precise experimental/theoretical values):

• Formula: HeH⁺ (habilitation: protonated helium). (Wikipedia)
• Bond length (rₑ): ≈ 0.77–0.79 Å (≈ 77–79 pm), from experimental Born-Oppenheimer potentials and high-level ab initio calculations. (PubMed)
• Dipole moment: nonzero — computed values in literature are around 2.3–2.8 Debye, which enables rotational transitions to be dipole-allowed and observable in emission/absorption. (Wikipedia)
• Rotational ground-state transition (observable signature): the J=1→0 line corresponds to wavelength ~149.14 µm (far-IR / terahertz), and this line was the one detected by SOFIA in NGC 7027. (Nature)

Why these numbers matter: bond length and dipole determine the rotational spectrum and transition strengths; the permanent dipole allows rotational emission in astrophysical plasmas, which is why HeH⁺ is spectroscopically accessible despite being an ion. The combination of small mass and significant dipole yields sharp transitions at far-IR/THz frequencies where modern terahertz spectrometers can operate — but only from high altitude or space because of atmospheric absorption. (Physical Review Links)

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4 — Chemistry: formation, destruction, and the role in primordial gas

Formation pathways (key reactions)
In astrophysical and early-universe contexts the dominant formation reactions for HeH⁺ are:

1. Radiative association:
   He + H⁺ → HeH⁺ + hν
   This is the canonical formation route in a low-density, metal-free environment (a proton attaches to helium and a photon carries off the binding energy). In the recombining primordial gas this slow radiative association is the primary path to make HeH⁺. (arXiv)
2. Ion–molecule routes: in some environments He⁺ + H₂ → HeH⁺ + H (depending on local conditions), or production linked to tritium decay in lab experiments (³H decay can create ³He⁺ that then grabs hydrogen). (Wikipedia)

Destruction pathways
HeH⁺ is extremely reactive and is destroyed rapidly by neutral hydrogen and other neutrals:

• HeH⁺ + H → He + H₂⁺ (followed by H₂⁺ + H → H₂ + H⁺) — this reaction sequence is crucial because it provides a route from HeH⁺ to H₂ formation in the early Universe: HeH⁺ + H → He + H₂⁺, then H₂⁺ + H → H₂ + H⁺. In this way HeH⁺ acts as a catalytic or intermediate species facilitating the first production of molecular hydrogen. (arXiv)

Net effect in cosmology: HeH⁺ forms when helium and protons are present and the gas cools enough for radiative association, then it reacts with neutral hydrogen to help produce H₂ — the first stable molecule that enables efficient cooling of gas and eventually star formation. Thus HeH⁺ is a stepping stone from atomic to molecular chemistry in a pristine, metal-free Universe. (arXiv)

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5 — Acidity, proton affinity, and a note on “strongest acid”

Language caution: “strongest acid” usually needs context. In solution-phase (aqueous) chemistry we measure pKₐ values relative to hydration energies, solvation, etc. HeH⁺ cannot exist in aqueous solution — it protonates essentially anything it meets, so gas-phase descriptors are more appropriate.

Gas-phase acid strength and numbers
HeH⁺ is often called the strongest Brønsted proton acid in the gas phase. Data compiled by evaluated reference tables list the proton affinity of helium (the energy released when helium accepts a proton) as ≈ 177.8 kJ·mol⁻¹ in the gas phase; converting reasonable thermochemical numbers into an aqueous-equivalent estimate yields an extremely negative effective pKₐ ≈ −63 (this is a notional comparison to convey the enormous tendency to donate a proton). That number is illustrative: it shows HeH⁺ is vastly more acidic than conventional superacids in gas-phase thermochemistry terms, but it does not imply you could bottle HeH⁺ acid like a superacid in the lab. The dramatic acidity is a consequence of the weak basicity of neutral helium in the gas phase and the large hydration energy of a proton. (cccbdb.nist.gov)

Practical takeaway: HeH⁺ will immediately protonate most molecules it contacts; it cannot be stored in bulk or placed in a container without instantly reacting. That’s part of why its chemistry must be studied in situ (ion beams, mass spectrometers, traps). (Wikipedia)

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6 — Laboratory methods: how HeH⁺ is made and studied on Earth

Early methods (ion beams & mass spectrometry)
The classic Hogness & Lunn experiments used discharge/ionization techniques on H/He mixtures and then analyzed the ion products with positive-ray / mass spectrometric methods, revealing peaks consistent with HeH⁺. Such gas-phase ionization experiments and ion-beam collision studies remain foundational for producing and characterizing HeH⁺. (Physical Review Links)

Modern techniques for precision spectroscopy
Because HeH⁺ is an ion and cannot be condensed, modern spectroscopy uses high-vacuum ion traps, radiofrequency (Paul) traps, cryogenic ion storage rings, and action spectroscopy (monitoring reaction products after laser excitation) to get high-resolution spectra and benchmark theory. Laboratory microwave and infrared spectroscopy, combined with high-precision quantum-chemical calculations, yields accurate molecular constants (rotational constants, vibrational frequencies, dipole moment) used to predict astrophysical line strengths and frequencies. High-accuracy Born-Oppenheimer potentials have been computed to spectroscopic precision for HeH⁺, which helps astronomers know exactly where to search in the far-IR. (Physical Review Links)

Challenges in the lab: the ion’s reactivity means experiments must be run at high vacuum and with careful timing; often researchers create HeH⁺ in a controlled beam or trap, probe it quickly, and measure decay/reaction channels rather than trying to isolate a bulk sample. (PubMed Central)

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7 — The 2019 SOFIA detection: what was seen and why it mattered

Where was HeH⁺ found?
The detection was reported toward the planetary nebula NGC 7027. Planetary nebulae are shells of ionized gas ejected by dying stars and can host the combination of hot radiation (producing He⁺ and H⁺) and enough neutral material that HeH⁺ can form in detectable column densities. The environment of NGC 7027 was theoretically a promising place to find HeH⁺, and SOFIA’s high-altitude vantage removed much of the water vapor that blocks the far-IR band at ~149 µm. (Nature)

How was the line identified?
The GREAT (German REceiver for Astronomy at Terahertz frequencies) spectrometer on SOFIA measured a line at the expected wavelength (~149.14 µm). The team demonstrated that the line’s strength, shape, and velocity profile matched the HeH⁺ prediction and were inconsistent with likely “interloper” features from other species. This constituted the first unambiguous astrophysical detection of HeH⁺ and closed a long chapter of theoretical expectation vs observational proof. The Nature paper and institutional press releases describe the line identification and the spectroscopic and modeling work behind the claim. (Nature)

Why it mattered (scientific significance)

• It validated the theoretical picture of primordial chemistry in which HeH⁺ plays a role as the first molecular bond after the Big Bang.
• It provided a practical example of how we can detect and model exotic ions in real astrophysical plasmas, informing models for radiative transfer and chemistry in ionized nebulae.
• The detection illustrated the value of terahertz/far-IR instrumentation (SOFIA/GREAT) and motivated follow-up observations with future space telescopes and ground-based submillimeter facilities. (Nature)

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8 — HeH⁺ beyond NGC 7027: where else to look and why it’s tricky

Candidate environments where HeH⁺ might appear in observable quantities:

• Other planetary nebulae with hot central stars and appropriate density/ionization structure.
• Helium-rich white dwarf atmospheres, where HeH⁺ can alter opacity and cooling behavior.
• Dissociative shocks (supernova remnants, fast stellar winds) where energetic conditions can produce He⁺ and H simultaneously.
• Early Universe / primordial gas — not observable directly at the same sensitivity, but HeH⁺ chemistry affects model predictions for early cooling and star formation. (Wikipedia)

Why detections are rare and hard:

• The key rotational line is in a portion of the spectrum heavily affected by atmospheric water absorption — so spaceborne or high-altitude platforms (SOFIA) are needed.
• Spectral confusion: some other astrophysical lines (e.g., certain CH radical lines) lie near the HeH⁺ transition, so high spectral resolution and careful modeling are required to exclude interlopers.
• Column densities of HeH⁺ in most environments are low because the species is rapidly destroyed by neutral H and other species; only in special ionization/density regimes will HeH⁺ accumulate to detectable amounts. (IRSA)

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9 — Theory and computation: pushing spectroscopic precision

HeH⁺ is a favorite benchmark problem in quantum-chemical methods because it is small enough for extremely accurate ab initio calculations yet nontrivial due to mass differences between the nuclei and the need for relativistic/adiabatic corrections for spectroscopic precision. Groups have calculated Born-Oppenheimer potentials and included non-adiabatic, relativistic, and quantum electrodynamic (QED) corrections to match experimental frequencies to high precision. Those theoretical line lists and potential energy surfaces are essential for astronomers to know exactly where to search and how to interpret line strengths and excitation. (Physical Review Links)

Selected theoretical takeaways:

• High-accuracy potentials allow prediction of rotational/vibrational transitions with uncertainties small enough for modern terahertz spectroscopy.
• Computed dipole moments and Einstein A coefficients let astronomers convert observed line intensities to column densities and excitation temperatures.
• Theory also models formation rates (radiative association cross sections) and destruction rates (reactive collisions) needed in chemical network simulations of nebulae and primordial gas. (Physical Review Links)

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10 — Isotopologues and cluster ions: chemistry is richer than HeH⁺ alone

HeH⁺ has several isotopologues depending on the isotopes of H and He (e.g., ³HeH⁺, HeD⁺, etc.). Laboratory work and theory show that heavier isotopologues have slightly shifted line positions and different zero-point energies, which can be useful both in laboratory spectroscopy and potentially in astrophysical searches if isotopic substitutions are relevant. In addition, larger helium–hydrogen clusters like He₂H⁺, He₃H⁺ … He₆H⁺ have been observed/theorized in mass-spectrometric and cold-ion studies; cluster stability depends on size and experimental conditions. These species are less astrophysically important but show the richness of helium–hydrogen ionic chemistry. (Wikipedia)

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11 — Broader implications: astrophysics, cosmology, and teaching

Astrophysics and cosmology

• The detection and study of HeH⁺ test and refine models of primordial chemistry and the timelines for molecule formation in the early Universe. Because HeH⁺ contributes to the pathway for H₂ formation, it indirectly affects predicted cooling rates for primordial gas and therefore the masses and formation epochs of the first stars. (arXiv)
• In local astrophysical contexts (planetary nebulae, white dwarfs), HeH⁺ can affect radiative transfer and opacity; accurate modeling of spectra for these objects needs to include HeH⁺ contributions where relevant. (PubMed Central)

Pedagogy and outreach value
HeH⁺ is an excellent storytelling molecule: it connects laboratory chemistry, quantum mechanics, instrumentation (terahertz spectroscopy), and big-picture cosmology (what was the very first molecule?). The 2019 detection is an attractive case study for courses in astrochemistry and spectroscopy. (Nature)

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12 — Current research frontiers & open questions

Where the field is actively working today:

1. More detections and surveys. After the SOFIA result, astronomers want to search other nebulae and environments and improve statistics on HeH⁺ incidence. High-sensitivity terahertz missions/platforms (and, in future, space far-IR telescopes) can expand the sample. (mpifr-bonn.mpg.de)
2. Precision chemistry under astrophysical conditions. Better experimental and theoretical rate coefficients for radiative association and destruction reactions (temperature/density dependence) are needed to reduce uncertainties in chemical network models. (arXiv)
3. Isotopic studies. Observing isotopologues could reveal clues about nucleosynthetic pathways or local isotopic ratios in extreme astrophysical sites. (EPFL Graph Search)
4. Role in exotic environments. Better modeling of HeH⁺ in shocks or plasma conditions and in helium-rich stellar atmospheres could reveal observational signatures not yet fully explored. (NSF Public Access Repository)
5. Laboratory techniques. New ion-trap, cryogenic storage, and pump-probe spectroscopic methods continue to improve the accuracy of molecular constants and reaction rate measurements. (PubMed Central)

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13 — Short technical appendix (reaction list, spectroscopic constants, quick reference)

Principal formation/destruction reactions (summary):

• Formation: He + H⁺ → HeH⁺ + hν (radiative association). (arXiv)
• Principal destruction: HeH⁺ + H → He + H₂⁺ (followed by H₂⁺ + H → H₂ + H⁺). (arXiv)

Notable spectroscopic line: J=1→0 rotational fundamental at ~149.14 µm (frequently used observational target). (Nature)

Representative molecular constants:

• Bond length rₑ ≈ 0.772 Å. (PubMed)
• Dipole moment ≈ 2.3–2.8 D (literature range). (Wikipedia)
• Proton affinity of He (gas phase): ≈ 177.8 kJ·mol⁻¹ → effective pKₐ estimate ≈ −63 (notional gas→aqueous conversion). (cccbdb.nist.gov)

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14 — Common misconceptions & FAQs

Q: Is helonium a “stable molecule” I can bottle?
A: No. HeH⁺ is highly reactive and exists only in isolation (high vacuum, ion traps, or transiently in plasmas). It will rapidly protonate any ordinary matter, so you can’t store bulk Helonium. (Wikipedia)

Q: Is HeH⁺ the same as HeH (neutral)?
A: No. HeH refers to hypothetical neutral diatomic helium hydride, which is not stable under normal conditions. HeH⁺ is the cationic form that is real and well characterized. (Wikipedia)

Q: Does the SOFIA detection mean HeH⁺ existed right after the Big Bang and we’ve now seen that primordial gas?
A: The SOFIA detection shows HeH⁺ exists in at least one local astrophysical environment (NGC 7027) and confirms key chemical pathways predicted for the early Universe. We cannot directly observe primordial HeH⁺ from recombination epochs (that signal is not directly accessible), but the detection strengthens the theoretical chain by which HeH⁺ would have formed in primordial gas. (Nature)

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15 — Conclusion: small ion, big story

Helonium / HeH⁺ is a supremely satisfying scientific object because it sits at the intersection of the very small (quantum molecular structure and extreme gas-phase acidity) and the very large (cosmology and star formation). The decades-long journey from initial lab reports in the 1920s to the 2019 SOFIA detection shows science at its best: theory predicted, technology matured, observers targeted a promising object, and the Universe delivered a clear signal. Today HeH⁺ remains a live subject for precision spectroscopy, astrochemical modeling, and planning for next-generation far-IR observatories. (Physical Review Links)

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Key references & suggested further reading

• R. Güsten et al., Astrophysical detection of the helium hydride ion HeH⁺, Nature, April 2019 — SOFIA/GREAT detection paper. (Nature)
• MPIfR (Max-Planck Institute) press release on the first astrophysical detection of HeH⁺. (mpifr-bonn.mpg.de)
• DLR / SOFIA press materials summarizing the detection and instrumentation. (dlr.de)
• T. R. Hogness & E. G. Lunn (1925) — early laboratory ionization experiments detecting HeH⁺. (Physical Review Links)
• NIST / CCCBDB listings for proton affinities and experimental constants. (cccbdb.nist.gov)
• LibreTexts / chemistry teaching notes on HeH⁺ acidity and proton affinity conversions. (Chemistry LibreTexts)
• J. A. Coxon & P. G. Hajigeorgiou, Experimental Born-Oppenheimer potential for HeH⁺, Journal of Molecular Spectroscopy, 1999 — molecular constants. (PubMed)
• K. Pachucki, Born-Oppenheimer potential for HeH⁺, Phys. Rev. A (high-precision calculations). (Physical Review Links)