Helium dimer

Key takeaways

Helium dimer belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Helium dimer to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Helium dimer from memory before moving on to harder problems.

Reference excerpt

A helium dimer is a molecule composed of two helium atoms. One type of helium dimer is a van der Waals molecule with formula He2 consisting of two helium atoms. This chemical is the largest diatomic molecule—a molecule consisting of two atoms bonded together. The bond that holds this dimer together is so weak that it will break if the molecule rotates or vibrates too much. It can only exist at very low cryogenic temperatures. Another type of helium dimer is two excited helium atoms bonded to each other. This form is called an excimer, and was discovered from a spectrum of helium that contained bands first seen in 1912. Written as He2* with the * meaning an excited state, it is the first known Rydberg molecule. Several dihelium ions also exist, having net charges of negative one, positive one, and positive two. Also two helium atoms can be confined together without bonding in the cage of a fullerene.

Molecule Based on molecular orbital theory, He2 should not exist, and a chemical bond cannot form between the atoms. However, the van der Waals force exists between helium atoms as shown by the existence of liquid helium, and at a certain range of distances between atoms the attraction exceeds the repulsion. So a molecule composed of two helium atoms bound by the van der Waals force can exist. The existence of this molecule was proposed as early as 1937. He2 is the largest known molecule of two atoms when in its ground state, due to its extremely long bond length. The He2 molecule has a large separation distance between the atoms of about 5,200 picometres (52 Å). This is the largest for a diatomic molecule without rovibronic excitation. The binding energy is only about 1.3 mK, 10−7 eV or 1.1×10−5 kcal/mol. Both helium atoms in the dimer can be ionized by a single photon with energy 63.86 eV. The proposed mechanism for this double ionization is that the photon ejects an electron from one atom, and then that electron hits the other helium atom and ionizes that as well. The dimer then explodes as two helium cations repel each other, moving with the same speed but in opposite directions. A dihelium molecule bound by Van der Waals forces was first proposed by John Clarke Slater in 1928.

Formation The helium dimer can be formed in small amounts when helium gas expands and cools as it passes through a nozzle in a gas beam. Only the isotope 4He can form molecules like this; 4He3He and 3He3He do not exist, as they do not have a stable bound state. The amount of the dimer formed in the gas beam is of the order of one percent.

Molecular ions He2+ is a related ion bonded by a half covalent bond. It can be formed in a helium electrical discharge. It recombines with electrons to form an electronically excited He2(a3Σ+u) excimer molecule. Both of these molecules are much smaller with more normally sized interatomic distances. He2+ reacts with N2, Ar, Xe, O2, and CO2 to form cations and neutral helium atoms. The helium dication dimer He22+ releases a large amount energy when it dissociates, around 835 kJ/mol. However, an energy barrier of 138.91 kJ/mol prevents immediate decay. This ion was studied theoretically by Linus Pauling in 1933. This ion is isoelectronic with the hydrogen molecule. He22+ is the smallest possible molecule with a double positive charge. It is detectable using mass spectroscopy. The negative helium dimer He2− is metastable and was discovered by Bae, Coggiola and Peterson in 1984 by passing He2+ through caesium vapor. Subsequently, H. H. Michels theoretically confirmed its existence and concluded that the 4Πg state of He2− is bound relative to the a2Σ+u state of He2. The calculated electron affinity is 0.233 eV compared to 0.077 eV for the He−[4P∘] ion. The He2− decays through the long-lived 5/2g component with τ~350 μsec and the much shorter-lived 3/2g, 1/2g components with τ~10 μsec. The 4Πg state has a 1σ2g1σu2σg2πu electronic configuration, its electron affinity E is 0.18±0.03 eV, and its lifetime is 135±15 μsec; only the v=0 vibrational state is responsible for this long-lived state. The molecular helium anion is also found in liquid helium that has been excited by electrons with an energy level higher than 22 eV. This takes place firstly by penetration of liquid He, taking 1.2 eV, followed by excitation of a He atom electron to the 3P level, which takes 19.8 eV. The electron can then combine with another helium atom and the excited helium atom to form He2−. He2− repels helium atoms, and so has a void around it. It will tend to migrate to the surface of liquid helium.

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Illustrations

Worked examples

Example 1 — a first encounter with Helium dimer

Start with the simplest possible case. Write down what Helium dimer claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Helium dimer before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Helium dimer ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Helium dimer

In research: Helium dimer appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Helium dimer in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Helium dimer is common in secondary-school and first-year university syllabi. It links to neighbouring topics Allotropes, Dimers (chemistry), Helium compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Helium dimer outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Helium dimer in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Helium dimer means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Helium dimer out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Helium dimer in simple terms?

A helium dimer is a molecule composed of two helium atoms. One type of helium dimer is a van der Waals molecule with formula He2 consisting of two helium atoms.

Why does Helium dimer matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Helium dimer?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Helium dimer.