Trihydrogen
Triatomic hydrogen or H3 is an unstable triatomic molecule containing only hydrogen. Since this molecule contains only three atoms of hydrogen it is the simplest triatomic molecule and it is relatively simple to numerically solve the quantum mechanics description of the particles. Being unstable the molecule breaks up in under a millionth of a second. Its fleeting lifetime makes it rare, but it is quite commonly formed and destroyed in the universe thanks to the commonness of the trihydrogen cation. The infrared spectrum of H3 due to vibration and rotation is very similar to that of the ion, H3+. In the early universe this ability to emit infrared light allowed the primordial hydrogen and helium gas to cool down so as to form stars.
The neutral molecule can be formed in a low pressure gas discharge tube.
A neutral beam of H3 can be formed from a beam of H3+ ions passing through gaseous potassium, which donates an electron to the ion, forming K+. Other gaseous alkali metals, such as caesium, can also be used to donate electrons. H3+ ions can be made in a duoplasmatron where an electric discharge passed through low pressure molecular hydrogen. This causes some H2 to become H2+. Then H2 + H2+→ H3+ + H. The reaction is exothermic with an energy of 1.7eV, so the ions produced are hot with much vibrational energy. These can cool down via collisions with cooler gas if the pressure is high enough. This is significant because strongly vibrating ions produce strongly vibrating neutral molecules when neutralised according to the Franck–Condon principle.
H3 can break up in the following ways:
The molecule can only exist in an excited state. The different excited electronic states are represented by symbols for the outer electron nLΓ with n the principal quantum number, L is the electronic angular momentum, and Γ is the electronic symmetry selected from the D3h group. Extra bracketed symbols can be attached showing vibration in the core: {s,dl} with s representing symmetrical stretch, d degenerate mode, and l vibrational angular momentum. Yet another term can be inserted to indicate molecular rotation: (N,G) with N angular momentum apart from electrons as projected on the molecular axis, and G the Hougen's convenient quantum number determined by G=l+λ-K. This is often (1,0), as the rotational states are restricted by the constituent particles all being fermions. Examples of these states are: 2sA1' 3sA1' 2pA2" 3dE' 3DE" 3dA1' 3pE' 3pA2". The 2p2A2" state has a lifetime of 700 ns. If the molecule attempts to lose energy and go to the repulsive ground state, it spontaneously breaks up. The lowest energy metastable state, 2sA1' has an energy -3.777 eV below the H3+ and e− state but decays in around 1 ps. The unstable ground state designated 2p2E' spontaneously breaks up into a H2 molecule and an H atom. Rotationless states have a longer life time than rotating molecules.
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