Decay modes and lifetimes of hydrogen metastable states

[snorkack]

Protium atom has two low lying excited states with long lifetimes.
These are:

1. 2s. Decay energy would be 121 nm, but forbidden (no angular momentum difference). Fastest allowed decay is two-photon emission, lifetime 0,15 s
2. Triplet 1s. Decay energy 211 mm. Prevalent decay single photon emission, lifetime 10 million years.

But diprotium molecule also has a lot of low lying excited states which lack dipole moments and whose decay is therefore forbidden. Which are their prevalent decay paths and lifetimes in vacuum, excluding collision-induced dipoles? The lower states being:

1. First vibrational excited state - decay energy 2,12 μm
2. Parahydrogen first rotational excited state, J=2 (due to Pauli principle, J=1 state cannot exist for parahydrogen) - decay energy 28,22 μm
3. Spontaneous decay of orthohydrogen - decay energy about 85 μm, but rather strongly forbidden. How strongly exactly - what´s fastest allowed decay mode, and the lifetime?

 

[BigJDubs]

The first vibrational excited state of the diprotium molecule has no dipole moment and is therefore forbidden to decay. The predominant decay path is through non-radiative processes such as collisional quenching. The lifetime in this case is very short, on the order of nanoseconds.The parahydrogen first rotational excited state (J=2) is also forbidden to decay, but the predominant decay path is through intersystem crossing via photon-induced spin-orbit coupling, with a lifetime of approximately 10-15 microseconds.Finally, the spontaneous decay of orthohydrogen is very strongly forbidden due to its large energy gap. The fastest allowed decay path is likely through Auger electron emission, with a lifetime of approximately 5-10 picoseconds.