Orthohydrogen to Parahydrogen: What particle?

[Chalmers]

What subatomic particle is emitted when a diatomic Hydrogen molecule transitions from Orthohydrogen to Parahydrogen? e.g. during cooling

And why can you cool a sample of Hydrogen to assure that all H2 molecules are para-, but you cannot heat to have any more than 3:1 ortho-:para- ?

 

[Drakkith]

What subatomic particle is emitted when a diatomic Hydrogen molecule transitions from Orthohydrogen to Parahydrogen? e.g. during cooling

I assume a photon.

And why can you cool a sample of Hydrogen to assure that all H2 molecules are para-, but you cannot heat to have any more than 3:1 ortho-:para- ?

Well, what's the difference between ortho- and para-?

 

[jfizzix]

And why can you cool a sample of Hydrogen to assure that all H2 molecules are para-, but you cannot heat to have any more than 3:1 ortho-:para- ?

The short reason is that in the limit of infinite temperature, any given hydrogen molecule is equally likely to be in anyone of its four spin spin states. Since there are three spin states for ortho hydrogen and only one for para hydrogen, you get a fixed proportion of 3:1 even in the limit of infinite temperature.

At lower than infinite temperatures, lower energy states will be more likely to be occupied. Still, since there are three times as many high energy spin states to low energy spin states, you can have equilibrium proportions near 3:1 even at room temperature.

At very low temperatures (relatively speaking), the lower energy state is so much more likely to be occupied than any higher energy state, that the proportion goes down and down, until at absolute zero, you get a proportion of 0:1, with all the hydrogen being para hydrogen.