What causes the drop of latent heat at lower temperatures in liquid helium?

• B
• snorkack
In summary, the article discusses the dependency of liquid helium's latent heat of evaporation on temperature, specifically the conspicuous maximum observed in the graph and table presented in Section 16. This maximum occurs at around 3.1 K and 94 J/mol for 4He. The article also mentions a similar graph for 3He, with a lower heat of evaporation and lower temperature for the maximum. The drop in latent heat at higher temperatures is a common occurrence in liquids due to thermal expansion and increased vapor density. However, the reason for the drop in latent heat at lower temperatures is still unknown and not commonly discussed. Comment 5 on page 48 states that at low temperatures, the latent heat can be calculated using the equation L
snorkack
The dependency of liquid He latent heat of evaporation on temperature shows a conspicuous maximum.
https://www.nist.gov/system/files/documents/srd/jpcrd551.pdf for
4He. Section 16, pages 1264-1266 on pages, 48-50 on pdf. From fig. 16.1 and table 16.3, the maximum is around 3,1 K and 94 J/mol. At lower temperatures, the latent heat drops sharply, though not to zero but 59,83 J/mol
3He has similar graph, though with lower heat of evaporation and lower temperature for maximum.

The drop of latent heat at higher temperatures is natural and common to liquids. With thermal expansion, the binding energy of liquid decreases; with increased vapour density, the binding energy of vapour increases and equalizes to the liquid binding energy at critical point.

What might be the reason for the drop of latent heat at lower temperature? I have not heard of it commented as any special property of liquid He.
Comment 5 at page 48 states that at low temperatures
L=L0+(5/2)RT
What is (5/2)R? Heat capacity of gas (the liquid´s being comparatively negligible)? Then which is the applicable one? He is monoatomic, so 3/2 would be constant volume, 5/2 constant pressure?

Lord Jestocost
Lambda point does make a notch in 4He latent heat, but that´s a completely different issue. Looks like the issue is that of the heat content of gaseous vs. condensed phases... and all substances should have a drop of latent heat of evaporation at low temperatures.

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