# Liquid He heat of evaporation

• B
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?