  We simulate atmospheric fractionation in escaping planetary atmospheres using
IsoFATE, a new open-source numerical model. We expand the parameter space
studied previously to planets with tenuous atmospheres that exhibit the
greatest helium and deuterium enhancement. We simulate the effects of
EUV-driven photoevaporation and core-powered mass loss on deuterium-hydrogen
and helium-hydrogen fractionation of sub-Neptune atmospheres around G, K, and M
stars. Our simulations predict prominent populations of deuterium- and
helium-enhanced planets along the upper edge of the radius valley with mean
equilibrium temperatures of 370 K and as low as 150 K across stellar types. We
find that fractionation is mechanism-dependent, so constraining He/H and D/H
abundances in sub-Neptune atmospheres offers a unique strategy to investigate
the origin of the radius valley around low-mass stars. Fractionation is also
strongly dependent on retained atmospheric mass, offering a proxy for planetary
surface pressure as well as a way to distinguish between desiccated enveloped
terrestrials and water worlds. Deuterium-enhanced planets tend to be
helium-dominated and CH4-depleted, providing a promising strategy to observe
HDO in the 3.7 um window. We present a list of promising targets for
observational follow-up.
