  To evaluate the feasibility of the recently proposed detection scheme of low
energy neutrinos released from the Sun and supernovae called CLEAN, Cryogenic
Low Energy Astrophysics with Noble Gases, which relies on the transparency of
noble-gas cryogenic liquids to VUV radiation produced by neutrinos, we analyze
theoretically VUV emission, self-absorption, and scattering of liquid helium
and neon, primary candidates for CLEAN. Owing to strong repulsion of noble-gas
atoms in the ground states at the equilibrium distance of the relevant excited
state, the emission spectrum is substantially shifted from the absorption
spectrum, and in principle the absorption is expected very small, allowing
building large detectors. Our analysis, however, shows that the self-absorption
and Rayleigh scattering are comparable to the size of the proposed detector.
  Our theoretical emission spectra are found in agreement with experimental
observations although some deviation exists due to binary-interaction
approximation, and our ab initio Rayleigh scattering lengths are found in
agreement with other calculations based on the extrapolation of experimental
refraction indices. The absorption process can result in either re-emission,
which conserves the number of photons but delays their escape from the liquid,
or in non-radiative quenching.
