In which state is matter more opaque to radiation? I'm talking about gaseous vs plasma. I know that opacity is a function of frequency in general, but I thought that, since a non-ionized gas has more degrees of freedom (since it has internal structure i.e. Electrons on shell) it has "more ways" to absorb or interact with Light, which would make it more opaque. But know studying cepheid variables it seems that Helll (doubly ionized helium) is more opaque than its less ionized partner Hell (singly ionized helium). Is this a particular case depending on the temperature of the radiation, or what am I getting wrong?
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In thinking about it you neglect the free electrons in a plasma. They can basically absorb photons of every energy while, as you said, those bound in atoms and ions cannot.
