In order for refraction to work, there has to be something for you to see refracted through the refractive object, so you really need that mesh background behind the head to get the glass effect. In fact you should probably have the head inside a mesh room that surrounds it completely. To get the alpha channel afterwards, do a second render with just the head (and you don't need the head to have any materials for this) then take the alpha channel from the second render and use it to isolate the head from the first render.

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