  Two different microscopic phases have been observed via inelastic neutron
scattering for superfluid helium confined within highly monodisperse, smooth,
and unidimensional silica pores. When the helium forms a thin film on the pore
walls, it supports a dramatically modified phonon-roton spectrum as well as a
2D surface roton. The energies of these modified phonon-roton modes are
consistent with predictions for a dilute, low-density film, while the energy of
the 2D surface roton corresponds to that of a dense film. The relatively smooth
and weak surface potential of the substrate permits the formation of a film
with large dilute regions and only small compressed regions. When the pores are
saturated with liquid, the modified phonon-roton spectrum disappears, and
bulk-like modes coexist with the 2D surface roton. Presumably, bulk-like liquid
occupies the core volume of the pores and high density liquid layers are
present at the liquid-solid interface. These findings clearly connect the
nature of the excitations to the local density of the liquid.
