  The properties of helium-filled nanopores in amorphous silicon are elucidated
by combining theoretical knowledge of helium electronic structure with the
results of Scanning Transmission electron microscopy/electron energy loss
spectroscopy (STEM/EELS). Two of the properties determined are the density and
pressure of the confined helium, these being key properties which are needed
for application. The experimental data consists, firstly of the shift of the
helium 1s^2-> 1s2p(^1P) excitation energy from that of a free atom upon
entering a condensed phase, and secondly, the intensities of both the
elastically and inelastically scattered electron beams. Analysis uniting theory
with the STEM/EELS measurements for both the helium-filled pores and earlier
similar studies of helium encapsulated as bubbles in solid silicon is combined
with fully trustworthy and closely related data for helium in its bulk
condensed phases. The non-empirical theory used, containing no free parameters,
is validated by the excellent agreement between the energy shifts predicted for
bulk condensed helium with the entirely independently measured values. The
above comparisons between the pores and bubbles with the bulk material show
that the helium behaviour is essentially the same in all three of these
environments. This means that, although the pressure is clearly temperature
dependent, the other important properties are governed solely by the helium
density. Although the energy of the electron beam used for the pore
measurements differs from that in the bubble experiments, it is shown that
scaling the results of either the pore or bubbles results using standard
scattering theory almost exactly reproduces the experimental results for the
other system. This provides confirmatory evidence that the behaviour of the
helium in the pores is essentially the same as that in the bubbles.
