  We report on Muonium (Mu) emission into vacuum following {\mu}+ implantation
in mesoporous thin SiO2 films. We obtain a yield of Mu into vacuum of (38\pm4)%
at 250 K temperature and (20\pm4)% at 100 K for 5 keV {\mu}+ implantation
energy. From the implantation energy dependence of the Mu vacuum yield we
determine the Mu diffusion constants in these films: D250KMu = (1.6 \pm 0.1)
\times 10-4 cm2/s and D100KMu = (4.2\pm0.5)\times10-5 cm2/s. Describing the
diffusion process as quantum mechanical tunneling from pore-to-pore, we
reproduce the measured temperature dependence T^3/2 of the diffusion constant.
We extract a potential barrier of (-0.3 \pm 0.1) eV which is consistent with
our computed Mu work-function in SiO2 of [-0.3,-0.9] eV. The high Mu vacuum
yield even at low temperatures represents an important step towards next
generation Mu spectroscopy experiments.
