  We present a new technique for measuring the critical temperature Tc in the
high pressure, high Tc electron-phonon-driven superconducting hydrides. This
technique does not require connecting leads to the sample. In the multiphonon
region of the absorption spectrum, the reflectance mirrors the temperature
variation of the superconducting order parameter. For an appropriately chosen
value of photon energy of order twice the gap plus 1.5 times the maximum phonon
energy, the temperature dependence of the reflectance varies much more rapidly
below T=Tc than above. It increases with increasing temperature in the
superconducting state while it decreases in the normal state. Examining the
temperature dependence of the reflectance at a fixed photon energy, there is a
cusp at T=Tc which provides a measurement of the critical temperature. We
discuss these issues within the context of the recently observed metallic phase
of hydrogen.
