  Hydrogen and helium are the most abundant elements in the universe and they
constitute the interiors of gas giant planets. Thus, their equations of states,
phase, chemical state, and chemical reactivity at extreme conditions are of
great interest. Applying Raman spectroscopy, visual observation, and
synchrotron X-ray diffraction in diamond anvil cells (DAC), we performed
experiments on H2-He 1:1 and D2-He 1:10 compressed gas mixture up to 100 GPa at
300 K. By comparing with the available data on pure bulk materials, we find no
sign of miscibility, chemical reactivity, and new compound formation.
