Metallic hydrogen: the question of metastability

We have studied the properties of MH under pressure in a diamond anvil cell. The reflectance was measured at five wavelengths in the near infrared, using narrow band light emitting diodes (LEDs) to establish that the sample was in the metallic phase at 480-500 GPa. MH has been predicted to possibly be metastable, i.e., it might remain in the metallic phase when the pressure is lifted. Theoretical predictions have scattered results, one predicting that the metastability pressure is ~ 200 GPa, and another that it is as low as 10-20 GPa. We have observed that MH is not metastable at zero pressure and possibly remains metallic until pressure was reduced below ~100 GPa. We shall discuss the techniques that were used for this determination.