6.7 ± 0.6 × 1016 s. These values differ appreciably
from a fit to the uncorrected data.
Because the diamond culet is stressed, we expect the index of refraction in the region of contact with the MH to change from the value at
ambient pressure, and this might lead to an
important uncertainty in the fitting parameters.
The index of the diamond under pressure and
uniaxial stress has been studied by Surh et al.
(34). For hydrostatic pressures up to 450 GPa,
the pressure dependence is rather weak; however, for uniaxial stressed diamond the change
of index can be substantial. We fitted data for
values of 2.12 for extreme uniaxial stress along the
 crystal direction and 2.45 along the 
and  directions, corresponding to a sample
pressure of 250 GPa (34). This resulted in an
uncertainty in the Drude parameters that was
much smaller than that because of the uncertainty in the measured reflectance (Fig. 3). We
fit the reflectance using a value ND = 2.41, yielding
the values for the Drude parameters shown in Fig.
3 and Table 1.
The pressure-temperature phase diagram of
hydrogen along pathway I is shown in Fig. 4,
focusing on the lower-temperature region. With
increasing pressure, hydrogen enters the phase
H2-PRE at 355 GPa, and this is followed by the
phase line based on the two points at temperature (T) = 83 and 5.5 K for the transition to MH.
(24). Because we changed the pressure in larger
Fig. 3. Reflectance as a function of photon energy. (A) The energy dependence of the normal incidence reflectance off MH and the Re gasket,
P = 495 GPa, at liquid-nitrogen and liquid-helium temperatures. We also
show our measured reflectance from a surface of Re at a pressure of 1 bar
at room temperature; this is in good agreement with literature values,
verifying our measurement procedure. The reflectances have been cor-
rected for absorption in the diamond. The lines through MH data are fits
with a Drude free-electron model; the lines through the Re data points are
guides to the eye. (B) Raw reflectance data without the diamond ab-
sorption correction. The uncertainties in the data points are from mea-
surement of the reflectance and the correction procedure and represent
Fig. 4. The T-P phase diagram of hydrogen along pathway I of Fig. 1. The data show the thermodynamic
pathway followed for our measurements. We also show other recent data for the phases at lower pressures
from Zha et al. (41), Howie et al. (23), Eremets et al. (25), and Dias et al. (24). A transition claimed by
Dalladay-Simpson et al. (42) at 325 GPa is plotted as a point, as is the earlier observation of black
hydrogen by Loubeyre et al. (28).