Intensity of Raman modes as a temperature gauge in fluid hydrogen and deuterium

The Raman spectra of liquid H 2 (D 2) have been collected in diamond anvil cell as a function of temperature at 3 GPa covering the range from 80 to ∼ 1000 K. Temperatures were measured using two independent methods: by thermocouple and from the relative intensity ratio of the present Raman modes. We find excellent agreement between the two methods in the low temperature regime (80 to 400 K) but observe discrepancies between these approaches at temperatures above 400 K. We attribute that the temperature difference between the two methods arises primarily from the proximity of the thermocouple relative to the heating elements and sample. Although not always available in high-pressure experiments, the metrology based on in situ physical properties of the sample is absolute and more reliable than the secondary gauges based on external devices.