Determination of H₂ Densities Over a Wide Range of Temperatures and Pressures Based on the Spectroscopic Characterization of Raman Vibrational Bands

Raman spectroscopy is a powerful method for determining the densities of gas species in fluid inclusions, especially for H₂-bearing inclusions in which the microthermometry approach is difficult to apply. The relationships between Raman peak position and H₂ density have been recorded in several previous studies. However, systematic discrepancies exist among these studies. In this study, the Raman spectral parameters (peak position, width, and intensity) of the vibrational bands of H₂ (Q₁(0), Q₁(1), Q₁(2), and Q₁(3)) were systematically measured at temperatures from 25 to 400 °C and pressures up to 150 MPa using a high-pressure optical cell. The variation in each parameter as a function of H₂ density was discussed. Several calibration polynomials derived from the measured peak positions and peak widths of these vibrational bands and the peak intensity ratios of Q₁(1) to Q₁(n = 0, 2, 3) were established to determine H₂ densities up to 0.062 g/cm³ at 25 °C. For natural fluid inclusions, the peak position of the Q₁(1) band is the best choice for density determination mainly because (i) Raman spectra derived from fluid inclusions are not always of applicable qualities and the strongest intensity Q₁(1) band could be obtained easier than others, and (ii) the peak position is insensitive to instrumental factors. The relationship between the peak position of Q₁(1) band and density can be represented by ΔQ₁(1) = 90,246.070 × ρ⁴ - 5471.203 × ρ³ + 770.944 × ρ²- 41.038 × ρ (r² = 0.999), where ρ is the density of H₂ in g/cm³; ΔQ₁(1) (cm^-1) is the difference between the obtained peak position of Q₁(1) band of H₂ and the known peak position of Q₁(1) band of H₂ at near-zero density. This polynomial is independent of instrumental factors and can be applied in any laboratory, as long as the peak position of H₂ with a near-zero density is known. The effects of temperature on the relationship between these spectral parameters and H₂ density were also examined.