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

Abstract

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.