Shift in the Raman symmetric stretching band of N2, CO2, and CH4 as a function of temperature, pressure, and density

Abstract

AbstractThe Raman spectra of pure N2, CO2, and CH4 were analyzed over the range 10 to 500 bars and from −160°C to 200°C (N2), 22°C to 350°C (CO2), and −100°C to 450°C (CH4). At constant temperature, Raman peak position, including the more intense CO2 peak (ν+), decreases (shifts to lower wave number) with increasing pressure for all three gases over the entire pressure and temperature (PT) range studied. At constant pressure, the peak position for CO2 and CH4 increases (shifts to higher wave number) with increasing temperature over the entire PT range studied. In contrast, N2 first shows an increase in peak position with increasing temperature at constant pressure, followed by a decrease in peak position with increasing temperature. The inflection temperature at which the trend reverses for N2 is located between 0°C and 50°C at pressures above ~50 bars and is pressure dependent. Below ~50 bars, the inflection temperature was observed as low as −120°C. The shifts in Raman peak positions with PT are related to relative density changes, which reflect changes in intermolecular attraction and repulsion. A conceptual model relating the Raman spectral properties of N2, CO2, and CH4 to relative density (volume) changes and attractive and repulsive forces is presented here. Additionally, reduced temperature‐dependent densimeters and barometers are presented for each pure component over the respective PT ranges. The Raman spectral behavior of the pure gases as a function of temperature and pressure is assessed to provide a framework for understanding the behavior of each component in multicomponent N2‐CO2‐CH4 gas systems in a future study.