Shift in the Raman symmetric stretching band of N2, CO2, and CH4 as a function of temperature, pressure, and density
The Raman spectra of pure N 2 , CO 2 , and CH 4 were analyzed over the range 10 to 500 bars and from −160°C to 200°C (N 2 ), 22°C to 350°C (CO 2 ), and −100°C to 450°C (CH 4 ). At constant temperature, Raman peak position, including the more intense CO 2 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 CO 2 and CH 4 increases (shifts to higher wave number) with increasing temperature over the entire PT range studied. In contrast, N 2 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 N 2 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 N 2 , CO 2 , and CH 4 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 N 2 -CO 2 -CH 4 gas systems in a future study.