Theoretical and experimental study on line intensities of CO2 and CO transitions near 1.5 μm at high temperatures

Abstract

Accurate spectroscopic parameters of probed species, especially the line strengths at high temperatures, are important for combustion diagnosis based on tunable diode laser absorption spectroscopy (TDLAS). However, most of the line strengths in databases are measured at normal atmospheric temperature and calculated at high temperatures. For example, the mostly used HITRAN database focuses on atmospheric conditions where the temperature ranging from 200-350 K. The high-temperature parameters in HITRAN database are obtained by calculation and the temperatures are limited to 3000 K. In this paper the line strengths of 30012-00001 transition band of CO2 and 3-0 transition band of CO at normal temperature (300 K) and some high temperatures (400-6000 K) are calculated using our calculated partition function and experimental transition moment squared and Herman-Waills factor coefficients. The total internal partition sums (TIPS) are calculated for CO2 and CO with the product approximation of the vibrational and the rotational partition functions. The vibrational partition function is calculated in harmonic oscillator approximation. For rotational partition sums, the centrifugal distortion corrections are taken into consideration. In order to validate the calculation, a high-temperature measurement system based on TDLAS is developed using a DFB diode laser near 1.573 μm. High-resolution absorption spectra of CO2 and CO can be recorded in a heated cell as a function of temperature and pressure. The 9 lines of CO2 30012-00001 band and 2 lines of CO 3-0 band have been measured by means of direct absorption spectroscopy in the temperature range of 300-800 K. The corresponding line strengths are inferred from the measured direct absorption spectrum. The calculated result and experimental data are compared with the HITRAN values. The calculated and measured data agree well with the existing databases (HITRAN 2012), the discrepancies being less than 3% for most of the probed transitions. All the results confirm the validity of the calculation of total partition function and the line strengths calculated. The variation of the line strength as a function of temperature for a certain transition is also discussed. While the lower state energy determines the equilibrium molecular population in the unabsorbing state as a function of temperature, how the line strength of a particular transition varies with temperature can also be controlled.