Density dependence of retrieved self-broadened CO2 line intensities from requantized molecular dynamics simulations

We report the first theoretical predictions of the density dependence of self-broadened CO 2 line intensities retrieved from fits of absorption spectra using an impact line-shape model. This effect, previously shown to arise from the breakdown of the impact approximation, results in the redistribution of a fraction of the line intensity from a narrow spectrum to a broad pedestal whose width is inversely proportional to the collision duration. Using requantized classical molecular dynamics simulations (rCMDSs), we predicted the spectral density of a CO 2 parallel band at three temperatures (212 K, 250 K, and 296 K) and various densities. For each temperature, the simulated spectra were fitted with the speed-dependent Nelkin–Ghatak profile including first-order line mixing. The results show a clear linear decrease in the retrieved line intensities with increasing density. The theoretical intensity depletion coefficients are in good agreement with previously measured values obtained from Fourier-transform spectra. At 296 K, the predicted intensity depletion ranges from 0.15 to 1.3 % amagat −1 , while the measured values range from 0.6 to 1.3 % amagat −1 . The effect increases at lower temperatures, reaching 1.5 to 3 % amagat −1 at 212 K. The results obtained at the three temperatures suggest a ( 2 9 6 𝑇 ) 3 . 4 temperature dependence for the intensity depletion coefficient. We also determined the continuum absorption - a broad pedestal resulting from the redistribution of intensity from the narrow line spectrum. The predicted continua are in excellent agreement with the measured ones at both 212 K and 296 K. Analysis of the time evolution of molecular populations obtained from rCMDSs clearly shows that the finite duration of collisions contributes to the observed intensity depletion and the associated continuum absorption.