Monitoring temperature dependence in the ultraviolet photoabsorption spectra of carbon dioxide

Photoabsorption cross sections of CO${}_2$ were investigated over a temperature range from 150 K to 800 K using the nuclear ensemble approach to evaluate its ability to capture temperature-dependent spectral changes. At each temperature, 1001 geometries were sampled from a Wigner distribution. For each geometry, the vertical excitation energies and oscillator strengths of the first 20 electronically excited states were computed and employed in the calculation of photoabsorption cross sections, considering only electronic transitions. The underlying electronic structure calculations were performed at the TD-DFT/CAMB3LYP/aug-cc-pVTZ and TD-DFT/$\omega$B97X/aug-cc-pVTZ levels of theory. The results reveal that, as temperature increases, the onset of the lower absorption band shifts to lower energies, while the intensity of the second band grows. These trends are in good agreement with previous experimental data from the literature Venot et al. (2018), highlighting the nuclear ensemble approach effectiveness in reproducing the temperature dependence of CO${}_2$’s photoabsorption cross sections.