Vibrational relaxation of carbon dioxide in water

The dynamics of vibrational relaxation of carbon dioxide in water has been studied using femtosecond mid-infrared pump–probe spectroscopy with excitation of the anti-symmetric stretching (ν₃) fundamental state of the solute. The relaxation dynamics were recorded at a constant pressure of 500 bars and in the temperature range between 300 and 600 K, thereby covering the liquid-to-near-critical region of the solvent. The excited state of the ν₃-mode is deactivated in two competing pathways: (i) direct relaxation to the ground state with resonant transfer of the excess vibrational energy into the bending–librational continuum of the water solvent and (ii) relaxation to the bending fundamental state with transfer into the intramolecular bending mode of H₂O. The rate of pathway (i) decreases with increasing temperature, from ∼1/(9 ps) at 300 K to ∼(1/16 ps) at 600 K and obeys Fermi’s golden rule strictly, provided that the spectral density of energy-accepting solvent states is derived from the stationary infrared absorption profile of H₂O. The rate of pathway (ii) is 1/(23 ps) and assumed to be temperature-independent within our data analysis. Finally, the bending fundamental of CO₂ can also relax to the ground state by resonantly transferring the remaining excess energy to the librational fundamentals of the solvent.