Spectroscopy of Hydrothermal Reactions. Part XI: Infrared Absorptivity of CO 2 and N 2 O in Water at Elevated Temperature and Pressure

Abstract

The temperature dependence of the infrared absorptivity of the asymmetric stretch of CO₂ and N₂O dissolved in H₂O was determined at 300–600 K under 275 atm. These results are essential for using these species as internal calibrants of the rate of many hydrothermal reactions by infrared spectroscopy. The absorptivity (band area) for ν₃(CO₂) at constant number density increases from 1.58 × 10⁴ cm/mmol at 300 K to 2.68 × 10⁴ cm/mmol at 600 K. The absorptivity of ν₃(N₂O) is 8.57 × 10³ cm/mmol at 300 K and 1.33 × 10⁴ cm/mmol at 525 K. The absorptivity is suppressed in the presence of H₂O by a factor of about 5 compared to results for the gas phase. The absorptivity increases, however, with increasing temperature in H₂O solution, which is opposite the trend for the gas phase. The Lorentzian line shape in H₂O solution provides a global relaxation time of <1 ps, which is more consistent with relaxation by vibrational energy transfer among associated molecules than by collisions or stochastic modulation by the surrounding H₂O field.