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 upsilon3 (CO2) at constant number density increases from 1.58 X 10 4 cm/mmol at 300 K to 2.68 X 10 4 cm/mmol at 600 K. The absorptivity of upsilon3 (N2O) is 8.57 X 10 3 cm/mmol at 300 K and 1.33 X 10 4 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.

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