The vibration–rotation of H2O and its complexation with CO2 in solid argon revisited

Fourier-transform infrared spectroscopy in the frequency range 400–4000 cm−1 has been used to investigate the absorption of H2O and H2O:CO2 complex isolated in solid argon. Thanks to the lowest temperature reached in our experiment, temperature effects and nuclear spin conversion studies allow us to propose a new assignment of the rovibrational lines in the bending band ν2 for the quasi-freely rotating H2O. An additional wide structure observed in this band shows two maxima around 1657.4 and 1661.3 cm−1, with nuclear spin conversion of the high-frequency part into the low-frequency one. This structure is tentatively attributed to a rotation–translation coupling of the molecule in the cage. However, the equivalent effect is not observed in the vibrational stretching bands ν1 and ν3. Finally, in double doping experiments with CO2 important new structures appear, allowing us to unambiguously extract the frequencies of the lines of the H2O:CO2 complex.