Nuclear Spin Symmetry Conservation in 1H216O Investigated by Direct Absorption FTIR Spectroscopy of Water Vapor Cooled Down in Supersonic Expansion
Résumé
We report the results of an experimental study related to the relaxation of the nuclear spin isomers of the water molecule in a supersonic expansion. Rovibrational lines of both ortho and para spin isomers were recorded in the spectral range of H2O stretching vibrations at around 3700 cm-1 using FTIR direct absorption. Water vapor seeded in argon, helium, or oxygen or in a mixture of oxygen and argon was expanded into vacuum through a slit nozzle. The water vapor partial pressure in the mixture varied over a wide range from 1.5 to 102.7 hPa, corresponding to a water molar fraction varying between 0.2 and 6.5%. Depending on expansion conditions, the effect of water vapor clustering was clearly seen in some of our measured spectra. The Boltzmann plot of the line intensities allowed the H2O rotational temperatures in the isentropic core and in the lateral shear layer probed zones of the planar expansion to be determined. The study of the OPR, i.e., the ratio of the ortho to para absorption line intensities as a function of Trot, did not reveal any signs of the OPR being relaxed to the sample temperature. In contrast, the OPR was always conserved according to the stagnation reservoir equilibrium temperature. The conservation of the OPR was found irrespective of whether water molecule clustering was pronounced or not. Also, no effect of the paramagnetic oxygen admixture enhancing OPR relaxation was observed.
Mots clés
Stretching
CRESU
Spin dynamics
Reservoirs (water)
Molecular physics
Water absorption
Water-molecule clustering
Vapor partial pressure
Rotational temperature
Nuclear spin isomer
Equilibrium temperatures
Water vapor
Rovibrational lines
Stretching vibrations
Supersonic expansions
Expansion
Fourier transform infrared spectroscopy
Isomers
Mixtures
Molecules
Oxygen
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Georges et al. - 2017 - Nuclear Spin Symmetry Conservation in 1H216O Inves.pdf (1.74 Mo)
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