Nuclear Spin Symmetry Conservation in ¹H₂¹⁶O Investigated by Direct Absorption FTIR Spectroscopy of Water Vapor Cooled Down in Supersonic Expansion

Abstract

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 HO stretching vibrations at around 3700 cm 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 HO 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 T, 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.

Quantum State-Resolved Characterization of a Magnetically Focused Beam of ortho-H2O

“…Fitting the spectrum using T rot = 22.0 ± 0.5 K yielded an OPR = 3.4 ± 0.3, that is, no smaller than 3 according to our experimental uncertainty (corresponding to a relative abundance of 75% ortho -H 2 O). As the equilibrium OPR is 1.92 at T = 22 K, this indicates that nuclear spin interconversion is much less efficient than rotational cooling under our experimental conditions, resulting in the NSIs populations remaining frozen in their high-temperature distribution during the free jet expansion (i.e., retaining an OPR = 3), in agreement with previous observations. − Contributions from ortho -H 2 O (blue shading) and para -H 2 O (red shading) to the simulated spectrum indicate that the majority (i.e., >75%) of water molecules occupy their respective fundamental rotational states at T rot = 22 K (i.e., J K a K c = 1 01 for ortho -H 2 O and 0 00 for para -H 2 O). A small but significant contribution due to (rotationally hot) water vapor from the residual background gas ( P H 2 O ≈ 1 × 10 –10 Torr) is also clearly present in the REMPI-TOF-MS spectrum acquired in the molecular beam (Figure b; dashed line).…”

Low-Energy Water–Hydrogen Inelastic Collisions

“…For H 2 O and H 2 in their ground vibronic states, the presence of two identical hydrogen atoms results in two nuclear spin states: ortho (K a + K c odd or j H2 odd) and para (K a + K c even or j H2 even). It is clear from our spectra, that the ortho/para ratio of water remained unchanged during the supersonic expansion, 29,31 at the value of 3:1. This is also the case for H 2 .…”

“…(a) Interaction time considering an isotropic distribution of the scattered water (black solid line) and a forward distribution: the threshold is at 18.6 cm –1 for H 2 O­(1 10 ) (blue dashed line), and f at 37.1 cm –1 or H 2 O­(1 11 ) (green dashed-dotted line). (b) Half-width at 1/e (HWE) of the collision energy distribution assuming velocity and collision angular spreads as Gaussian functions. − …”

Observation of different reactivities of para and ortho-water towards trapped diazenylium ions

“…However, nuclear-spin-symmetry interconversion has been observed in a variety of polyatomic molecules and has been rationalized to occur via doorway states with a mixed nuclear-spin character 2 , 4 , 5 . For water in the vapor and condensed phases, the para/ortho- interconversion rates reported in the literature vary widely and remain a controversial topic 6 – 8 .…”