Nuclear spin symmetry conservation and relaxation of water (H₂¹⁶O) seeded in supersonic jets of argon and oxygen: measurements by cavity ring-down laser spectroscopy

ABSTRACT

We have investigated nuclear spin symmetry conservation and relaxation of water seeded in argon or molecular oxygen using supersonic jet expansions probed by high-resolution continuous-wave laser cavity ring-down spectroscopy. The probing of the R-branch of the 2 band (above 7500 cm¹) was used to complement our previous investigation. We were able to further cool down the water samples (down to 17 K., i.e. 7 K colder than before) and double the data set (regarding argon as carrier gas). We confirm our first observations: at the lowest rotational temperatures and low partial pressures of HO, nuclear spin symmetry is conserved, in agreement with theoretical expectation for inelastic collisions. For high concentrations of water in the gas mixture, we obtained higher rotational temperatures and were able to observe nuclear spin symmetry relaxation. This can be related to the formation of water clusters at the early stage of the supersonic jet expansion. We also present the analogous series of measurements performed using molecular oxygen (O) as carrier gas: we observed similar behaviour for low and high concentrations of water in the gas mixtures when cooling the samples to 16 K. The conservation of nuclear spin symmetry at low temperature and low concentration using oxygen as carrier gas indicates that the paramagnetic collisional partner does not play a significant role regarding the possible nuclear spin symmetry conversion of water. We discuss possible mechanisms related to our observations of apparent nuclear spin symmetry relaxation.

GRAPHICAL ABSTRACT

KEYWORDS:

• Water (H₂O)
• nuclear spin symmetry conservation
• supersonic jets
• high-resolution infrared spectroscopy
• cavity ring-down spectroscopy

Acknowledgements

The authors gratefully acknowledge fruitful discussions with Veronika Horka Zelenkova, Ondrej Votava (also during a seminar at ETH Zurich), Frédéric Merkt, Jeremy Richardson and Georg Seyfang as well as discussions and correspondence with Takeshi Oka. The authors also enjoyed many years of fruitful interactions with Michel Herman on high-resolution molecular spectroscopy in general and of supersonic jets in particular. This paper is thus justly dedicated to him.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Our work was supported financially by the Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, by an Advanced Grant of the European Research Council, ERC, as well as the ETH Zurich (Laboratory for Physical Chemistry).