Abstract : In cometary atmospheres, nuclear spin isomers' abundance ratios of molecules like H2O, NH3, CH4, and more recently CH3OH, have been evaluated. They are not in thermodynamic Boltzmann's equilibrium and the question is : is this non equilibrium the signature of the temperature of the primordial medium in which molecules have been formed?
As a matter of fact, different nuclear spin isomeric forms can be identified for molecules which have equivalent protons of non-zero nuclear spin in symmetrical positions. The conversion between these isomers, which is forbidden at the first order of approximation in the gas phase, is very slow and depends upon the neighbouring of the molecules.
The aim of the present work, both experimental and theoretical, is to identify some relevant parameters involved in the nuclear spin conversion of water molecules trapped in rare gas matrices.
In such media, an enrichment of ortho or para nuclear spin isomers is obtained by a very fast change of the temperature of the sample. Then, FTIR spectra recorded successively in the Ν2 and Ν3 vibrationnal modes of water, allow a measurement of the nuclear spin conversion time.
Varying water dilution in argon matrix (from [Ar/H2O]=5000 to 50) at 4,2 K, shows that two types of conversions are present. For dilutions higher than 1000, the conversion times (~670 minutes) are independent of water dilution. We show that this is not due to unwanted effects (molecular impurities, IR source exposure time, copper-gold surface influence,...). It is then due to an intramolecular coupling process, accelerated by the matrix.
On the other hand, below a dilution of 1000, a strong acceleration of the conversion is observed (conversion time of ~180 minutes, at a dilution of 50). This acceleration of conversion is clearly of intermolecular origin.
We have developed a model based on intermolecular nuclear spin (of protons) magnetic interactions and energy exchange with the matrix. In this model, the conversion depends on the energy difference between the lowest rotational levels and on a collisional relaxation rate of the molecule with the matrix atoms. Intermolecular conversion at 4,2 K is allowed via a conversion channel between the two first rotational levels of the molecule. The presence of a second channel, that is consistent with the acceleration of the intermolecular conversion when increasing temperature, is found to be due to a population effect.
Matrix change confirms that the intermolecular conversion mechanism is more efficient in the matrix with the smallest interatomic distances, in accordance with our model.