Seminar of François Robert

Abstract:

The first doubts on the general character of the mass dependent isotopic fractionation theory (MDF) appeared in 1983 with the synthesis of ozone (1). Oxygen isotopes are often used to illustrate the MDF: a relative variation of 1% in the ¹⁷O/¹⁶O isotopic ratio must be accompanied by 2% in the ¹⁸O/¹⁶O because the mass difference between ¹⁷O and ¹⁸O is ≈ 2. The origin of the MDF is rooted in the most fundamental principles of quantum mechanics. It is ubiquitous, observed for isotope exchange reactions and for 2-body chemical reactions. It was theorized in 1947 by H. Urey (2).
On the contrary, ozone results from a 3 body reaction (O+O₂ + X → O₃ + X where X is the third body stabilizing the complex O₃*) and shows equal relative variations in ¹⁷O/¹⁶O and ¹⁸O/¹⁶O (referred to as the MIF effect). This result reproduce the variations observed in meteorites (3) and more generally in the whole solar system. Just as the physical origin of the MIF effect is still an open question in quantum mechanics, its possible application to the formation of the solar system seems difficult to judge.
The seminar contains 3 parts: (1) theoretical, (2) experimental and (3) analytical. In part 1, I will show that the results on ozone can be fully reproduced if the fundamental quantum mechanical requirement that, for indistinguishable isotopes, the two possible reaction pathways (elastic scattering and particle exchange i.e. isotopic exchange and non-exchange) are superimposed(4,5). In part (2) I will discuss how the equations that govern the effect in ozone led us to consider condensation experiments in plasma. The part (3) is dedicated to the results obtained for Ti, Mg, O and N isotopes using mixture of TiCl 4 +Pentane, MgCl3 +Pentanol and CO₂ or N₂O+Pentane (6,7). Large (in the range 1000-10,000 ‰) and mass independent isotope fractionation are observed in micrometric size carbonaceous particle. These results are reasonably well accounted by the model built for ozone. As proposed by (1) , they imply that the MIF effect becomes a serious candidate to explain the origin of solids in the solar system. They nevertheless need to be documented experimentally under physical and chemical conditions similar to those modeled in cosmochemistry.

(1) M.H. Thiemens, J.E. Heidenreich III, Science 219 (1983) 1073–1075. (2) H.C. Urey, J. Am. Chem. Soc. 15 (1947) 562–581 (3) R.N. Clayton, L. Grossman, T.K. Mayeda, Science 182 (1973) 485–488. (4) P. Reinhardt, F. Robert, Chem. Phys. 513 (2018) 287–294. (5) F. Robert, P. Reinhardt. Chem. Phys. Impact 4, (2022) 100073. (6) F. Robert, M. Chaussidon, A. Gonzalez- Cano, S. Mostefaoui, PNAS 118 (2021) 52. (7) F. Robert, R. Tartese, G. Lombardi, P. Reinhardt, M. Roskosz, B. Doisneau, Z. Deng, M. Chaussidon, Nat. Astron 4 (2020) 762–768.