Magnetic field control of molecular dissociation energies

Abstract

We show that it is possible to control the dissociation energies of molecules with an external magnetic field. We focus our interest on the lowest dissociation channel for which the two atomic and/or molecular products are formed in their ground state. The crucial requirement is the paramagnetic character of at least one of the two dissociation products. Then, an external magnetic field lowers the energy of the paramagnetic species in its lowest Zeeman component and, possibly, the corresponding energy of dissociation of the parent molecule. This it true for diatomic molecules when at least one of the atoms has an odd number of electrons. This is also true for oxygen and phosphorus atoms which have a 3P2 ground state. The Zeeman energy shift of paramagnetic species is always of the order of 1 cm−1 per tesla. The main theoretical difficulty is to determine the correlation diagram existing between the bound states of the parent molecule and the states of the products, or equivalently, how the energy evolves as a function of the internuclear distance corresponding to the dissociation coordinate. Little is known about this evolution, except for diatomic molecules, because the large internuclear distances are difficult to observe experimentally. The main part of the information come from ab initio calculations. For diatomic molecules, the dissociation coordinate is also the unique internuclear distance while for polyatomic molecules, the potential energy surface has 3N-6 coordinates and multidimensional effects should be considered. In any case, the singlet–triplet–quintet, etc… (or doublet–quartet, etc...) interactions should play an important role in the correlation diagram because crossings are expected between singlet and triplet potential energy curves (from short to long internuclear distances) and these interactions transform the crossings into anticrossings. The specific examples of alkali diatomic molecules (Li2, Na2, etc…), of NO2 and of (O2)2 are analyzed in details. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 571–580, 1997