Bond dissociation energies of transition metal and lanthanide borides with resonant two photon ionization spectroscopy*

Metal boride compounds have unique properties that make them as chemically interesting as they are relevant in a multitude of disciplines. As more applications are discovered for metal borides, improved chemical models are needed to accurately predict the behavior of these species. To assist in this effort, we have developed a method for the precise and accurate measurement of bond dissociation energies (BDEs) and have applied it to the diatomic transition metal borides (MB), triatomic transition metal diborides (MB2), and triatomic lanthanide metal borides (LnB2). For the vast majority of these species, none have had their BDEs experimentally studied previously before.

The method relies on the fact that in the open d-subshell and open f-subshell metal boride molecules, there is a large density of electronic states present at energies about the molecule’s dissociation limit. Spin-orbit and adiabatic couplings among the large number of potential energy curves in this region enable the molecules to hop from curve to curve, finding their way to dissociation as soon as sufficient energy is available for this process. In our resonant two-photon ionization experiments, the high density of states in this region leads to a quasi-continuous spectrum below the dissociation energy, followed by a sharp drop to baseline when the molecules are excited above the dissociation limit. The sharp drop in signal occurs at a wavelength that corresponds to the BDE of the molecule. Using this method, the BDEs of 18 diatomic borides and 10 triatomic boride molecules have been measured to high accuracy with the majority of these species having no previous experimental data or observations recorded. These data provide important benchmarks for the development and testing of improved computational methods for these species which will be helpful for modeling larger metal boride clusters and nanoscopic materials. Of equal importance, a comprehensive set of bond dissociation energies for these species allows for a quantitative and qualitative landscape of the transition metal-boron bond to be elucidated.