Chemical pathways in ultracold reactions of SrF molecules

We present a theoretical investigation of the chemical reaction SrF + SrF $\to$ products, focusing on reactions at ultralow temperatures. We find that bond swapping, SrF + SrF $\to$ Sr${}_2$ + F${}_2$, is energetically forbidden at these temperatures. Rather, the only energetically allowed reaction is SrF + SrF $\to$ SrF${}_2$ + Sr, and even then only singlet states of the SrF${}_2$ trimer can form. A calculation along a reduced reaction path demonstrates that this abstraction reaction is barrierless, and proceeds by one SrF molecule “handing off” a fluorine atom to the other molecule.