Separation of \textit{cis} and \textit{trans} geometric isomers by Coulomb explosion imaging*

Isomers, i.e. molecules with the same chemical formula but different chemical structure, play an important role in many biological processes [1]. Recently, it was shown that it is possible to identify different isomers of a chiral molecule by Coulomb explosion imaging [2]. Here, we show that by imaging the Coulomb explosion of C$_{\mathrm{2}}$H$_{\mathrm{2}}$Br$_{\mathrm{2}}$ molecules after inner-shell photoionization, we are able to separate a mixture of \textit{cis} and \textit{trans} structures using the momentum correlation between ionic fragments measured in coincidence. Furthermore, we used this capability to investigate the isomer-selective photoionization and fragmentation dynamics of C$_{\mathrm{2}}$H$_{\mathrm{2}}$Br$_{\mathrm{2}}$ after Br (3d) ionization. Coulomb explosion simulation results for momentum correlation as well as kinetic energies match closely the experimental results. [1] B. Levine \textit{et al, }Annu. Rev. Phys. Chem. \textbf{58}:613--34 (\textit{2007}) [2] M. Pitzer \textit{et al}, Science \textbf{341}, 1096-1100 (2013).