Core electron binding energy shifts of AlBr3 and Al2Br6 vapor

The Al 2p and Br 3d inner-shell photoelectron spectra of aluminum tribromide monomer and dimer vapor were measured at 90 and 95 eV photon energy, respectively, to determine the core electron binding energies of the atoms in the two molecular species. While AlBr3 has three identical Br atoms, Al2Br6 exhibits four terminal and two bridging Br atoms. The species are identified by their distinct valence photoelectron spectra. Comparison of the observed Al 2p1/2 and Al 2p3/2 electron binding energies of AlBr3 with those of Al2Br6 shows that there is a chemical shift of (0.15 ± 0.03) eV to lower energy in the dimer. In Al2Br6, an assignment is proposed in which the Br 3d3/2 and Br 3d5/2 binding energies of terminal Br atoms are (1.18 ± 0.03) eV lower than those of bridging Br atoms. This assignment assumes that both types of Br atoms have similar cross-sections for ionization. With this result, the Br 3d3/2 and Br 3d5/2 binding energies of Br atoms in AlBr3 are (0.81 ± 0.03) eV lower than those of bridging Br atoms of the dimer but (0.37 ± 0.03) eV higher than those of terminal Br atoms of the dimer. The obtained chemical shifts are considered in terms of the binding relations and electron density distributions in both molecules. Chemical shifts that are larger than a few hundred millielectron volts, as observed in the Al2Br6/AlBr3 system, offer potential to study the dissociation dynamics of the dimer in a femtosecond visible or ultraviolet-pump/XUV-probe experiment.