The boron K photoabsorption spectra of BF₃, BCl₃, and BBr₃

The photoabsorption cross sections of BF₃, BCl₃, and BBr₃ were obtained in the region between 190 and 280 eV using synchrotron radiation. Above the B K ionisation threshold, a broad band due to shape resonance was observed for BF₃, while no counterpart was observed for BCl₃ and BBr₃. Instead, a new strong band was found below the B K ionisation threshold in the spectra of BCl₃ and BBr₃. The origin of the shape resonances is discussed by comparing the spectra of these three molecules. Assignments of the B K spectra of BF₃, BCl₃ and BBr₃ and of the Cl L spectrum of BCl₃ are given with the aid of ab initio calculations.