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

The photoabsorption cross sections of BF 3 , BCl 3 , and BBr 3 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 BF 3 , while no counterpart was observed for BCl 3 and BBr 3 . Instead, a new strong band was found below the B K ionisation threshold in the spectra of BCl 3 and BBr 3 . The origin of the shape resonances is discussed by comparing the spectra of these three molecules. Assignments of the B K spectra of BF 3 , BCl 3 and BBr 3 and of the Cl L spectrum of BCl 3 are given with the aid of ab initio calculations.