Threshold electron impact excitation of HBr

A high-resolution electron monochromator combined with an efficient detector of near-zero-energy electrons is used to study threshold electron impact excitation of the HBr molecule. A spectrum is obtained in the energy region from 8 to 11.5 eV corresponding to the excitation of Rydberg states below the ground state X 2 pi i of the ion. At low excitation energies threshold electron impact spectra of HBr can be easily correlated with optical absorption data except for large differences in relative intensities. At high excitation energies not investigated by optical absorption experiments a number of new structures are observed.