A high energy (keV), large angular range, precision electron spectrometer for atomic and molecular targets

An electron impact spectrometer for operation in the keV incident−electron energy range is described. The spectrometer operates between 20 and 60 keV incident−electron energy, utilizing both a low−resolution silicon surface barrier detector (5 keV FWHM), and a high−resolution (0.7 eV FWHM) Möllenstedt energy analyzer over a 1.7 keV energy loss range. It features a large angular scan range (0⩽ϑ<120°), high target gas throughput (i.e., 19000 l/sec for He and 13000 l/sec for N2 through a tube of 0.15 mm diam), and a wide range of incident−electron beam currents (0.1−400 μA) with a focused electron beam size of less than 200 μ FWHM over most of the current range. Careful measurements of the gas density profile of the target gas emanating from the nozzle are also reported. Results are presented for the Bethe surface of He and N2. The latter example is reported here for the first time. In addition, the observation of the angular dependence of the Lyman−Birge−Hopfield band in N2 (dipole forbidden) is compared with the dependence of a neighboring dipole allowed transition.