Atomic beam spectrometer using a LiF analyzer crystal

An energy analyzer has been constructed and operated in UHV for the purpose of analyzing the energy of neutral atoms scattered from solid surfaces. The analyzer consists of a LiF single crystal located at an angle close to the normal to the sample crystal so that the diffraction pattern obtained by scanning the LiF crystal yields the energy of the scattered atoms. Two designs which have been used are described. The temperature of both sample and analyzer crystal is near 20 K and once cleaned they can be maintained in the state of initial preparation for many weeks. The sample was a (001) Cu surface in this case. Such an energy analyzer can only be used, in most cases, for He atom scattering although Ne atoms could be used if the scattered intensities were adequate. The detector developed in this study is able to detect about 2×105 atoms/s. The resolution of the spectrometer depends on the incident energy of the atom and is about 1 meV at an incident energy of 23 meV. This resolution can be improved by a factor of 3 to 4 by cooling the nozzle to a temperature lower than 77 K and using variable size slits which can be inserted into the beam path.