Low-energy positron-diffraction study of NaF and LiF

Using a magnetically transported beam of slow positrons, we have measured the elastic backscattering probability versus energy for positrons normally incident on air-cleaved and vacuum-cleaved (100) surfaces of NaF and LiF. We observe enormous first-order Bragg peaks and several higher-order peaks, the positions of which imply an inner potential of -0.5 +- 0.2 and -0.1 +- 0.2 eV for NaF and LiF, respectively. The atomic plane spacings are equal to the bulk values within 1/2%. There are only slight differences between the data obtained using the freshly air-cleaved and the vacuum-cleaved surfaces. Surface disorder may be responsible for the presence of some weak Bragg reflections corresponding to directions that would otherwise be inaccessible to the incident positron beam. The temperature dependence indicates that the disorder would not have a thermal origin