Temperature dependence of normal-emission photoelectron diffraction and analogies with extended x-ray-absorption fine structure

Abstract

The temperature dependence of normal-emission photoelectron diffraction (NPD) of the prototype adsorbate-substrate system Se-Ni (001) was studied. Two interesting observations emerged. Thermal diffuse scattering yielded a decreasing peak-to-valley contrast ratio in NPD with increasing temperature characterized by an effective temperature ${\Theta }_{\mathrm{eff}}\sim 135$ K. Also, a new low-temperature form of $p(2\mathrm{}\times 2)$ selenium structure was observed, with $d_{\perp }$ estimated to be larger by ∼(0.06-0.1) Å than the room-temperature form. The new form, which is probably undissociated ${\mathrm{H}}_2$Se, is apparent through a systematic NPD peak shift reminiscent of extended x-ray-absorption fine structure (EXAFS) spectra. It is noted that NPD, though usually associated with low-energy-electron diffraction, in fact has strong similarities to EXAFS. This is particularly evident in the importance of an extended $k$-space data set and in the temperature sensitivity.