The influence of hydrogen on the growth of Y on Ni(001) and Ni(110) at
room temperature is studied by full-hemispherical X-ray photoelectron
diffraction and X-ray photoelectron spectroscopy. We find that Y star
ts to form a hydride at a thickness of about 25 Angstrom when evaporat
ing Y under a 10(-6) mbar hydrogen partial pressure. At the same time
a weak ordering of the hydride on Ni is observed in the form of a pref
erential orientation. No ordering occurs for evaporation without hydro
gen in ultra-high vacuum.