The spin polarization of photoelectrons from ErFe 2 was found to go from near zero to ∼ − 20% upon exposure to 1000 L H 2 . Exposure to 1000 L O 2 yielded zero spin polarization. We conclude that no hydrogen or oxygen induced surface segregation occurs at room temperature in ErFe 2 (unlike LaNi 5 or FeTi). It is indicated that hydrogen itself causes activation of the ErFe 2 surface for hydrogenation.
