Infrared reflection absorption spectroscopic study for CO adsorption on molecular beam epitaxially grown Fe films on Cu(1 1 1)

Adsorption of CO at 90 K on molecular beam epitaxially grown Fe films on Cu(1 1 1) has been investigated using IR reflection absorption spectroscopy (IRRAS). Reflection high energy electron diffraction images reveal Fe films up to 4 ML (monolayer) thickness have an fcc(1 1 1) structure and a mixture of fcc(1 1 1) and bcc(1 1 0) structures for 4-8 ML thick films. A bcc(1 1 0) structure dominates the films beyond 8 ML in thickness. The C-O stretching frequency at saturation coverages observed by IRRAS represents these structural differences clearly. For 1-4 ML thick fee Fe films C-O stretch is located at 2000 cm^-1 and the 6-8 ML thick films yield two C-O stretch bands at 2000 and 2032 cm^-1, the latter of which is dominant for films more than 10 ML thick.