We present temperature programmed desorption data and inverse photoemission (IPE) spectra that directly demonstrate the effects of quantum confinement of electrons in an ultrathin metal film on molecular chemisorption at the film’s surface. The prototypical system of CO adsorbed on the $n$-monolayers-$\mathrm{Cu}∕\mathrm{fcc}\mathrm{Fe}∕\mathrm{Cu}\left(100\right)[2<n<15]$ was explored. For all films, the CO peak desorption temperature $\left(T_{\mathrm{des}}\right)$ is lower than that of Cu(100). Moreover, oscillations in $T_{\mathrm{des}}$ are correlated with oscillations in the IPE intensity at the Fermi level $\left(E_{\mathrm{F}}\right)$ caused by metallic quantum well states in the Cu overlayer passing through $E_{\mathrm{F}}$ as function of film thickness.

• Received 3 March 2004

DOI:https://doi.org/10.1103/PhysRevB.70.165420