Site selective probe of the desorption of CO from Pd(111) by sum frequency generation and Fourier transform IR: a comparison of thermal and laser desorption

The laser induced desorption of CO from Pd(111) at 308 nm and 532 nm is compared to the thermal desorption by recording FTIR and SFG spectra of the CO molecules remaining on the surface. There is no photodesorption from a fully ordered CO layer. However, when the CO layer is adsorbed below 270 K, extra sites (assigned to antiphase domain boundaries) appear in addition to the normal sites (assigned to domains), and the photodesorption occurs selectively from the extra sites. The relative stability of the various sites is similar under heating and under laser exposure, but the numerical calculation of laser heating and of the corresponding thermal desorption shows that laser heating cannot account for the observed photodesorption. (C) 1999 Elsevier Science B.V. All rights reserved.