Mechanisms of laser interaction with metal carbonyls adsorbed on Si(111)7×7: Thermal vs photoelectronic effects

Abstract

Ultrahigh vacuum studies of the interaction of 514 nm radiation from a cw Ar ion laser and its second harmonic at 257 nm with mono- and multilayer coverages of Mo(CO)6, W(CO)6, and Fe(CO)5 adsorbed on Si(111)7×7 at 90 K using thermal desorption spectroscopy (TDS), laser induced desorption spectroscopy, high resolution electron energy loss spectroscopy (HREELS), and Auger electron spectroscopy were performed. A model for the temperature rise of the sample due to cw laser heating is developed. By directly measuring the substrate temperature, these experiments were able to distinguish between photoelectronic and thermal effects active in the decomposition and desorption mechanisms of the adsorbed carbonyls. Results from TDS and HREELS show that Mo(CO)6 and W(CO)6 are molecularly adsorbed, while Fe(CO)5 partially dissociates upon adsorption. The decomposition of adsorbed Mo(CO)6 is caused by electronic excitation due to direct absorption of the 257 nm radiation. Irradiation with 514 nm radiation results in no photochemistry. The same mechanism is dominant for adsorbed W(CO)6 and Fe(CO)5; however, new excitation mechanisms are available to these molecules that lead to bonding changes in W(CO)6 and Fe(CO)5 with 514 nm irradiation. The photodecomposition products of the adsorbed carbonyls are found to be different from the gas-phase decomposition products. The surface stabilizes the adsorbed carbonyls, preventing complete removal of all the CO ligands. Desorption of CO due to photoelectronic excitation is found to occur via sequential single photon absorption and extraction of CO ligands. Evidence of clustering of carbonyl fragments was observed after 257 nm irradiation.