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.

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Adsorption of CO and the coadsorption of CO with low coverages of K on Fe(111) at temperatures as low as 83 K have been studied with low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), and high resolution electron energy loss spectroscopy (HREELS). The results indicate that CO fills three different adsorption sites and is mobile enough to access all binding sites after striking the surface at 83 K. A previously unobserved, very low CO stretch vibration is reported at 1325 cm−1 on the unpromoted surface. In addition, a missing metal–CO vibration is reported for an unpromoted surface. The screening mechanism for the missing Fe–CO stretch vibration is discussed in relation to promoted metal surfaces and to other adsorbates which display similar behavior.

Articles you may be interested inGrowth of metal oxide thin films by laserinduced metalorganic chemical vapor deposition We report the results of a series of experiments studying metal film growth by low-power UVlaser photolysis of metal carbonyls. Small-area Mo, W, and Cr films were grown on Si substrates by photolysis in a background pressure of 0.1-0.2 Torr of the carbonyl. The different metals exhibited vastly different deposit morphologies. Elemental depth profiling by Auger electron spectroscopy (AES) revealed the presence of large amounts of carbon and oxygen in the films. Ultrahigh vacuum studies of the interaction of 257-and 514-nm radiation with multilayer coverages of Moe CO)" and W (CO)/) adsorbed on SiC 111)7 X 7 using highresolution electron energy-loss spectroscopy, laser-induced desorption, and AES were performed to clarify the mechanisms of adsorbed metal carbonyl decomposition and impurity incorporation into the films. Decomposition of both the adsorbed and gas-phase carbonyls proceeds through direct electronic excitation of the molecule by absorption of the 257-nm radiation; however. the photodecomposition products of the adsorbed carbonyls are different from the gas-phase decomposition products. The evidence suggests that both the surface and the previously deposited photofragments stabilize the partially photolyzed carbonyls, preventing removal of all the CO ligands from the adsorbed species. 996

Articles you may be interested inMechanism of the CO oxidation reaction on surfaces studied with near-edge x-ray absorption fine structure spectroscopy Interaction of water with clean and gallium precovered Fe(111) surfaces J.Desorption kinetics on an alkali metalprecovered surface: CO and K on Pt(111) J. Chem. Phys. 90, 6018 (1989); 10.1063/1.456367The kinetics and mechanisms of alkali metalpromoted dissociation: A time resolved study of NO adsorption and reaction on potassiumprecovered Rh(100)The adsorption of N2 on K-precovered Fe( 111) at 74 K has been studied with low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), and high resolution electron energy loss spectroscopy (HREELS). The presence of low precoverages ofK ( < 1.4 X 10 14 em -2) dramatically enhances the sticking coefficient of a 1-N 2' the 1T-bonded precursor to dissociation, and causes an increase in the maximum population of this species. We conclude that the effects ofK on this system are primarily mediated by long range interactions; we have modeled the nonloca1 K-induced changes of the adsorption and desorption ofN z for temperatures <;;430 K and found that by slightly decreasing the r-N2 adsorption energy and increasing the a l -N 2 adsorption energy we can quantitatively account for both the increase of the a 1-N 2 sticking coefficient at 74 K and the increase of the dissociative sticking coefficient at 430 K previously reported by Ert!, Lee, and Weiss [Surf. Sci. 114, 527 (1982) 1. The promoted a I state has an N-N stretch frequency less than 20 cm-I lower than that of un promoted a-N 2 , as expected for a weak long range interaction with K, indicating that the N-N bond is not appreciably perturbed. This is significant, as the catalytic effects of K have been previously attributed to bond weakening in the dissociation precursor. At higher K precoverages, local N2-K interactions dominate, characterized by broad N-N vibrations at 1600-1800 cm -I. The HREELS spectra of H2 and H 2 0, common vacuum contaminants, are also reported for adsorption on a K-precovered surface at 83 K.