AVS1996 Session SS2-MoM: Photochemical Processes at Surfaces

Monday, October 14, 1996 8:20 AM in Room 106A/B

Monday Morning

Start	Invited?	Item
8:20 AM		SS2-MoM-1 Visible-Light-Induced Etching of Br/GaAs(110) C. Cha, J. Brake, B. Han, J. Weaver (University of Minnesota) STM topographs reveal light-induced etching of Br adsorbed on GaAs(110) at 300 K. Prior to photon excitation, Br atoms were chemisorbed in ordered islands of 2x1/c(2x2) symmetry. Irradiation with 532 nm photons from a pulsed Nd-YAG laser created single-layer deep etch pits with footprints correponding to the shape of the Br chemisorption islands. The laser energy was kept below 1mJ per pulse to minimize thermal effects. For comparison, we annealed an identically prepared Br overlayer on GaAs(110) to 700 K. Analysis of the different etching morphologies suggests an electronic pathway for the light-induced reactions. We deduce the etching activation is substrate-mediated. Possible reaction mechanisms are discussed.
8:40 AM		SS2-MoM-2 Photo-Dynamics and Molecular Orientation of Ethyl Bromide on GaAs(110) K. Khan, D. Slater, P. Lasky, R. Osgood, Jr. (Columbia University) Angle-resolved time-of-flight (TOF) and temperature programmed desorption (TPD) spectroscopies have been employed in a study of the surface dynamics, reactivity and orientation of adsorbed ethyl bromide on GaAs(110). TPD spectroscopy indicates that this adsorbate is weakly bound and desorbs intact at low temperatures. Following irradiation, thermal desorption of ethyl radicals, ethene from \beta\-hydride elimination in adsorbed ethyl groups and gallium bromide resulting from photodecomposition of the overlayer is observed. Irradiation also results in the production of energetic ethyl radicals that are directly ejected into the gas phase. For these species, TOF spectroscopy reveals, at 1 ML coverage, two peaks with differing characteristic kinetic energies that vary only slightly with large changes in the photon energy of the exciting radiation (0.90-0.65 eV and 0.30 eV), suggesting that a substrate mediated, dissociative electron attachment, mechanism is operative. At coverages \<=\ 1 ML angle resloved measurements in the plane defined by the surface normal and the [01] direction show that the two features of the TOF spectra also exhibit characteristic angular distributions with the fast feature peaking sharply at 30\super o\ in the [0 -1] direction and the slow peak exhibiting a more isotropic, cosine-like distribution. This angular distribution indicates that the R-Br bond axis is tilted at 30\super o\ with respect to the surface normal in the [0 -1] direction. The slower, isotropic feature in the TOF spectrum is discussed in terms of fragments scattered by collisions with adjacent adsorbed molecules and possible 'trapping-desorption-like' scattering.
9:00 AM		SS2-MoM-3 Vibrational Excitation Dynamics in Photodesorption. F. Zimmermann (Rutgers University) Insight into the dynamics of photochemical processes at surfaces can be obtained by studying the internal and translational state distributions of desorbing product molecules. Vibrational state populations are of particular interest because of the information they contain about the lifetime and nature of the electronic excited state responsible for desorption. Vibrational distributions measured with laser induced fluorescence spectroscopy of CO photodesorbed from NiO(111) [1] and from Si(100) [2] are presented and compared with other non-dissociative photodesorption processes of diatomics from metal, semiconductor, and oxide surfaces. The measured vibrational distributions tend to resemble Boltzmann distributions with temperatures of 800-1200 K for desorption from metal surfaces, and 1700-2300 K for semiconductors and oxides. Quantum dynamics calculations of the vibrational excitation process show that the observed vibrational distributions can be reproduced only if the intramolecular equilibrium bondlength in the electronic excited state is remarkably similar to that of the ground state. For example, if the intramolecular potential change upon excitation is taken to be the same as for the transition from the neutral (gas phase) molecule to the anion, reproducing the observed vibrational distributions requires residence times in the excited state which are much too short to cause desorption (unless one assumes unrealistically steep potentials in the desorption coordinate). Implications for the nature of the excited state are discussed. [1] F. M. Zimmermann, M. Asscher, L. L. Springsteen, P. L. Houston, and W. Ho, to be published. [2] F. M. Zimmermann, P. L. Houston, and W. Ho, to be published.
9:20 AM		SS2-MoM-4 Hot Electron Femtochemistry at Surfaces using Tunnel Junctions J. Gadzuk (National Institute of Standards & Technology) Femtochemistry is founded on the proposition that the basic "control- lable event" in such processes occurs over the natural timescale for periodic intra-molecular motion. As usually conceived, control is achieved by use of ultrafast, temporally shaped and sequenced laser pulses. A promising alterna- tive realization of femtochemistry at solid surfaces, based on inelastic reso- nant scattering of hot electrons by the reactants, achieves control of the intramolecular dynamics required for select chemical processes by controlling the energy distribution of the hot-electron flux upon the surface. A methodology is presented here which is based on a novel application of a solid state (M-I-M)tunnel junction. Theory for the tunnel junction device has been developed in terms of an independent 4-step model: i) field emission from bottom metal to tilted insulator conduction band; ii) transport through insula- tor with possible phonon/polaron losses; iii) hot electron transport and atten- uation across top metal film; iv) resonant inelastic hot electron scattering from adsorbate/reactants at the top metal-vacuum surface where the controlled femtochemistry occurs. Theoretical expressions for reaction rates as a func- tion of applied voltage provide predictive capabilities. By appropriate choice of device parameters such as barrier heights, film thicknesses, and top metal species, it is demonstrated how one can produce an electron source optimally suited for a particular femtochemical process.
9:40 AM		SS2-MoM-5 Femtosecond Laser-induced Desorption of CO from Cu(100): Comparison of Theory and Experiment L. Struck, L. Richter, S. Buntin, R. Cavanagh, J. Stephenson (National Institute of Standards & Technology) Femtosecond (fs) laser excitation has been reported to cause novel desorption dynamics for molecules on metal surfaces, attributed to the high laser-induced transient electronic temperature at the surface. Details of the desorption mechanism remain in question. We have used 2+1 REMPI (through the CO B\super 1\\Sigma\\super +\) to characterize the energy partitioning in CO following desorption of top site CO at a coverage \Theta\=0.5 ML from Cu(100) using a 400 nm desorption laser with a pulse duration of about 160 fs. The nonlinear laser fluence (F) dependence on desorption yield (Y) is described by a power law with an exponent of 8\+-\1. The average rotational (E\sub R\), translational (E\sub T\), and vibrational (E\sub V\) energies of the desorbed CO are characterized by "temperatures" of 225 K, 230 K, and 1330 K, respectively (E\sub R\ and E\sub T\ are slightly non-Boltzmann and E\sub T\ increases with E\sub R\ and F). The timescale for desorption was measured by a 2-pulse correlation experiment; the correlation had a FWHM of 3 ps. The observed E\sub R\, E\sub T\, and E\sub V\ are compared to theoretical predictions for the CO/Cu(100) system. Reasonable agreement is found with an electronic friction theory where the coupling terms are extracted from ab initio calculations. In addition, the results can be calculated for a simple friction model with coupling strengths taken from published experimental observations. Comparison between experiment and both theories for the CO/Cu(100) system show that purely frictional models account for the data.
10:00 AM		SS2-MoM-6 Rotational and Translational Energy Distributions of CO Photodesorbed from Ag(111) R. Beuhler, L. Fleck, M. White (Brookhaven National Laboratory) The photodesorption dynamics of neutral CO molecules from cooled Ag(111) surfaces have been studied utilizing the high sensitivity of VUV (1 + 1`) resonant multiphoton ionization (REMPI) in combination with time resolved mass spectrometry to measure the translational and internal energy of the desorbed CO. Studies were confined to monolayers of CO by maintaining the crystal temperature high enough to prevent multilayer formation. Photodesorption dynamics were measured as a function of wavelength and power of the photodesorption laser utilizing wavelengths of 1064, 532, and 355 nm. In the case of 1064 nm radiation, the power dependence was studied over a thousand fold range with yields of CO detectable with powers extending down to approximately 0.1 mJ/cm\sup 2\. These results contradict the accepted model for photodesorption induced by laser heating of the surface followed by evaporation. Our experiments indicate that for weakly bonded systems there is a non-thermal mechanism operative at low photon energies which is most likely associated with photoexcited charge carriers near the surface.
10:20 AM		SS2-MoM-7 Thermal and Photon-induced Desorption of NH\sub 3\ and ND\sub 3\ from Ag(111) G. Szulczewski, J. White (University of Texas, Austin) We have studied the thermal desorption of ammonia from Ag(111) with temperature programmed desorption (TPD) and the photoinduced desorption with time-of-flight (TOF) measurements. Using very slow heating rates (0.5 K/sec) TPD reveals adsorption is reversible with three desorption peaks at 145, 120, and 104 K. Irradiation of ammonia with 193 nm light results in the desorption of molecular ammonia from the surface; no evidence for dissociation was observed in TPD. A strong coverage dependence was observed in the photo-induced desorption yield for NH\sub 3\ and ND\sub 3\. Below 3 monolayers (ML) the cross section for photodesorption is less than ~ 1 x 10\super -20\ cm\super + 2\ for both isotopes, and above three monolayers the desorption yield increased linearly with coverage. TOF distributions were recorded as a function of coverage (3-50 ML) and characterized by two channels with average translational energies of 2500 K and 800 K. Both channels are believed to originate from direct absorption of incident photons by ammonia molecules in the condensed phase, since the average flux-weighted kinetic energy is independent of laser pulse energy and the yield increases linearly with laser power. A photodesorption model involving the excitation of Rydberg states in the multilayer is proposed to explain the observed results.
10:40 AM		SS2-MoM-8 Physisorption and Photodesorption of Formyl Fluoride on Ag(111) P. Howe, H. Dai (University of Pennsylvania) The adsorption and photochemistry of formyl fluoride (HFCO) on Ag(111) has been studied using temperature programmed desorption, electron energy loss spectroscopy and time-of-flight mass spectroscopy. Formyl fluoride is physisorbed on Ag(111) at temperatures below 108 K with attractive interactions between neighbouring molecules. Irradiation of submonolayer formyl fluoride with 266 nm photon leads to desorption of formyl fluoride. The photodesorption process was determined to be nonthermal as the yield is linearly proportional to the integrated light intensity. No photodissociation was detected. Despite the similar gaseous photochemical and adsorption kinetics behavior as formaldehyde (H2CO), its photochemical behavior on Ag contrasts with the dissociation/polymerization observed for formaldehyde.
11:00 AM		SS2-MoM-9 Translational Energy Measurements from Bimolecular Photoreactions on Pt(111) R. Pelak, W. Ho (Cornell University) Bimolecular photoreactions on metal surfaces have been much studied recently as model systems for catalytic reactions. To better understand energy transfer within such systems, we have measured the yield and translational energy distributions of reactant and product molecules from two bimolecular systems: \super 18\O\sub 2\ and \super 14\N\super 16\O, and \super 18\O\sub 2\ and \super 13\C\super 18\O on Pt(111). Data was collected at several wavelengths in the visible and near ultraviolet wavelength regimes using a pulsed laser (nanosecond time scale) and time of flight mass spectrometry. An isotope exchange product, \super 14\N\super 18\O was found to be produced in the NO-O\sub 2\ system with an average translational energy that decreases with photon energy, while extremely hot (approx. 2800 K) carbon dioxide was produced in the CO-O\sub 2\ system. From both systems, photodesorbed reactants were found to have average translational energies considerably greater than that from when the reactants are adsorbed individually. Comparison with our recent studies using femtosecond laser pulses provides unprecedented understanding of surface photochemical dynamics.
11:40 AM		SS2-MoM-11 H\sub 2\O Resonant Desorption from Ice Multilayers using the Mark III Infrared Free Electron Laser A. Krasnopoler, S. George (University of Colorado, Boulder) The resonant desorption of H\sub 2\O from ice multilayers has been studied as a model for photoablative surgery using the Mark III Infrared Free Electron Laser (IRFEL) at the Duke Free Electron Laboratory. The IRFEL was used to excite the O-H stretching vibration in H\sub 2\O using infrared wavelengths between 2.8-3.3 \micron\. The H\sub 2\O multilayers were deposited on single-crystal Al\sub 2\O\sub 3\ substrates in ultrahigh vacuum. The resonantly desorbed H\sub 2\O was measured versus wavelength, pulse energy and multilayer thickness. The resonant desorption spectrum agreed well with the infrared absorption spectrum of ice at wavelengths longer than the peak absorbance at ~3.09 \micron\. At shorter wavelengths, the desorption yield was much greater than expected from the absorption spectrum. The enhanced resonant desorption yield was attributed to the dynamic absorption coefficient for H\sub 2\O. This dynamic absorption coefficient results from the absorbance shift of the O-H stretching vibration to shorter wavelengths versus temperature in hydrogen-bonded networks. The resonant desorption yield was also measured for consecutive IRFEL pulses versus pulse energy and ice multilayer thickness at wavelengths of 2.94 \micron\ and 3.09 \micron\. For an ice multilayer thickness of 5 microns and IRFEL macropulse energy of 1mJ, the desorption yield was nearly constant until the ice multilayer thinned to ~3 microns. An etch depth of 0.5-0.7 microns per IRFEL pulse was measured in this regime. For ice multilayer thicknesses <3 microns or macropulse energies >1 mJ, the desorption yield progressively decreased after the first IRFEL pulse.