Steven A. Buntin

Laser-excited hot-electron induced desorption: A theoretical model applied to NO/Pt(111)

Abstract We present a theoretical model for stimulated desorption due to the interaction of energetic substrate carriers with molecular adsorbates. The model is based on the premise that optically excited hot electrons scatter into an unoccupied valence electron resonance of the adsorbate, thus forming a temporary negative molecular ion which then experiences an enhanced attraction towards the substrate. Neutralization of the ion returns the adsorbed molecule to one of the continuum states of the molecule/substrate potential energy surface, possibly in an internally excited state. The consequences of such a model are worked out using semiclassical wave packet dynamics which, in the short time limit relevant to the present situation, can be brought to an analytic realization. The model provides considerable insight into recent experiments on the laser-induced desorption of NO from Pt(111).

State‐resolved evidence for hot carrier driven surface reactions: Laser‐induced desorption of NO from Pt(111)

State‐specific diagnostics are used to characterize the laser‐induced desorption of NO from Pt(111). Two desorption channels are observed; one is consistent with thermal activation, while the other is driven by adsorbate interactions with hot carriers. For this latter channel, the observed dependence of the desorption yield on the wavelength of the incident laser pulse (1907, 1064, 532, and 355 nm) and the wavelength dependence of the kinetic energy distributions establish the nonthermal nature of the excitation process. The inverted spin–orbit population, the non‐Boltzmann rotational state distributions, and the vibrational state population are interpreted in terms of a desorption mechanism involving a temporary ion resonance.

DYNAMICS OF THE H ATOM ABSTRACTION OF D ADSORBED ON SI(100)

Product HD kinetic energy distributions are reported for the incident gas phase H atom abstraction of D adsorbed on a monodeuteride-terminated Si(100) surface. The H atoms are generated by laser photolysis of HI and have well-defined kinetic energies in the range of 1–3 eV. For an incident H atom average kinetic energy of 〈EH〉=1.1 eV, the HD product kinetic energy distribution has a mean value of 〈EHD〉=1.2–1.3 eV and extends up to the nominal available-energy limit, providing dynamical evidence for a direct Eley–Rideal mechanism for this abstraction reaction. For 〈EH〉=1.5 and 3.2 eV, the HD product kinetic energy distribution broadens relative to that for 〈EH〉=1.1 eV while 〈EHD〉 remains unchanged, suggesting that energy loss to the substrate becomes more significant and the reaction becomes less Eley–Rideal-like for these higher energies. The results are compared with recent classical trajectory calculations.

HYPERTHERMAL H ATOM INTERACTIONS WITH D/SI(100) : EFFECTS OF INCIDENT H ATOM KINETIC ENERGY ON THE REMOVAL OF ADSORBED D

The interactions of H atoms having hyperthermal energies with a monodeuteride‐terminated Si(100) surface are investigated. H atoms having mean kinetic energies of 1.0 and 2.9 eV are generated by 248 and 193 nm laser photolysis, respectively, of a pulsed, free‐jet expansion of HI. Full characterization of the laser photolysis conditions allows the determination of the relative, as well as absolute, H atom exposures for these two kinetic energies. The depletion probability of adsorbed D per incident H atom is identical for species having incident kinetic energies of 1.0 and 2.9 eV and has an absolute value of 0.3±0.2.

State-resolved studies of the laser-induced desorption of NO from Si(111)7×7 : low coverage results

The results of a quantum‐state‐resolved study of the laser‐induced desorption (LID) of NO from Si(111) 7×7 at a surface temperature of 100 K are reported. All aspects of the LID are found to be sensitive to the initial coverage. The coverage dependence indicates that there are two desorption mechanisms, one operative at low coverages that is quenched with increasing NO exposure, and one operative at high coverage. This report characterizes the low coverage channel. Most of the energy in the desorbed NO occurs as vibration and translation, with the rotations substantially cooler. The desorption is selective for production of the ground spin–orbit state. The energy partitioning shows strikingly little change as the desorption‐laser wavelength was varied from 1907 to 355 nm. This, coupled with a quantitative study of the yield over the same photon energy range and selective coadsorption experiments, establishes that the desorption is specifically due to an interaction involving photogenerated holes in the re...

Non-Boltzmann rotational and inverted spin--orbit state distributions for laser-induced desorption of NO from Pt(111)

The internal state distributions of NO desorbed from a Pt(111) surface by visible and near‐visible laser radiation (355, 532, and 1064 nm) were measured by laser‐induced fluorescence. Non‐Boltzmann rotational state distributions and inverted spin–orbit populations were observed and both were found to be relatively insensitive to the desorption‐laser wavelength. It is suggested that the internal state distributions arise from the charge exchange processes occuring during desorption via a short‐lived negative‐ion resonance intermediate.

H atom abstraction of D adsorbed on Si(100): dynamical evidence for an Eley-Rideal mechanism

Abstract Kinetic energy distributions are reported for the HD product from the reaction of incident gas phase H atoms with D adsorbed on Si(100). For an incident kinetic energy of nominally 1 eV, the HD product kinetic energy distribution extends up to the nominal available-energy limit and has a mean value of 1.2–1.3 eV, providing dynamic evidence for a direct Eley-Rideal mechanism. For higher incident H atom kinetic energies, the HD product kinetic energy distribution broadens while the mean value remains unchanged, suggesting that energy loss to the substrate becomes more significant and the reaction becomes less Eley-Rideal-like.

Constraints on the Use of Polarization and Angle of Incidence to Characterize Surface Photoreactions

Abstract It is demonstrated that for substrates with a large absolute value of the dielectric constant, comparison between surface photoreaction experiments and calculations of both the substrate absorption and the surface electric fields cannot distinguish between a substrate-mediated mechanism and a direct excitation mechanism characterized by a transition moment predominantly parallel to the surface. These two mechanisms cannot be distinguished since the rate for a direct excitation process is related to the incident-beam irradiance, while the rate for a substrate-mediated process is related to the irradiance intercepted by the surface.

Adsorption and photodecomposition of Mo(CO)6 on Si(111) 7×7: An infrared reflection absorption spectroscopy study

The adsorption and photodecomposition of Mo(CO)6 adsorbed on Si(111) 7×7 surfaces has been studied with Auger electron spectroscopy, temperature programmed desorption,low energy electron diffraction and infrared reflection absorption spectroscopy in a single external reflection configuration. The external‐reflection technique is demonstrated to have adequate sensitivity to characterize submonolayer coverages of photogenerated Mo(CO) x fragments. It is proposed that the first layer of Mo(CO)6 adsorbs in ordered islands with a Mo(CO)6 atop each adatom of the 7×7 reconstructed Si surface.UV irradiation of these islands produces a carbonyl fragment, identified as chemisorbed Mo(CO)5. The Mo(CO)5 thermally decarbonylates via two subcarbonyl intermediates with little CO dissociation. Photolysis of thicker layers results in the formation of Mo x (CO) y dimers/polymers, as evidenced by the appearance of bridging CO, which is attributed to a facile association reaction. The dimer/polymer species correlate with deposition of C and O on the surface.

Laser-induced desorption of NO from Si(111): effects of coverage on NO vibrational populations

Abstract Laser-induced desorption of NO from Si(111) has been investigated using state-specific detection techniques. The observed energy partitioning in the desorbed NO varies significantly with the NO coverage. Characterization of the energy partitioning with desorption-laser wavelengths in the range 355–1907 nm indicates that different substrate electronic excitations are responsible for the desorption at low NO coverage vs saturation coverage. The relationship between the substrate electronic excitation and observed vibrational-state-population distributions is explored.