Effect of beam energy and surface temperature on the dissociative adsorption of H2 on Si(001)

Survival of rotational alignment in H2 scattering from Si(100)

We report a state-prepared, state-resolved study of rotational scattering of a diatomic molecule from a solid surface. Specifically, H₂ molecules with 80 meV kinetic energy are rotationally aligned in the j = 3 rotational state via stimulated Raman pumping and then scattered from a Si(100) surface at normal incidence. The rotational alignment of the scattered molecules is determined by measuring, for both the incident and scattered molecules, the ionization yield of a probe laser, tuned to selectively ionize molecules in the j = 3 rotation level, as the probe laser polarization is rotated. The measurement is performed for two initial rotational alignments: a "helicoptering" alignment with the bonds constrained to lie primarily parallel to the surface and a "cartwheeling" alignment with the bonds lying primarily normal to the surface. For both initial alignments, the modulation of the probe ionization yield with laser polarization for the scattered molecules is pronounced, although significantly weaker than the modulation measured for the incident molecules. This indicates a significant modification but not a complete elimination of the initial alignment. The modulation is found to be stronger for the scattered molecules originating in the cartwheeling alignment than for the helicoptering alignment. These results contribute toward an improved understanding of the role of rotational motion in molecule-surface dynamics.

Dynamics of proton transfer reactions on silicon surfaces: OH-dissociation of methanol and water on Si(001)

The reaction dynamics of methanol and water on Si(001) were investigated by means of molecular beam techniques. The initial sticking probability s₀ was determined as a function of the kinetic energy of the incoming molecules, E_kin, and surface temperature, T_s. For both, methanol and water, a nonactivated reactional channel was observed; the dynamics were found to be determined by the reaction into the datively bonded intermediate state. A low conversion barrier was deduced for the conversion from this intermediate into the final state. It is attributed to the reaction mechanism, which proceeds via proton transfer from the OH-group of the datively bonded molecules to a Si surface atom. Despite this low conversion barrier, adsorption into the intermediate and further reaction via proton transfer were found to be largely decoupled.

Binding Energy and Dissociation Barrier: Experimental Determination of the Key Parameters of the Potential Energy Curve of Diethyl Ether on Si(001)

The key parameters of the potential energy curve of organic molecules on semiconductor surfaces, binding energy of the intermediate state and dissociation barrier, were experimentally investigated for the model system of diethyl ether (Et2O) on Si(001). Et2O adsorbs via a datively bonded intermediate from which it converts via ether cleavage into a covalently attached final state. This thermally activated conversion into the final state was followed in real-time by means of optical second-harmonic generation (SHG) at different temperatures and the associated energy barrier ϵa = 0.38 ± 0.05 eV and pre-exponential factor νa = 10(4±1) s(-1) were determined. From molecular beam experiments on the initial sticking probability, the difference between the desorption energy ϵd and ϵa was extracted and thus the binding energy of the intermediate state was determined (0.62 ± 0.08 eV). The results are discussed in terms of general chemical trends as well as with respect to a wider applicability on adsorbate reactions on semiconductor surfaces.

Antiferromagnetic spin ordering in the dissociative adsorption of H2 on Si(001): density-functional calculations

The dissociative adsorption of an H(2) molecule on the Si(001) surface, which has been experimentally identified in terms of dissociation on one side of two adjacent Si dimers, is investigated by spin polarized density-functional calculations within the generalized-gradient approximation. In contrast to the prevailing nonmagnetic configuration of charge ordering, we propose a new ground state where the two single dangling bonds (DBs) created by H(2) dissociation are antiferromagnetically coupled with each other. Such a spin ordering is found to be energetically favored over the previously proposed charge ordering. In the latter configuration, the buckling of the two DBs amounts to a height difference (Delta h) of 0.63 A, caused by a Jahn-Teller-like distortion, while in the former configuration, their buckling is almost suppressed to be Delta h=0.03 A as a consequence of spin polarization.

Evidence for hydrogen desorption through both interdimer and intradimer paths from Si(100)-(2 x 1)

Despite intensive work there are still controversial issues about desorption and adsorption of hydrogen on Si(100)-(2 x 1). In particular, the relative importance of the various interdimer- and intradimer-desorption paths is not clear. Nanosecond-pulse-laser desorption data have been used to argue that the 4H interdimer path is important, while data from thermal-desorption time-of-flight measurements suggest a large translationally hot contribution which cannot arise from the 4H interdimer path. The observation of a translationally hot desorption fraction at low to medium coverage can be accounted for by including the 2H interdimer path in quantum dynamical calculations. In this paper we investigate this issue further and present evidence that supports the inclusion of the intradimer path. Specifically, our results show that the intradimer and 3H interdimer paths provide the major contributions to the translationally hot fraction in the desorbate. Our conclusions are based on density-functional calculations of hydrogen translational excitation, mean-field analysis of thermal-desorption experiments over a range of ramp rate, and Monte Carlo simulations of nanosecond-pulse-laser experiments.

Desorption dynamics of deuterium molecules from the Si(100)-(3x1) dideuteride surface

We measured polar angle (theta)-resolved time-of-flight spectra of D2 molecules desorbing from the Si(100)-(3x1) dideuteride surface. The desorbing D2 molecules exhibit a considerable translational heating with mean desorption kinetic energies of approximately 0.25 eV, which is mostly independent of the desorption angles for 0 degrees<or=theta<or=30 degrees. The observed desorption dynamics of deuterium was discussed along the principle of detailed balance to predict their adsorption dynamics onto the monohydride Si surface.

Coverage dependent desorption dynamics of deuterium on Si(100) surfaces: Interpretation with a diffusion-promoted desorption model

We studied coverage dependence of time-of-flight (TOF) spectra of D2 molecules thermally desorbed from the D/Si(100) surface. The mean translational energies Et of desorbed D2 molecules were found to increase from 0.20+/-0.05 eV to 0.40+/-0.04 eV as the desorption coverage window was decreased from 1.0 ML> or =thetaD> or =0.9 ML to 0.2 ML> or =thetaD> or =0 ML, being consistent with the kinetics switch predicted in the interdimer mechanism. The measured TOF spectra were deconvoluted into 2H, 3H, and 4H components by a curve fitting method along the principle of detailed balance. As a result, it turned out that the desorption kinetics changes from the 4H to the 3H situation at high coverage above thetaD=0.9 ML, while the 2H desorption is dominant for a quite wide coverage region up to thetaD=0.8 ML. A dynamic desorption mechanism by which the desorption is promoted by D-atom diffusion to dangling bonds was proposed.

Molecular beam investigation of hydrogen dissociation on Si(001) and Si(111) surfaces

The influence of molecular vibrations on the reaction dynamics of H2 on Si(001) as well as isotopic effects have been investigated by means of optical second-harmonic generation and molecular beam techniques. Enhanced dissociation of vibrationally excited H2 on Si(001)2 x 1 has been found corresponding to a reduction of the mean adsorption barrier to 390 meV and 180 meV for nu=1 and nu=2, respectively. The adsorption dynamics of the isotopes H2 and D2 show only small differences in the accessible range of beam energies between 50 meV and 350 meV. They are traced back to different degrees of vibrational excitation and do not point to an important influence of quantum tunneling in crossing the adsorption barrier. The sticking probability of H2 on the 7 x 7-reconstructed Si(111) surface was found to be activated both by H2 kinetic energy and surface temperature in a qualitatively similar fashion as H2/Si(001)2 x 1. Quantitatively, the overall sticking probabilities of H2 on the Si(111) surface are about one order of magnitude lower than on Si(001), the influence of surface temperature is generally stronger.

Quantum Monte Carlo calculations of H2 dissociation on Si(001)

The dissociative adsorption of H2 on the Si(001) surface is theoretically investigated for several reaction pathways using quantum Monte Carlo methods. Our reaction energies and barriers are at large variance with those obtained with commonly used approximate exchange-correlation density functionals. Our results for adsorption support recent experimental findings, while, for desorption, the calculations give barriers in excess of the presently accepted experimental value, pinpointing the role of coverage effects and desorption from steps.

Translational heating of D(2) molecules thermally desorbed from Si(100) and Ge(100) surfaces

The translational energies of D(2) molecules thermally desorbed from the Si(100) and Ge(100) surfaces under a heating rate of 6 K/s have been measured. In contrast to the previous laser desorption study, results show a considerable translational heating; the observed translational temperature is about 3 times higher than the desorption temperature for both surfaces. This fact indicates that energy barriers for adsorption are present even in the desorption pathway. Detailed balance is applicable to the adsorption and desorption dynamics of hydrogen on the Si(100) surface.

Probing high-barrier pathways of surface reactions by scanning tunneling microscopy

The ability of scanning tunneling microscopy to probe the pathways of thermally activated high-barrier surface processes is frequently limited by competing low-barrier processes that can confuse measurement of the true initial and final configuration. We introduce an approach to circumvent this difficulty by driving the surface process with nanosecond laser heating. The method is applied to determine the pathway of recombinative desorption in the H/Si(001) system. The observed configuration of dangling bonds after laser heating reveals that the desorbed hydrogen molecules are not formed on single dimers, but rather from neighboring silicon dimers via an interdimer reaction pathway.

Stereochemistry on Si(001): angular dependence of H2 dissociation

The angular dependence of the dissociative adsorption of molecular hydrogen at terrace and step sites of vicinal single-domain Si(001) surfaces was investigated by means of molecular beam techniques and optical second-harmonic generation. A strongly anisotropic behavior was observed for terrace adsorption with polar distributions of cos3theta and cos12theta parallel and perpendicular to the dimer, respectively. The D(B)-steps show enhanced reactivity under glancing incidence in the upwards direction. The results are traced back to the directionality of the covalent surface bonds.

Real-space study of the pathway for dissociative adsorption of H2 on Si(001)

Dissociative adsorption of molecular hydrogen on clean Si(001) surfaces has been investigated by means of scanning tunneling microscopy. The dissociated hydrogen atoms are found to occupy Si atoms of adjacent dimers. In addition to this interdimer configuration associated with the adsorption of isolated hydrogen molecules, pairs of adjacent doubly occupied dimers are readily formed. They arise from the enhanced reactivity of partially occupied dimers following the initial H2 adsorption step. The results are considered in light of recent adsorption and desorption measurements.

Structure Sensitive Reaction Channels of Molecular Hydrogen on Silicon Surfaces

The ability of the Si(001) surface to adsorb H2 molecules dissociatively increases by orders of magnitude when appropriate surface dangling bonds are terminated by H atoms. Through molecular beam techniques the energy dependent sticking probability at different adsorption sites on H-precovered and stepped surfaces is measured to obtain information about the barriers to adsorption, which decrease systematically with an increase in coadsorbed H atoms. With the help of density functional calculations for interdimer adsorption pathways, this effect is traced back to the electronic structure of the different adsorption sites and its interplay with local lattice distortions.

Interaction of H2 with Si(001)-(2 x 1): solution of the barrier puzzle

The sticking probability of H2 on Si(001) is immeasurably small at room temperature, indicating the presence of a large energy barrier to adsorption. Surprisingly, the final state energy distributions of H2 molecules desorbing from Si(001) show no signs of having traversed such a barrier, in apparent contradiction with microscopic reversibility. Here we report experimental and theoretical evidence resolving this long-standing puzzle. Adsorption and desorption proceeding along two distinct, microscopically reversible pathways can explain all observations.