ALBERT

Notes: We examined energy levels and stationary states of a quantum-mechanical particle adsorbed on a rough and disordered surface. The specific system examined consists of an H atom (or a D atom) adsorbed on an amorphous ice cluster (H2O)115. Two kinds of stationary states of the adsorbate particle were obtained: states localized in cavities on the cluster surface, and states occupying corridors of low potential energy present on the cluster surface. Zero-point energy effects were found to be very significant; thus the ordering of ground state energies in the different cavities does not follow at all the ordering of minimum potential energies in the cavities. Significant localization was demonstrated of the calculated eigenstates within small portions of connected energetically accessible regions on the cluster surface. The localization seems to be associated with disorder in the three-dimensional potential energy surface, which includes strongly varying well depth, and sharp turns in potential contours. Phase matching of the adsorbate wave function is not easily achieved in the different parts of the potential well, the result being localization of the eigenstates.

Notes: We investigated the potential energy surface for the H2O⋅⋅⋅H interaction in the van der Waals well region. Calculations were carried out using the Møller–Plesset second- and fourth-order perturbation theory in a [12s,7p,2d]→(6s,5p,2d) basis set for the O atom, and [6s,2p,1d]→(5s,2p,1d) for the H atoms. Basis set and superposition error effects were analyzed to gauge the reliability of the calculated potential. The potential was investigated in five physically distinct directions. The deepest potential well was found in the H2O molecular plane 3.30–3.45 A(ring) from the H2O center of mass, near the H end of the OH bond. The following parameters are suggested for the spherically averaged potential: well depth 53±6 cm−1; minimum distance from the center of mass 3.25–3.40 A(ring).

Notes: A microscopic model is presented for anharmonic vibrations of ethylidyne, 3/4 CCH3, chemisorbed on the Pt(111) surface. The model includes 24 vibrational modes of the adsorbate and of the solid. A quantum-mechanical calculation based on second-order perturbation theory is used to interpret experimental data on vibrations of 3/4 CCH3/Pt(111) and 3/4 CCD3/Pt(111). The measured temperature dependence of the CC infrared fundamental and of the umbrella mode fundamental can be accounted for by anharmonic coupling between the CC stretch and the three PtPt stretch coordinates at the base of the adsorbate. Line shapes calculated using classical molecular dynamics disagree significantly with quantum-mechanical results, the apparent reason being overestimation of vibrational energy transfer in the classical calculation. A semiclassical approximation is suggested, in which all the high frequency adsorbate modes except the infrared absorbing mode are frozen; the remaining modes are treated by classical mechanics. The semiclassical calculation agrees much better with the quantum-mechanical results, and can be extended to higher dimension in a straightforward fashion.

Notes: Condensation dynamics and structure of low temperature ≤100 K amorphous ices is studied using the classical trajectory simulation technique. Specifically, classical equations of motion are solved for a sequence of H2O molecules impinging at random on an initial nucleus of ten water molecules. The sticking probability to the cluster is found to be unity. Using this technique we accumulated three amorphous clusters of 100–300 water molecules. The analysis of the results included collision dynamics and molecular structure. Upon collision with the cluster, the impinging H2O molecule forms typically two hydrogen bonds. The two new bonds are almost always asymmetric, i.e., one bond is via an O atom of the new molecule, and the other via one of its H atoms. We identified two distinct types of collision trajectories—"simple'' trajectories and "complex'' ones. In a simple trajectory, a new molecule is attached to the surface, without disrupting the preexisting hydrogen bond network in the cluster. In a complex trajectory, the preexisting bond network is modified significantly during the collision. Up to ∼15 hydrogen bonds can be changed (formed or destroyed) during such a trajectory; and the spatial extent of the changes can be as large as ∼20 A(ring).The complex trajectories comprise 60%∼70% of all the collision trajectories. Thus, the hydrogen bond network evolves continuously during the growth of the ice sample; the net trend being towards the four coordinated molecular configuration around each molecule. The average coordination number of a molecule in the final clusters is 3.5∼3.7, despite the fact that most of the water molecules are on the exposed surface of the cluster. The main features of the local molecular structure in the clusters are (a) ordering of OO and OH distances within hydrogen bonds (b) very significant disorder in O–O–O angles between adjacent hydrogen bonds; the width of the O–O–O angle distribution is ∼40°. The molecular structure seems to be closely related to that of liquid water. The experimental electron diffraction patterns of amorphous clusters are reproduced very well by our models. Comparison was also made with the radial distribution functions derived for bulk amorphous ice from x-ray and neutron diffraction experiments. Reasonable agreement was obtained within the range (approximately-less-than)4 A(ring), while at larger distances finite size effects become important.

Notes: This study focuses on van Hove-type spikes in the sticking probability of light particles on crystalline surfaces. The spikes result from singularities in the density of surface phonons subject to the constraints of the energy and the momentum conservation. The sharp features in sticking probability as a function of incoming angle and energy are derived and demonstrated for a model H2/Cu(100) system.

Notes: Interpretation is provided for the recently measured stretch spectra of the dangling OH and OD bonds in amorphous ice deposits. The spectral bands are doublets; the higher and the lower frequency feature of each doublet is assigned respectively to 2- and 3-coordinated water molecules on the surface. The assignment is based on combined computational and experimental evidence. Annealing of 2-coordinated surface molecules is demonstrated in a simulated amorphous H2O cluster.

Notes: This investigation is focused on dynamics and structure of an amorphous water cluster (H2O)116. The cluster was obtained in a previous study by simulation of the condensation of amorphous ice. The time evolution of the cluster is studied on a ∼400 ps time scale, using the classical trajectory technique at different temperatures. Two distinct structural and dynamical domains are identified. At 36 K the cluster displays glass-like behavior. There is also a broad distribution of molecular states extending from 1- to 5-coordinated water molecules. Upon heating to 100 K the cluster undergoes a transition to a more compact structure. The molecular energy distribution narrows down and is dominated by 3- and 4-coordinated molecules. After the transition the cluster displays liquid-like characteristics. All these changes are not reflected by the normal frequency distribution of structures along the trajectory.

Notes: Infrared spectra are reported for thin films of deuterated microporous amorphous ice formed at 12 K and saturated with absorbed molecular hydrogen. This paper focuses on both the influence of the surface-bound H2 on the absorption bands of the OD groups that dangle from the micropore surfaces and the behavior of the induced infrared bands of the stretching mode of H2 itself. Both structural changes and the relaxation of ortho-H2 to para-H2 are apparent from variations in the observed spectra with time and temperature. A reasonably detailed interpretation of the complex spectral behavior has been possible through simulation of spectra for H2 interacting with the surface of amorphous ice clusters generated previously in a classical trajectory study of the cluster growth through the accumulation and relaxation of individual water molecules. Potential minima were calculated with respect to H2 coordinates on the cluster surface and a qualitative interpretation of adsorbate–surface bonding provided via the partitioning of the H2⋅⋅D2O interactions into relatively weak van der Waals interactions, and stronger electrostatic interactions between the H2 quadrupole and the H2O dipole and quadrupole. The net binding of H2 to the surface is dominated by several van der Waals interactions with neighboring D2O molecules, but in most of the calculated minima, H2 also forms a single electrostatic bond to a surface molecule. While such electrostatic bonds provide only a small fraction of the binding energy, they appear to influence strongly the observed spectra. Observed H2-induced shifts in the dangling OD bands appear to be caused by electrostatic bonding of H2 to a D atom of a dangling OD and a significant fraction of the H2 intensity is proposed to originate from H2 molecules which are electrostatically bonded to dangling oxygen atoms on the surface. Calculations (which do not contain adjustable parameters) reproduce quite well several features of the measured spectra, including the splitting of the dangling OD band, the shift of this band due to binding of H2, and the frequency and the width of the H2 band.

Notes: Application of the diffusion Monte Carlo method is discussed for ground states of molecular systems in which molecules are treated as rigid bodies. The method is applied to the investigation of the para-H2...H2O and ortho-H2...H2O clusters. Significant differences are demonstrated in the bonding of para-H2 and ortho-H2 to the water molecule.