V. Buch

Time‐dependent self‐consistent field approximation for intramolecular energy transfer. I. Formulation and application to dissociation of van der Waals molecules

The time‐dependent self‐consistent field method (TDSCF) is formulated and applied to the study of intramolecular dynamics and unimolecular decomposition processes. The method is illustrated by calculations on vibrational predissociation in van der Waals molecules, such as I2(v)Ne→I2(v′)+Ne. The TDSCF has the advantage of achieving formal separability of the modes by associating a time‐dependent Hamiltonian with each mode, while permitting (possibly extensive) energy transfer among the modes via the time‐dependent mean potential which acts on each mode. We present quantal, semiclassical, and classical versions of the method; a proper classical limit of the quantum TDSCF replaces the averages over wave functions by averages over self‐consistently obtained bundles of trajectories. In all three versions, considerable computational economy is retained in comparison with full dynamics calculation. A detailed study is made of those properties which can be correctly obtained in such a time‐dependent mean field theory. Attention is drawn to problems such as the occurrence of spurious states in the asymptotic region, and a simple method for avoiding them is suggested. We find for the I2(v)Ne vibrational predissociation that the TDSCF compares well with corresponding full dynamics calculations for average single mode properties such as dissociation lifetimes and the translational energy release. Moreover, comparison with classical trajectory calculations shows that the TDSCF method reproduces the essential dynamical mechanism of the dissociation (in‐phase impulsive I⋅⋅⋅Ne collision following several ineffective vibrations). The self‐consistent bundle trajectories and the time‐dependent mean fields are analyzed, and provide insight into the process dynamics. It is concluded that the TDSCF approach in both quantal and quasiclassical versions is a potentially powerful tool in the study of intramolecular energy transfer and unimolecular dissociation.

Path integral simulations of mixed para‐D2 and ortho‐D2 clusters: The orientational effects

A recently suggested computational scheme [Buch and Devlin, J. Chem. Phys. 98, 4195 (1993)] is extended to describe orientational phenomena in systems containing assemblies of odd‐J and even‐J hydrogen and deuterium molecules. Nuclear symmetry and rotational effects are incorporated in the path integral Monte Carlo algorithm. The scheme is employed in a study of the (para‐D2)3(ortho‐D2)10 and (para‐D2)13 clusters in the 1–3 K temperature range. In the (para‐D2)3(ortho‐D2)10 cluster at 1 K, the three J=1 molecules form a ‘‘cluster within cluster,’’ held together by the anisotropic quadrupole–quadrupole interactions. At 3 K the three molecule J=1 cluster is dissolved within the (D2)13 cluster. The orientational structure of the (para‐D2)13 cluster at 1 K is composed of two distinct distorted‐T pair configurations of neighboring J=1 molecules. Upon heating to 3 K the orientational structure changes, and a clear separation between the two pair geometries is no longer obtained. Moreover at 3 K thermal distorti...

Treatment of rigid bodies by diffusion Monte Carlo: Application to the para‐H2⋅⋅⋅H2O and ortho‐H2⋅⋅⋅H2O clusters

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.

Solid water clusters in the size range of tens-thousands of H2O: A combined computational/spectroscopic outlook

A joint computational and experimental effort was directed towards the understanding of large solid water clusters. The computations included structure optimizations and calculations of OH stretch spectra for select sizes in the range n = 20–931. The measurements focused predominantly on OH stretch spectroscopy as a function of mean cluster size. FTIR spectra are discussed for the size range of tens to hundreds-of-thousands of molecules. Photofragment spectroscopy in molecular beams is shown to be a sensitive probe of the outer cluster surfaces. The crucial element of the different experimental approaches is the control and the estimation of the mean cluster sizes. The combined experimental and computational results are consistent with the physical picture of quasi-spherical nanocrystals with disordered reconstructed surface layers. The surface reconstruction can be viewed as the outcome of recombination of surface dangling atoms, to increase the number of hydrogen bonds. The hydrogen bonds within the mostly crystalline subsurface layer are stretched by the interaction with the disordered component. The emergence of the (strained) crystal core occurs at a size of a few hundred H2O. Smaller clusters are described as compact-amorphous.

Spectra of dangling OH bonds in amorphous ice: Assignment to 2‐ and 3‐coordinated surface molecules

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.

Vibrational spectra of water complexes with H2, N2, and CO

Ab initio calculations are carried out on the H2O...N2, H2O...H2, and H2O...CO complexes. Infrared spectra of the complexes are investigated, with an emphasis on the effect of weak bonding on the frequencies and the infrared intensities of the monomers. Connections are explored between the computational results and the experimentally measured infrared spectra of ice surfaces covered by H2, N2, and CO adsorbate. Additional issues addressed include the influence of the counterpoise correction on the equilibrium geometry of the complexes, and the analysis of the different contributions (exchange, dispersion, electrostatic) to the weak bonding, and to the frequency shifts.

Infrared spectra of ice surfaces and assignment of surface‐localized modes from simulated spectra of cubic ice

The use of a new method of preparing micron‐thick deposits of nanocrystals of ice for Fourier transform infrared sampling, with the nanocrystals supported on a vertical infrared window, has greatly improved the signal‐to‐noise levels of the spectra available for large ice clusters. High quality spectra of modes of the surface molecules are reported, even for regions that underlie the intense bands of the bulk ice modes. These experimental features are most clearly displayed through the use of difference spectra. For example, the difference between spectra obtained for nanocrystals, before and after an annealing cycle that significantly increases the average cluster size, reflects the decrease in number of surface groups and the corresponding increase in number of interior molecules. Similarly, differences between spectra of bare and adsorbate‐covered nanocrystals, obtained at the same temperature for the same ice sample, show the significant shifts of ‘‘surface‐localized’’ ice modes caused by the adsorbat...

A new interpretation of the OH-stretch spectrum of ice

A new interpretation is presented of the vibrational band shapes of the proton disordered ice Ih and Ic in the OH and OD stretch region. The vibrationally excited states are shown to be related to those of a periodic system, whose unit cell is composed of four oscillating dipoles in a tetrahedral arrangement around an O-atom. Analysis of the excitations in this periodic system and of the perturbation induced by disorder is used to provide a detailed assignment of the band shapes of the measured spectra.

Experimental and simulated vibrational spectra of H2 absorbed in amorphous ice: Surface structures, energetics, and relaxations

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 part...

Ab initio study of the intermolecular potential of the water–carbon monoxide complex

The combination of the supermolecular Mo/ller–Plesset scheme with the perturbation theory of intermolecular forces is applied in the analysis of the potential energy surface (PES) of the H2O...CO complex. We located three low‐energy configurations on the potential energy surface corresponding to two isomeric H‐bonded complexes OC...HOH (C structure), CO...HOH (O structure), and a T‐shaped structure with CO bonded to the O atom of H2O. The absolute minimum corresponds to the C configuration OC...HOH, involving a nonlinear C...H–O bond. The tilt from the linearity is 11 deg, in agreement with the value derived from the experimental data. The computed binding energies on the fourth‐order perturbation theory level are 651 cm−1 for the C configuration, 301 cm−1 for T, and 256 cm−1 for O. The anisotropy of the potential energy surface is analyzed using the perturbation theory. The absolute minimum results from the attractive electrostatic contribution and dispersion energy, which overcome considerable exchange ...