Volume 112, Issue 5, 01 February 2000
 COMMUNICATIONS


Observation of spinpolarized atomic photofragments through the Dopplerresolved Faraday technique
View Description Hide DescriptionThe angular distribution of spinoriented Rb photofragments produced by photodissociation of RbI molecules using circularly polarized UV light at 266 nm has been investigated for the first time applying the method of Dopplerresolved paramagneticFaraday rotation. The observed signals could successfully be explained as the sum of two contributions being due to incoherent and coherent excitation of the RbI molecules. For the ratio of the two corresponding anisotropy parameters we obtain a value of −0.33 as a preliminary result.
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 THEORETICAL METHODS AND ALGORITHMS


A study of water clusters using the effective fragment potential and Monte Carlo simulated annealing
View Description Hide DescriptionSimulated annealing methods have been used with the effective fragment potential to locate the lowest energy structures for the water clusters with 8, 10, 12, 14, 16, 18, and 20. The most successful method uses a local minimization on each Monte Carlo step. The effective fragment potential method yielded interaction energies in excellent agreement with those calculated at the ab initio Hartree–Fock level and was quite successful at predicting the same energy ordering as the higherlevel perturbation theory and coupled cluster methods. Analysis of the molecular interaction energies in terms of its electrostatic,polarization, and exchangerepulsion/chargetransfer components reveals that the electrostatic contribution is the dominant term in determining the energy ordering of the minima on the potential energy surfaces, but that differences in the polarization and repulsion components can be important in some cases.

Determining point charge arrays that produce accurate ionic crystal fields for atomic cluster calculations
View Description Hide DescriptionIn performing atomic cluster calculations of local electronic structuredefects in ionic crystals, the crystal is often modeled as a central cluster of 5–50 ions embedded in an array of point charges. For most crystals, however, a finite threedimensional repeated array of unit cells generates electrostatic potentials that are in significant disagreement with the Madelung (infinite crystal) potentials computed by the Ewald method. This is illustrated for the cubic crystal We present a novel algorithm for solving this problem for any crystal whose unit cell information is known: (1) the unit cell is used to generate a neutral array containing typically 10 000 point charges at their normal crystallographic positions; (2) the array is divided into zone 1 (a volume defined by the atomic cluster of interest), zone 2 (several hundred additional point charges that together with zone 1 fill a spherical volume), and zone 3 (all other point charges); (3) the Ewald formula is used to compute the site potentials at all point charges in zones 1 and 2; (4) a system of simultaneous linear equations is solved to find the zone 3 charge values that make the zone 1 and zone 2 site potentials exactly equal to their Ewald values and the total charge and dipole moments equal to zero, and (5) the solution is checked at 1000 additional points randomly chosen in zone 1. The method is applied to 33 different crystal types with 50–71 ions in zone 1. In all cases the accuracy determined in step 5 steadily improves as the sizes of zones 2 and 3 are increased, reaching a typical rms error of 1 μV in zone 1 for 500 point charges in zone 2 and 10 000 in zone 3.

Intrinsic cascading contributions to the fifth and seventhorder electronically offresonant Raman spectroscopies
View Description Hide DescriptionThe cascading polarization contributing to the femtosecond fifth and seventhorder coherent Raman scattering processes is obtained in terms of the third and fifthorder Raman response functions. It is found that four cascading processes contribute to the fifthorder Raman scattering signal. In the case of the seventhorder Raman scattering, there are 34 possible cascading processes contributing to the seventhorder Raman scattering signal, and they are classified as  and types depending on the involved loworder nonlinear optical processes. Due to the complicated interference among these polarization fields, it may not be possible to experimentally measure the pure seventhorder Raman scattering signal without contamination from the cascaded contributions.

Reduced dynamics in spinboson models: A method for both slow and fast bath
View Description Hide DescriptionWe study a model for treating dissipative systems, a one dimensional quantum system coupled to a harmonic bath. The dynamics of such a system can be described by Feynman’s path integral expression for the reduced density matrix. In this formulation the interaction of the system with the environment is stored in the influence functional. Recently we showed that fast environmental modes that give rise to correlations in the influence functional which are short range in time can be treated efficiently by a memory equation algorithm, which is a discretized version of a master equation. In this work we extend this approach to treat slow environmental modes as well, thereby efficiently linking adiabatic and nonadiabatic regimes. In this extended method the long range correlations in the influence functional arising from slow bath modes are taken into account through Stock’s semiclassical selfconsistentfield approach.

Subspace iteration techniques for the calculation of resonances using complex symmetric Hamiltonians
View Description Hide DescriptionWe investigate and compare the performance of the Arnoldi and blockDavidson approaches for the calculation of selected eigenstates of complex symmetric Hamiltonians arising in the study of resonances. In the context of the blockDavidson scheme, both the “natural” complex symmetric subspace projection and the unsymmetric orthogonal projection are studied. The latter is found to possess the best convergence properties in realistic examples, clearly outperforming the other methods.

On the adiabatic to diabatic states transformation near intersections of conical intersections
View Description Hide DescriptionA Hamiltonian, “rigorously” diabatic in the vicinity of a point of conical intersection, is constructed using secondorder degenerate perturbation theory. Near an on a seam of conical intersection of two states of different symmetry, may exhibit a confluence with a seam of conical intersection of two states of the same symmetry. Thus by construction, there exists a “rigorous” diabatic representation of the vicinity of this confluence. A procedure for defining a unique linear combination of the degenerate states at a conical intersection is found to be useful for determining the parameters for and for identifying approximate symmetries in situations where point group symmetry is rigorously absent.

Nuclear dynamics in the vicinity of the crossing seam: Theory and application to vibrational spectrum of
View Description Hide DescriptionBased on the equivalence of the gauge transformations for the nuclear and electronic wave functions, the physical meaning of the geometric phase effect has been disclosed for a two coupledstate system. It is found that the geometric phase is defined by the argument of the complex electronic vector state in the complex plane spanned by the two realvalued electronic components. Such an angle is identical (up to a constant) to the mixing angle γ(R). Novel generalized Born–Oppenheimer equations for the two coupledstate problem in the vicinity of the crossing seam have been derived, and numerical calculations of vibrational spectra done for The results demonstrate significant differences in relation to those obtained from the assumption that

Relations among the modern theories of diffusioninfluenced reactions. II. Reduced distribution function theory versus modified integral encounter theory
View Description Hide DescriptionIt is shown that exactly the same results of the modified integral encounter theory (MET) of diffusioninfluenced reactions can be derived from the nonequilibrium reduced distribution function (RDF) theory. The method of solution of the RDF theory is much simpler than that of the MET, and shows clearly that the sophisticated approximation scheme employed in the latter is effectively equivalent to a simple truncation approximation adopted in the former. We evaluate the relative accuracy of the various theories by comparing the theoretical results with the results of an accurate Brownian dynamics simulation and the exact theory that are available for simple reaction models.
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 GAS PHASE DYNAMICS AND STRUCTURE: SPECTROSCOPY, MOLECULAR INTERACTIONS, SCATTERING, AND PHOTOCHEMISTRY


Theoretical electronic absorption and natural circular dichroism spectra of (−)transcyclooctene
View Description Hide DescriptionUsing the random phase approximation and coupled cluster singles and doubles linear response theory in conjunction with two basis sets of augmented doublezeta quality and two nuclear geometries, we have calculated electronic absorption and natural circular dichroismspectra of (−)transcyclooctene. We present a density functional theory optimized nuclear geometry whose ground state electric dipole moment and harmonic vibrational spectrum compare well with experimental data. The coupled cluster results obtained with this nuclear geometry are in good agreement with experimental electronic spectra, although the original interpretation of the most intense lowlying band as a transition is contradicted.

Very high rotational excitation of CO in a cooled electric discharge through carbon monoxide
View Description Hide DescriptionInfrared emission from and excited in the cathode region of a discharge tube immersed in liquid nitrogen, was recorded by Fouriertransform spectrometry at a resolution of 0.005 cm^{−1}. The sequence bands recorded in the 2500–1800 cm^{−1}spectral interval, indicate the existence of three different rotational populations; (i) molecules in the zeroground level with K (responsible for reabsorption of part of the 1–0 emission band); (ii) molecules with K (maximum intensity for in each band, K for K for (iii) molecules with limited to 6, for which Rrotational lines are observed for values between 50 and 120 nonBoltzmannian population distribution). The fullwidth at halfmaximum (FWHM) of all the observed lines is less than 0.007 cm^{−1}. A Doppler width of 0.005 cm^{−1} and translational temperature K can be deduced. Such highJ levels of the CO molecule had never been observed in the laboratory. In the absorptionspectrum of the Sun photosphere, the same lines present FWHM values 5–8 times larger. The best available Dunham coefficients are checked to reproduce the highJ lines wave numbers to at least 0.001 cm^{−1}. Dissociative recombination of the dimer cation, which is likely to be formed in our experimental conditions, is discussed as a possible mechanism to produce CO fragments with very high rotational excitation, while keeping vibrational excitation limited to

Ab initio study of interactions in methylamine clusters. The significance of cooperative effects
View Description Hide DescriptionMethylamine clusters consisting of up to four molecules were studied by employing Hartree–Fock, density functional theory, and Moller–Plesset calculations with the basis set. The dimer was found to exhibit two minima with similar interaction energies (−13 kJ/mol) and involving a hydrogen bond. The dipole moment for the dimer differs by up to 20% from the vector addition of the dipole moments for the individual molecules by effect of the interaction; also, the N–H bond distance in the group involved in the hydrogen bond is lengthened by up to 0.006 Å as a result. The minima identified for the trimer and tetramer possess cyclic structures that differ in the position of the methyl groups with respect to the plane containing the hydrogen bonds. The contribution of nonadditivity to the interaction in these structures is quite significant (12%–18% of the overall interaction energy). N–H distances in the donor molecule are longer than in the dimer and increase with increasing cluster size. Likewise, the hydrogen bondingenergy increases with cluster size. These results expose the significance of cooperative phenomena in the interactions between methylamine molecules. The computations predict slight shifts in the C–N stretching frequencies, which are consistent with their experimental counterparts. The N–H stretching and the wagging modes undergo large shifts, with a magnitude that increases as the size of the cluster grows.

Control of photofragment velocity anisotropy by optical alignment of
View Description Hide DescriptionA molecule placed in an electric field can be aligned because of the anisotropy of its polarizabilitytensor. As an alternative to high static electric fields, the electric field of a pulsed laser can be used for alignment. The alignment can be demonstrated by measurement of the control of the anisotropy of photodissociated fragments generated by polarized light. Sakai et al. have recently reported such an experiment on aligned iodine molecules [J. Chem. Phys. 110, 10235 (1999)]. This paper reports an independent finding of the same phenomenon with General equations are presented for the degree of alignment and its control of the photodissociation amplitude. The control parameter for a symmetric top or linear molecule is where α is the mean polarizability, is the element of the polarizabilitytensor along the symmetry axis, and χ is the angle between the E vectors of the aligning and dissociating lasers. The fragment velocity anisotropy can be increased or decreased by varying this control parameter.

The 1silaketenyl radical (HSiCO): Ground and first excited electronic states
View Description Hide DescriptionThe two lowestlying and electronic states and lowest linear stationary point of the 1silaketenyl radical (HSiCO) have been investigated systematically using ab initioelectronic structure theory. The lowest linear stationary point possesses two distinct imaginary vibrational frequencies along the HSiC bending coordinates, indicating a strong Renner–Teller interaction. The ground and first excited states of HSiCO are found to have transplanar bent structures and they are more distorted from linearity but less polar than the corresponding states of HCCO. Specifically, the structure features a small HSiC bond angle of 84°. With our most reliable method, ccpVQZ CCSD(T), the classical splitting has been predicted to be 35.7 kcal/mol The barriers to linearity were determined to be 53.5 kcal/mol (2.32 eV, 18 700 cm^{−1}) for the state and 17.8 kcal/mol (0.77 eV, 6240 cm^{−1}) for the state. The ground state of HSiCO was found to be relatively stable thermodynamically against the two dissociationreactions and Due to the large infrared (IR) intensities of some of the vibrational modes, IR spectroscopic investigation of the HSiCO radical may be feasible. HSiCO is the global minimum for these four atoms, lying energetically below SiCOH (38.5 kcal/mol), HCSiO (40.7 kcal/mol), and CSiOH (76.3 kcal/mol) at the configuration interaction with single and double excitations (CISD) level of theory.

Microsolvation of the methyl cation in neon: Infrared spectra and ab initio calculations of and
View Description Hide DescriptionRotationally resolved infrared photodissociationspectra of the degenerate asymmetric C–H stretch vibration of the and ionic complexes have been recorded. The rotational structure and vibrational frequencies are consistent with πbound cluster geometries, where the Ne ligands are attached to either side of the orbital of the central C atom of the methyl cation, leading to and symmetric structures for the dimer and trimer. The intermolecular bonds in the ground vibrational state are characterized by averaged separations of in the dimer and 2.34 Å in the trimer. The origins of the band are blueshifted by 11.5 and 21.5 compared to the monomer frequency, indicating that vibrational excitation is accompanied by a small and additive destabilization of the intermolecular bond.Ab initio calculations at the level confirm that the πbound configurations correspond to the global minimum structures for both the dimer and the trimer The calculated intermolecular potential energy surface of the dimer is characteristic for a diskandball complex and reveals significant angularradial coupling, which accounts for the large discrepancy between the vibrationally averaged and calculated equilibrium intermolecular separations, The comparison of the ionic dimers (Rg=He, Ne, Ar, Kr, Xe) with the isoelectronic molecules (X=H, F, Cl, Br, I) reveals that chemical bonding onsets with and increases with the size of the Rg atom.

Photochemistry of hydrogen peroxide in Kr and Xe matrixes
View Description Hide DescriptionUVphotolysis of hydrogen peroxide in various raregas matrixes is comparatively studied. The photorecovery of from the tight complex is observed in Kr and Xe matrixes, in addition to this reaction in an Ar matrix found previously. The similarity of spectral position and efficiency of the photorecovery reaction in various raregas solids indicates its fundamental character, supports chargetransfer excitation of as its origin, and preserves promises to find this photoreaction in media of environmental importance. In UVphotolysis of the relatively small concentration of isolated OH radicals is found in a Kr matrix, and no OH radicals appear in a Xe matrix, and this trend is discussed in terms of delayed cage exit. Moreover, additional species photogenerated from in a Xe matrix as well as the absence of OH radicals might be connected with participation of some hidden intermediates (HOXeOH, HXeOOH, etc.) in the dynamics, thus, catalyzing new photodissociation channels. Among the photolysis products, the loose complex is suggested to be stabilized in Kr and Xe matrixes. This loosely bound complex is quasistable and decomposes at relatively low temperatures (below 20 K) quantitatively forming the known tight structure. This lowtemperature process offers one additional example of shortrange atomic mobility introduced recently in the literature.

Ab initio potentialenergy surface for the interaction and bound rovibrational states
View Description Hide DescriptionAdiabatic potentialenergysurfaces for the and states of the complex have been calculated at the restricted coupled cluster level of theory including single, double, and noniterated triple excitations [RCCSD(T)]. The potentialenergysurface (PES) of the state has three minima: for the Tshaped geometry, barely skewed toward oxygen and and for two collinear forms. In contrast, PES of the state has two minima, both related to Tshaped forms: (i) A global minimum, with He shifted toward nitrogen and and a shoulderlike minimum, with He shifted toward oxygen and The collinear forms of the state correspond to saddle points. The surface lies above the surface, except in the vicinity of perpendicular arrangement. The interaction energies have been analyzed via perturbation theory of intermolecular forces. The shape and location of the minima is determined primarily by the anisotropy of the exchange component, and stronger repulsion of is due to repulsion between the singly occupied antibonding orbital and He. A variational calculation of the bound rovibrational states supported by the potential suggests that the HeNO complex is bound by about 7 cm^{−1}. The bound levels correlate with NO rotational levels with and and are well described as nearly freerotor states in which j (NO rotation) and L (endoverend rotation of R) are nearly good quantum numbers. Excited intermolecular stretching vibrations are not predicted to be bound.

Rotational spectra and dynamics of the van der Waals adducts of neon and argon with 1,1difluoroethylene
View Description Hide DescriptionThe free jet millimeter wave spectra of the complexes of Ne and Ar with 1,1difluoroethylene are reported. Information on the equilibrium configurations, dynamics, and dissociation energies of these weakly bonded complexes are obtained from their nonrigidity. The vibrational doubling (0.78 MHz) of the rotational transitions of 1,1difluoroethylene–Ne has been useful in scaling the ab initio potential energy function calculated for the tunneling of Ne above and below the plane of 1,1difluoroethylene. This motion is hindered by a barrier of about 36 cm^{−1} at the planar configuration.

Observation of the singlet–triplet pair of the Rydberg state and assignment of the Rydberg series of
View Description Hide DescriptionThe triplet Rydberg state of with an origin at 78 245.8 cm^{−1}, 526 cm^{−1} below the previously detected singlet state origin, has been observed in a twocolor resonantly enhanced multiphoton ionization experiment. The bending vibrational levels with intervals of has been identified and reported. Based on the symmetry, bending frequencies, quantum defects, and term values, this state singlet–triplet pair was assigned to the sulfur state belonging to the series converging to the state. The same procedure also allows the reassignment of the Rydberg state previously observed in the 111.5–115 nm region to the sulfur orbital (converging to the state) and the one in the 106.3–110.4 nm region to the orbital (converging to the state). The spectral patterns also indicate that the states have a detectable increase in the dissociation rate above the level. The triplet state near the 193 nm region is dissociative with a homogeneous width of ∼13 cm^{−1}.

Tunneling splittings in water trimer by projector Monte Carlo
View Description Hide DescriptionWe demonstrate the Monte Carlo calculation of multidimensional tunnel splittings in the water trimer without the use of explicit nodal approximations. The calculations are made for the threedimensional torsional model of the nonbonded hydrogen atom motions. We employ the projection operator imaginary time spectral evolution (POITSE) method, which requires specification of a set of projector operators. The permutation symmetry leading to exchange tunneling is incorporated into these projectors directly. We analyze the optimal choice of the real space vibrational coordinates with a comparative study of local vs nonlocal symmetryadapted nuclear coordinates. Extraction of the lowest four sets of tunneling states yields good agreement with available discrete variable representation calculations.
