bluuebunny/crossref_metadata_2025 huggingface dataset:
Photodissociation dynamics of the S1(nπ*) state of formic acid
Authors: Ebata Takayuki , Amano Taro , Ito Mitsuo
Published: July 2002

Formic acid monomer was excited to the S1(nπ*) region from 220 to 250 nm. The generated OH fragment was studied by the fluorescence excitation spectrum of the A 2∑+–X 2∏ transition under a high resolution and with different laser polarization geometries. The internal state distribution, translational energy and angular distribution of the OH fragment were determined by the observed results. No vibrational excitation was observed ( fv<0.009) for the OH fragment. Though the rotational excitation is also low ( fr<0.06), the fraction increases with the dissociation energy. The spin–orbit components are roughly in equilibrium. The degree of alignment of the OH fragment is very small and its π-lobe lies preferentially in the plane of rotation. The translational energy of the OH fragment is relatively high ( ft∼0.34) but the Doppler line shape does not show any clear sin2 θ or cos2 θ dependence. These results indicate that the S1 state dissociates directly through a pyramidal structure or that the dissociation is not direct dissociation but is predissociation. Compared to the OH fragment, the HCO fragment is thought to be internally very excited ( fint∼0.3) as a result of the character of S1(nπ*) state.
Infrared chemiluminescence studies of H atom reactions with Cl2O, ClNO, F2O, CF3OF, ClO2, NO2, and ClO
Authors: Wategaonkar S. J. , Setser D. W.
Published: July 2002

Infrared chemiluminescence from the HF, HCl, and OH products has been used to measure the nascent vibrational distributions and the rate constants at 300 K for the title reactions in a fast flow reactor. The ClO reaction was observed as a secondary step in the ClO2 and Cl2O systems. The reactions proceeding by a direct mechanism, Cl2O, ClNO, OF2, and CF3OF release relatively small fractions, ∼0.37, of the available energy as HCl or HF vibrational energy with narrow distributions, in accord with the dynamics associated with the H atoms on a repulsive potential surface. The ClO2 and NO2 reactions, which proceed by short lived intermediates, release a larger fraction of vibrational energy to OH and with broad distributions. Although the data for the ClO reaction are not definitive, the OH formation channel is the more important by a factor of 4–5. The NO2 reaction was studied in direct comparison with Cl2 to choose the best Einstein coefficients of OH by comparing the OH and HCl formation rate constants.
Influence of an external uniform electric field on harmonic vibrational frequencies. Analytic energy second derivatives for closed-shell restricted Hartree–Fock wave functions with an applied uniform electric field
Authors: Duran Miquel , Andrés José Luis , Lledós Agustí , Bertrán Juan
Published: July 2002

Expressions are given for analytic determination of energy second derivatives of closed-shell restricted Hartree–Fock (RHF) wave functions under the effect of an external uniform electric field. In this way, harmonic vibrational frequencies can be computed. Application of the theoretical method to the methane molecule is presented, and comparison with the induced changes in vibrational bands of its experimental infrared spectrum is carried out.
Connectivity and percolation of randomly centered spheres: Correction to the Percus–Yevick approximation
Authors: Chiew Yee C. , Stell G.
Published: July 2002

The connectivity and percolation behavior of a system of one-component randomly centered spheres is studied based on a new approximation scheme which goes beyond the Percus–Yevick closure. This new closure corrects the Percus–Yevick direct-connectedness function. The percolation density determined from this approximation agrees with the simulation result to within 4% (compared with a 43% deviation in the Percus–Yevick approximation). Good agreement for the pair-connectedness function is also obtained.
Stimulated emission spectroscopy of the ground state of Na3
Authors: Broyer M. , Delacrétaz G. , Ni G.-Q. , Whetten R. L. , Wolf J.-P. , Wöste L.
Published: July 2002

The vibronic level spectrum of the ground state of the alkali trimer Na3 has been observed via stimulated emission spectroscopy of a supercooled metal cluster beam. Results reported herein use the C state as an intermediate, and reveal strong transitions to combinations of the symmetric stretch and bending modes, along with highly detailed fine structure.
Photodissociation dynamics of water in the second absorption band. II. A
b i
n
i
t
i
o calculation of the absorption spectra for H2O and D2O and dynamical interpretation of ‘‘diffuse vibrational’’ structures
Authors: Weide Klaus , Schinke Reinhard
Published: July 2002

We calculated the absorption spectra of H2O and D2O in the second absorption band around 128 nm using a two-dimensional ab initio potential energy surface for the B̃(1A1) electronic state. Nonadiabatic coupling to the lower states Ã and X̃ and the vibrational degree of freedom of the OH fragment are completely neglected. Despite these limitations the agreement with the measured spectra is very satisfactory. The overall shape, the width, and the energetical position of the maximum are well described. Most important, however, is the reproduction of the diffuse vibrational structures superimposed on the broad background. It is demonstrated that this structure is not caused by pure bending-excitation in the B̃ state with associated bending quantum numbers ν′2=1,2,3,... as originally assumed. Because the equilibrium HOH bending angle and the equilibrium H–OH distance are very different in the ground and in the excited state, the main part of the spectrum and especially the diffuse structures occur at high energies within the continuum of the B̃ state potential energy surface. Within the time-dependent approach, based on the autocorrelation function and simple classical trajectories, it is shown that the diffuse structures originate from the temporary excitation of a large amplitude bending and stretching oscillation embedded in the continuum (short lived quasiperiodic orbits). The vibrational period of this mode is approximately 40 fs and the lifetime of the trapped trajectories is on the average one vibrational period.
Electron-stimulated desorption and surface species conversion: The observation of a desorption resonance for atomic oxygen on the Pd(111) surface
Authors: Hoffman Alon , Guo Xingcai , Yates John T. , Gadzuk J. W. , Clark C. W.
Published: July 2002

The influence of electron bombardment on an oxygen-covered Pd(111) surface at 87 and at 300 K has been studied by temperature programmed desorption. It is found that electron bombardment of an O2(a) adlayer at 87 K leads to the depletion of O2(a) and the conversion to O(a) with about the same cross sections, ∼1×10−17cm2 (Ve =200 eV). The O2(a)-depletion process is preferred at higher O2(a) coverage as the availability of open sites limits the O2(a) conversion. The new channel for conversion to O(a) due to electron bombardment results in a maximum increase of O(a) coverage by 88% and an additional desorption feature at 704 K compared to the usual temperature programmed desorption behavior due to O(a) recombination. The cross sections for desorption and conversion of O2(a) exhibit different electron energy dependence, suggesting that different primary electron excitations are involved in the two processes. The cross section for the electron-stimulated desorption of atomic oxygen is observed to be strongly dependent on electron energy, with its maximum value being ∼1.2×10−17 cm2 at an electron energy of 10±2 eV. The behavior of the cross section as a function of electron energy suggests the involvement of a surface resonance near 10 eV. A possible mechanism for resonance enhanced desorption is suggested, based on semiclassical wave packet propagation over potential energy curves which are consistent with the excitation spectrum of oxygen negative ions.
Rotational relaxation of a molecule trapped in a three-dimensional crystal. III. Environmental effects and relaxation channels
Authors: Delgado V. , Breton J. , Alvira E. , Plata J. , Girardet C.
Published: July 2002

The stochastic classical trajectory method is used to calculate the energy relaxation of a highly excited diatomic rotor trapped in rare gas crystal at T=20 K. The friction kernels, which appear in the generalized Langevin equations characterizing the motions of the molecule and of nearest neighbor crystal atoms, are expressed in terms of the interaction potentials. The influence of the surrounding crystal on the relaxation mechanism and the efficiency of the various dissipation channels are analyzed by changing the rare gas species and by artificially switching off some channels. Within the limits of the model (classical two-dimensional rotation of the diatomic molecule, coupled on the one hand to a restricted number of first shell atoms themselves coupled to the bulk crystal and on the other hand to the other first shell atoms considered as pertaining to the bath), the results of the calculations show that, in the present case, rotational relaxation is a rapid process, over the picosecond scale, and that the local mode connected to the motions of the molecular center of mass plays a major role in the mechanism. This local mode is responsible, at short times t≤0.5 ps, for the relaxation of 95%, 75%, and 60% of the rotational energy excess in Ar, Kr, and Xe crystals, respectively.The remaining energy is then dissipated over longer times via the local mode or directly towards the crystal modes. A striking energy saturation phenomenon of the local mode is exhibited in xenon crystal.
Quantum dynamics for vibrational and rotational degrees of freedom using Gaussian wave packets: Application to the three-dimensional photodissociation dynamics of ICN
Authors: Henriksen Niels E. , Heller Eric J.
Published: July 2002

We present an approach to quantum dynamics, based entirely on Cartesian coordinates, which covers vibrational as well as rotational motion. The initial state is represented in terms of multidimensional Gaussian wave packets. Rotational adaptation to angular momentum eigenstates is done by using angular momentum projection operators. This gives an initial state represented as a weighted superposition of Gaussians with different average orientation in space. It is shown that the subsequent dynamics can be determined from the dynamics of Gaussians corresponding to just one of these orientations. An application to the 3D photodissociation dynamics of ICN is presented. All six degrees of freedom which describe the internal motion of the triatomic are included, the only approximation introduced in the present calculation being the thawed Gaussian wave packet approximation for the dynamics. The total absorption spectrum out of vibrational–rotational eigenstates of ICN as well as fully resolved final product distributions are calculated.
Bath-induced vibronic coherence transfer effects on femtosecond time-resolved resonant light scattering spectra from molecules
Authors: Ohtsuki Y. , Fujimura Y.
Published: July 2002

Effects of vibronic coherence transfer induced by the heat bath on ultrafast time-resolved resonant light scattering (RLS) spectra are theoretically investigated within the master equation approach. The vibronic coherence initially created by a coherent optical excitation transfers to other vibronic coherent states due to inelastic interactions between the vibronic system concerned (the relevant system) and the heat bath. The vibronic coherence transfer results in the quantum beats in the time-resolved RLS spectra. The bath-induced vibronic transition operator is derived in the double space representation of the density matrix theory. Model calculations of the femtosecond (fs) time-resolved RLS spectra are performed to demonstrate the effects of the bath-induced vibronic coherence transfer.
Triplet dipoles in the absorption spectra of dense rare gas fluids. II. Long range interactions
Authors: Guillot Bertrand
Published: July 2002

A model calculation is presented to evaluate the long range dispersion dipole occurring when three nonoverlapping dissimilar atoms interact. Deviation from pairwise additivity is taken into account by a simplified theory based on electrostatic arguments. By comparing the long range triplet dispersion dipole with the one resulting from exchange overlap effects previously investigated [J. Chem. Phys. 90, 650 (1989)], it is shown that a cancellation between both contributions occurs in the vicinity of the van der Waals separation. The far infrared spectra of dense rare gas fluids are next generated by molecular dynamics simulation including long range dispersion and short range overlap dipoles. An estimate of the expected absorption in liquid krypton is given, αmax =3.6×10−9 cm−1 am−2, which might give some hope to experimentalists. As far as rare gas mixtures are concerned, the model calculation reproduces surprisingly well the experimental absolute spectral density of a Kr–Ar liquid mixture (15% Kr, 135 K) over a large domain of frequency but fails to reproduce the low frequency dip. Nevertheless, irreducible three-body dipoles contribute significantly to the spectrum. The difficulty of obtaining accurate results from an approximate model is emphasized and a guideline for further improvements is sketched.
Treatment of rotational isomers in free energy calculations. II. Molecular dynamics simulation study of 18-crown-6 in aqueous solution as an example of systems with large numbers of rotational isomeric states
Authors: Straatsma T. P. , McCammon J. A.
Published: July 2002

The evaluation of free energy differences using the perturbation method or thermodynamic integration method requires special caution if multiple rotational isomeric states may exist in the system under investigation. In this article a recently suggested procedure to properly treat rotational isomeric states is illustrated with a molecular dynamics simulation of an aqueous solution of uncomplexed 18-crown-6 crown ether, as an example of a system in which large numbers of isomeric states may exist. By using very long molecular dynamics simulations, thermodynamic perturbation methods and symmetry arguments, the free energy of host organization into the conformation required to form the complex with K+ is estimated to be 2.6 kJ mol−1. It has also been found that the lowest energy conformations of 18-crown-6 in vacuo do not necessarily correspond to the most populated conformations in aqueous solution.
Exotic dielectric behavior of polar liquids
Authors: Chandra Amalendu , Bagchi Biman
Published: July 2002

We present a report of our analysis of wave vector dependence of the static dielectric function of a dipolar liquid obtained by a microscopic calculation. At low values of the wave vector (k), the longitudinal dielectric function ε(k) increases with k, in contradiction to some assumptions reported in the literature. As the value of k is increased, ε(k) diverges at a critical value kc which depends on the value of the long wavelength static dielectric constant (ε0) of the liquid. The dielectric function is negative for values of k greater than kc. At large values of k, the calculated ε(k) fails to attain the limiting value of unity. We attribute this result to the failure of the point dipole assumption made in the evaluation of the polarization correlation function required by the theory. The behavior of ε(k) for the dipolar liquid is compared with that of one component plasma for which reliable results can be obtained over the full range of wave vectors. For both systems, the stability conditions are fulfilled at all values of k. A plausible explanation of this rather exotic behavior of ε(k) is given.
Viscometric titration of a linear polyelectrolyte
Authors: Hodgson Donald F. , Amis Eric J.
Published: July 2002

Viscosities of dilute solutions of linear poly(2-vinyl pyridine) with molecular weights ranging from 3.0×104 to 1.0×106 have been measured under conditions where the polymer can be charged by protonation. By employing weakly acidic solvents, large reduced viscosities typical of polyelectrolytes in low-salt solutions are observed. They exhibit a pronounced maximum occurring at a concentration which is independent of molecular weight. Separate pH titrations are used to calculate polyion charge density and electrostatic screening lengths. The origin of the viscosity maximum is explained by consideration of changing electrostatic persistence lengths on dilution, resulting in polyion coil expansion and contraction. Investigations of limiting conditions of low charge density and full electrostatic screening with basic and high acid or salt solutions reveal expected normal polymer behavior in concentration extrapolations to the intrinsic viscosity.
Thermo- and photo-induced reactions of NO on Si(111)7×7. I. Adsorption and chemical reactions
Authors: Ying Z. C. , Ho W.
Published: July 2002

The adsorption of NO on Si(111)7×7 at 90 K and the subsequent reactions induced by thermal heating and photon irradiation have been investigated in detail, using high-resolution electron energy loss spectroscopy and mass spectrometry. It is found that, in addition to molecular and dissociative adsorption of NO, N2O is produced from NO on Si(111)7×7 at 90 K. The product N2O physisorbs on the surface and, at 110 K, partially desorbs and partially dissociates into N2 and O. Molecular adsorption of NO exists in two independent, presumable bridge and atop configurations. There is no observable conversion between the two forms of molecularly adsorbed NO. At 147 K, the bridge NO is thermally activated, which leads to simultaneous NO desorption and, to a much larger extent, N2O synthesis. Dissociation of bridge NO also occurs at about 147 K, at least at low NO exposures. The atop NO is not involved in the thermal reactions at 147 K, but dissociates between 200 and 300 K. Photon irradiation in the UV, visible, and IR induces desorption and dissociation of NO on Si(111)7×7 at 90 K via nonthermal mechanisms. Furthermore, evidence for photosynthesis of N2O on the surface is observed. It is suggested that the N2O synthesis during thermal heating and photon irradiation proceeds via reaction between an adsorbed NO molecule and a hot N atom produced from NO dissociation.
Theoretical study of low-lying states of H3O
Authors: Talbi Dahbia , Saxon Roberta P.
Published: July 2002

The first two doublet and quartet states of H3 O have been surveyed by multiconfiguration self-consistent field/first-order configuration interaction (MCSCF/FOCI) calculations in C3v symmetry. Geometries of the minima on the doublet surfaces have been optimized by MCSCF gradient techniques and energies obtained by large-scale multireference single and double excitation CI calculations. The correlation diagram linking the minima to different dissociation limits has been established. A local minimum in C3v symmetry of ion-pair character is shown to be unstable with respect to dissociation to H2 +H+O. The lowest state, 1 2 A1 in C3v , (1 2 A′ in Cs ) may be characterized as an H3 O+ core surrounded by an oxygen 3s Rydberg electron. Transition states for dissociation of the 1 2 A′ state to H2 O+H and to OH+H2 have been investigated. An extremely low barrier height, 3.58 kcal/mol without vibration, 0.4 kcal/mol for H3 O, an estimated 1.3 kcal/mol for D3 O, with zero-point energy, is found for dissociation of the 1 2 A′ state to H2 O+H. Within the uncertainty of the calculation it is not possible to predict whether the lowest state of H3 O should be observable experimentally. The relationship of these results to experimental observations for H3O is discussed.
Electron transmission through layers of H2O and Xe in the ultrahigh vacuum photoreduction of CH3Cl on Ni(111)
Authors: Gilton Terry L. , Dehnbostel Claus P. , Cowin James P.
Published: July 2002

The photoreduction of CH3Cl was used to detect the transmission of electrons through layers of H2O and Xe on Ni(111) under ultrahigh vacuum (UHV) conditions. At a laser wavelength of 248 nm with H2O spacers, the electron intensity exhibited a nearly exponential decay to zero signal with the 1/e point at about 2 monolayers. At 193 nm with H2O spacers, the signal decayed with the 1/e point at about 4 monolayers to a constant value due to the direct photofragmentation of the CH3 Cl possible at this wavelength. At 248 nm with Xe spacers, the signal decayed by a factor of 4 in the first 2 monolayers, after which the signal was nearly constant and nonzero for at least 64 monolayers, indicating a long electron mean free path. CH3Br is shown to behave similarly with H2 O spacers at 248 nm.
Experimental and theoretical study of the relaxation of vibrationally excited HF by NO and CO
Authors: Rensberger K. J. , Blair J. T. , Weinhold F. , Crim F. F.
Published: July 2002

We employ laser double resonance techniques to measure the rates of gas-phase collisional deactivation of vibrationally excited HF by NO molecules, in order to compare the relaxation efficiency of this free radical species with CO and other closed-shell molecules whose vibrational dynamics are well known. Although the near-resonant energy gaps for HF collisional energy transfer are less favorable for NO than for CO, we find that NO is as much as an order of magnitude more efficient than CO in relaxation HF vibrations. Since the NO and CO collision partners have rather similar dipole moments (0.153 vs 0.112 D), rotational constants (1.17 vs 1.93 cm−1), and molecular weights, the disparity in vibrational relaxation efficiency may come from chemical factors, particularly the open- vs closed-shell electronic character, associated with long-range interactions. Ab initio calculations and natural bond orbital (NBO) analysis of the structure and energetics of NO:HF and CO:HF complexes indicate that the NO monomer is better able to form effective n→σ* donor–acceptor H bonds to HF over a wide range of nonlinear ‘‘acceptance angles.’’ Compared to CO, NO presents a significantly attractive potential to HF over an appreciably wider range of collision orientations, leading to structures in which HF stretching couples to other internal modes of the transient complex and vibrational excitation is efficiently quenched. Our results strongly suggest the important role that chemical factor can play in the dynamics of fast vibrational relaxation processes.
Molecular dynamics simulations of reactions in solids: Photodissociation of Cl2 in crystalline Xe
Authors: Alimi R. , Brokman A. , Gerber R. B.
Published: July 2002

The photodissociation of isolated Cl2 impurities in a Xe crystal was investigated by molecular dynamics simulations. The calculations were carried out for a photodissociation energy of 3.775 eV, and for several temperatures in the range from 10 to 150 K (the melting point is 162 K). The focus was on the physical mechanisms whereby the product atoms exit from the cage, on the properties of the final sites occupied by the Cl atom, and on the temperature dependence of the processs. The main findings were: (1) exit of a Cl atom from the original reagent cage, when it occurs, is always direct upon photodissociation, and does not involve multiple collisions with the surrounding cage walls. This is in qualitative contrast with the dynamics of cage exit in the case of HI photodissociation in Xe at very low temperatures, found in a previous study. (2) The occurrence of product exit from the cage depends entirely on whether the reagent molecule has been oriented at the direction of a transition state for the exit at the instant of photodissociation. (3) The temperature threshold of Cl exit from the cage is 95 K, and essentially coincides with onset of free rotation for the reagent molecules in the host crystal. (4) The temperature dependence of the probability for cage exit is strongly nonmonotonic: The probability increases as T increases from 95 to 110 K, falls off to 0 around 125 K, then increases again as T approaches melting. (5) At the photodissociation energy used, the only site that the Cl atoms occupy in the new cage is the octahedral interstitial site. Various aspects of reaction dynamics in crystalline solids are discussed in the light of the above results and by their comparison with findings of a previous study on photodissociation of HI in Xe.
Infrared multiphoton excitation of CF3I. I. Transient ultraviolet absorption study of after-pulse dissociation and excited state populations
Authors: Abel B. , Herzog B. , Hippler H. , Troe J.
Published: July 2002

Transient UV absorption spectra of CF3I were recorded during the IR multiphoton excitation of this molecule. Short laser pulses (20–50 ns) without longer tails were employed. Pressure effects were analyzed in the 10–100 mTorr range. Unimolecular dissociation rates (in the 106–107 s−1 range) after the pulse were identified unambiguously by time-resolved low-pressure observations and collisional quenching experiments. Marked laser intensity effects on the rates are attributed to intensity-dependent changes of rotational state distributions of the reacting molecules. Excited state distributions have been resolved in direct observations. Bimodal distributions were identified.
Electronic spectroscopy and energy transfer pathways of matrix isolated iodine
Authors: Macler Michel , Nicolai Jean-Philippe , Heaven Michael C.
Published: July 2002

Visible laser excitation (460–725 nm) of dilute rare gas/I2 (2000:1) matrices resulted in emission from the I2A 3Π(1u) state. Reanalysis of the A→X spectra provided revised molecular constants for matrix isolated I2. A state lifetimes of 70±20, 80±20, and 110±30 μs were observed in Ar, Kr, and Xe hosts, respectively. Excitation spectra for the A state closely followed the I2 continuum absorption spectrum, indicating that transfer from the B 3Π(0+u) and 1Π(1u) states was effective in populating I2(A). At dilution ratios of 600:1 or lower the I 2P1/2–2P3/2 transition was observed in conjunction with the A–X bands. Excitation studies showed that isolated I atoms, trapped during the deposition process, were excited by energy transfer from nearby I*2 molecules. A vibronic progression, similar to the A–X bands, but shifted to longer wavelengths, was noted in concentrated Rg/I2(300:1) matrices. This system, which was emitted with a lifetime of about 10 ms, most probably originated from perturbed I2 A′ 3Π(2u). Intermolecular energy transfer was observed in matrices that contained I2 codeposited with O2. Electronic excitation of I2 resulted in a long-lived emission from O2 a 1Δg. Matrices containing high concentrations of iodine also exhibited O2(a)→I(2P1/2) transfer.
The adsorption of hydrogen on Si(111)-7×7 as studied by multiple internal reflection spectroscopy
Authors: Jansson U. , Uram K. J.
Published: July 2002

Multiple internal reflection (MIR) infrared spectroscopy has been used to study the adsorption of hydrogen on the Si(111)-7×7 surface over the temperature range of 100–400 K. The high resolution of this technique has enabled us to observe three different silicon monohydride stretch modes at 2076, 2086, and 2095 cm−1. Using the polarization selection rules for infrared spectroscopy we have assigned these modes to a silicon monohydride whose bond axis is parallel to the surface normal and two monohydrides whose bond axes are tilted with respect to the surface normal. We have also observed the temperature dependent formation of surface dihydride and trihydride species. The formation of both the dihydride, observed at 2119 cm−1, and the trihydride, seen at 2143 cm−1, is favored by low adsorption temperatures. Finally, we also report a temperature dependent competitive formation between the two higher hydrides with the formation of the dihydride species maximizing near 200 K and lower temperatures favoring the surface trihydride.
Carbon-13 nuclear magnetic resonance of ferroelectric liquid crystals with off-magic-angle spinning
Authors: Poon Chi-Duen , Fung B. M.
Published: July 2002

The orientational ordering of three ferroelectric liquid crystals 4′-(3-methyl-2-chloropentanoyloxy)-4-alkyloxybiphenyls has been studied by 2D carbon-13 nuclear magnetic resonance (NMR) with variable angle sample spinning. The three homologs have hexyloxy (C6), heptyloxy (C7), and octyloxy (C8) chains, respectively, at the 4 position. They exhibit smectic A (SA) and chiral smectic C (S*C; ferroelectric) phases and have unusually large spontaneous polarization of the electric dipoles. In the SA phase, macroscopic alignment of the molecular director along the spinning axis can be achieved by cooling the sample rapidly from the isotropic liquid. For the S*C phase, the molecular alignment is different for the three compounds. At a magnetic field of 7.05 T, the helical structures of C6 and C8 unwind and the molecular directors align along the spinning axis at a spinning rate of 1 kHz. However, under the same conditions, the helical structure of C7 is retained and its carbon-13 NMR spectrum shows a partial powder pattern. On the other hand, the racemate of C7 has a normal smectic C (SC) phase without a helical structure, and the molecular director can be easily aligned for the NMR study. The racemate of C8 was also synthesized and studied for comparison. The 2D NMR technique used was separated local field (SLF) spectroscopy with an efficient proton–proton dipolar decoupling sequence BLEW-48 in the evolution period. The carbon–proton dipolar coupling constants were determined and the order parameters for different molecular segments of the liquid crystal were calculated. The carbon-13 chemical shifts were also measured as functions of temperature and the data were used to probe the temperature dependence of order parameters. The results show that the C–H bond at the first chiral center in these compounds has an unusually large negative order parameter. This could be due to restricted rotation of the molecular segment at the chiral center, which would be related to the high spontaneous polarization of these compounds.
One- and two-photon-resonant multiphoton ionization spectra of CS2 from 45 500 to 48 100 cm−1
Authors: Donaldson D. J.
Published: July 2002

One- and two-photon-resonant multiphoton ionization spectra of the low energy region of the intense 1 B2 state of room temperature CS2 are reported. A comparison of these with one another, as well as with absorption spectra, yields a frequency of 46 247 cm−1 for the electronic origin of the transition. Most of the intense features in this region can be assigned as transitions to excited state bending levels. The frequency of this mode in the excited state is found to be 429 cm−1, in excellent agreement with results from the higher energy region of the spectrum. Two-photon excitation is used to observe, for the first time, the excited state antisymmetric stretch mode. Its frequency is 1567 cm−1, close to the corresponding ground state value.
Dissociative decay of NeH and NeD Rydberg states: A strong isotope effect
Authors: Devynck P. , Graham W. G. , Peterson J. R.
Published: July 2002

Radiative and predissociative decay of the n=2 Rydberg states of NeH and NeD is examined using translational spectroscopy to determine the c.m. kinetic energies of the dissociation fragments. The n=2 states are produced from 5 keV NeH+ and NeD+ by electron capture in Cs vapor. Expected radiative dissociation of the B 2Π state is observed in NeH, as is predissociation of the A 2Σ+. However, a surprisingly strong suppression of predissociation occurs in NeD. It is attributed primarily to a reduced overlap of the bound and continuum nuclear wave functions, and results in radiative decay. The results for both species support calculations of Theodorakopoulos et al. [J. Phys. B 20, 5335 (1987)].
Variational phase-space theory studies of silicon-atom diffusion on reconstructed Si(111)-(7×7) surfaces
Authors: Agrawal Paras M. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

The dynamics of silicon-atom diffusion on a reconstructed Si(111)-(7×7) surface have been investigated using variational phase-space theory methods with a previously described [J. Chem. Phys. (to be published)] potential-energy surface. A four-layer lattice model of the Binnig et al. (7×7) reconstruction containing 291 atoms is employed for the surface. Canonical Markov walks with importance sampling incorporated are used to evaluate the flux across both right-circular and right-elliptical cylindrical dividing surfaces separating adsorption sites. This flux is minimized with respect to the parameters of the dividing surface to obtain the best estimate of the classical jump frequencies. The minimum jump frequencies so obtained are corrected for recrossings of the dividing surface by the calculation of trajectories that start from phase-space points obtained in the random walk that lie within a specified distance w of the dividing surface. The corrected jump frequencies are then used as input to a set of 225 diffential equations that describe the diffusion rates across the (7×7) surface. Diffusion coefficients D are computed from the slope of plots of the time variation of the root-mean-square displacements obtained from the solution of the rate equations. Arrhenius plots of results obtained at 300, 600, and 1000 K yield D=2.15×10−3 exp (−1.51 eV/kBT) cm2/s. The calculated activation energy of 1.51 eV is in excellent accord with the result obtained by Farrow from molecular-beam pyrolysis data on SiH4 deposition. An examination of the details of the diffusion shows that it is not isotropic on the (7×7) surface. We find that preferential directions of flow exist. These directions correspond to ‘‘gateways’’ at three of the four corners of the (7×7) unit cell. The results suggest that diffusion rates are a sensitive function of the geometry of the (7×7) reconstruction so that careful measurements of diffusion rates and associated activation energies may be able to serve as a means of differentiating different proposed models of the Si(111)-(7×7) reconstruction.
Structure and stability of Mg+n and Mg++n clusters (n≤7)
Authors: Durand G.
Published: July 2002

We present a nonempirical model Hamiltonian of the valence bond type for singly and doubly charged clusters of atoms which have a s2(1S) ground state configuration. This model includes delocalization, instantaneous polarization energies, interaction between induced dipoles, and Coulombic repulsion of the holes in doubly charged clusters; up to four-body terms are included. The model is applied to magnesium clusters. Comparisons with ab initio (configuration interaction) calculations show overall agreement for singly charged clusters, whereas doubly charged clusters suffer from the lack of collective p-type contributions in model calculations. For singly charged magnesium clusters, ground state geometries are found to be neutral atoms surrounding either a charged atom or a charged dimer. Considerations of shape and energy show that clusters are easily obtained by aggregation processes and analysis of vertical ionization potentials shows that they can also be obtained by direct ionization of neutral species.
Theoretical study of the dipole moment function of OH(X 2Π)
Authors: Langhoff Stephen R. , Bauschlicher Charles W. , Taylor Peter R.
Published: July 2002

We present a theoretical study of the sensitivity of the dipole moment function (DMF) of the X 2Π ground state of OH to basis set saturation and to refinements in the correlation treatment. Emphasis is placed on determining the slope of the DMF at re and the r value at which the maximum occurs. We consider the effect of oxygen polarization functions up through h type, expansion of the active orbital space to include the O 3dδ orbital, the effect of higher excitations using the averaged coupled-pair functional method, and the effect of evaluating the dipole moment as an energy derivative rather than as an expectation value. Our theoretical DMFs, which should be the most accurate to date, differ markedly from an empirical DMF of Turnbull and Lowe that is based on experimentally derived intensity ratios. The theoretical DMFs agree better with a recently published DMF of Nelson et al., but suggest that this empirical DMF is also inaccurate for r>2.3 a0.
Critical point shifts in binary fluid mixtures
Authors: Jacobs D. T.
Published: July 2002

An empirical observation is made of an apparent universal shift in the consolute point (Xc=critical composition, Tc=critical temperature) of binary fluid mixtures when the system identity is perturbed. The shift in a ‘‘pure’’ system’s critical point (Xco, Tco) when perturbed becomes (Xc,Tc) which seems to obey (Tc−Tco)/Tco= (Xc−Xco)/Xco. This relation has been observed to hold in a wide range of systems including closed-loop coexistence curves (guaiacol–glycerol–water, or tertiary butyl alcohol in secondary butyl alcohol and water), deuterated systems (methanol–cyclohexane, or isobutyric acid–water), impurities added to methanol–cyclohexane, the molecular weight dependence in polystyrene–methylcyclohexane, and the pressure dependence of methanol–cyclohexane.
Variable curvature coordinates for molecular vibrations
Authors: Colbert Daniel T. , Sibert Edwin L.
Published: July 2002

A class of curvilinear coordinates for describing molecular vibrations is presented. The coordinates are parametrized by their curvature. Eigenvalues and eigenfunctions for three model Hamiltonians have been calculated in these coordinates. We investigate how the separability of the eigenfunctions varies as the curvature of the coordinates is changed, and we introduce a quantitative measure of separability using natural modal expansions. For the C–H stretch–bend interaction in CHD3, improved separability is found when coordinates are used which have a greater curvature than bond-angle coordinates. We predict and explain the results of Sibert et al. [J. Phys. Chem. 87, 2032 (1983)], who found the Fermi coupling between the bend and symmetric stretch in CO2 to be the same in rectilinear and curvilinear coordinates.
A test particle approach to the zero separation theorems of molecular distribution functions
Authors: Lee Lloyd L. , Shing Katherine S.
Published: July 2002

It is shown that the potential distribution of a strong test particle leads to the zero separation values of the cavity distribution functions yab(0) in a mixture. This relation furnishes a direct means of computing by Monte Carlo simulations the coincidence values of the cavity function ln yab(0) and the potential distribution 〈exp[−βΨa−βΨb]〉. Test particle simulations have been carried out for mixtures of Lennard-Jones molecules differing considerably in size [(σab/σbb)3 =0.25, 0.5, 0.75, 1.00, 1.25, 1.50, 1.75, and 2.00] and in strength of interaction (εab/εbb =0.5, 1.0, 1.5, and 2.0). Alternative Monte Carlo methods are employed to check the statistics. In order to predict the behavior of the potential distribution, a distribution function theory, the reference hypernetted chain (RHNC) equation, is solved based on the universality of the bridge functions. Hard sphere mixtures are taken as reference fluids. The criteria recently proposed by Rosenfeld and Blum are used to select the equivalent hard sphere diameters. Close agreement with MC results is attained for most of the states considered. This provides further evidence that the RHNC equation is a reliable theory for mixtures of simple fluids.
Collisionally cooled spectroscopy: Pressure broadening below 5 K
Authors: Willey Daniel R. , Goyette Thomas M. , Ebenstein William L. , Bittner D. N. , De Lucia Frank C.
Published: July 2002

New results from pressure broadening measurements between 4.3 and 1.7 K for the J=2←1 and 3←2 transitions of CO broadened by helium are reported. Unlike the results of helium pressure broadening experiments at higher temperature, significant variations in cross section occur with changes in temperature. These measurements are compared with the results of close coupling calculations and show both the effects of resonances in the collision channels and energy defects at the very low collision energies. Room-temperature measurements of helium pressure broadening have also been made on CO, NO, CH3F, and H2S. These provide a normalization for our previous studies of these species at very low temperatures and a basis for the qualitative comparison of the diverse results we have obtained.
Reaction of chemisorbed CH and H on nickel
Authors: Yang Hong , Whitten Jerry L.
Published: July 2002

The reaction of chemisorbed CH and H on the [111] surface of Ni is treated using a many-electron embedding theory to describe the electronic bonding, modelling the lattice as a 28-atom, three layer cluster. Ab initio valence orbital CI calculations carried out on a local surface region permit an accurate description of bonding at the surface. Both CH and CH2 species are found to bind strongly to the Ni(111) surface at threefold and bridge sites. Adsorption energies are between 2.9 and 3.1 eV for CH adsorption, and 2.5 and 2.9 eV for CH2 adsorption. Atop Ni adsorption sites are more than 1 eV higher in energy. The Ni–C bond lengths range from 2.00 to 2.04 Å for CH and CH2 on the surface. Vibrational frequencies for HC-surface and H2C-surface vibrations are between 400 and 600 cm−1; CH stretching frequencies are around 3050 cm−1 in both CH and CH2. The reaction of CH+H→CH2 on the nickel surface is 1.45 eV exothermic, however, a barrier exists to moving H to a threefold site adjacent to one occupied by CH. The geometries of CH and CH2 and the nature of the bonding to the surface are discussed.
Experimental results on the rate of nucleation in supersaturated n-propanol, ethanol, and methanol vapors
Authors: Peters F. , Paikert B.
Published: July 2002

The homogeneous nucleation of three supersaturated alcohol vapors carried in argon is investigated experimentally. A gasdynamic shock tube method serves to subject the vapors to short supersaturation pulses. The growing nuclei are detected in number and size by Mie-light scattering. The results provide the supersaturation over a wide range of temperature for the nucleation rate of 108 cm−3 s−1. Comparison with the classical nucleation theory (CNT) exhibits increasing divergence with decreasing temperature.
An exact analysis of the optical Kerr effect for an arbitrary field strength including nonlinear effects
Authors: Lee Young Hee , Kim Do , Lee Seung Hee , Jung Won Gi
Published: July 2002

An optical Kerr effect, which is the transient electric birefringence of isotropic molecular fluids when the molecules align themselves with external field, is investigated. Calculations have been carried out on the rotational diffusion of rod-like molecules in the presence of an arbitrary field strength. We will provide an exact recurrence relation with nonlinear effects and this recurrence relation is solved numerically to see how the nonlinearity affects the answer for various field strengths.
Spectroscopic analysis of transition state energy levels: Bending–rotational spectrum and lifetime analysis of H3 quasibound states
Authors: Zhao Meishan , Mladenovic Mirjana , Truhlar Donald G. , Schwenke David W. , Sharafeddin Omar , Sun Yan , Kouri Donald J.
Published: July 2002

We report converged quantum mechanical calculations of scattering matrices and transition probabilities for the reaction of H with H2 with total angular momentum 0, 1, and 4 as functions of total energy in the range 0.85–1.15 eV on an accurate potential energy surface. These calculations show energy dependences that may be attributed to dynamical resonances with vibrational quantum numbers (100 0) and (111 0). The resonance structure is illustrated with Argand diagrams, and we present state-to-state reactive collision delay times and lifetimes. For J=0, 1, and 4, we found the lowest-energy H3 resonance at total energies of 0.983, 0.985, and 1.01 eV, respectively, with lifetimes of about 16–17 fs. For J=1 and 4 there is a higher-energy resonance at 1.10–1.11 eV; for J=1 the lifetime is about 4 fs and for J=4 it is about 1 fs.
Surface photochemistry. VIII. CH3Cl and coadsorbed CD3Br–CH3Cl
Authors: Roop B. , Lloyd K. G. , Costello S. A. , Campion A. , White J. M.
Published: July 2002

The photochemical carbon–halogen bond cleavage of CH3 Cl and CD3 Br, adsorbed on Pt(111) at 100 K, has been studied. These two adsorbates, which do not decompose thermally, are readily photodissociated, either alone or coadsorbed, when irradiated with the full spectrum of a 100 W high-pressure Hg arc. Just as in the gas phase, the dissociation products are methyl groups and halogen atoms. These species are identified using high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), and Auger electron spectroscopy (AES). With wavelengths below 300 nm removed with a cutoff filter, irradiation of coadsorbed layers breaks only C–Br bonds. Using substrate temperatures of 65 K and coadsorbed Xe, the absence of significant local heating effects is clearly demonstrated. These results show that the photodissociation process is molecule specific and that the time scale for excited state quenching is, for methyl chloride and methyl bromide, not significantly faster than the time scale for significant bond elongation.
Observation of an infrared frequency of the C4 molecule
Authors: Shen L. N. , Graham W. R. M.
Published: July 2002

A Fourier transform infrared isotopic study of the products from the vacuum ultraviolet photolysis of diacetylene and 1,3-butadiene trapped in solid argon has enabled the identification of the 1543.4 cm−1 frequency as a vibration of the C4 molecule. The assignment is confirmed by the observation of the frequencies for seven carbon-13 substituted isotopomers of the molecule.
Geometries and energy separations of 14 electronic states of Au4
Authors: Balasubramanian K. , Feng Ping Yi , Liao M. Z.
Published: July 2002

Multiconfiguration SCF (MCSCF) followed by multireference configuration interaction calculations are carried out on 14 electronic states of Au4 . The low-lying electronic states are studied further using multireference singles + doubles CI (MRSDCI) calculations including excitations from the d shells. The ground state of Au4 is found to be a closed shell 1 A1 state with an equilibrium geometry of rhombus. Two excited states of 3 B2 symmetry with rhombus and square geometries were found to lie 0.98 and 1.12 eV above the ground state. The spin–orbit effects were included using the relativistic configuration interaction method (RCI). Accurate MRSDCI calculations which included up to 1.5 million configurations on the ground states of Au2 and Au3 are also performed with the objective of comparing the binding energies of Au2 –Au4 . The gold tetramer is found to be much more stable in comparison to the trimer and dimer.
Resonance Raman spectra of t
r
a
n
s-1,3,5-hexatriene in solution: Evidence for solvent effects on excited-state torsional motion
Authors: Ci Xiaopei , Pereira Marco A. , Myers Anne B.
Published: July 2002

Resonance Raman spectra of trans-1,3,5-hexatriene in cyclohexane, hexane, methanol, and perfluorohexane are compared with the corresponding vapor phase spectra. The absolute cross sections in cyclohexane indicate that the solvation induced electronic spectral breadth is partly homogeneous (amplitude level in the Raman process) and partly inhomogeneous. Overtones and combination bands involving torsional modes, particularly the central double bond torsion, are dramatically reduced in intensity upon solvation, the reduction being greatest in solvents that generate the largest red shift of the absorption. Quantitative modeling of the cyclohexane data shows that these intensity changes can be attributed only in part to the preferential damping of low-frequency overtones induced by the increase in electronic homogeneous linewidth upon solvation. The remaining intensity reduction may arise either from a stiffer excited-state potential surface for double bond twisting in solution or from coordinate-dependent dephasing in the upper electronic state. Additionally, time-dependent wave packet propagation techniques are employed to estimate the barrier to double bond twisting in the excited state from the experimental ratio of four-quantum to two-quantum Raman overtone intensities.