bluuebunny/crossref_metadata_2025 huggingface dataset:
Reinvestigation of the HCP electronic spectrum: Experimental determination of D for the X̃ state and observation of hyperfine quantum beats in the B̃ state
Authors: Mason Martin A. , Lehmann Kevin K.
Published: July 2002

In the laser-induced fluorescence spectrum of HCP, a sharp decrease in fluorescence intensity was observed at 41 680 cm−1. This diminuation in emission intensity is due to a rotationally mediated coupling of the excited vibronic level with the dissociative continuum of the X̃ state; this has allowed us to place an upper limit on D0 for the ground electronic state. A lower limit for D0 can be placed at 41 650 cm−1 since strong emission was observed below this excitation energy. Using the assignments of Johns et al. [Can. J. Phys. 47, 893 (1969)] as a guide, fluorescence decays were recorded from the two B̃ state vibronic bands to determine lifetimes. In addition to determining an unusually long lifetime for both states, the decays were seen to be modulated at four frequencies resulting from the coherent excitation of four nuclear hyperfine levels. This increase in lifetime and magnetic hyperfine interaction arises from a mixing of three singlet and triplet states. The contributing states are tentatively identified.
Highly vibrationally excited HCN/HNC: Eigenvalues, wave functions, and stimulated emission pumping spectra
Authors: Bentley Joseph A. , Huang Chang-Ming , Wyatt Robert E.
Published: July 2002

We report the large scale calculation of accurate vibrational eigenstates of X̃ state HCN/HNC up to ∼37 000 cm−1. The discrete variable representation-distributed Gaussian basis (DVR-DGB) approach is used. In the final major computational step, large dense Hamiltonian matrices (≥11 000 rows or columns) must be diagonalized. The use of new software for out-of-core eigenproblems is described in some detail. Representative wave functions of high lying vibrational eigenstates near the isomerization barrier and at energies up to 39 313 cm−1 are presented. Theoretical stimulated emission pumping (SEP) spectra for several initial (Ã) vibrational states are presented along with calculations of the quantum survival probability for these spectra. The spectra range from being dominated by localized eigenstates to being dominated by delocalized eigenstates, depending on the geometry of each initial state. In addition, time-dependent expectation values of the coordinates of the evolving wave packet are plotted.
An investigation of three-body effects in intermolecular forces. III. Far infrared laser vibration–rotation–tunneling spectroscopy of the lowest internal rotor states of Ar2HCl
Authors: Elrod M. J. , Loeser J. G. , Saykally R. J.
Published: July 2002

The c-type intermolecular out-of-plane bend of Ar2HCl has been observed at 45.2 cm−1, completing the high resolution far infrared measurements of the three lowest-lying Ar2HCl bending states which correlate to the j=1 internal rotational state of the HCl monomer. The rotational and nuclear quadrupole hyperfine structures indicate the existence of a Coriolis perturbation. The perturbing state is postulated to be a heavy-atom stretching overtone that is very nearly degenerate with the out-of-plane bend. A partial reassignment of the previously reported [J. Chem. Phys. 95, 3182 (1991)] Ar2HCl in-plane bend is presented and a treatment of Coriolis effects between the in-plane and Σ bends is discussed. Comparison with dynamically rigorous calculations presented in the accompanying paper [J. Chem. Phys. 98, 5337 (1993)] indicate substantial three-body contributions to the intermolecular potential, which should be determinable from the data presented in this paper.
A global potential energy surface for ArH2
Authors: Schwenke David W. , Walch Stephen P. , Taylor Peter R.
Published: July 2002

We describe an analytic representation of the ArH2 potential energy surface which well reproduces the results of extensive ab initio electronic structure calculations. We also give an empirical modification of the function designed to improve agreement with experimental estimates of the van der Waals minimum. The analytic representation smoothly interpolates between the H+H and strong bonding H2 limits. In the fitting process, an accurate reproduction of regions of the potential expected to be important for high temperature (∼3000 K) collision processes is emphasized. Overall, the analytic representation well reproduces the anisotropy and H2 bond length dependence of the input data.
Rydberg spectroscopy of OCS: New assignments in the 70 000–74 000 cm−1 energy region
Authors: Weinkauf R. , Boesl U.
Published: July 2002

The first Rydberg states of OCS have been reinvestigated by applying a combination of several spectroscopic methods. The comparison of (2+1)- and (3+1)-multiphoton ionization (MPI) and one photon absorption spectra reveals quite different intensities indicating different symmetries of the involved Rydberg states. Assuming propensity rules for the ionization process and by measuring the ion ground state population with (2+1)-MPI photoelectron spectroscopy, it was possible to find new assignments for the spin–orbit components of the ion core and vibrational structures in the Rydberg states. Characteristic isotopic shifts in the (2+1)-MPI spectra confirm these assignments and deliver additional arguments for the identification of transitions to bending vibrations. In particular, for the bending vibration split by Renner–Teller interaction, ambiguities in the MPI photoelectron (PE) spectra could be solved by photodissoziation spectroscopy of the OCS+ ions prepared by (2+1)-MPI using the Rydberg transitions as intermediate resonances.
Structure of propadienylidene, H2CCC
Authors: Gottlieb C. A. , Killian T. C. , Thaddeus P. , Botschwina P. , Flügge J. , Oswald M.
Published: July 2002

The millimeter-wave rotational spectra of four isotopic species of the cumulene carbene propadienylidene (H213CCC, H2C13CC, H2CC13C, and D2CCC) were measured and the same set of rotational and centrifugal distortion constants used to describe the normal species [Vrtilek et al., Astrophys. J. Lett. 364, L53 (1990)] were determined, allowing the r0 and rs structures to be derived. Vibration–rotation coupling constants calculated ab initio in the CEPA-1 approximation were combined with the experimental rotational constants for the four isotopic species and the normal species to yield the equilibrium geometry: re(HC(1))=1.083±0.001 Å, re(C(1)C(2))=1.3283±0.0005 Å, re(C(2)C(3))=1.291±0.001 Å, and ∠ (HC(1)H)=117.6±0.2°. The calculated spectroscopic properties may aid forthcoming high-resolution IR spectroscopy of H2CCC.
Dynamics of high n molecular Rydberg states with application to mass analyzed threshold ionization spectroscopy
Authors: Zhang Xu , Smith Jonathan M. , Knee J. L.
Published: July 2002

Studies are performed which measure the lifetime of very high lying molecular Rydberg states which play a central role in high resolution threshold ionization spectroscopy. The molecular systems investigated are styrene, phenol, and phenanthrene as well as the atomic iron system for comparison. It is shown that the lifetimes of the highest lying levels are determined not by intramolecular nonradiative processes but by intermolecular Rydberg–ion or Rydberg–Rydberg interactions. Under low fluence laser excitation, the highest lying Rydberg states are observed to have a lifetime greater than 25 μs but under high fluence the observed lifetime is ∼4 μs. The long Rydberg lifetimes are utilized to implement a simplified spectrometer for mass analyzed threshold ionization (MATI) spectroscopy.
Comment on: The relation between intensity and dipole moment for bending modes in linear molecules
Authors: Gwaltney Steven R. , Bartlett Rodney J.
Published: July 2002

In a recent paper [J. Chem. Phys. 97, 4616 (1992)] that examines the relationship between experimental and computed intensities for bending modes of linear molecules, So/rensen and Jo/rgensen state that ‘‘most recent quantum mechanical computations predict band intensities of the bending modes in linear molecules that are approximately a factor of 2 bigger than observed intensities, and it is shown that the problem relates to the ab initio treatment.’’ It is further stated that ‘‘the disturbingly big discrepancies...are real, and the cause of the discrepancies has to be sought in the ab initio computation of the dipole moment.’’ This apparent failing of theory is addressed here by demonstrating that coupled-cluster calculations provide intensities for the bending modes of acetylene, carbon dioxide and nitrous oxide, of 177, 55, and 8 km/mol which compare favorably with the experimental values of 177, 55, and 7; while that for HCN is 70 compared to an experimental value of 58. The discrepancy in the latter might recommend some reconsideration of the experimental value, including the role of electrical and mechanical anharmonicity.
The calculation of exchange forces: General results and specific models
Authors: Scott T. C. , Babb J. F. , Dalgarno A. , Morgan John D.
Published: July 2002

In order to clarify questions about the calculation of the exchange energy of a homonuclear molecular ion, an analysis is carried out of a model problem consisting of the one-dimensional limit of H2+. It is demonstrated that the use of the infinite polarization expansion for the localized wave function in the Holstein–Herring formula yields an approximate exchange energy which at large internuclear distances R has the correct leading behavior to O(e−R) and is close to but not equal to the exact exchange energy. The extension to the n-dimensional double-well problem is presented.
The drift velocity in reptation models for electrophoresis
Authors: Kooiman A. , van Leeuwen J. M. J.
Published: July 2002

The drift velocity (diffusion constant) in the Rubinstein–Duke model with periodic boundary condition is calculated analytically to lowest order in the applied electric field and numerically for the whole scaling regime. The model is modified by restricting the polymer-storing capacity of the cells and for this case again the diffusion constant is determined. The periodic boundary condition decouples the different tube configurations. Thus, with the process of tube renewal removed, only the diffusion of length defects through the tube remains. The effect of the periodic boundary condition on the value of the diffusion constant and the behavior of the scaling function is discussed on the basis of numerical results for both models with free endpoint motion. The results strongly suggest that to linear order in the field the drift velocity is unaffected by the process of tube renewal, i.e., is only determined by the transport of reptons along the tube.
A note about the comparison of the WCA and self-consistent WCA perturbation methods
Authors: Masson Thierry , Kazandjian Luc
Published: July 2002

The self-consistent WCA method is generally regarded as more accurate than the WCA method at high densities, which may seem surprising in view of the orders of approximation expected. It is shown in this note that, in fact, the relative ranking of the two methods depends on the representation chosen for the indirect correlation function of the reference hard-sphere fluid.
Rotationally resolved single-photon ionization of HCl and DCl
Authors: Drescher M. , Brockhinke A. , Böwering N. , Heinzmann U. , Lefebvre-Brion H.
Published: July 2002

The rotationally resolved photoionization yields of jet-cooled HCl and DCl have been measured in the energy range between their spin–orbit split 2Π ionic thresholds. For single-photon excitation, narrow-band vuv radiation is generated by resonant frequency mixing. The spectra are complex due to an interaction of autoionizing resonances belonging to series converging to different rotational states of the ion core. This is taken into account using the multichannel quantum defect theory in a treatment capable to handle a transition in angular momentum coupling from Hund’s case (c) to case (e). Even for Rydberg orders n≳35 the experimental data cannot be explained using a pure case (e) picture.
Kinetic analysis of computer experiments on electron hydration dynamics
Authors: Keszei E. , Nagy S. , Murphrey T. H. , Rossky P. J.
Published: July 2002

Based on a careful analysis of recent nonadiabatic dynamical simulations, a new mechanism is proposed to describe the hydration of electrons in pure water, and the solution of the corresponding set of differential equations is given. According to this mechanism, a thermalization via spontaneous de-excitations across a manifold of delocalized excited states is followed by a branching between a two-step hydration and a direct trapping path leading to the ground state of the hydrated electron. From a kinetic perspective, the most important results of the analysis derive from the temporal evolution of the electronic states in two different sets of simulation trajectories; one in which the electron initially possesses about 2 eV excess energy in the unrelaxed solvent, and another at an initial excess energy higher about 0.5 eV. Estimated characteristic rates for energy loss during thermalization are found to be identical for the two cases. However, branching ratios and characteristic times for the solvation steps show considerable differences, depending on the initial energy of the hot electrons injected into the water.
Small hydrophobic organic ions in aqueous solutions
Authors: Kunz Werner , Calmettes Patrick , Cartailler Thierry , Turq Pierre
Published: July 2002

Small-angle neutron scattering experiments were performed at 25 °C on solutions of tetraphenylphosphonium chloride and sodium tetraphenylborate in deuterated water and in deuterated dimethylformamide. The results show a strong attraction between either the organic cations or the organic anions when the salts are dissolved in water. On the contrary, in the organic solvent the attraction between the equally charged organic ions is found to be very weak. This is the first structural evidence of hydrophobic association of small organic ions in water.
Formation of ArCl(B,C), Ar(3P2), and Cl* by the three-body ionic-recombination reaction of Ar+(2P3/2)+Cl−+He
Authors: Tsuji Masaharu , Ide Masahiro , Muraoka Toshihiko , Nishimura Yukio
Published: July 2002

The three-body ionic-recombination reaction of Ar+(2P3/2)+Cl−+He has been studied by observing ArCl(B–X,C–A) and Cl(4s,4s′,4p,4p′,4d,5p,6s) emissions in the He flowing afterglow. CCl4 or CFCl3 is used as a source of Cl−. The ArCl(B–X,C–A) emissions enhance relative to the Cl* bands with increasing the CCl4 pressure. It is explained as due to the secondary Ar(3P2)+CCl4 reaction, which is used to estimate the relative formation rate of Ar(3P2). The relative formation rates of ArCl(B,C), Ar(3P2), and Cl* are determined to be 0.0087±0.0009, 0.64±0.06, and 0.35±0.04, respectively. The small branching ratio of ArCl(B,C) is attributed to fast predissociation into Ar(3P2) and Cl*. The favorable Cl* states are 4s′ populated by direct reaction and 4p populated by direct reaction and/or radiative cascade from the 3d and 5s states. The electronic state distribution of Cl(4p′,4d,5p,6s) is nonstatistical without any strong preference for J. Although both doublet and quartet states are found for the Cl[5p(P,D)] states, only quartet states are observed for the Cl[5p(S),4d(D,F)] states.
Valence electronic states of SiH2+ by ab initio effective valence shell Hamiltonian
Authors: Park Jong Keun , Sun Hosung
Published: July 2002

The ab initio second order effective valence shell Hamiltonian which is based on quasidegenerate many-body perturbation theory has been applied to the SiH2+ dication. From the characteristic properties of Hv, all the valence states are determined with a same accuracy. The four low lying quasibound states (X 2Σ+, A 2Π, a 4Π, C 2Σ+) are found for the dication. It is verified that the existence of some quasibound states is due to the interaction of an attractive state from ion-neutral pair asymptote with a same symmetry repulsive state from ion–ion pair asymptote. Since the experimental data are scarce, this work provides all theoretical spectroscopic properties of these quasibound states. Also the repulsive valence states are examined.
Dielectric relaxation studies of glass formation in some esters of phthalic acid
Authors: Nayak S. K. , Murthy S. S. N.
Published: July 2002

Dielectric relaxation studies have been made on a series of dialkyl esters of phthalic acid over a frequency range of 20 Hz to 1 MHz and over a temperature range of 77 to 573 K. Studies have also been made above room temperature (up to 363 K) at microwave frequencies of 5.3, 10, and 32 GHz. All the samples in their supercooled liquid state reveal yet another process in addition to the primary or α process. Careful measurements have been made to resolve the secondary or β process from the α process at temperatures above Tg. The activation energy of the β process decreases from 39 KJ/mole to 20 KJ/mole with increase in the size of the alkyl group. It is also noted that the bifurcation of liquid relaxation into α and β processes occurs at much higher temperature with increase in the alkyl group. A sharp increase with temperature in the magnitude of the β process in the vicinity of the glass transition Tg temperature, is noted. The relevance of these observations to glass formation in liquids is discussed.
Theoretical integrated vibrational band intensities of water vapor
Authors: Gabriel Wolfgang , Reinsch Ernst-Albrecht , Rosmus Pavel , Carter Stuart , Handy Nicholas C.
Published: July 2002

Using variational rotational–vibrational wave functions and ab initio electric dipole moment functions, rotational–vibrational line strengths and integrated vibrational band intensities of water vapor have been calculated. The theoretical line strengths are in excellent agreement with existing experimental data.
Experimental study of ion-induced nucleation by radon decay
Authors: He F. , Hopke P. K.
Published: July 2002

In the environment, the presence of ions from natural radioactivity may increase the rate of new particle formation through ion-induced nucleation. A thermal diffusion cloud chamber (TDCC) has been built to experimentally study ion-induced nucleation where the ions are produced by gaseous radioactive sources. The critical supersaturation values and nucleation rates for methanol, ethanol, 1-propanol, and 1-butanol vapors on ions produced within the volume of the chamber by alpha decay of 222Rn have been measured quantitatively at various radioactivity concentrations and supersaturations. The presence of ion tracks and the effect of an external electric field were also investigated. The alpha tracks and ion-induced nucleation formed by 222Rn decay become visible at the critical supersaturation that is below the value needed for homogeneous nucleation. At this supersaturation, the nucleation rates increase substantially with increasing 222Rn at low activity concentrations, but attain limiting values at higher concentrations. The experimental results indicate that the ionization by radon decay will promote ion-cluster formation and lower the free energy barriers. The formation of visible droplets is strongly dependent on the supersaturation. This study also confirms that the external electric field has a significant effect on the observed rates of nucleation.
Emission spectroscopy of H2O dissociating in the B̃ 1A1 state: Rapid bending motion manifested through excitation of high bending states of H2O (X̃)
Authors: von Dirke Michael , Heumann Bernd , Schinke Reinhard , Sension Roseanne J. , Hudson Bruce S.
Published: July 2002

We present a theoretical and experimental investigation of the emission spectrum of dissociating water after excitation in the second absorption band (X̃ 1A1→B̃ 1A1). The calculations are performed in the time-dependent wave packet formalism employing an ab initio potential energy surface. All three degrees of freedom (the two OH stretching modes and the HOH bending mode) are taken into account. The B̃ 1A1 potential energy surface depends strongly on the HOH bending angle which leads to very fast opening of this angle after the water molecule is promoted to the excited electronic state. As a consequence, we observe, both experimentally and theoretically, the excitation of high bending states in the X̃ ground state. According to the wave packet study the emission spectrum is determined in the first ten femtoseconds of the motion in the excited state. The agreement with the measured spectrum for an excitation wavelength of 141.2 nm is good.
Statistical properties of surfaces covered by large spheres
Authors: Wojtaszczyk P. , Schaaf P. , Senger B. , Zembala M. , Voegel J. C.
Published: July 2002

Statistical properties of surfaces covered by particles deposited under the influence of gravity are investigated by means of optical microscopy and image analysis. The radial distribution function of the particle configurations is determined over a wide coverage range. Special attention is paid to the fluctuations of the number of particles on small surfaces which contain information upon the deposition process. A first analysis of these fluctuations is presented within a mean field approximation. All results are compared to the ballistic process which proves to be a good model. On the contrary, the observations are not compatible with a simple random sequential adsorption model.
Invariant imbedding and full collision matrix methods: Incorporation of closed channels, complex potentials, and determination of bound state energies
Authors: Tuvi I. , Band Y. B.
Published: July 2002

We demonstrate the relationship between the full collision matrix [J. Chem. Phys. 88, 2309 (1988)] and the invariant imbedding [J. Chem. Phys. 77, 1942 (1982)] methods for quantum scattering. We show how to include closed channels into scattering calculations employing these methods, yet retaining the inherent stability of the methods. We develop techniques for using these methods for (single and) multichannel bound state problems to determine the bound state energy eigenvalues. The method is generalized to include complex potentials in order to simulate decay.
Vibrational levels for the lowest-lying triplet and singlet states of CH2 and NH+2
Authors: Barclay V. J. , Hamilton I. P. , Jensen Per
Published: July 2002

The 30 lowest vibrational energy levels are calculated for the X̃ 3B1 and ã 1A1 states of CH2 and NH+2 for various potential energy surfaces. We include a new surface for the NH+2 triplet state, with predicted fundamentals ν1=3059, ν2=845, and ν3=3360 (expt 3359.9) cm−1. Where possible, results are compared with vibrational levels calculated using two other methods and with experiment. At low energy, the differences between the calculated vibrational levels are due primarily to the choice of potential energy surface. Of secondary importance is the method used to calculate the vibrational energy levels. The differences resulting from the method used are most apparent for cases with a diffuse vibrational wave function or for cases where the bond distance along the minimum energy path depends strongly on the bending angle.
Ellipsometric measurement of a surface amplitude ratio near a critical end point
Authors: Smith Dan S. P. , Law Bruce M.
Published: July 2002

We present the experimental determination of a universal surface amplitude ratio near a critical end point. From ellipsometric measurements on five critical binary liquid mixtures, three with an upper critical point and two with a lower critical point, we obtain a value for the modified critical adsorption amplitude ratio of Rma=1.18±0.13, where the error represents three standard deviations. Reasonable agreement is obtained with the theoretical results of H. W. Diehl and M. Smock [Phys. Rev. B 47, 5841 (1993)], from which we obtain Rma=1.33.
A local dynamical scaling behavior of the Ostwald ripening in nonuniform systems
Authors: Nakahara Akio , Kawakatsu Toshihiro , Kawasaki Kyozi
Published: July 2002

The Ostwald ripening of droplets of precipitates in nonuniform systems is investigated by using dynamical scaling assumptions and by performing numerical simulations. First, we analytically study the Ostwald ripening in nonuniform systems by using dynamical scaling assumptions. To examine validity of the dynamical scaling assumptions, we numerically solve the basic model evolution equations for both the supersaturation of the solute and the size distribution functions of droplets for the nonuniform system where several homogeneous cells are coupled together by the diffusion of the solute. We found an important effect of the initially large droplets, which govern the late stage dynamics of the coarsening process. Next, to investigate the nonuniform system which consists of many cells, we perform a reduction of the degrees of freedom of the size distribution function and construct a simplified model (a reduced model). In deriving this reduced model, we take into account the importance of the large size droplets by dividing the size distribution function of droplets into two parts; one is for smaller droplets and the other for the larger droplets. We perform numerical simulations of the reduced model and study the formation of the spatial structure of precipitations. An appreciable mass transport is induced by an initial spatial inhomogeneity in the size distribution function of large droplets which dominates the system at later times. Depending on the spatial inhomogeneity of the system, the size distribution function of droplets is locally characterized by a new scaling function, which is related to the changing rate of the total volume of droplets in the region and is different from the Lifshitz–Slyozov–Wagner (LSW) universal scaling function for uniform systems.
High order and local structure of water determined by microwave dielectric study
Authors: Mashimo Satoru , Miura Nobuhiro
Published: July 2002

Water mixture with tert-butanol shows two critical points if the logarithm of dielectric relaxation time is plotted against the mol fraction of water XW. One point at XW∼0.97 is suggested to indicate the break point of a high order structure of water and another point XW=0.83 is that of a local structure. Existence of the high order structure was already indicated by a recent dielectric study on water–glucose mixture. The structure is a less ordered lattice of ordinary ice type. It is also shown that water mixture with treharose or maltose exhibits one relaxation peak as well as the glucose mixture. This manifests that the diameter of the water cluster is nearly the same as the length of treharose or maltose, at least, smaller than that of maltotriose which shows two relaxation peaks. The critical point XW∼0.97 corresponds to about 30 water molecules included in the high order structure, which is very reasonable to compare the result of the glucose mixture.
Rovibronic interactions in NO2 around 17 700 cm−1 observed by Zeeman effect and anticrossing experiments
Authors: Delon Antoine , Dupre Patrick , Jost Rémy
Published: July 2002

We have observed the Zeeman effect on N=1, K=0 rotational levels of seven vibronic levels of NO2 located between 17 438 and 17 842 cm−1. We have used a supersonic jet, (Trot≊4 K) located inside a 5 MW Bitter coil of 100 mm bore which allows magnetic field scans up to 8 T. CW monomode ring dye laser excitation allows a resolution of about 300 MHz limited by the residual Doppler effect. We have observed the evolution of the Zeeman energy levels versus the field. The values of high field Landé factors range from 1.80 to 1.98, significantly lower than the free spin value (2.0023). The standard perturbation theory of Curl [Mol. Phys. 9, 585 (1965)], which relates Landé factor and the spin-splitting constant ε̄, does not fit the observed results. In addition, 54 anticrossings due to rovibronic interactions have been observed. The corresponding matrix elements range from about 50 MHz (limited by field inhomogeneities) up to 15 GHz, (0.5 cm−1). The expected number of anticrossings in the magnetic field range scanned (from the known rovibronic density of state and from first-order ‘‘spin–rotation’’ interaction selection rules), is only 27. We explain the additional anticrossings by higher order interactions. In fact, the distribution of observed matrix elements is smooth, without any gap between first order and higher order matrix elements. In this case, we have assumed that the first order matrix elements are the larger ones.

With this assumption, we have determined the average reduced matrix element of first-order spin–rotation interaction: 0.73±0.15 cm−1. These off diagonal spin–rotation interactions are expected to be roughly independent of the N rotational quantum number. This contrasts with the diagonal electronic-spin interactions (spin splittings) which increase linearly with N but which are significantly weaker than off-diagonal interactions at least for the N=1, K=0 levels studied here. We show that these rovibronic interactions (by both first order and higher order) induce the numerous irregularities previously observed in the zero field jet cooled excitation spectrum of NO2. Moreover, the average reduced matrix element of first order spin–rotation interaction observed in the zero field spectrum from 16 500 to 18 500 cm−1 is about 0.76±0.25 cm−1 in agreement with the above-mentioned high field measurement.
A comparison of time-dependent and time-independent quantum reactive scattering—Li+HF→LiF+H model calculations
Authors: Balint-Kurti Gabriel G. , Gögtas Fahrettin , Mort Steven P. , Offer Alison R. , Laganà Antonio , Gervasi Osvaldo
Published: July 2002

Reactive scattering probabilities are computed over a wide range of collision energies for a model system based on the Li+HF→LiF+H reaction using both grid based time-dependent and time-independent quantum mechanical methods. The computations are carried out using a fixed Li–F–H angle which is chosen to be that at which the barrier to the chemical reaction is lowest. The calculated reaction probabilities for this system display many sharp features as a function of energy which are ascribed to scattering resonances. The time-independent calculations have been carried out on a very dense energy grid, thus permitting detailed comparison between time-independent and time-dependent methods (in the latter case, a single computation of the wave packet dynamics provides information on the energy dependence over a given energy range). The results show that the time-dependent calculations are capable of reproducing even the sharpest resonance features computed using the time-independent method. The time-dependent techniques are conceptually very simple and therefore easily implemented. The results presented also demonstrate that the grid based time-dependent quantum mechanical methods used here are able to describe threshold energy dependence of reaction probabilities where the exit channel kinetic energy is effectively zero. The nature of some of the resonance structures are investigated by computing the time-independent continuum wave functions at the ‘‘resonance’’ energies thus mapping out the nodal structure of the wave functions. The good agreement between time-independent and time-dependent methods is shown to be maintained when a centrifugal barrier is added to the potential to simulate the effect of nonzero orbital angular momentum.
Persistent infrared holeburning in poly (ammonium styrene sulfonate)
Authors: Cho H.-G. , Strauss H. L.
Published: July 2002

The ND stretching bands of the title compound have been burned. The polymer is doped with deuterium and contains a small amount of NH3D+ ion. The resulting holes are similar to those produced in inorganic ammonium salts, but the hole burning efficiency of the burning process is lower in the polymer.
Pair connectedness functions and percolation in highly charged electrolyte solutions
Authors: Bresme F. , Abascal J. L. F.
Published: July 2002

The structure of highly charged electrolyte solutions is investigated through a percolation approach. The systems, simulated by standard Monte Carlo methods, include several 2:2 aqueous solutions with concentrations ranging from 0.25 to 2 M and one 1:1 electrolyte at 1 M concentration. The ion–ion pair connectedness functions and related quantities have been computed. Two ions are considered connected if they are closer than a predefined distance. Besides this geometric criterion, an energetic condition (i.e., only particles with unlike charge signs are allowed to form directly connected links) has also been employed. The clusters obtained via the geometric and the energetic conditions are substantially different for the 1:1 electrolyte while the 2:2 solutions show a cluster structure almost independent on the criterion used. The percolation thresholds exhibit a nonmonotonic variation with concentration. A minimum in the percolation threshold has been observed at the higher concentrations investigated for the 2:2 systems; the shape of the curve and the limiting values for infinitely dilute solutions suggest the appearance of a maximum at concentrations lower than those studied in this paper. In addition, the critical exponents α, γ, and τ have been computed. The result for α significantly differs from the values reported for three dimensional lattices but it agrees with those obtained for the Lennard-Jones fluid. In contrast, the γ and τ exponents for 3D lattice systems and for our electrolyte solutions seem to be coincident.
Near threshold photoionization of silicon clusters in the 248–146 nm region: Ionization potentials for Sin
Authors: Fuke Kiyokazu , Tsukamoto Keizo , Misaizu Fuminori , Sanekata Masaomi
Published: July 2002

Photoionization thresholds for silicon clusters in the size range Si2 to Si200 have been examined by laser photoionization with detection by reflectron time-of-flight mass spectrometry. Stimulated Raman scattering light of narrow bandwidth 193 nm radiation was used as the ionization light source down to 146 nm. A characteristic size dependence of ionization potentials (IPs) was found for clusters smaller than 22 atoms, featuring major maxima at n=10 and 20. We also found a large gap in IPs between n=20 and 22. This gap was ascribed to the structural transition of neutral clusters in analogy with that found recently for small silicon cluster ions. The IPs for larger clusters with n=100–200 silicon atoms were found to be 5.0–5.17 eV, which are lower than the ionization energy but still higher than the work function of bulk Si(111) surface. This difference was discussed with relation to the nature of surface states for both phases.
Determination of the structure of Ar H2CO
Authors: Novick Stewart E.
Published: July 2002

The rotational spectrum of the weakly bound complex Ar H2CO has been measured using a pulsed beam Fourier transform microwave spectrometer. Both Ar H2CO and Ar D2CO isotopomers were studied. The rotational constants of the ground nuclear spin state of the protonated isotopomer of the complex are A–ΔK=40 265.189 MHz, B=2204.453 MHz, and C=2078.690 MHz. The argon atom sits almost directly above the center of mass of formaldehyde with an Ar–cm distance of 3.649 Å and an Ar–cm–O angle of 87°. It is likely that the hydrogen atoms of the formaldehyde execute hindered internal rotation about the C–O axis and thus their positions within the complex are not well specified. As a best effective structure, we take the geometry of the complex to be planar. A second, similar, structure is determined for the complex involving the excited nuclear spin state of formaldehyde.
Vibrational spectroscopy and intramolecular relaxation of benzene
Authors: Iachello Francesco , Oss Stefano
Published: July 2002

By making use of Lie algebraic methods, we construct the complete vibrational spectrum of benzene. We use this construction to study the process of intramolecular vibrational energy relaxation in the first and second overtone of the CH stretching mode.
Intermolecular Raman bands in the ground state of benzene dimer
Authors: Venturo Vincent A. , Felker Peter M.
Published: July 2002

Mass-selective, ionization-detected stimulated Raman spectroscopies have been applied to a study of low-frequency structure in the ground states of benzene dimer isotopomers. The results reveal two Raman bands below 10 cm−1, as well as structure in the range of 47 to 53 cm−1. Tentative assignments for the bands are proposed.
FNO: Another challenging molecule for correlation methods
Authors: Dibble T. S. , Francisco J. S.
Published: July 2002

Although FNO has long been considered a computationally challenging molecule, no thorough study of this species has been performed with methods of electron correlation beyond second-order Mo/ller–Plesset perturbation theory (MP2) and the singles and doubles configuration interaction (CISD) method. We have investigated the equilibrium structure, vibrational spectra, and relative energetics of FNO using high-levels of ab initio theory. Methods include third and fourth-order Mo/ller–Plesset perturbation (MP3 and MP4), singles and doubles coupled-cluster (CCSD) theory, and the CCSD(T) method, which incorporates a perturbational estimate of the effects of connected triple excitations. The quadratic configuration interaction method including single, double, and triple excitations [QCISD(T)] method is also used. Computations using these methods are performed with medium and large basis sets. Geometries calculated at the Hartree–Fock level are grossly in error, while going to the CISD, MP3, MP4, and CCSD methods produces increasingly better results. The CCSD(T) and QCISD(T) methods yield quite accurate structures; surprisingly, the MP2 results are almost as good. The quality of the calculated relative energetics closely follows that of the structure predictions, except that the MP2 level of theory now performs slightly worse than MP3, MP4, and CCSD methods.
15N and 18O kinetic isotope effects in the thermal decomposition of N2O catalyzed by bromine
Authors: Lesar Antonija , Senegačnik Marjan
Published: July 2002

15N and 18O kinetic isotope effects (KIEs) in the thermal decomposition of N2O catalyzed by bromine were experimentally determined in the temperature range 773–873 K, resulting in KIE (15N)=−2.07+4020/T and KIE (18O)=−0.41+3290/T. For theoretical interpretation, based on the Bigeleisen formalism, the following planar transition states were taken into account: trans (N–N–O–Br)‡, trans (Br–N–N–O)‡, and branched (N–N<OBr)‡. In addition to KIE, activation energy according to the Sanderson bond-energy–bond-order relationship and pre-exponential factor were calculated as subsidiary parameters in selecting an appropriate transition state among all probable configurations. The result reveals that it is meaningless to speculate whether the Br atom approaches the central or the terminal N atom of the N2O molecule, since both transition states could generate acceptable values of the kinetic isotope effects, activation energy, and pre-exponential factor.
Role of the resonance function in the rotational relaxation of molecules
Authors: Odashima Hitoshi , Ikeda Hirohiko , Minowa Tatsuya , Shimizu Tadao
Published: July 2002

The dependence of the cross section of the rotational relaxation of molecules on the energy discrepancy (ΔE) in collision is clearly observed in various types of collisional interactions, such as those of dipole–dipole, quadrupole–dipole, and London dispersion. It is found that the ΔE dependence is quite different according to the order of collisional interaction. This result is explained by the conception of the resonance function in the Anderson–Tsao–Curnutte and the quantum Fourier transform theories. A new physical interpretation of the resonance function is also given on the basis of both theories.
An exact quantum Monte Carlo calculation of the helium–helium intermolecular potential
Authors: Anderson James B. , Traynor Carol A. , Boghosian Bruce M.
Published: July 2002

We report ‘‘exact’’ ab initio calculations of potential energies for the interaction of two helium atoms. The quantum Monte Carlo method used is exact in that it requires no mathematical or physical approximations beyond those of the Schrödinger equation. As in most Monte Carlo methods there is a statistical or sampling error which is readily estimated. For the equilibrium internuclear distance of 5.6 bohr, the calculated electronic energy is −5.807 483 6±0.000 000 3 hartrees and the corresponding well depth (ε/k) is 11.01±0.10 K. The calculated total energies are approximately 0.004 hartrees or 1200 K below the most recent variational calculations of Liu and McLean [J. Chem. Phys. 92, 2348 (1989)]. The calculated interaction energies are in excellent agreement with the interaction energies of Liu and McLean and with a recent experimental/theoretical compromise potential energy curve of Aziz and Slaman [J. Chem. Phys. 94, 8047 (1991)] which successfully predicts a variety of experimental measurements. The error bars of the ‘‘exact’’ quantum Monte Carlo interaction energies straddle the Liu–McLean and Aziz–Slaman results. The Monte Carlo results support the existence of a bound dimer state.
An improved representation for the high-density structure of Lennard-Jones systems: From liquid toward glass
Authors: Gazzillo D. , Della Valle R. G.
Published: July 2002

We show that a modified hypernetted-chain (MHNC) integral equation with a properly chosen hard sphere bridge function can yield an excellent representation for the high-density structure and thermodynamics of monatomic Lennard-Jones (LJ) systems, continuously from fluid to supercooled liquid and glassy states. In particular, the theory is able to reproduce the gradual development of the second peak splitting in the radial distribution function. The LJ bridge function is approximated with a hard sphere bridge function calculated through a slight modification of a formula due to Malijevsky and Labik (ML). To select the equivalent hard sphere diameter d, several methods are tested. First, a criterion proposed by Rosenfeld and Blum and, second, a best fit of structural and thermodynamic simulation data; finally, an empirical parametrization for d as a function of density and temperature. For stable fluid states the predictions of the MHNC-ML theory are successfully compared with a wide set of simulation results from the literature. For supercooled liquid and glassy states the comparison is made with a molecular dynamics simulation of an isochoric quench, which we have performed.
Interaction-induced light scattering near the glass transition: A computer simulation study
Authors: Watson S. , Madden P. A.
Published: July 2002

The relationships between dipole-induced dipole (DID) light scattering spectra and other dynamical properties of a fluid close to the glass transition were studied in a computer simulation of a highly supercooled binary atomic fluid. The predictions of the mode coupling theory (MCT) of the glass transition, which relates spectral features in the DID spectra to the relaxation of Fourier components of the density, were examined in detail. The MCT account of the origin of the α and β peaks in the spectra was confirmed, thereby corroborating recent experimental findings. Other issues examined included the role played by compositional fluctuations, the coupling of the DID polarizability to shear waves, and the influence of the range of the intermolecular polarizability on interaction-induced spectra near to the glass transition.
Absorption spectrum calculations using mixed quantum-Gaussian wave packet dynamics
Authors: Haug Kenneth , Metiu Horia
Published: July 2002

We calculate the absorption spectrum of a cluster using a computational method in which classical-like degrees of freedom for the nuclei are described by Gaussian wave packets while the valence electrons are treated quantum mechanically. We examine the spectral features in comparison to an even simpler mixed quantum-classical model in which the nuclear motion is treated by purely classical mechanics. Anomalous features (such as negative absorption) in the absorption spectrum which can arise from mixed quantum-classical methods are examined and the Gaussian wave packet nuclear dynamics is found to substantially reduce these anomalous features. This method is applied to a two-coordinate model problem in which exact numerical results can be obtained and we find that the method works fairly well. We also apply the method to the valence electronic absorption spectrum for a KXe6 cluster. The method does not suffer from the dramatic failure seen when Xe motion is treated classically. The method is used to calculate the vibrational width and the vibrational structure of the electronic absorption spectrum.
The vacuum ultraviolet photodissociation of silane at 125.1 nm
Authors: Glenewinkel-Meyer Th. , Bartz J. A. , Thorson G. M. , Crim F. F.
Published: July 2002

We observe the photodissociation of SiH4 in a pulsed molecular beam after excitation with 125.1 nm vacuum ultraviolet light generated via resonant four-wave mixing in mercury vapor. Ultraviolet radiation from a Nd:YAG/dye laser combination ionizes the neutral photodissociation fragments and a time-of-flight mass spectrometer detects the ions. The photodissociation signal consists entirely of silicon atoms and silylidyne (SiH) in its first electronically excited state. We see no silylene (SiH2) or silyl radicals (SiH3). Thus, the photodissociation cleaves almost all of the silicon–hydrogen bonds, but the energetics of the dissociation require the production of at least one hydrogen molecule per dissociation event. These results imply that the high energy content of the initially excited Rydberg state prevents formation of the silylene and silyl radicals in stable vibronic states and that dissociation pathways exist that connect the Rydberg state directly to the corresponding silicon atom and silylidyne asymptotes. These pathways are likely to exist because of Jahn–Teller distortion from the initial Td symmetry. Very little of the available energy appears as kinetic energy of the fragments but rather as electronic excitation of the products. Our results differ from those of earlier studies that concluded silylene and silyl are the principle products.
The electrical double layer in wall–wall hypernetted chain approximation with bridge functions
Authors: Attard Phil , Miklavic S. J.
Published: July 2002

The wall–wall Ornstein–Zernike equation is formulated for the case of charged walls in a primitive model electrolyte. Numerical results are presented for the modified hypernetted chain (HNC) closure, which includes the effects of ion size and correlations. The bare HNC gives attractions between like-charged surfaces rather too readily, but this is ameliorated by the inclusion of the first bridge diagram. The singlet approach is shown to be accurate in divalent electrolyte, and in the large separation asymptotic regime. The method is less reliable in dilute monovalent electrolyte.