bluuebunny/crossref_metadata_2025 huggingface dataset:
Singlet gerade Rydberg states of molecular hydrogen
Authors: Rottke H. , Welge K. H.
Published: July 2002

We investigate the singlet gerade Rydberg series with low total angular momentum J=0,1,2 of molecular hydrogen near the ionization threshold. They are excited by starting at selected rotational levels of the B 1Σ+u (v′=0) state. The spectra show Rydberg series converging at the different rovibrational levels of the X 2Σ+g electronic ground state of H+2 with the Rydberg electron in an s or d-electronic orbital. An identification of the final states is done with the help of a model multichannel quantum defect theory (MQDT) calculation, which is also used to calculate quantum defects from a small set of parameters. Nearly n-independent quantum defects are derived from the experiment for the different Rydberg series with the Rydberg electron in the s-orbital. In the d-Rydberg series perturbations through l-uncoupling are small. An electronic s–d interaction between Rydberg series is present, which is partly induced by the doubly excited gerade configuration (1σu)2. Decay processes of the Rydberg states through autoionization, predissociation, or electric field ionization are found in the excitation spectra.
Rotationally resolved fluorescence as a probe of molecular photoionization dynamics
Authors: Kakar Sandeep , Choi Heung-Cheun , Poliakoff E. D.
Published: July 2002

We present rotationally resolved data for the v′=0 and v′=1 levels of N2+(B 2Σu+) produced via 2σu−1 photoionization of N2. The data are obtained over a broad photon energy range (19≤hνexc≤35 eV). This is made possible by using synchrotron radiation excitation in conjunction with dispersed fluorescence detection. The results exhibit both resonant and nonresonant effects.
Connectivity of critical lines around the van Laar point in T, X projections
Authors: van Pelt A. , de Loos Th. W.
Published: July 2002

It is shown that the SPHCT equation of state describes qualitatively the same types of binary phase behavior around the van Laar point as the van der Waals and the Redlich–Kwong equation of state. A continuous transition is possible from a binary system with a van Laar point into binary systems that show type II, III, IV and IV* phase behavior. The global phase diagram turns out to be a helpful tool in the exploration of these different types of phase behavior. Special attention is paid to the link between moving across the global phase diagram and the resulting changes in the phase behavior of the corresponding binary mixtures. We will focus on the temperature-composition behavior of the critical lines in the binary mixtures. This temperature-composition behavior is very important if one wants to make an experimental verification of the van Laar point or type IV* phase behavior.
Deuteron relaxation and anisotropic reorientation of a nematic liquid crystal
Authors: Goetz Jon M. , Hoatson Gina L. , Vold Robert L.
Published: July 2002

2H-NMR relaxation experiments have been used to investigate molecular motion of the nematogen 2-fluorenyl-4′-tetradecycloxy benzoate-d9 (FLOC14-d9). The molecule is perdeuterated in the rigid fluorenyl moiety and in the magnetically oriented nematic phase gives spectra with seven quadrupole doublets. An inversion recovery experiment was used to measure Zeeman order relaxation times T1Z, and a separate broadband Jeener–Broekaert experiment was used to measure quadrupolar relaxation times T1Q. This combination yields spectral densities, J1i(ω0) and J2i(2ω0), for each site i, on the liquid crystal molecule. The spectral density data have been interpreted by fitting the experimental results with a composite diffusion model of molecular motion of nematic liquid crystal molecules. The temperature and frequency dependence of the rotational correlation times is examined and discussed. The rotational motion is found to be statistically correlated with fluctuations of the nematic director, in such a fashion that director fluctuations make negligible contribution to the observed spectral densities.
Vibrational excitation of methylamine by electron impact in the 4.5–30 eV energy range
Authors: Motte-Tollet F. , Hubin-Franskin M.-J. , Collin J. E.
Published: July 2002

Vibrational excitation of gaseous methylamine induced by 4.5–30 eV energy electrons has been investigated by the electron energy loss spectroscopy. The ratios of the differential cross sections for excitation of the vibrational modes and for elastic scattering measured as a function of the electron kinetic energies show that at 15 and 30 eV, the vibrational excitation occurs mainly through a direct mechanism. The absolute vibrationally elastic and inelastic differential cross sections have been measured at these impact energies. The cross sections for the inelastic scattering are strongly dependent on the vibrational mode which is excited.
Critical comparison of approximate and accurate quantum-mechanical calculations of rate constants for a model activated reaction in solution
Authors: McRae Robin P. , Schenter Gregory K. , Garrett Bruce C. , Haynes George R. , Voth Gregory A. , Schatz George C.
Published: July 2002

Accurate quantum-mechanical calculations of rate constants for a model of reaction in solution are used as benchmarks for two approximate methods: variational transition-state theory with semiclassical corrections for reaction coordinate motion, and the path-integral centroid density method. The reaction model corresponds to a single solute coordinate coupled to a harmonic bath mode. When the harmonic frequency of the bath oscillator is sufficiently high, the results of the approximate methods agree well with the accurate quantum-mechanical ones. For the lowest-frequency bath oscillator considered, the agreement is not as good, but still satisfactory; the worst discrepancies are a factor of 2.0 for the centroid density methods and a factor of 3.3 for variational transition-state theory with semiclassical tunneling corrections. Applications of the approximate methods to models including up to ten bath oscillators indicate that a single bath oscillator provides a reasonable model of a converged harmonic bath.
Kinetic phase diagrams for the monomer–dimer surface reaction: Unification of mean-field and lattice–gas behavior
Authors: Evans J. W.
Published: July 2002

We provide a unified discussion of kinetic phase transitions for mean-field and lattice–gas treatments of the monomer–dimer surface reaction. For high surface mobilities, kinetics is typically well described by mean-field rate equations. These reveal bistability over a range of monomer adsorption rates which diminishes with increasing nonreactive monomer desorption rate d, and vanishes at some critical d=dc. Relative stability in the bistable region is determined from the behavior of chemical waves corresponding to the displacement of one stable state by the other. Their evolution is determined via appropriate reaction–diffusion equations. Conventional diffusion terms are modified here to reflect the influence of the presence of one species on diffusion of the other. We determine equistability (i.e., discontinuous transition) points for d≤dc, and thus construct a kinetic phase diagram including a spinodal line. For lower surface mobilities, analysis of lattice–gas models reveals qualitatively analogous behavior. The key difference is that for lower mobilities, spontaneous fluctuations are effective in automatically selecting the most stable state, i.e., in reducing metastability, and thus producing discontinuous transitions.
Optical interferometric effects in photofragmentation of physisorbed molecules
Authors: Schwarz W. N. , Yang Q. Y. , Chen D. L. , Osgood R. M.
Published: July 2002

The photodissociation yield at the top of a condensed CH3Br layer on GaAs(110) is observed to oscillate as the layer thickness increases. The oscillation is successfully explained and fitted by optical interference theory. The method can reveal the dissociation mechanism and the molecular arrangement near a condensed layer surface.
The scattering of evanescent light waves from oriented, tethered rod polymers
Authors: Rosa Epaminondas , Dahler John S.
Published: July 2002

The theory of Gao and Rice is used to investigate the scattering of evanescent light waves by rod polymers tethered to a flat interface. Calculations of the structure factor are performed for a distribution of rod orientations about the normal to the interface and for rods tilted from the normal. The structure factor is found to vary significantly with this tilt angle and with the width of the orientational distribution as well.
The interaction between water and the liquid-mercury surface
Authors: Sellers Harrell , Sudhakar Pamidighantam V.
Published: July 2002

The energy surface for the interaction between water and the close-pack Hg surface was computed at the relativistic core potential Hartree–Fock+second-order many-body perturbation theory level. The binding energies were found to be 13.1, 12.2, and 11.6 kcal/mol for the binding of a water molecule to the ontop, bridging, and hollow sites, respectively. The equilibrium surface-to-oxygen distances were found to be 5.33, 4.89, and 4.86 bohrs for the ontop, bridging, and hollow sites, respectively. The water molecule physisorbs with the hydrogens pointing away from the surface. The mechanism of the physisorption bonding and physical explanation of the binding-site preference is also presented.
Spectroscopic interrogation of heterocluster isomerization. II. Spectroscopy of (9,10 dichloroanthracene)⋅(rare gas)n heteroclusters
Authors: Ben-Horin Narda , Bahatt Dar , Even Uzi , Jortner Joshua
Published: July 2002

In this paper we report on the electronic spectroscopy of mass-resolved heteroclusters of 9,10-dichloroanthracene (DCA) with rare-gas atoms, i.e., DCA⋅Arn (n=1–54), DCA⋅Krn (n=1–20), and DCA⋅Xen (n=1–14), which were interrogated by resonant two-photon two-color ionization in conjunction with mass spectrometric detection. We measured the spectral shifts δν and the linewidths (FWHM) Δ of the S0→S1 transition, the ionization potential shifts δI from S0, and the change Δτr in the pure radiative lifetime of S1, relative to the corresponding values for the bare DCA molecule. We were able to provide a global assignment of the structure of isomers of DCA⋅An (A=Ar,Kr) heteroclusters, which rests on six independent sources of information, i.e., the application of experimental combination rules and on the comparison between the results of the molecular dynamics (MD) simulations and the experimental data for δν, δI, and Δτr. The size dependence and the isomer specificity of δν, δI, and Δτr are well accounted for by microscopic theories for these spectroscopic observables in conjunction with MD simulations. The size dependence of the experimental linewidths was analyzed in terms of finite temperature MD simulations, which incorporate the effects of homogeneous and inhomogeneous line broadening. Our analysis provides an adequate description of the qualitative difference between the gradual line narrowing of DCA⋅Arn (n=20–34) and the abrupt onset of line narrowing of DCA⋅Krn (n≥16) heteroclusters, which are attributed to the reduction of inhomogeneous line broadening originating from the combined effects of homogeneous line broadening and of the isomer specificity of the spectral shifts.

The temperature dependence of the homogeneous linewidths provides means for internal cluster thermometry. The isomerization phenomena inferred from MD simulations of nuclear dynamics could not be identified by the cluster size dependence of δν, δI, Δτr, and Δ, in accord with the conclusions drawn from the MD simulations of these spectroscopic observables. Finally, we explored the evolution of condensed matter properties with increasing the heterocluster size, advancing cluster size equations, which quantitatively accounts for the smooth ‘‘transition’’ of the spectral shift and of the homogeneous line broadening of DCA⋅Arn heteroclusters to those of DCA in bulk solid Ar.
Intermolecular HF motion in ArnHF micromatrices (n=1,2,3,4): Classical and quantum calculations on a pairwise additive potential surface
Authors: McIlroy Andrew , Nesbitt David J.
Published: July 2002

The availability of pairwise additive ‘‘two-body’’ potentials for van der Waals systems from near-IR, far-IR and microwave data permits detailed prediction of librational behavior for isolated HF chromophores solvated by successive numbers of rare gas Ar atoms. This paper describes theoretical calculations of ArnHF equilibrium structures and intermolecular HF vibrational frequencies based on an ArnHF ‘‘two-body’’ potential energy surface developed from previously determined Ar–Ar and Ar–HF potentials. Isomeric structures are predicted from local minima on these multidimensional surfaces, and are found to be in excellent qualitative agreement with near-IR observations of ArnHF clusters with n=1,2,3, and 4 Ar atoms. Quantum mechanical calculations are performed for the HF librational and van der Waals stretching modes against a rigid Arn frame. These pairwise additive potentials predict a strongly increasing angular anisotropy for the HF bending coordinate with number of Ar atoms (for small n), and provide predictions of HF intermolecular van der Waals bend and stretch vibrational frequencies. Fourier transform (FT)-microwave and near-IR data, on the other hand, demonstrate only a minor dependence of the anisotropy on n; this suggests the pairwise additive potentials may systematically overestimate the angular anisotropy for HF bending. Selected cuts through these potential surfaces indicate significant coupling between the Arn–HF stretch, Ar–Ar stretch, and Ar–Ar bending vibrations. This strong vibrational coupling indicates that a full quantum treatment of all intermolecular coordinates may be required in order to make quantitative comparison with van der Waals vibrational data. In the limit of sufficient Ar atoms to fill the first coordination sphere around the HF, the calculations indicate a nearly perfect cancellation of angular anisotropy for HF librational motion, consistent with the nearly free internal rotation of the HF observed in cryogenic Ar matrices.
Effect of triplet correlations on the adsorption of a dense fluid onto a crystalline surface
Authors: Huckaby Dale A. , Blum Lesser
Published: July 2002

The adsorption of a fluid of hard spheres of diameter σ onto a planar surface containing a triangular lattice of adsorption sites, spacing σ, was studied using a model which is equivalent to a lattice gas with n-body interactions that are related to the n-body correlation functions of the fluid. The present paper extends our previous work, which included only pairwise interactions, to also include triplet interactions. We discovered a simple but accurate analytical approximation to the triplet correlation function for three spheres in mutual contact, which, when combined with a Müller–Hartmann and Zittarz approximation of the critical point in the equivalent lattice gas, yields the estimate ρcσ 3=0.8409 of the fluid density at the critical point of the condensation transition which occurs at the fluid–crystal interface. This estimate, which includes the effects of both pair and triplet correlations in the fluid, is significantly higher than the value ρcσ 3=0.6678 obtained if only the effects of pair correlations are included in the calculation.
Methyl rotor effects on acetone Rydberg spectra. I. The 1A2(3p←n)←1A1 transition
Authors: Kundu Tapanendu , Thakur Surya N. , Goodman Lionel
Published: July 2002

Two-photon spectra of the acetone 1A2(3px←n)←1A1 Rydberg transition obtained by injection seeded excitation determines a2(ν12) and b1(ν17) acetone 1A2 Rydberg state torsional fundamental frequencies as 70.8 cm−1 (ν′12) and 129.4 cm−1 (ν′17) (compared to the ground-state values 77.8 and 124.5 cm−1, respectively). Corresponding values in (CD3)2CO are 49.1 and 98.2 cm−1, compared to 53.4 and 96.0 cm−1 in the ground state. The 1A2 state 2ν′12, 3ν′12, and ν′12 + ν′17 energies are also assigned allowing determination of excited-state potential constants. There are large changes (65–200 cm−1) in these constants from those for the ground state causing significant perturbation in the shape of the 1A2 state potential from that of the ground state. The 1A2 constants are V3=551, V33=349, V′33=−221, and V6=0 cm−1. The effective potential barrier height to internal rotation (202 cm−1) is decreased from the ground state by about 15%, i.e., by ≊35 cm−1; but the barrier height for eclipsed–eclipsed→staggered–staggered synchronous-rotation is increased by ≊300 cm−1 (i.e., ≊35%) to 1103 cm−1.
Assignment of peaks in photodetachment spectra using predicted densities of reactive states: Application to H2F− and D2F−
Authors: Kress J. D. , Hayes E. F.
Published: July 2002

Accurate cumulative reaction probabilities and densities of reactive states have been obtained for the reactions F+H2→H+HF and F+D2→D+DF using the hyperspherical (APH) coordinate scattering method developed by Pack and Parker. Results for scattering energies ranging from threshold to 0.7 eV above threshold have been obtained using two different potential energy surfaces that have been proposed for this system. The predicted peaks in the densities of reactive states are due to the presence of scattering resonances. The characteristic steps in the cumulative reaction probabilities are used to identify the resonance peaks due to quantized dynamical bottlenecks. The trapped-state or Feshbach-type resonances are identified by narrow oscillations in the density of reactive states. The positions of these resonance peaks in the density of reactive states are in good agreement with the peaks in the Franck–Condon factors for the photodetachment of H2F− and D2F−. Significant changes in resonance types are found for the T5a and 5SEC potential energy surfaces for H2F. While the assignment of resonance peaks presented is in reasonable agreement with the assignments suggested by Hahn and Taylor based on their classical mechanical analysis, there are some differences that will require further study.
Correlation in first-row transition metal atoms using generalized Mo/ller–Plesset perturbation theory
Authors: Murphy Robert B. , Messmer Richard P.
Published: July 2002

Correlation effects in the lowest lying states arising from the 4s23dn, 4s3dn+1, and 3dn+2 configurations of the first-row transition metal atoms, Ti, Cr, Fe, and Ni are studied using a generalized Mo/ller–Plesset perturbation theory applicable to multiconfigurational reference wave functions. Reference wave functions including configurations describing the dominant 4s2 angular and 3d radial correlation effects are found to be essential for a perturbative treatment of these atoms. Generalized Mo/ller–Plesset theory applied to such multiconfigurational reference wave functions gives relative energies comparable to corresponding multireference configuration interaction (CI) calculations at a lower computational cost.
A time-dependent golden rule wave packet calculation for vibrational predissociation of D2HF
Authors: Zhang Dong H. , Zhang John Z. H. , Bačić Zlatko
Published: July 2002

Calculations of the total and partial decay widths are presented for the vibrational predissociation of D2HF. The calculation is done for the total angular momentum J=0 of the molecule and for the dissociation process D2HF(v=1)→D2(v=1)+HF(v=0), which is the dominant decaying channel. A time-dependent wave packet approach in the golden rule approximation is employed in the numerical calculation for the decay widths. Our computed total and partial decay widths are in good agreement with those from a time-independent scattering calculation. The time-dependent golden rule wave packet method, which does not solve coupled channel scattering equations, offers an efficient alternative to the time-independent scattering method for computing the decay widths in vibrational predissociation for relatively large molecules. The total time of propagation required in the golden rule wave packet calculation is determined by the duration time of the final state interaction between the fragments on the vibrationally deexcited adiabatic potential surface. This interpretation clearly explains the fact that only a short time is needed in our wave packet propagation, regardless of how long the lifetime of the predissociating state may be.
Calculation of molecular magnetic properties within the Landau gauge in hydrogen fluoride, ammonia, and methane molecules
Authors: Ferraro M. B. , Herr T. E. , Lazzeretti P. , Malagoli M. , Zanasi R.
Published: July 2002

The magnetic susceptibility and the nuclear magnetic shielding tensors of HF, NH3, and CH4 molecules in the presence of a spatially uniform time-independent magnetic field have been calculated within the Landau gauge for the vector potential. The random-phase approximation has been used, adopting flexible Gaussian basis sets of large size. Magnetic susceptibilities are quite harder to evaluate accurately in the Landau gauge than in Coulomb gauge. Calculated nuclear magnetic shieldings are close to the Hartree–Fock limit. Sum rules for charge conservation and origin independence of magnetic susceptibility and nuclear magnetic shieldings are fairly well satisfied by basis sets of extended size.
All-electron molecular Dirac–Hartree–Fock calculations: Properties of the group IV monoxides GeO, SnO, and PbO
Authors: Dyall Kenneth G.
Published: July 2002

Dirac–Hartree–Fock calculations have been carried out on the electronic ground states of the group IV monoxides GeO, SnO, and PbO. Geometries, dipole moments, and infrared data are presented. For comparison, nonrelativistic, first-order perturbation and relativistic effective core potential calculations have also been carried out. Where appropriate, the results are compared with the experimental data and previous calculations. Spin–orbit effects are of great importance for PbO, where first-order perturbation theory including only the mass–velocity and Darwin terms is inadequate to predict the relativistic corrections to the properties. The relativistic effective core potential results show a larger deviation from the all-electron values than for the hydrides and confirm the conclusions drawn on the basis of the hydride calculations.
Molecular dynamics simulations in zeolites: From deterministic to random motion
Authors: El Amrani S. , Kolb M.
Published: July 2002

Molecular dynamics (MD) simulations of argon in silicalite were performed in order to study the transition from the deterministic motion at short times to the random motion at long times. A characteristic length λ (time τ) is associated with this change in dynamics. At a temperature T=229(14) K, we find λ=0.45(8) Å [τ=0.21(6) ps]. A detailed analysis of the trajectories shows that the optimal choice for the elementary time step Δt in MD is slightly below τ. Little is gained by decreasing Δt as two trajectories with almost identical initial conditions diverge exponentially fast with time. A MD algorithm with variable Δt is proposed and used to show that the size of the zeolite channels and cavities influences how the asymptotic Gaussian random process is reached.
A direct method of determining the coupling between internal molecular motions and transport properties: Application to liquid n-butane
Authors: Travis Karl P. , Brown David , Clarke Julian H. R.
Published: July 2002

The coupling between internal molecular motions and transport properties is investigated with particular reference to the influence of torsional motions on the coefficients of self-diffusion and shear viscosity in n-butane. A novel technique, frozen distribution sampling (FDS), is designed to allow comparison of the dynamic properties of two fluids, one in which torsional motions are controlled by a well defined torsional potential, the other in which an equilibrium distribution of torsion angles are held completely rigid. It is shown that, as expected, the two fluids have exactly the same static equilibrium properties. The importance of satisfying the former criteria is illustrated by well chosen counter examples where related models of n-butane were studied in which the intramolecular potential was changed. In these cases interpretation of the relative values of transport properties is ambiguous since alterations of the potential cause significant changes in the equilibrium state. Our results show that there are no significant coupling effects between torsional motion and self-diffusion or shear viscosity at the state point considered.
A theoretical study of the HgAr2(3P1←1S) vibronic spectrum
Authors: Zúñiga José , Bastida Adolfo , Requena Alberto , Halberstadt Nadine , Beswick J. Alberto
Published: July 2002

A quantum mechanical calculation of the vibronic spectrum of the HgAr2 van der Waals cluster in the region of the Hg(3P1←1S0) electronic transition is presented and compared with experiments. The potential energy surfaces for the ground and excited states are obtained using available empirical Ar–Ar and Ar–Hg potentials. For the ground electronic state, the potential is written as a sum of pairwise Ar–Ar and Ar–Hg(1S0) interactions. On the contrary, for the electronically excited states correlating to Hg(3P1), an axis switching rotation has to be applied to each individual Ar–Hg(3P1; Ωe=0, ±1) interactions in order to define a common quantization axis. This results in a nonpairwise additive diabatic matrix Hamiltonian which after diagonalization provides the adiabatic excited potential energy surfaces. The vibrational wave functions associated to the ground and the first two (Ã and B̃+) adiabatic electronically excited states are obtained by variational techniques using basis sets along Jacobi coordinates. Analysis of the wave functions reveals that the normal mode description is valid only for the very few first levels and that strong mode mixing prevails through all the rest of the spectrum. For the B̃+ state, the calculated and observed spectra compare well, while for the Ã state, some discrepancies are found. Possible reasons for these differences are discussed.
Low-energy vibrational modes of the monolayer adsorbate CO2/NaCl(001)
Authors: Lange G. , Toennies J. P. , Vollmer R. , Weiss H.
Published: July 2002

Dispersion curves of surface vibrational modes of monolayer CO2/NaCl(001) have been measured by inelastic He atom scattering along the ΓX azimuth. Eight phonon modes could be followed across almost the entire Brillouin zone. The identification of the modes is discussed based on previously calculated normal mode energies at the zone origin.
Reduction of the two-dimensional master equation to a Smoluchowsky type differential equation with application to CH4→CH3+H
Authors: Robertson S. H. , Shushin A. I. , Wardlaw D. M.
Published: July 2002

A master equation (ME) approximation describing the effect of vibrational/rotational (V/R) relaxation on unimolecular reactions in the gas phase is presented. It is demonstrated that some forms of ME can be transformed exactly to an effective two-dimensional Smoluchowsky type differential equation (SE) in the V/R energy variables, V and R. The SE allows interpretation in a unified way of both weak and strong collision limits. Analytical expressions for the unimolecular reaction rate coefficient ku(T) are derived for simple models of the energy dependence of the microscopic reaction rate k(V,R) and V/R density of states. For realistic k(V,R) (obtained from flexible transition state theory for the reaction CH4→ArCH3+H) the SE is solved numerically. Both numerical and analytical calculations show that the collisional rotational energy transfer can, in principle, affect the rate coefficient ku(T). However, for the particular reaction considered, ku is found to be less sensitive to changes in the average rotational energy transfer than to changes in the average vibrational energy transfer. Furthermore, the strong collision limit is achieved only for unrealistically large values of the average rotational energy transfer. This means that it is a good approximation in this case one can treat V/R energy transfer and dissociation by a one-dimensional master equation in total energy.
An iterative Bloch approach to the resonance state problem
Authors: Périé Jeanne , Jolicard Georges , Killingbeck John P.
Published: July 2002

This paper proposes a new Gauss–Seidel Bloch formulation of the degenerate eigenvalue problem. The algorithm is designed to be applicable to large vector spaces; it only requires the presence in core memory of the few vectors which constitute the degenerate subspace. The theory is applied to the resonance states of the linear van der Waals complexes I2–X(X=Ar,Ne,He). Partial widths and branching ratios are determined by analyzing the asymptotic outgoing flux transported by the quasibound states in the various open channels. The comparison with previous close-coupling results reveals the efficiency of the method for resolving the resonance eigenvalue problem.
The experimental determination of vibrational transition moments for HF dimer
Authors: Laush Curtis , Lisy James M. , Huisken Friedrich , Kulcke Axel
Published: July 2002

The vibrational transition moments for the proton acceptor and proton donor HF stretches in (HF)2 have been measured by vibrational predissociation spectroscopy under saturated conditions in a molecular beam using mass spectrometric detection. The proton acceptor transition moment shows only slight enhancement (∼10%) over the HF monomer value of 0.0997 D. The proton donor transition moment is substantially enhanced by a factor of ∼2, as a result of hydrogen bonding. The enhancements are in excellent agreement with predictions from ab initio calculations.
Pulsed and continuous wave electron nuclear double resonance patterns of aquo protons coordinated in frozen solution to high spin MN2+
Authors: Tan Xiaoling , Bernardo Marcelino , Thomann Hans , Scholes Charles P.
Published: July 2002

For the water protons that coordinate to Mn2+, the frozen solution ENDOR (electron nuclear double resonance) spectra are made complex by the anisotropic electron–proton hyperfine interaction and by multiple contributions of the electron spin 5/2 manifold. A spin 5/2 Mn2+ ion having magnetic quantum numbers Ms=±1/2, ±3/2, ±5/2 and small zero-field splittings has overlapping electron spin EPR transitions. Proton hyperfine couplings to each of these electron spin states have yielded overlapping ENDOR patterns whose interpretation is nontrivial, even in so simple a system as Mn2+ ion having hexaaquo coordination. We have experimentally obtained and theoretically explained these proton ENDOR patterns and in so doing have laid the foundation for interpreting and sorting out frozen solution ENDOR patterns in more complex (enzyme) environments. Pulsed and cw ENDOR experiments showed features of metal-coordinated water protons occurring not only within a few MHz of the free proton frequency (as will happen for an electron spin 1/2 system) but extending over a range of up to 35 MHz. The EPR line of the Mn2+ S=5/2 manifold was broadened by zero-field splitting for hundreds of Gauss away from g=2.00, and the relative intensity of different ENDOR features reflected couplings to differing Ms spin states at varying fields across this EPR line.

An expression was derived to show the dependence of proton ENDOR frequencies on the electron spin quantum number, Ms, upon the principal values of the intrinsic proton hyperfine tensor, upon the direction of the magnetic field, and upon the free proton frequency. This expression provided the starting point for powder simulations of the overall ENDOR pattern. These ENDOR powder pattern simulations were not elementary extensions of first-order theory as often applied to single-crystallike ENDOR spectra obtained at frozen solution EPR extrema. These simulations addressed the interpretation and the conditions for obtaining consistent hyperfine information from nuclei hyperfine coupled to Mn2+ where all Ms levels of the Mn2+ ion can furnish contributions to ENDOR and EPR. The aquo protons yielded an isotropic coupling As=0.8 MHz and a dipolar coupling Ad=3.4 MHz, where A∥=2Ad+As (=7.6 MHz) and A⊥=As−Ad (=−2.6 MHz). Such couplings are in agreement with those obtained by proton ENDOR of [Mn(H2O)6]2+ in single crystals [R. DeBeer, W. DeBoer, C. A. Van’t Hof, and D. Van Ormondt, Acta Cryst. B29, 1473 (1973)].
Constant pressure–constant temperature simulations of liquid water and carbon dioxide
Authors: Palmer Bruce J. , Garrett Bruce C.
Published: July 2002

Rigid and flexible models of liquid water and carbon dioxide are simulated using Andersen–Nosé constant pressure–constant temperature molecular dynamics. Both equilibrium and dynamical properties are investigated and compared with constant energy simulations of the same models. The results of the constant pressure–constant temperature simulations are in very close agreement with the constant energy simulations, even for dynamical quantities.
Microwave spectrum of toluene⋅SO2: Structure, barrier to internal rotation, and dipole moment
Authors: Taleb-Bendiab Amine , Hillig Kurt W. , Kuczkowski Robert L.
Published: July 2002

The microwave spectrum of toluene⋅SO2 was observed with a pulsed beam Fourier-transform microwave spectrometer. The spectrum displays a-, b-, and c-dipole transitions. The transitions occur as doublets arising from the internal rotation of the methyl group. The transitions were assigned using the principal-axis method (PAM) internal rotation Hamiltonian with centrifugal distortions. Assuming a threefold symmetry for the internal rotation potential, the barrier height was determined as V3=83.236(2) cm−1. The torsional–rotational spectra of toluene-CD3⋅SO2 and toluene-d8⋅SO2 were also assigned. Additional small splittings of the c-dipole transitions for the normal species and toluene-CD3⋅SO2 suggest a reorientation tunneling motion of SO2 with respect to the aromatic plane. The moment of inertia data show that the two monomer units are separated by Rcm=3.370(1) Å, with the SO2 located above the aromatic ring. The projection of the C2 axis of SO2 on the aromatic plane makes an angle of τ=47.0(1)° with the C3 axis of toluene. The dipole moment of the complex is μT=1.869(27) D.
Effect of pressure on molecular photodissociation in matrices: Molecular dynamics simulations of Cl2 in Xe
Authors: Alimi R. , Apkarian V. A. , Gerber R. B.
Published: July 2002

A theoretical study is presented on the photodissociation dynamics of Cl2 in crystalline xenon at 100 K, and within a range of pressure between 0 and 100 kbar. Temperature/pressure ensemble molecular dynamics simulations were carried out. The potentials used were accurate enough to reproduce the experimental equation of state of solid xenon. The results show that the photodissociation quantum yield varies strongly with pressure, falling from 30% at zero pressure, to 2% at 12.5 kbar, and 0% at higher pressures. These yields are in good agreement with experimental measurements. This behavior is found to be due to the strong effect of pressure on the librational (rotational) amplitudes of the Cl2 molecule and to the sharp dependence of the photodissociation yield on the molecular orientation in the reagent cage.
NH3D+ ions in ammonium perchlorate: The N–D stretching bands of different sites
Authors: Weier Jane E. , Strauss Herbert L.
Published: July 2002

The N–D stretching region of the infrared spectrum of doped NH4ClO4 has been reexamined at low temperatures. The spectra are found to be still changing at 2 K, indicating the existence of energy differences of this order. The N–H spectrum in ND4ClO4 was also examined. Comparison of the two sets of spectra led to an assignment of the small energy differences as due to tunneling among the distinct positions the NX3Y+ ions can assume in the lattice. The distinct orientations of NH3D+ and the ND3H+ ions have slightly different energies and this, plus the tunneling, accounts for all the eight observed lines. A complete assignment of the tunneling levels will, however, require more information.
Large CO2 clusters studied by infrared spectroscopy and light scattering
Authors: Disselkamp Robert , Ewing George E.
Published: July 2002

Large CO2 clusters were formed by introducing room temperature gaseous mixtures of CO2 in argon into a cryogenic cell at 77 K. Rapid cooling of each mixture resulted in a highly supersaturated CO2 concentration, giving rise to homogeneous nucleation and thus cluster formation [F. F Abraham, Homogeneous Nucleation Theory, Advances in Theoretical Chemistry, Supplement 1 (Academic, New York, 1974), and references therein]. Experimental results will be presented here for CO2 in argon dilutions of 1:104, 1:2×105, and 1:106. Light scattering and infrared absorption techniques have been combined to estimate an average cluster radius of 0.20 μm for the 1:104 dilution sample, and an upper limit in cluster radius of 0.10 μm for the 1:2×105 dilution sample. Therefore, the higher dilution CO2:Ar mixtures led to the formation of smaller cluster sizes. Infrared structure in the ν3-asymmetric stretching region of the clusters will be discussed.

The quantum mechanical exciton model and the classical Mie model are only partially successful in explaining these experimental observations. Weak absorption features have been assigned to the naturally abundant 13C16O2 and 12C16O18O isotopes. The infrared structure attributed to these minority isotopes is relatively invariant with cluster size formed, and can be explained by the exciton model. This analysis suggests that, whatever overall shape the clusters have assumed, the CO2 molecules within the clusters have separations and orientations like those in the bulk crystal. Infrared spectra were collected at regular intervals over a period of 4 h for each sample. A monotonic decrease in the integrated infrared absorbance of the clusters with time was observed, with a characteristic half-life of 65, 180, and 230 min for the 1:104, 1:2×105, and 1:106 dilution samples, respectively. The average cluster radii obtained from light scattering and infrared absorption measurements have been used to calculate a sedimentation half-life of 60 and 180 min for the 1:104 and 1:2×105 dilution samples, respectively, which agreed well with the observed disappearance times. A cluster radius of 0.09 μm for the 1:106 dilution experiment is consistent with its sedimentation half-life of 230 min.
Atom–cluster interaction potentials and thermal collision rates
Authors: Hu Xiche , Martens Craig C.
Published: July 2002

A point-continuum model for evaluating atom–cluster interaction potentials is developed and applied to the calculation of capture cross sections and thermal association rate constants for atom–cluster association in the framework of the Langevin model. The approach is illustrated by treating the initial capture step in the association reaction of an iodine atom and iodine-doped van der Waals cluster I+I(Ar)n→I2+n Ar. Two cluster sizes n=12 and 54 are considered. The cluster structures correspond to an initial icosahedral geometry of Ar atoms surrounding a central I impurity, denoted Ar6(I)Ar6, and Ar27(I)Ar27 for n=12 and 54, respectively. For I+Ar6(I)Ar6 at lower relative translational energies, Langevin capture cross sections predicted by the point-continuum model agree with classical trajectory results within statistical uncertainty. However, as the relative translational energy increases, the predicted capture cross sections start to deviate slightly from the classical trajectory values. For the larger cluster size (n=54), the predicted capture cross sections and collisional rate coefficient are in quantitative agreement with the predictions of classical trajectory simulation, illustrating the emergence of bulk-like continuum behavior as cluster size is increased.
The electronic spectrum of chlorofluorocarbene
Authors: Karolczak J. , Joo D. L. , Clouthier Dennis J.
Published: July 2002

A vibronically and partially rotationally resolved electronic spectrum of chlorofluorocarbene has been observed using the technique of pyrolysis jet spectroscopy. A vibrational analysis has established the origin of the electronic transition to be at 25 277.8 cm−1. The pattern of isotope effects for the CF35Cl and CF37Cl species was used to confirm the vibrational assignments. The rotational analysis of three bands established that the transition moment is perpendicular to the molecular plane and can be assigned as Ã 1A″–X̃ 1A′. The ground and excited state geometries were derived from the rotational analysis and compared to ab initio predictions. The transition was found to be the direct analog of the 600 nm band system of dichlorocarbene.
Intermolecular bonding and vibrations of 2-naphthol⋅H2O (D2O)
Authors: Schütz Martin , Bürgi Thomas , Leutwyler Samuel , Fischer Thomas
Published: July 2002

A combined experimental and theoretical study of the 2-naphthol⋅H2O/D2O system was performed. Two different rotamers of 2-naphthol (2-hydroxynaphthalene, 2HN) exist with the O–H bond in cis- and trans-position relative to the naphthalene frame. Using Hartree–Fock (HF) calculations with the 6-31G(d,p) basis set, fully energy-minimized geometries were computed for both cis- and trans-2HN⋅H2O of (a) the equilibrium structures with trans-linear H-bond arrangement and Cs symmetry and (b) the lowest-energy transition states for H atom exchange on the H2O subunit, which have a nonplanar C1 symmetry. Both equilibrium and transition state structures are similar to the corresponding phenol⋅H2O geometries. The H-bond stabilization energies with zero point energy corrections included are ≊5.7 kcal/mol for both rotamers, ≊2.3 kcal/mol stronger than for the water dimer, and correspond closely to the binding energy calculated for phenol⋅H2O at the same level of theory. Extension of the aromatic π-system therefore hardly affects the H-bonding conditions. The barrier height to internal rotation around the H-bond only amounts to 0.5 kcal/mol. Harmonic vibrational analysis was carried out at these stationary points on the HF/6-31G(d,p) potential energy surface with focus on the six intermolecular modes. The potential energy distributions and M-matrices reflect considerable mode scrambling for the deuterated isotopomers. For the a′ intermolecular modes anharmonic corrections to the harmonic frequencies were evaluated. The β2 wag mode shows the largest anharmonic contributions. For the torsional mode τ (H2O H-atom exchange coordinate) the vibrational level structure in an appropriate periodic potential was calculated. On the experimental side resonant-two-photon ionization and dispersed fluorescence emission spectra of 2HN⋅H2O and d-2HN⋅D2O were measured. A detailed assignment of the bands in the intermolecular frequency range is given, based on the calculations. The predicted and measured vibrational frequencies are compared and differences discussed.
Finite range scattering wave function method for scattering and resonance lifetimes
Authors: Jang Hyo Weon , Light John C.
Published: July 2002

A generic expression for the scattering wave function in terms of the full discrete spectral Green’s function on a finite range is used to obtain the ‘‘finite range scattering wave function (FRSW)’’ which is accurate over a finite range of the scattering coordinate. We show that the representation of the FRSW in a finite basis set can be used to compute the scattering matrix and related quantities when the interaction potential is also restricted to this range. Comparisons of numerical results for several model problems with those of other methods and with analytical results indicate that the FRSW method is very accurate when converged and requires comparable or less computation than other methods. The main difference between the present method and other variational scattering methods is that the real Green’s function is used and that the scattering wave function itself is calculated nonvariationally. Thus the FRSW can be used to solve quantum mechanical problems involving scattering wave functions over a finite range such as scattering theory, resonance studies, and photodissociation. Results of two implementations are presented. Both require only one representation of the real Green’s function in a finite basis. One requires energy dependent matrix elements, while the other does not.
Adsorption/desorption kinetics of H2O on GaAs(100) measured by photoreflectance
Authors: Carlson C. R. , Buechter W. F. , Che-Ibrahim F. , Seebauer E. G.
Published: July 2002

The mechanism of H2O adsorption on GaAs(100) has been elucidated by an adaptation of the photoreflectance (PR) technique for surface kinetic measurements. Being an optical method, PR is especially well-suited for probing weakly bonded adsorption systems where the pressures required for significant interaction (≳10−5 Torr) preclude the use of traditional electron or ion spectroscopies. H2O adsorbs through a physisorbed state. This species can desorb or react to a chemisorbed form, which in turn can desorb. Both the physisorbed and chemisorbed species are undissociated. We interpret exceptionally low values for the prefactors associated with the chemisorbed state in terms of an adsorbate-induced surface reconstruction.
One-electron and electron pair densities of first-row hydrides in momentum space
Authors: Wang Jiahu , Smith Vedene H.
Published: July 2002

The one-electron density, and intracule (relative motion p=p1−p2) and extracule [center of electron mass P=1/2(p1+p2)] pair densities for the first-row hydrides (LiH–FH) in their ground states have been calculated from both self-consistent-field (SCF) and configuration interaction (CI) wave functions. For each molecule, the anisotropic, radial, and projected one-electron and pair densities along (longitudinal) and perpendicular (transverse) to the molecular axis are displayed and analyzed. The moments of the one-electron and pair distributions in momentum space are also discussed. The effect of electron correlation on these momentum densities and moments have been studied.