bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of the chemical environment on the electronic structure and spectroscopic properties of Er3+ doped Cs3Lu2Cl9, Cs3Lu2Br9, and Cs3Y2I9
Authors: Lüthi Stefan R. , Güdel Hans U. , Hehlen Markus P.
Published: July 2002

Energies and intensities of 114, 101, and 76 f–f absorption transitions of Er3+ are determined by high-resolution spectroscopy in the closely related host lattices Cs3Lu2Cl9, Cs3Lu2Br9, and Cs3Y2I9, respectively. The observed trends in the energy-level structure reflect the increasing covalency and the length of the Er3+–X− bond. The decreasing Coulomb repulsion of the 4f electrons, spin–orbit coupling, and crystal-field potential reduces the energy splittings of the SL, SLJ, and SLJMJ states by 0.5%, 0.5%, and 25%, respectively, along the series Cl–Br–I. Energy-level calculations that include crystal-field and correlation crystal-field terms in the effective Hamiltonian, reproduce most of the experimentally found trends. Root-mean-square standard deviations of 18.0, 19.2, and 21.9 cm−1 are reached in least-squares fits to the experimental crystal-field energies. The f–f transition intensities increase along the series Cl–Br–I as a result of the decreasing energy of the f–d bands. In the iodide compound, where the first f–d bands are as low as 30 000 cm−1, this influence is especially pronounced for the f–f absorptions at higher energy. The quality of the wavefunctions obtained in the energy-level calculations is not sufficient to reliably calculate the relative absorption intensities of individual crystal-field components within a given multiplet transition. This deficiency is ascribed to small deviations of the actual coordination geometry of Er3+ from the C3v point group symmetry that was assumed in the calculation. Intensities are analyzed on the level of multiplet-to-multiplet transitions using the Judd–Ofelt formalism.
The rotational spectrum of chloryl chloride, ClClO2, in its ground vibrational state
Authors: Müller Holger S. P. , Cohen Edward A. , Christen Dines
Published: July 2002

Rotational spectra of the four main isotopomers of ClClO2 which together span the quantum numbers 10⩽J⩽77 and 0⩽Ka⩽34 have been studied in selected regions between 10 and 417 GHz. The in situ syntheses using reactions between FClO2 and either HCl or BCl3 are described. Rotational and centrifugal distortion constants have been derived and used for structure and harmonic force field calculations. The quadrupole coupling constants for both Cl nuclei have been determined and the molecular dipole moment has been derived from low field Stark effect measurements in the submillimeter region. The results are discussed in relationship to published results from a matrix-isolation study, from theoretical calculations, and from studies of related molecules.
Diffusion mechanisms of normal alkanes in faujasite zeolites
Authors: Clark Louis A. , Ye George T. , Gupta Amit , Hall Linda L. , Snurr Randall Q.
Published: July 2002

Molecular dynamics simulations of C1 through C14 n-alkanes in a siliceous faujasite zeolite are used to elucidate diffusion mechanisms. Additional simulations of the bulk liquids are conducted to compare the liquid and adsorbed phases. Heats of adsorption, diffusivities, and activation energies are compared with experimental values and agree well. Particularly good agreement is found between predicted self-diffusivities and those measured with pulsed field gradient nuclear magnetic resonance (NMR). Analysis of the chain conformations and motional frequencies indicates that the liquid and adsorbed phases are quite similar, most likely due to the open nature of the faujasite structure. Insights into the influence of local environment on molecular relaxation are also given. Siting results reveal an interesting progression of adsorption sites as the chain length is increased. Shifts of preferred adsorption sites are due to matching between the length scales of sorbate and adsorbent. Alkanes below C6 adsorb preferentially above the 4-rings and 6-rings inside the supercages. For longer chains, the preferred sites shift more toward the center of the supercages. For C8 and above, the molecules spend much of their time in the window regions. These window sites are separated from the supercage sites by unexpected orientational barriers. Consequently, diffusion of longer alkanes in faujasite type structures is not characterized by simple supercage-to-supercage hopping but by hopping between supercage and window sites.
Real time observation of hydrogen transfer: Femtosecond time-resolved photoelectron spectroscopy in the excited ammonia dimer
Authors: Farmanara P. , Radloff W. , Stert V. , Ritze H.-H. , Hertel I. V.
Published: July 2002

The energy flow in ammonia dimers excited to the electronic Ã state is analyzed by combining the femtosecond pump–probe technique and the photoelectron–photoion coincidence detection. We use ∼140 fs laser pulses (200 nm for excitation and 267 nm for ionization). For the dimer ion the photoelectron spectra change drastically from a rather broad shape (≳1 eV) at small delay times between pump and probe pulse to a rather narrow peak (0.25 eV) at some picoseconds. This is explained by the dynamics of an internal H-atom transfer in the electronic Ã state to an NH4…NH2 configuration. The measured photoelectron energies are consistent with ab initio potential energy surface calculations. The observed picosecond lifetime of the hydrogen-transfer state NH4…NH2 can be understood by a conical intersection with the charge-transfer state NH4+…NH2−.
Anions made of cations and dianions: [CsC9]− and [CsC7]−
Authors: Dreuw A. , Cederbaum L. S.
Published: July 2002

Recently, C92− dianions have been observed as products in the collision of 48 keV [CsC9]− ions with hydrogen or helium. Motivated by this observation we examined the structure of [CsC9]− anions in a detailed theoretical study. The geometries of the finally detected C92− dianions should be closely related to the intermediately produced [CsC9]− anions. We found stable isomers of [CsC9]− which possess “lightly embracing” structures reflecting the geometrical characteristics of the theoretically prognosticated stable C92− dianions. The examination of the smaller [CsC7]− anions confirmed these results: Lightly embracing isomers of [CsC7]− have been found to form stable anions.
Translational–rotational coupling in supercooled liquids: Heterodyne detected density induced molecular alignment
Authors: Hinze G. , Francis R. S. , Fayer M. D.
Published: July 2002

We present a new time domain technique for studying molecular orientational relaxation in viscous liquids. A molecular velocity gradient (acoustic disturbance) associated with a density change induced by weak absorption of a 1.06 μm excitation pulse, causes molecular alignment through translational–rotational coupling. Using an optical heterodyne detection method, molecular orientational relaxation is monitored. An eightfold experimental cycle, analogous to phase cycles in NMR, is used to separate the DIHARD signal (density induced heterodyne amplified rotational dynamics) from optical Kerr effect (OKE) contributions and thermal lensing effects. Calculations combining the Navier–Stokes equation with translational–rotational coupling are presented that describe the nature of the method. The method is analyzed theoretically and demonstrated with experiments on supercooled salol (phenyl salicylate). DIHARD experiments on salol combined with heterodyne detected OKE experiments are used to examine long time scale orientational relaxation over a wide range of times and temperatures. While OKE experiments measure the time derivative of an orientational correlation function, it is shown that DIHARD directly measures the time dependence of an orientational correlation function. The experimental results are compared to those previously reported in the literature, which were obtained with other methods.
The structure and electronic state of photoexcited fullerene linked with a nitroxide radical based on an analysis of a two-dimensional electron paramagnetic resonance nutation spectrum
Authors: Mizuochi Norikazu , Ohba Yasunori , Yamauchi Seigo
Published: July 2002

An electron paramagnetic resonance (EPR) study of 3,4-fulleropyrrolidine-2-spiro-4′[2′,2′,6′,6′-tetramethyl]piperidine-1′-oxyl (1) was performed on the photoexcited quartet state in toluene glass. The spectrum of the |S,Ms〉=|3/2,±3/2〉⇔|3/2,±1/2〉 transitions was observed selectively by using a two-dimensional (2D) nutation method and analyzed with a spectral simulation in a randomly oriented system. A position of the nitroxide moiety was determined with respect to the zero-field splitting (zfs) axes of excited triplet fullerene (3C60) by taking into account of the dipolar–dipolar interaction between the radical and 3C60, the hyperfine coupling, the anisotropic g-value of the nitroxide radical, and the zfs of the 3C60 moiety. It was found that none of the zfs axes of the 3C60 moiety coincide with the local C2 axis of the molecule which is defined by the position of addition. A symmetry of the electronic structure in 3C60 is discussed on the basis of the result.
Fourier transform microwave spectroscopy of jet-cooled ZrO2 produced by laser vaporization
Authors: Brugh Dale J. , Suenram Richard D. , Stevens Walter J.
Published: July 2002

The pure rotational spectrum of the asymmetric top ZrO2 has been collected using a Fourier transform microwave spectrometer that employed a laser ablation molecular beam source. Four rotational transitions for each of five Zr16O2 isotopomers have been recorded. The rotational constants of the 90Zr16O2 isotope were determined to be A=19 881.352 MHz±0.068 MHz, B=7693.895 MHz±0.021 MHz, and C=5533.111 MHz±0.036 MHz. The r0 structure was determined to possess a Zr–O bond length of 1.7710 Å±0.0007 Å, and an O–Zr–O bond angle of 108.11°±0.08°. The electric dipole moment has been measured for the 90Zr16O2 isotope and found to be μb=26.02×10−30 C⋅m±0.07×10−30 C⋅m [7.80 Debye±0.02 Debye]. The nuclear quadrupole hyperfine structure for the 91Zr16O2 isotope has also been recorded and analyzed, yielding χaa=115.94 MHz±0.16 MHz, χbb=−37.55 MHz±0.33 MHz, and χcc=−78.39 MHz±0.16 MHz. High-level density functional theory calculations yield a structure that agrees well with the values determined experimentally. Several new transitions assigned to Zr16O were also recorded in the course of this study, and these were analyzed to extract values of Be, αe, and γe for four isotopomers.
Analysis of exchange energy at long range for states of alkali diatomic molecules correlating to two ground state atoms
Authors: Zemke Warren T. , Stwalley William C.
Published: July 2002

Recently experimentally-based asymptotic exchange energies ΔVE between ground state atoms in Li2, Na2, K2, and NaK have been determined from spectroscopically-determined potential energy curves. These empirical results are shown to be well fit by the expression −CRαe−βR of Smirnov and Chibisov, where C is a positive constant and α and β can be readily calculated from the atomic ionization energies. The expression should be useful for estimating the exchange energy in other alkali dimers.
A coupled-cluster study of the HOBr→HBrO transition state
Authors: Li Zhuangjie , Francisco Joseph S.
Published: July 2002

The structural and energetic properties of the HOBr→HBrO transition state are examined using the single and doubles coupled-cluster method that includes a perturbational estimate of the effect of connected triple excitations [CCSD(T)]. The energy change for the isomerization reaction is best estimated to be 56.5 kcal mol−1 endothermic, and the activation energy for the process is 75.0 kcal mol−1.
Electronic isomers in [(CO2)nROH]− cluster anions. II. Ab initio calculations
Authors: Saeki Morihisa , Tsukuda Tatsuya , Iwata Suehiro , Nagata Takashi
Published: July 2002

Ab initio MO calculations have been performed for the [(CO2)nROH]− (R=H and CH3) anions with n=1 and 2. Three stable structures are found for [(CO2)H2O]−, and two structures for [(CO2)CH3OH]−. All the [(CO2)ROH]− structures are characterized by the charge localization on the CO2 moiety, which interacts with ROH through an O–H⋯O linkage. It is also revealed that the addition of ROH to CO2− leads to the formation of a potential barrier against autodetachment higher than that of a bare CO2−, which results in the increasing stability of [(CO2)ROH]− species. For n=2 the calculations predict the existence of two types of isomers having different degrees of the excess electron localization: CO2−⋅ROH(CO2) and C2O4−⋅ROH isomers. These “electronic isomers” are calculated to be close in energy, while their calculated vertical detachment energies (VDEs) differ by more than 1 eV. The ab initio results are discussed in comparison with recent experimental ones derived from photoelectron spectra of [(CO2)nROH]−.
Quantum-classical molecular dynamics simulation of femtosecond spectroscopy on I2 in inert gases: Mechanisms for the decay of pump–probe signals
Authors: Ermoshin V. A. , Kazansky A. K. , Engel V.
Published: July 2002

A mixed quantum–classical method is proposed to describe the dynamical behavior of a diatomic molecule in a gas environment. The vibrational coordinate is treated quantum-mechanically and all other degrees of freedom classically. Within the present approach the classical equations for the rotational motion have no singularities. A symplectic, energy conserving and time-reversible algorithm is used for the propagation. As an application we treat the dynamics of I2 molecules excited by femtosecond laser pulses moving in collision-free and high pressure rare-gas environments. For freely rotating I2 molecules, the thermal average over rotational states leads to the decay of the pump–probe signal. For I2 in inert gases, we show that dephasing by collisions with the buffer gas is a weak effect in comparison with the decay of the signal due to the anharmonicity of the potential energy curve. Therefore the oscillating structure of the pump–probe signals depends weakly on the mass and the pressure of the solvent, in agreement with experimental data.
Molecular model of hydrophobic solvation
Authors: Silverstein Kevin A. T. , Haymet A. D. J. , Dill Ken A.
Published: July 2002

The physical basis for the “hydrophobic effect” is studied using a simple statistical mechanical model of water, the “MB” model, in which water molecules are represented as Lennard-Jones disks with hydrogen bonding arms. Using a four-state framework developed by Muller [Acc. Chem. Res. 23, 23 (1990)], and extended by Lee and Graziano [J. Am. Chem. Soc. 118, 5163 (1996)], we find the model reproduces the fingerprints of hydrophobicity, namely, the large positive heat capacity, and temperatures TH and TS at which the enthalpy and entropy of transfer, respectively, are zero. Further, the behavior can be interpreted readily in terms of hydrogen bonds that are either made or broken in the bulk or in the first solvation shell around a nonpolar solute. We find that inserting a nonpolar solute into cold water causes ordering and strengthening of the H bonds in the first shell, but that the reverse applies in hot water. This provides a physical interpretation for the crossover temperatures TH and TS.
The chemistry of superheavy elements. III. Theoretical studies on element 113 compounds
Authors: Seth Michael , Schwerdtfeger Peter , Fægri Knut
Published: July 2002

The chemistry of element 113 is investigated by theoretical methods. The results of fully relativistic calculations for (113)H and (113)F are compared with those derived by other techniques to obtain an indication of the accuracy of the more approximate models as well as the importance of including scalar and/or spin–orbit relativistic effects. Both of these effects are found to be important. The spin–orbit coupled pseudopotential approximation yields results of satisfactory accuracy, but the two relativistic methods that do not include spin–orbit coupling (Douglas–Kroll and scalar relativistic pseudopotential method) do not agree so well with each other. The calculated properties of (113)H and (113)F and a number of other hydrides and halides of element 113 are compared with the properties of the equivalent compounds of the lighter group 13 elements. In general, element 13 exhibits behavior that is consistent with its placement in group 13 of the periodic table. Some of its properties are found to be somewhat unusual however, e.g., the element is relatively electronegative, the molecules (113)H3, (113)F3, and (113)Cl3 are predicted to be T-shaped rather than trigonal planar, and the 6d electrons of element 113 participate to a significant extent in chemical bonding. Compounds where element 113 is present in the +5 oxidation state are considered as well but are predicted to be thermodynamically unstable.
Electrical response in chemical potential equalization schemes
Authors: Chelli Riccardo , Procacci Piero , Righini Roberto , Califano Salvatore
Published: July 2002

In this paper we compare the polarization response given by two different chemical potential equalization schemes to be applied to molecular dynamics simulations: the standard fluctuating point charge model (FQ) and the atom–atom charge transfer model (AACT). We have tested the transferability of FQ and AACT parameters, fitted to the polarizability of small size alkanes and polyenes, to large size homologues. We show that the FQ scheme is not adequate for the n-alkanes as it strongly overestimates the polarizability tensor components as the number of carbon atoms increases. The FQ approach has been found more predictive for highly conjugated systems like polyenes, although still unsatisfactory. The AACT parameters tuned on ethane are instead perfectly transferable to alkanes of any length and conformation. The AACT scheme satisfactorily reproduces the polarization response also for highly conjugated systems.
Mapping volume scale for overlapping clusters
Authors: Reiss Howard , Bowles Richard K.
Published: July 2002

The use of the observed frequency of appearance of a cluster in a small simulation cell, immersed in a macrosystem, to specify the number of such clusters in that macrosystem is discussed, for the special case in which the cluster consists of all the molecules in the cell and the volume of the cell is one of the defining parameters of the cluster. In particular, clusters consisting of ideal gas molecules are considered. Furthermore, the clusters are allowed to “overlap” in the sense that the same molecules may be shared by more than one cluster of the same or different type. Recently, the mapping of simulation cell frequencies onto the macrosystem, for this purpose, has been discussed for the case of rare nonideal, nonoverlapping clusters of the type of interest to nucleation theory and a difference of opinion has arisen with respect to the specification of a volume scale to be used in the mapping process. Two scales have been suggested. One scale is simply the volume of the simulation cell. The other is specified by Eq. (1) of the present paper. In this paper we prove that the scale for the overlapping ideal clusters is that of Eq. (1). Since with overlapping clusters it is easy to perform computer experiments that provide a direct count of the clusters in the macrosystem, the system, although having limited physical value, is still of considerable analytical value in deciding between the two scales and, as a corollary, between the two theories underlying these scales. The indicated experiments have been performed and are reported in a following paper. They confirm the applicability of Eq. (1).
Parallel wave-packet simulations of electron transmission through water
Authors: Peskin Uri , Edlund Åke , Bar-On Ilan
Published: July 2002

The dynamics of electron tunneling through water layers embedded between two metal plates is studied by electron wave-packet simulations. The tunneling flux is shown to increase by orders of magnitude due to resonances when the thermal motion of the water nuclei is “frozen” and transient molecular nanocavities dominate the tunneling mechanism. This enhancement is observed even when the energy width of the wave-packet is larger than the resonance width, and the transmission probability does not show resonance peaks as a function of the impact electron energy. The wave-packet simulations are based on a parallel solution of the multidimensional time-dependent Schrödinger equation, in which the N-dimensional Hilbert space is distributed into subspaces associated with an N-dimensional hypercube of processors. The propagated wave function is fully distributed at all times and the computation rate can increase linearly with the number of processors. The significant advantage of the present algorithm over serial algorithms is in the ability to increase the size of the propagated wave-functions without increasing the computation time by adding more processors.
Photodissociation of hydrogen halides in rare gas matrices, and the effect of hydrogen bonding
Authors: Lorenz Martin , Kraus Dieter , Räsänen Markku , Bondybey Vladimir E.
Published: July 2002

Spectra of hydrogen halides, their oligomers and complexes with the heavier rare gas atoms, xenon and krypton, and their photolysis were investigated. The monomers exhibit rotational spectra with a prominent broad R(0) line, and very sharp, weak Q(0) line. Their rotational constants are considerably lowered compared with the gas phase. The YH species hydrogen bond readily to form rigid oligomers or complexes with Xe or Kr, which in general exhibit sharp zero phonon line spectra of nonrotating molecules, with clearly resolved halogen isotopic structure. The nonrotating polymers photolyse at 193 nm more efficiently, but the YH⋯Rg complexes less efficiently than the monomers, with the photolysis of the dimers yielding BrH⋯Br “hydrogen dihalide” products. Annealing and diffusion result in formation of the HXeY rare gas compounds, and partial recovery of the YH monomers, but only very weak reformation of the (YH)n oligomers. The mechanisms of the photolysis, annealing, and diffusion processes are discussed.
Response to “Comment on ‘Quantum Monte Carlo study of the dipole moment of CO’ ” [J. Chem. Phys. 112, 4419 (2000)]
Authors: Schautz F. , Flad H.-J.
Published: July 2002

In our previous publication we have mistakenly claimed that the applicability of the Hellmann–Feynman theorem in fixed-node quantum Monte Carlo calculations is not subject to the manner how the nodal boundary depends on an external parameter. This statement is not correct in general, except where the Hellmann–Feynman force is calculated for a nodal boundary which coincides with that of the unconstrained exact eigenfunction. We point out the error in our arguments and present an explicit expression for the correction term which supplements the Hellmann–Feynman force. This term can be interpreted as the linear response of the energy with respect to variations of the nodal region.
Shielding polarizabilities via continuous transformation of the origin of the current density in the set of small molecules: H2O2, F2, H2C2, H2CO, NH3, HCN, and HNC
Authors: Caputo M. C. , Ferraro M. B. , Lazzeretti P.
Published: July 2002

A procedure, based on a continuous transformation of the origin of the (quantum mechanical) current density that sets the diamagnetic contribution to zero (CTOCD-DZ ) all over the molecular domain, is applied to determine shielding polarizabilities to first order in a perturbing electric field. In any calculations relying on the algebraic approximation, irrespective of size and quality of the (gaugeless) basis set employed, all the components of the CTOCD-DZ magnetic shielding polarizability are origin independent, and the constraints for charge and current conservation are exactly satisfied. The effects of a static uniform electric field on the nuclear magnetic resonance (NMR) shielding of H2O2, F2, H2C2, H2CO, NH3, HCN, and HNC molecules have been investigated within the CTOCD-DZ method, and compared with the conventional results evaluated via the same basis sets, and with theoretical results taken from the literature.
Reversible fluorescence quenching: Generalized Stern–Volmer equations on the basis of self-consistent quenching constant relations
Authors: Naumann Wolfgang
Published: July 2002

For irreversible fluorescence quenching in solution, it is shown that the kinetic prediction of the Smoluchowski approach, which is exact under target model conditions, can also be alternatively formulated in terms of well-defined non-Markovian rate equations. For the well-known superposition approximation, it is demonstrated that the definition of an approximate quenching constant by a self-consistent relation can also be formally transferred to the reversible quenching processes if only the low-density limits of the net forward rate kernels in the generalized rate equations are known. Fluorescence quenching by reversible excimer formation and by reversible excitation transfer meet this requirement due to the recent findings of several authors. It is demonstrated that the proposed quenching constant approximation procedure leads to nonlinear plots with positive curvature which correct the zeroth-order linear plots in the higher quencher concentration region. The influence of the yield-reducing back reaction effect is discussed.
Imaging organic device function with near-field scanning optical microscopy
Authors: McNeill Jason D. , O’Connor Donald B. , Barbara Paul F.
Published: July 2002

Recent developments in the use of near-field scanning optical microscopy-based techniques to monitor and image physical processes related to device function in thin film organic materials are surveyed. By combining subwavelength optical probes with electro-optical spectroscopic techniques, methods have been developed for measuring the dynamics, spatial variation, and diffusion of energy and charge carriers in organic electro-optical devices. Materials investigated include polymer thin films, polycrystalline and amorphous (glassy) organic films, liquid crystalline materials, and molecular semiconductor heterostructures.
Photoionization and photofragmentation of gaseous toluene using 80-fs, 800-nm laser pulses
Authors: Müller A. M. , Uiterwaal C. J. G. J. , Witzel B. , Wanner J. , Kompa K.-L.
Published: July 2002

This paper presents ion yields resulting from multiphoton ionization and fragmentation of gaseous toluene (C7H8) in the focus of an 80 fs Ti:sapphire laser beam (λ=800 nm) with a sufficiently small B-integral [Siegman, Lasers (University Science Books, Mill Valley, CA, 1986)]. The peak intensity was varied between 1.9×1013 and 2.8×1014 W cm−2, and both linear and circular polarization were used. Over the whole range of intensities studied, only the singly charged parent ion and its fragment, C7H7+, are found. Although the Keldysh adiabaticity parameter equals 0.86 for the saturation intensity of ∼1×1014 W cm−2, there is no indication of tunneling. The parent ion yield is found to be effectively proportional to the sixth power of the peak intensity. This is shown to be in good agreement with a multiple lowest-order perturbation multiphoton ionization model which takes into account successive channel closing for increasing peak intensities and orders up to 11 inclusive. On the assumption that the excess energy acquired by the toluene cation as a result of the interaction with the electromagnetic field is of the order of the ponderomotive energy for the intensity prevailing at the moment of the ionization, the internal energy distribution of the toluene cations created that is brought about by this multiple-order multiphoton ionization model is calculated. This internal energy distribution is in perfect agreement with the measured C7H7+ yield, if the rate-energy curve for the fragmentation of excited toluene cations as given by Golovin et al. [Sov. J. Chem. Phys. 2, 632 (1985)] is moderately reduced by a factor of 4.5.
Properties of atoms in molecules: Transition probabilities
Authors: Bader R. F. W. , Bayles D. , Heard G. L.
Published: July 2002

The transition probability for electric dipole transitions is a measurable property of a system and is therefore, partitionable into atomic contributions using the physics of a proper open system. The derivation of the dressed property density, whose averaging over an atomic basin yields the atomic contribution to a given oscillator strength, is achieved through the development of perturbation theory for an open system. A dressed density describes the local contribution resulting from the interaction of a single electron at some position r, as determined by the relevant observable, averaged over the motions of all of the remaining particles in the system. In the present work, the transition probability density expressed in terms of the relevant transition density, yields a local measure of the associated oscillator strength resulting from the interaction of the entire molecule with a radiation field. The definition of the atomic contributions to the oscillator strength enables one to determine the extent to which a given electronic or vibrational transition is spatially localized to a given atom or functional group. The concepts introduced in this article are applied to the Rydberg-type transitions observed in the electronic excitation of a nonbonding electron in formaldehyde and ammonia. The atomic partitioning of the molecular density distribution and of the molecular properties by surfaces of zero flux in the gradient vector field of the electron density, the boundary condition defining the physics of a proper open system, is found to apply to the density distributions of the excited, Rydberg states.
Photothermodissociation of selenium clusters
Authors: Bréchignac C. , Cahuzac Ph. , Kébaïli N. , Leygnier J.
Published: July 2002

The dissociation of selenium cluster ions containing from 7 up to 30 atoms has been studied using unimolecular decay of photothermoexcited clusters. Clusters containing more than 14 atoms evaporate Se6, Se7, and Se8 species, whereas smaller clusters with 7–10 atoms dissociate manly by Se2 loss. When the size increases, Se6 becomes the predominant channel showing a dissociation which evolves toward the bulk behavior. The changes in the observed channels correlate to changes in the dissociation energies which are deduced from the measured fractional dissociation rates and from the entropy contribution. Together with the previous study of tellurium clusters, this work provides comparative behavior of dissociation energies of group VI clusters and their convergence toward bulk properties.
Theoretical investigation of Ca⋅RG, Ca+⋅RG, and Ca2+⋅RG (RG=Ar and Ne) complexes
Authors: Kirschner K. N.
Published: July 2002

The ground state structure, harmonic frequency, and dissociation energy for Ca⋅RG, Ca+⋅RG, and Ca2+⋅RG (RG=Ar and Ne) complexes are computed at four theoretical levels [HF, B3LYP, MP2, and MP2(full)] using three different basis sets. The most rigorous method employed was Møller–Plesset second order perturbation with valence plus core electron correlation using 183 basis functions for the calcium–neon complexes and 187 basis functions for the calcium–argon complexes. Correcting the dissociation energies, bond distances, and frequencies for basis set superposition error (BSSE) were done at the most rigorous level of theory by fitting the Morse function to the potential energy curves generated by the counterpoise procedure. At this level of theory, proceeding from the neutral to the doubly charged complexes, the calcium–neon bond distances range from 5.40 to 2.45 Å with dissociation energies (De) from 0.03 to 5.86 kcal/mol. Likewise, the calcium–argon bond distances range from 5.00 to 2.70 Å with dissociation energies from 0.23 to 16.80 kcal/mol as the metal charge increases. Good theoretical agreement is obtained with experimental data when available, while the remaining results can aid in the interpretation of future experiments. In all comparable cases where the calcium–rare gas complexes possess equivalent charge, the argon atom is bound tighter to the metal than the neon atom due to its larger atomic polarizability. An examination of the relationship between dispersion and charge-induced dipole forces is done using these calcium–rare gas complexes.
A theoretical study of the reaction of Ti+ with ethane
Authors: Moc Jerzy , Fedorov Dmitri G. , Gordon Mark S.
Published: July 2002

The doublet and quartet potential energy surfaces for the Ti++C2H6→TiC2H4++H2 and Ti++C2H6→TiCH2++CH4 reactions are studied using density functional theory (DFT) with the B3LYP functional and ab initio coupled cluster CCSD(T) methods with high quality basis sets. Structures have been optimized at the DFT level and the minima connected to each transition state (TS) by following the intrinsic reaction coordinate (IRC). Relative energies are calculated both at the DFT and coupled-cluster levels of theory. The relevant parts of the potential energy surface, especially key transition states, are also studied using multireference wave functions with the final energetics obtained with multireference second-order perturbation theory.
Laser probing of rotational-state-dependent velocity distributions of N2+ (ν″=0,J) drifted in He
Authors: Anthony Eric B. , Bastian Michael J. , Bierbaum Veronica M. , Leone Stephen R.
Published: July 2002

Rotational state-resolved velocity component distribution functions of N2+ (ν″=0) drifted in helium are measured in a flow-drift apparatus by the technique of single-frequency laser-induced fluorescence (LIF). A single-mode ring dye laser is used to probe Doppler profiles of various rotational lines in the R-branch of the (ν′,ν″)=(0,0) band of the B 2Σu+–X 2Σg+ system at 390 nm, with the laser propagation direction set either parallel or perpendicular to the drift field. A small but definite (3%) increase in ion mobility with increasing rotational state from J=13.5 to J=22.5 is observed at a fixed field strength of 12 Td. Mobilities of J=15.5 measured over the range of 1.5–16 Td yield a K0(0) of 22.0 cm2 V−1 s−1 and are in good agreement with earlier arrival-time measurements. Parallel translational temperatures are found to be significantly higher than perpendicular temperatures; a difference of at least 140 K between these temperatures is measured for J=15.5 at 16 Td. No discernible difference between the parallel translational temperatures for different rotational states is observed. There is evidence for a small degree of positive skewness (third central moment) in the parallel velocity component distributions, the first observation of such an effect in a molecular ion-atomic buffer system. Previous results that indicated poor agreement between CO+–He pulsed-field arrival-time and LIF mobilities are discussed; the LIF results were most likely hampered by space-charge effects.
Picosecond polarized fluorescence studies of anisotropic fluid media. I. Theory
Authors: Bain A. J. , Chandna P. , Bryant J.
Published: July 2002

Recent work has demonstrated the production of substantial molecular alignment in a free ethylene glycol jet. Using variably polarized photoselection, a range of initial nonequilibrium orientational distributions can be prepared and their subsequent relaxation monitored via polarization resolved time correlated single photon counting. The imposition of order in a fluid is seen to have a profound effect on molecular motion and a strong anisotropy in θ and φ diffusion is indicated. In this work (Papers I and II) we describe a detailed investigation of this phenomenon. Here (Paper I) we develop the formalism necessary to describe the interaction of polarized laser pulses with an anisotropic medium, allowing a full description of the initially photoselected distribution. The relaxation of the nonequilibrium distributions is considered via a perturbation treatment of the anisotropic rotational diffusion equation. New “selection rules” for orientational relaxation can be deduced from symmetry arguments and the form of the cylindrically symmetric (θ diffusion) and asymmetric alignment decays (φ diffusion) are predicted.
Excited states dynamics of polydiacetylenes: An ab initio and femtosecond spectroscopic investigation of the change from the acetylenic to the butatrienic structure
Authors: Turki Mohamed , Barisien Thierry , Bigot Jean-Yves , Daniel Chantal
Published: July 2002

The configuration change from the acetylenic form =(RC–C≡C–CR′=)x (x=1,2,3.5) to the butatrienic form –(RC=C=C=CR′–)x (x=1,2,3), considered as model systems for the two alternative structures of polydiacetylene chains, has been investigated through complete active space self-consistent field and second-order perturbation approach CASSCF/CASPT2 calculations. The character and energetics of the low-lying excited states of both structures are reported. The excited states properties of the oligomers are compared to those of the three-membered ring forms –(C≡C–CH=CH–)3 and –(CH=C=C=CH–)3. A qualitative interpretation of the femtosecond time-resolved molecular dynamics of the polydiacetylene backbone is proposed on the basis of wave packet propagations on associated potential energy curves connecting the electronic ground and excited states of the two structures in the cyclic form.
Effect of substrate vibrations on the sticking of atoms at surfaces: A critical comparison of different propagation methods for the H/Cu(100) system
Authors: Klamroth Tillmann , Saalfrank Peter
Published: July 2002

Several effects due to the coupling of the translational motion of a gas phase atom (H), to the vibrations of a substrate [Cu(100)], are investigated here with time-dependent wave packet methods. Three different propagation techniques, namely, reduced-dimensionality but “exact” wave packet propagation, the time-dependent-self-consistent-field (TDSCF) method, and the “mean-field” (Ehrenfest) mixed quantum-classical-molecular-dynamics scheme (QCMD), are tested against each other and compared with classical trajectory results, and with rigid-surface calculations. Our key findings are: (1) The Cu(100) substrate is very “open” for impinging H atoms, leading to large subsurface and bulk absorption yields; (2) the H atoms can be “hot” for several picoseconds after hitting the surface before they finally settle down in adsorption or absorption sites; (3) while classical mechanics agrees reasonably well with the exact quantum calculation, the mixed quantum-classical and TDSCF approaches which both rely on a single-configuration ansatz for the total nuclear wave function, grossly underestimate the coupling between the H atom and the surrounding Cu atoms; (4) all (approximate) methods agree in the fact that by taking more substrate vibrations into account, the reflection probability diminishes while sticking increases.
Relaxation in a supercooled polymer melt within the dynamically disordered Rouse model
Authors: Ilan Boaz , Loring Roger F.
Published: July 2002

Dielectric and rheological measurements on polymer melts have demonstrated that rates of dynamical processes on short length scales decrease more rapidly than relaxation rates associated with longer length scales as the melt is cooled towards its glass transition temperature. We demonstrate here that the dynamically disordered Rouse model is consistent with these observations and provides a simple physical picture of molecular motions in supercooled melts. In this coarse-grained model of the dynamics of a single molecule, the beads of a harmonic chain are characterized by mobilities that fluctuate between zero and a finite value at a rate that is determined self-consistently. We present calculations of two observables whose temperature dependence near the glass transition has revealed the inadequacies of the Rouse model: Je, the steady-state compliance, and ωs/ωn, the ratio of segmental and normal mode peak frequencies in the dielectric spectrum of a polar polymer.
Driven flow and pinning of molecular aggregates in a heterogeneous medium
Authors: Foo Grace M. , Pandey R. B.
Published: July 2002

Aggregation and flow of polymer chains (each of length Lc) on a heterogeneous surface are studied in presence of a field E using a hybrid simulation. Effects of field, heterogeneity (i.e., the barrier concentration pb), and temperature T on aggregation and desegregation of chains with low molecular weight is found to be different from that with high molecular weight. For low Lc, at low T=0.2, the impurity barriers act as seeds for pinning the growth of molecular aggregates that lead to larger aggregates at lower pb. At high temperature (T=1.0), in contrast, larger aggregates appear at higher pb where pinning of aggregates is augmented by cluster of clustering. For large Lc, orientational ordering with a molecular bridging occurs at low pb, while a nearly isotropic network of chains anchored by the barriers emerges at higher pb. The rms displacement of chain ranges from drift-like for short chains at low barrier concentration to strongly subdiffusive for long chains at high fields. A linear response of the flow rate density j to field j∼E is observed over low to moderate fields (E⩽1.0), high temperature (T⩾1), and low barrier concentrations (pb⩽0.1). The variation of the effective linear permeability φm of polymer with the field is nonmonotonic over the range 0.0<E<1.0, with decreasing trend at higher values of E. In the low field regime (E<0.2), the monomer permeability shows a power-law decrease with chain length φm∼Lc−α, α≃0.25–0.37.
Tunneling and decay dynamics of H̄ 1Σg+ outer well states in hydrogen
Authors: Reinhold E. , Hogervorst W. , Ubachs W.
Published: July 2002

We present an experimental study of the decay dynamics of rovibrational states confined in the outer minimum of the HH̄ 1Σg+ potential of the hydrogen molecule. Specific (v,J) quantum states are populated using a two-step excitation process involving a pulsed extreme ultraviolet laser source. Decay products are selectively probed with additional laser pulses, using the timing of these pulses to determine lifetimes. The competition between dissociation and ionization, occurring at short internuclear distance after tunneling through the potential barrier, is addressed. Observed lifetimes and decay channels of H̄ levels are consistent with a semiclassical description of tunneling of the nuclear motion through the potential barrier towards small internuclear distance. For the HD isotopomer breaking of the g−u symmetry is found to strongly affect the decay dynamics.
Crystallization of dipolar spheres: A discussion of second-order density functional theory
Authors: Klapp S. H. L. , Patey G. N.
Published: July 2002

We investigate the application of second-order density functional theory to the crystallization of dipolar hard and soft spheres. It is shown that the results are highly sensitive to the repulsive part of the pair potential, and to the symmetry of the fluid phase considered in the density functional theory. In general, the theoretical results for dipolar soft spheres (with r−12 repulsion) are found to be physically more reasonable than those obtained for dipolar hard spheres. In particular, the fluid–solid density gap is greatly reduced and lies in the range expected on physical grounds and from simulations. However, theories constructed by expanding about the isotropic fluid phase overestimate the stability of the ferroelectric solid for both models, and fail to predict the stable ferroelectric fluids found in computer simulations. We have carried out calculations using ferroelectric dipolar fluids with both perfect and fluctuating orientational order as “starting points” in the density functional expansion. The results demonstrate the high sensitivity of the theory to the underlying fluid state, and strongly indicate the existence of stable ferroelectric fluids in accord with computer simulations.
Electron-impact ionization of the chlorine molecule
Authors: Calandra Pietro , O’Connor Caroline S. S. , Price Stephen D.
Published: July 2002

Relative partial ionization cross sections for the formation of Cl2+, Cl+ and Cl2+ from molecular chlorine have been recorded as a function of the ionizing electron energy. In these measurements particular attention has been paid to the efficient collection of fragment ions with high translational energies and the minimization of any mass-dependent discrimination effects. The cross sections show that at electron energies above the double ionization threshold the yield of fragment ions can be comparable with the ion yield of nondissociative ionization. Further analysis shows that at electron energies above 50 eV the yield of fragment ions from multiple ionization is comparable with the yield of fragment ions from single ionization: dissociative multiple ionization contributes 14% of the ion yield at 50 eV electron energy and 26% at 100 eV. The decay of Cl22+ by heterolytic cleavage to form Cl2+ is a result of approximately 5% of the dissociative double ionization events. This heterolytic process has a threshold of 41.8±1.5 eV. Electron-impact induced triple ionization to form long-lived Cl23+ ions has been detected for the first time. This nondissociative triple ionization process makes up approximately 2% of the triple ionization events and triple ionization is responsible for approximately 2% of the ion yield above 100 eV. The threshold for dissociative triple ionization is determined to be 65.3±1.5 eV, a value in good agreement with a trication precursor state energy derived from the kinetic energy release for the fragmentation of Cl23+ to Cl2+ and Cl+, which provides the first experimental estimate of the triple ionization energy of molecular chlorine.
Coupled cluster investigation of Sternheimer shieldings and electric field gradient polarizabilities
Authors: Coriani Sonia , Halkier Asger , Jørgensen Poul , Gauss Jürgen , Christiansen Ove , Rizzo Antonio
Published: July 2002

A coupled cluster (CC) investigation is presented for the (generalized) Sternheimer shieldings and the electric field gradient (EFG) polarizabilities which describe the effect of external electric fields and field gradients on the electric field gradient at the nuclei. Calculations are performed for the linear molecules N2, CO, HF, C2H2, HCl, HCN, and HNC. Correlation effects are monitored by employing a hierarchy of CC models consisting of CCS, CC2, CCSD, and CC3. The effect of tight basis functions and core correlation is investigated by carrying out CCSD calculations with core-valence basis sets. Accurate theoretical estimates for EFGs, Sternheimer shieldings, and EFG polarizabilities are given and the effects of vibrational corrections are discussed. Our final estimates for the considered EFG properties can be used, for example, in simulations of electric field effects on the EFG at the nuclei in interacting molecules.
Analysis of the ultrafast photodissociation of electronically excited CF2I2 molecules by femtosecond time-resolved photoelectron spectroscopy
Authors: Farmanara P. , Stert V. , Ritze H.-H. , Radloff W.
Published: July 2002

Applying the femtosecond pump–probe technique combined with the photoelectron–photoion coincidence detection we have studied the time-resolved photoelectron spectra of CF2I2 and its fragments after excitation with 4.65 eV photons. The time-dependent photoion signals reflect the complete dissociation of the CF2I2 molecules with a time constant of (100±30) fs which is preceded by an ultrafast relaxation process with (30±10) fs. The analysis of the electron spectra reveals that three electronic states with different vibrational energies are populated by one photon excitation during the pump pulse. Furthermore, the number of absorbed pump and probe photons for higher order excitation, the ionization potential of CF2I2 and its binding energies in the ionic state have been determined by the electron spectroscopy. Both the ion signals as well as the electron spectra demonstrate that the observed products CF2, I2, and I are formed by dissociation of the excited CF2I2 molecules, but no CF2I has been detected in all experiments with widely spread laser parameters. Thus, we conclude the concerted reaction mechanism to be the dominant dissociation channel while the sequential decay with the CF2I intermediate is negligible. The measured long-living signals for I2+ are suggested as due to molecular detachment after absorption of two pump photons. The detected electron spectra for I+ at longer delay times reflect the formation of highly excited neutral iodine atoms by absorption of at least three pump photons.