bluuebunny/crossref_metadata_2025 huggingface dataset:
Evidence for polyicosahedral structure in ammoniated iron, cobalt, and nickel clusters
Authors: Parks E. K. , Winter B. J. , Klots T. D. , Riley S. J.
Published: July 2002

Experimental evidence is presented that small (19- to 34-atom) clusters of iron, cobalt, and nickel tend to adopt polyicosahedral structure when saturated with ammonia. Clusters in the polyicosahedral series (having 19, 23, 26, 29, 32, and 34 atoms) often saturate with 12 NH3 molecules, the expected number for polyicosahedral structure. Other clusters in this size range, which do not have an obvious number of NH3 binding sites, generally do not show a clear saturation. Adsorbate-induced structural changes are also seen, and suggest that for larger clusters in the series there is a transformation to an alternate structure that binds more than 12 NH3 molecules.
Quantum molecular interpretation of the absorption spectra of Na5, Na6, and Na7 clusters
Authors: Bonačić-Koutecký V. , Pittner J. , Scheuch C. , Guest M. F. , Koutecký J.
Published: July 2002

The configuration-interaction study of the excited states of the most-stable structures of Na5, Na6, and Na7 clusters employing ab initio effective-core potential corrected by the core polarization potential predicts spectroscopic patterns which are in good agreement with the measured depletion spectra. A comparison of the transition energies and the oscillator strengths with the experimental data makes possible tentative structural assignments. Planar Na5 and Na6 structures and the three-dimensional pentagonal bipyramid for Na7 appear responsible for the recorded spectra. The full agreement between theory and experiment is present for Na6 and Na7. The measured cross sections and calculated oscillator strengths for Na5 compare better for lower transition energies than for higher ones. A many-electron description of the excited states of Na5, Na6, and Na7 yields a complete quantum molecular interpretation of the absorption spectra. From a comparison of the experimental and theoretical results the conclusion has been drawn that a transition from planarity to the three dimensionality takes place for Na7.
Concentration and molecular weight regime diagram of salt-free polyelectrolyte solutions as studied by light scattering
Authors: Sedlák Marián , Amis Eric J.
Published: July 2002

Static and dynamic light-scattering measurements were carried out on salt-free aqueous sodium poly(styrene sulfonate) solutions. The concentration dependence of the solution behavior was investigated in the range c=0.01– 45.6 g/L for three samples with Mw=5000, 100 000, and 1 200 000. A critical concentration ccr≂0.5 g/L was experimentally established. Above ccr, two diffusion coefficients Df (fast) and Ds (slow) were measured. Df, corresponding to the coupled diffusion of polyions and counterions, is independent of concentration. Ds, corresponding to the presence of multichain domains in solution, increases upon dilution. The angular dependence of the scattering intensity as it reflects the size of domains becomes less pronounced upon dilution and, at c=ccr, the scattering intensity is independent of angle. The concentration dependence of the excess scattering intensity is stronger than linear (the reduced intensity I/c increases with concentration). Below ccr, Df decreases upon dilution and finally merges with Ds. The scattering intensity is first independent of angle, then upon dilution a broad maximum appears. The concentration dependence of the excess scattering intensity is weaker than linear (the reduced intensity I/c decreases with concentration). These experimental data indicate that, for cccr, the solution structure and dynamics are predominantly influenced by coupled polyion-counterion dynamics and the presence of attractive forces giving rise to domains. The critical concentration ccr is independent of molecular weight. A concentration molecular weight regime diagram is constructed combining the present data with previous results on the molecular weight dependence and with literature data on very dilute solutions.
An a
b i
n
i
t
i
o study of the structure and infrared spectrum of Si3C
Authors: Rittby C. M. L.
Published: July 2002

The ground state geometry and vibrational spectrum of the previously not studied silicon–carbon cluster Si3C has been investigated. Several possible isomeric structures are studied at the Hartree–Fock level. The energetically most favorable isomer is found to be a rhomboidal C2v structure with two equivalent silicon atoms and a transannular Si–C bond. Harmonic frequencies, isotopic shifts, and infrared intensities are calculated using many-body perturbation theory to second order and found to be in excellent agreement with recent experimental results supporting the identification of a new tetra-atomic silicon–carbon cluster.
Vibrational spectra of tetra-atomic silicon–carbon clusters. I. Rhomboidal Si3C in Ar at 10 K
Authors: Presilla-Márquez J. D. , Graham W. R. M.
Published: July 2002

The vibrational spectrum of Si3C has been observed for the first time in a Fourier transform infrared study of the products of the vaporization of carbon/silicon mixtures trapped in Ar at 13 K. Five of the six fundamental modes have been assigned: the symmetric breathing vibration, ν1(a1)=658.2 cm−1; the Siβ–Siα–Siβ symmetric deformation vibration, ν2(a1)=511.8 cm−1; the Siβ–C–Siβ symmetric deformation vibration, ν3(a1)=309.5 cm−1; the Siβ –C antisymmetric stretching vibration, ν5(b2)=1101.4 cm−1; and the Siα–Siβ antisymmetric stretching vibration, ν6(b2)=357.6 cm−1. The assignments are supported by 13C, 29Si, and 30Si isotopic data and are in excellent agreement with the predictions of an ab initio study carried out by Rittby in collaboration with this work. The results of force constant adjustment calculations are consistent with the ground state geometry established by the ab initio calculation, a rhomboidal structure of C2v symmetry, with carbon–silicon transannular bonding between the two equivalent Siβ atoms.
Theoretical study of the electronic structure of Ar++2
Authors: Cachoncinlle C. , Pouvesle J. M. , Durand G. , Spiegelmann F.
Published: July 2002

The electronic states of the ionic excimer Ar++2 are calculated using ab initio multireference configuration interaction and effective core pseudopotentials. Among states dissociating into Ar+(2P)+Ar+(2P), all are found to be repulsive, except the ground state, which occurs to be quasibound near Re=4.1a0 with a well depth of ≥230 cm−1. All states originating from Ar++(3P,1D,1S)+Ar are bound with dissociation energies in the range 3200–4500 cm−1 and equilibrium distances between 5.6a0 and 6a0. Simulation emission spectra from bound excited states are derived from the calculated potentials and the possible contribution of the Ar++2 ion to the third continuum fluorescence is discussed.
Magic numbers in (NO)+mArn heteroclusters produced by two-photon ionization in a supersonic expansion
Authors: Desai Sunil R. , Feigerle C. S. , Miller John C.
Published: July 2002

Mixed clusters of the form (NO)mArn(m ≤ 4,n ≤ 22)are produced in a supersonic expansion and photoionized by nonresonant two-photon absorption of 266 nm photons. The ions are subsequently separated and detected by time-of-flight mass spectrometry. Anomalously large relative intensities are observed for the cluster ions, NO+Arn(n= 12,18,22) and (NO)2+Arn(n = 17,21), and are attributed to extra stability of these ions. These ‘‘magic numbers’’ at (m+n)=13,19,23 are compared to those observed in rare gas clusters and other M+Arn heteroclusters and assigned to icosahedral structures. Other cluster ions of the form (NO)+mNO2, (NO)+mN2O, N2O+Arn, and (NO)+mH2O are observed and briefly discussed.
Toluene–water clusters: Ion fragmentation and chemistry
Authors: Li Shijian , Bernstein Elliot R.
Published: July 2002

Toluene/water cluster ion fragmentation is studied for isolated cold clusters by means of one- and two-color mass resolved excitation spectroscopy, time resolved pump (S1←S0) probe (I←S1) spectroscopy on the nanosecond time scale, and nozzle/laser delay timing experiments. These experiments lead to an identification of parent clusters for all fragment ion clusters observed. Fragmentation reactions depend on cluster size and on the energy deposited in the ion by the two photon I←S1←S0 excitation sequence. Fragments identified by these techniques include (H2O)xH+ (x=3,...,6) and toluene+(H2O)n−1 for toluene(H2O)n clusters and (H2O)xD+, (H2O)xH+, and toluene–d+3(H2O)n−1 for toluene–d3(H2O)n clusters. For n≤3 the preferred cluster fragmentation pathway is loss of a single H2O molecule, while for n≥4 the preferred cluster fragmentation pathway is generation of (H2O)nH+. Cluster ion fragmentation is prevalent in this system because of product stability (i.e., solvated protons and the benzyl radical) and because the I←S1 transition leaves the cluster ion in a very highly excited vibrational state (Δv≫0 for the I←S1 transition). The fragmentation of toluene+(H2O)3 to generate (H2O)3H+ and a benzyl radical takes place by two distinct pathways with generation times τ1<60 ns and τ2=480 ns. The toluene+(H2O)2 fragmentation from toluene+(H2O)3 has a generation time of τ<60 ns. The possible energetics, kinetics, and mechanisms for these fragmentations are discussed.
Strong asymmetry induced ΔK
a=3 transitions in the CH2 b̃ 1B1→ã 1A1 spectrum: A study by Fourier transform emission spectroscopy
Authors: Hartland Gregory V. , Xie Wei , Qin Dong , Dai Hai-Lung
Published: July 2002

Dispersed fluorescence spectra of the CH2b̃ 1B1→ã 1A1′2116 band were recorded by Fourier transform emission spectroscopy for a series of rotational levels in the b̃1 B1 (0,160,0) state. Strong ΔKa=3 transitions were observed with their intensity increasing as J increases in the ã 1A1 (0,1,0) state. The observed intensities could be well reproduced by a calculation based on a rigid asymmetric rotor Hamiltonian, in which CH2 was assumed to be linear in the b̃ 1B1 state and bent in the ã1A1 state. The calculation shows that the intensity of the ΔKa=3 transitions arises from the asymmetry of the ã 1A1 state.
Femtosecond multiphoton ionization of ammonia clusters
Authors: Wei S. , Purnell J. , Buzza S. A. , Stanley R. J. , Castleman A. W.
Published: July 2002

Herein, we report on femtosecond time-resolved experiments in ammonia clusters. The mechanisms of their ionization and the subsequent formation of the protonated ammonia cluster ions are studied using a femtosecond pump-probe technique at 620 nm. It is found that an intermediate corresponding to C′ states of the monomer is responsible for the ionization of ammonia clusters. Femtosecond pump–probe studies show that the lifetime of the intermediate to the formation of the protonated cluster ions (NH3)nH+ (n=1–5) is the same as that leading to the formation of the unprotonated cluster ions (NH3)m+ (m=2–5). The results provide the first direct experimental proof that formation of the protonated cluster ions takes place through an absorption–ionization–dissociation mechanism.
Multiconfigurational time-dependent Hartree study of complex dynamics: Photodissociation of NO2
Authors: Manthe U. , Meyer H.-D. , Cederbaum L. S.
Published: July 2002

The multiconfigurational time-dependent Hartree (MCTDH) approach is applied to an example showing very complex dynamics: the wave-packet dynamics on the three-dimensional B2 potential-energy surface of NO2. The ability of the MCTDH scheme to describe accurately the severe splitting of the wave packet on a saddle-shaped surface is demonstrated. Internal checks of the MCTDH calculation enable us to assess the degree of convergence without the need to resort to a numerically exact wave-packet calculation. As a representative observable the photodissociation spectrum is calculated and discussed. The A1/B2 vibronic coupling is neglected in our study, but the dynamics on the diabatic B2 surface is treated in its full three dimensionality.
Molecular bonding in SF6 measured by elastic electron scattering
Authors: Miller J. D. , Fink M.
Published: July 2002

Elastic differential cross-section measurements of gaseous SF6 were made with 30 keV electrons in the range of 0.25 bohrs−1≤s≤10 bohrs−1. Structural parameters derived in this study closely matched those found in an earlier total (elastic plus inelastic) scattering investigation. Multiple-scattering effects were incorporated in the structural refinement. The discrepancies between the independent atom model and the measured differential cross section reproduce earlier total scattering results for momentum transfers of greater than 5 bohrs−1. By extending the measurements to smaller s values, a closer examination of a Hartree–Fock calculation for SF6 was possible. It was found that the difference curve obtained from the Hartree–Fock calculation matched the experimental data in this region. A more quantitative analysis was performed using the analytic expressions of Bonham and Fink to compute moments of the molecular charge distribution from the differential cross-section data. Comparison of these results with similar fits to the Hartree–Fock calculation confirmed the good agreement between the Hartree–Fock calculation and the current elastic data.
H+O2→OH+O: Excitation function and absolute reaction cross sections
Authors: Keßler K. , Kleinermanns K.
Published: July 2002

The reaction H+O2 was investigated using translationally excited H atoms from HI photolysis in the range 227–300 nm. An increase in the reaction probability with increasing collision energy up to 175 kJ/mol followed by a decrease at higher E was observed. Rotational distributions were obtained at 240, 250, 254, 262.5, 270, 280, and 290 nm. Absolute reaction cross sections at different collision energies were measured with a calibration method using HNO3 photolysis as a reference: σR (115.7 kJ/mol)=0.18±0.10 Å2, σR (145.1 kJ/mol) =0.24±0.13 Å2, σR (171.9 kJ/mol)=0.65±0.36 Å2, and σR (179.2 kJ/mol)=0.45 ±0.25 Å2. The experimental values are compared with cross sections from trajectory calculations on an ab initio H+O2 potential energy surface. Good agreement was obtained at low and very high energies while in the intermediate range 145–210 kJ/mol the measured cross sections were considerably higher than the calculated ones. Possible reasons for this discrepancy are discussed.
Time-resolved dynamics of cluster isomerization
Authors: Heidenreich A. , Jortner Joshua , Oref I.
Published: July 2002

In this paper we report on the time-resolved dynamics of the cube → ring isomerization of the Na4Cl4 cluster, which was interrogated by constant energy molecular dynamics simulations. The isomerization was induced by several excitation modes of the nuclear motion, i.e., nonselective, bond selective, ion selective, and normal-mode selective vibrational excitations. The nuclear excitation was conducted from a cluster equilibrated state at 600 K (total vibrational energy Ev=7930 cm−1 ) to total energies in the range Ev=10 610 cm−1 (cube temperature T=800 K) to Ev=30 730 cm−1 (T=2300 K). The reaction rates for isomerization were initially obtained from the mean first passage times for the ring formation. Concurrently, we have simulated the time evolution of the concentrations of the cube, ladder, and ring isomers by the thermal quenching method. From the time-dependent concentrations for nonselective excitation, we have obtained the Ev dependent four rate constants for the isomerization scheme cube ⇄ ladder ⇄ ring, establishing the relations between the results of the first passage time calculations and the detailed kinetic analysis. The rates in the energy domain Ev<20 000 cm−1 (T<1500 K) exhibit no appreciable dependence on the initial excitation mode, and deviations from statistical behavior are negligible. We have also explored the intracluster vibrational energy redistribution (IVR) times, their dependence on the excitation mode and on the excitation energy. In the energy domain Ev<20 000 cm−1 (T<1500 K), the separation of the time scales between fast IVR and slow isomerization is applicable, whereupon the kinetics exhibits a statistical behavior. This conclusion is compatible with the vibrational level structure of cubic Na4Cl4, where no frequency mismatch prevails. Deviations from statistical behavior are manifested by the breakdown of the conventional kinetic scheme at high energies (Ev>26 000 cm−1 ), when both the IVR and the isomerization time scales approach their limiting values of a vibrational period.
Pair-correlation energy and successive ionization potentials of atoms He–Zn
Authors: Vijayakumar M. , Gopinathan M. S.
Published: July 2002

The successive ionization potentials (IP’s) of atoms He–Zn are calculated using the relativistic and correlated local-density RCΞ method. The contribution of correlation energy to IP’s of these atoms are reported. It is found that these correlation contribution to IP’s are different for different IP’s of the same atom. It is also different for a given IP for different atoms. This behavior is qualitatively explained on the basis of the results of pair-correlation energy. A simple approximate expression to calculate the pair-correlation energy proposed earlier is discussed.
Mass spectrometric identification of novel gaseous calcium oxide bromide and calcium oxide iodide complexes
Authors: Hilpert K. , Miller M.
Published: July 2002

The vaporization of CaBr2–CaO and CaI2–CaO mixtures was investigated by the use of Knudsen effusion mass spectrometry at temperatures of 1246 and 1048 K, respectively. The ions Ca+, CaX+, CaX2+, Ca2X3+, Ca3OX3+, Ca4OX5+ (X=Br,I), I+, Ca2OI+, I2+, and Ca3OI2+ originating from the vapor species were identified by the use of an electron impact energy of 18 eV. Relative ion intensities, ionization efficiency curves, and appearance potentials were determined. The existence of the novel calcium oxide halide complexes Ca4OX6(g) and Ca3OX4(g) (X=Br,I) was concluded by a discussion of these data in addition to the existence of the gaseous species CaX2(g) and (CaX2)2(g) already known.
Ab initio study on the effect of attaching a hydrogen molecule to the (H5O2)+ ion cluster
Authors: Bosch Enric , Moreno Miquel , Lluch José M.
Published: July 2002

Ab initio energy calculations using the Mo/ller–Plesset perturbation theory up to fourth order with the 6-31G(D,P) basis set have been performed for the H5O2+ ion cluster with and without a hydrogen molecule attached to it. In agreement with experimental results, a C2 structure which gives rise to only two high O–H stretching frequencies is predicted to be the only minimum of the isolated ion, whereas when H2 is present, again only one minimum is found which can be roughly assigned to be of the Cs type. This structure is predicted to have four active O–H stretching frequencies, again in agreement with the experimental spectrum.
Electric field splitting of the octatetraene 1 1A
g→2 1A
g transition in n-hexane
Authors: Gradl Gerhard , Kohler Bryan E. , Westerfield Curtis
Published: July 2002

Octatetraene can be photoisomerized even when it is incorporated in a low temperature n-hexane crystal. When this is done by irradiating the zero-phonon component of the S0→S1 origin band with a single frequency laser, very narrow (∼10 MHz) persistent holes can be burned. We have determined the effect of an externally applied electric field on these persistent holes and fit the observed data with a simple quantum mechanical model parameterized in the S0→S2 and S1→S2 transition dipoles and the magnitude of a molecular field intrinsic to the n-hexane site occupied by the octatetraene guest. The magnitude of the molecular field (∼106 V/cm) is consistent with only one of the three possible n-hexane sites that could have accommodated the octatetraene guest.
Ab initio study of low-energy electron-ethane scattering
Authors: Sun Weiguo , McCurdy C. W. , Lengsfield B. H.
Published: July 2002

We report the results of the first ab initio study of low-energy electron-ethane scattering which includes the effect of target polarization. The complex-Kohn method is used in scattering calculations that employ both static-exchange and polarized-self-consistent-field (polarized-SCF) trial functions. Integral, momentum transfer, and differential elastic cross sections are reported for both staggered and eclipsed conformations. Overall agreement between our studies and the most recent experimental data is very good. For staggered ethane, our integral cross section shows a Ramsauer–Townsend minimum at 0.18 eV. This is only the second theoretical study to find such a minimum in a molecule that possesses a nonzero quadrupole moment. We find that a polarized-SCF wave function is needed to obtain reliable cross sections in the vicinity of the Ramsauer–Townsend minimum and that this polarized-SCF wave function also provides a good description of the 7.5 eV f-wave shape resonance. The low-energy cross sections we obtain for staggered and eclipsed C2H6 differ very little from each other near the Ramsauer–Townsend minimum, but significant differences are found at higher energies.
The transition probability of electron loss from anions in the gas phase: The lifetime of O2 ⋅−
Authors: Yu Dake , Armstrong David A. , Rauk Arvi
Published: July 2002

The unimolecular primary chemical process, A−→A+e−, is investigated from a theoretical and computational point of view. A ‘‘standard’’ procedure for calculating the lifetime of A− directly is proposed based on the quantum mechanical transition probability. Specific application of the standard procedure is made to electron loss from O ⋅−2. The lifetime of O ⋅−2(ν′=4) calculated with the proposed method is 36×10−12 s, in good agreement with experimental data. An improvement to the standard procedure yields the best theoretical estimate for the lifetime, 125×10−12 s. The lifetimes of O ⋅−2 at higher vibrationally excited states are also calculated and agree well with those of previous calculations. The dependence of the calculated lifetime on the geometric and vibrational parameters of O2 and O ⋅−2 is discussed.
Inertial and attractive potential effects on rotation of solitary water molecules in apolar and polar solvents
Authors: Nakahara Masaru , Wakai Chihiro
Published: July 2002

Deuteron nuclear magnetic resonance spin–lattice relaxation times T1 have been measured for solitary water molecules (D2O) at very low concentrations (8–70 mM) in a series of solvents over a wide range of temperatures; the solvents studied are carbon tetrachloride (10 to 40 °C), benzene (10 to 60 °C), chloroform (−40 to 50 °C), acetonitrile (−40 to 50 °C), and acetone (−40 to 50 °C). The orientational correlation times τ2R for D2O with extremely small moments of inertia (I) in CCl4, C6H6, CHCl3, CH3CN, and (CH3)2CO at 30 °C are, respectively, 96, 224, 230, 625, and 826 fs, which are all by far smaller than the bulk value (2210 fs). The τ2R value is proportional to the proton chemical shift of water in each solvent which is taken as a measure of the strength of solute–solvent interactions in the short range. On the other hand, τ2R is almost inversely proportional to solvent viscosity in disagreement with the simple hydrodynamic friction model. The correlation time (τ*) scaled by the free rotator correlation time √I/kBT is 1.20, 1.01, and 0.89 in CCl4, respectively, at 10, 30, and 40 °C, suggesting a strong inertial effect on the water rotation in the weakly coupled solvent cage. In other solvents, the quantity τ* increases with an increase in the water proton chemical shift, and the larger the τ* value the stronger the temperature dependence. The usefulness of the solvent cage model is illustrated and limitations of primitive models so far often used are discussed.
The temperature-independent paramagnetism of MnO4−: A coupled Hartree–Fock study
Authors: Fowler P. W. , Steiner E.
Published: July 2002

Coupled Hartree–Fock calculations of electric and magnetic properties of the isolated closed-shell permanganate anion show it to be paramagnetic, in agreement with experimental measurements on KMnO4. Lattice effects on the computed value are small, but the computed paramagnetizability is highly sensitive to changes in the Mn–O bond length, moving from near perfect agreement with the estimated experimental value of ξ p=75.9 a.u. for the self-consistent-field bond length (R=2.93 a0) to over twice that value at the experimental distance (R=3.078 a0). Analysis of the perturbed wave function leads to a physical rationalization of the anomalous sign of the magnetizability: Ligand-field splitting of the Mn d valence space leads to near-degenerate occupied and virtual orbitals coupled by induced 2-center σ↔π rotations. The improvements in magnetic properties that would result from use of a valence complete active space wave function are discussed.
Structure and order in partially oriented solids by three-dimensional magic angle spinning nuclear magnetic resonance spectroscopy
Authors: Titman Jeremy J. , Féaux de Lacroix Stefan , Spiess Hans W.
Published: July 2002

A new rotor-synchronized carbon-13 three-dimensional magic angle spinning NMR experiment for the extraction of orientational distribution functions from partially ordered solid materials is presented. Problems with overlap of two-dimensional spinning sideband patterns inherent in earlier methods are overcome by extension to three dimensions, allowing the degree of order for each resolvable carbon site to be measured. The new technique is demonstrated on a frozen smectic liquid-crystalline side group polymer which shows an order gradient from the aligned mesogen to the disordered polymer main chain.
Single-electron transfer in organic molecules
Authors: Malinsky J. , Magarshak Y. , Smolyar A. , Lipman A.
Published: July 2002

The rate of electron transfer was studied in typical organic molecules. The approach is based on a Green’s function technique which allows one to account for multiscattering electron tunneling. Quantum-chemical calculations are used to obtain the parameters of dynamical Dyson’s equations for relevant double-time retarded Green’s function. The parameters are used to estimate electron tunneling rate including neutral and mononegative ions of the α, ω-diphenylalkanes with different numbers of methyl groups. The role of bridge as the conductor of an extra electron is demonstrated from results of the calculations.
The structure of small clusters: Multiple normal-modes model
Authors: Franke G. , Hilf E. R. , Borrmann P.
Published: July 2002

A simple model for the structural fluctuations, isomerizations, and phase transitions of small rare-gas clusters is defined (multinormal-model) which studies the statistical equilibrium of several isomers making use of the normal mode expansion of the free energy. It is evaluated classically and quantum mechanically and its results are compared to those of extensive (path integral) simulation calculations.
A theoretical investigation of the geometries, vibrational frequencies, and binding energies of several alkali halide dimers
Authors: Dickey Robert P. , Maurice David , Cave Robert J. , Mawhorter Richard
Published: July 2002

Results are presented from ab initio calculations on the symmetrical alkali halide dimers made up of Li, Na, K, F, and Cl. We examine the sensitivity of representative monomer and dimer geometries to the variation of the basis set with and without polarization and diffuse functions. The geometries are then compared with available experimental results. We have also calculated vibrational frequencies at the restricted Hartree–Fock level and examined the changes in geometry brought about by correlation using second-order Mo/ller–Plesset perturbation theory. It is found that Hartree–Fock theory in a modest basis set with diffuse and polarization functions yields results comparable to much larger sp basis sets and that the theoretical results are in good agreement with the experimental results for the Li and Na dimers. Our best results for the K-containing species tend to have bond lengths that are too long for the monomers and this error is carried over for the dimers. We also find a nearly uniform expansion of the M–X bond length in proceeding from monomer to dimer of 0.16±0.03 Å, independent of the alkali or halide involved. The calculated dimer dissociation energies are in excellent agreement with experiment. Inclusion of correlation appears to have a minimal effect on the computed geometries and a modest effect on the binding energies. The vibrational frequencies for the monomers are in excellent agreement with experimental gas-phase results and reasonable agreement is obtained with the available experimental frequencies for the dimers. Finally, a reanalysis of the electron diffraction data for Na2F2 in light of the current ab initio results leads to a significant change in the experimental value of the bond angle.
Acetylene: Synergy between theory and experiment
Authors: Lundberg James K. , Field Robert W. , Sherrill C. David , Seidl Edward T. , Xie Yaoming , Schaefer Henry F.
Published: July 2002

Six anomalous vibronic feature states [∼2 cm−1 full-width at half-maximum (FWHM), each consisting of ∼20 partially resolved eigenstates] have been observed in stimulated emission pumping (SEP) spectra of C2D2. Of the two plausible assignments for these features, the one most consistent with spectroscopic observations would imply that the lowest energy cis-bent triplet state of acetylene has T0≤25 820 cm−1, which is inconsistent with previous ab initio predictions. New higher level ab initio quantum mechanical methods have been used to predict the energy difference between X̃ 1Σg+ ground state and the cis-bent ã 3B2 lowest triplet state of acetylene. In conjunction with a triple zeta plus double polarization plus f function (TZ2Pf) basis set, the coupled cluster including single, double, and linearized triple excitations CCSD(T) method yields T0=ΔE(ã 3B2–X̃ 1Σg+)=30 500 cm−1. The true value of T0 for the ã 3B2 state is estimated to be ∼500 cm−1 higher. At the same level of theory the zero-point levels of the lowest triplet state of the trans-bent (ã 3Bu) and vinylidene (ã 3B2) isomers lie at still higher energies. This result conclusively rules out any triplet assignment for the anomalous feature states. The alternative assignment, as highly excited vibrational levels of the X̃ 1Σg+ state, is surprising in view of the Franck–Condon selectivity, dynamical stability, and nonselective relaxation of this special class of ‘‘bright states’’ observed in the SEP spectra.

Such an assignment would be implausible in the absence of the present ab initio calculations. Previous experimental observations [Lisy and Klemperer, J. Chem. Phys. 72, 3880 (1980) and Wendt, Hippler, and Hunziker, J. Chem. Phys. 70, 4044 (1979)] of acetylene triplet states are discussed and shown to be completely consistent with each other and with the present ab initio ordering of the cis and trans isomeric minima on the T1 potential energy surface: cis ã 3B2 below trans ã 3Bu .
Rotational-RKR inversion of intermolecular stretching potentials: Extension to linear hydrogen bonded complexes
Authors: Nesbitt David J. , Child Mark S.
Published: July 2002

A Rydberg–Klein–Rees (RKR)-based method is described which determines effective 1D intermolecular stretching potentials for polyatomic linear complexes from high precision rotational data alone. This extends the ‘‘rotational RKR’’ inversion method from pseudodiatomic van der Waals clusters with only two nonhydrogenic atoms to much larger complexes with several heavy atoms. Sample inversion of rotational eigenvalues generated from a model 1D potential reproduces the model potential to ≲0.13 cm−1 accuracy and correctly predicts harmonic frequencies, force constants, and dissociation energies to ≲0.1%. In contrast, the commonly used ‘‘pseudodiatomic’’ approximation lead to quite significant (10%–20%) errors, even for exact model potentials for which these approximations were developed. The method is further tested on high resolution near IR spectroscopic data of 14N14N–HF, which determines the vibrationally averaged hydrogen bond stretching potential from 3.39≲Rcm≲3.85 Å. The RKR data yield a hydrogen bond length of RN–H=2.106 Å (2.079 Å) and predict a van der Waals stretching frequency of 86.9 cm−1 (90.7 cm−1) for vHF=0 (vHF=1). RKR fits that incorporate electrostatic models of long-range behavior also permit estimates of the hydrogen bond dissociation energies and vibrational red shift for the vHF=0 and vHF=1 states, respectively. The range of D0 values agree reasonably well with previous ab initio calculations, and the difference in D0 values between vHF=0 and 1 is in good agreement with the experimentally observed red shift.
Evanescent wave light scattering study of a diblock copolymer adsorbed at the air/water interface
Authors: Lin Binhua , Rice Stuart A.
Published: July 2002

The in-plane angular distribution of light scattered from a water supported monolayer of a diblock copolymer of polystyrene and polymethyl methacrylate has been measured. These data lead to the inference that the polymer molecules aggregate into discs with diameter 3800 Å, sensibly independent of surface coverage in the range 6.0–18.0 Å2/monomer. There is no sign of this aggregation in the monolayer isotherm.
N2(a″ 1Σg+) metastable collisional destruction and rotational excitation transfer by N2
Authors: Wedding A. B. , Borysow Jacek , Phelps A. V.
Published: July 2002

Quenching and rotational coupling rate coefficients have been measured for the J=4–10, v=0 levels of the a″ 1Σg+ metastable state of N2 in collisions with ground state N2. Laser absorption is used to monitor the population of rotational levels of the a″ 1Σg+ state following depletion of the population of one or more levels by optical pumping to other states. The observed time dependence of the recovery of population of the perturbed level and the collision induced growth and decay of the populations of adjacent levels are interpreted in terms of quenching to other electronic levels and excitation exchange among adjacent rotational levels. For the J=6, v=0 level of the a″ 1Σg+ state the rate coefficients extrapolated to zero discharge current at 300 K are 2.3±0.1×10−16 m3/s for electronic quenching and 1.1±0.6×10−16 m3/s for excitation transfer to the J=4 and J=8 levels in collisions with N2. Very similar rate coefficients were obtained for the J=4, 7, and 8 levels.
Approximate path integral methods for partition functions
Authors: Messina Michael , Schenter Gregory K. , Garrett Bruce C.
Published: July 2002

We review several approximate methods for evaluating quantum mechanical partition functions with the goal of obtaining a method that is easy to implement for multidimensional systems but accurately incorporates quantum mechanical corrections to classical partition functions. A particularly promising method is one based upon an approximation to the path integral expression of the partition function. In this method, the partition-function expression has the ease of evaluation of a classical partition function, and quantum mechanical effects are included by a weight function. Anharmonicity is included exactly in the classical Boltzmann average and local quadratic expansions around the centroid of the quantum paths yield a simple analytic form for the quantum weight function. We discuss the relationship between this expression and previous approximate methods and present numerical comparisons for model one-dimensional potentials and for accurate three-dimensional vibrational force fields for H2O and SO2.
Mass spectrum and sublimation pressure of sodium oxide vapor: Stability of the superoxide molecule NaO2
Authors: Hildenbrand D. L. , Lau K. H.
Published: July 2002

The total sublimation pressure of Na2O(c) has been measured in the range 1000–1050 K by the torsion-effusion method, and the mass spectra of vapors over Na2O(c) and Na2O2(c) have been examined in order to resolve several issues regarding the thermochemical stability of the superoxide molecule NaO2. Measured torsion-effusion pressures are completely consistent with the primary sublimation process Na2O(c)=2Na(g)+0.5 O2(g) and set an upper limit of 200 kJ mol−1 for the bond strength D0(Na–O2). The ionization efficiency curves of Na+ and O2+ in the mass spectrum of vapor over Na2O(c) show no evidence for fragmentation contributions from NaO2. In addition, the absence of a detectable Na+/NaO2 fragment ion signal in vapor over Na2O2(c) can be used to derive an even tighter upper bound of 180 kJ mol−1 for D0(Na–O2). These results are in conflict with various kinetic analyses yielding D0(Na–O2) values of 202 to 243 kJ mol−1, but are in accord with rigorous theoretical calculations giving values near 160 kJ mol−1.
Comment on: Study of C−6 and C6 with threshold photodetachment spectroscopy and autodetachment spectroscopy
Authors: Liu Ruifeng , Zhou Xuefeng
Published: July 2002

Multiconfigurational ab initio calculations on the ground state linear C6 molecule indicate that the symmetric stretching modes ν2 and ν3 are significantly higher than the recent experimental assignment of Arnold et al., which suggests the results of the threshold photodetachment and autodetachment study are in error.
Reply to Comment on: Study of C−6 and C6 with threshold photodetachment spectroscopy and autodetachment spectroscopy
Authors: Arnold Caroline C. , Neumark Daniel M.
Published: July 2002

This reply offers further support for the assignments of the vibrational frequencies of C6 made in our original paper [C. C. Arnold, Y. Zhao, T. N. Kitsopoulos, and D. M. Neumark, J. Chem. Phys. 97, 6121 (1992)]. Two of our experimental frequencies are quite different from the corresponding ab initio values, and the resolution of this discrepancy clearly requires further theoretical work.
Millimeter-wave spectrum of vibrationally excited cyclopropenylidene, c-C3H2
Authors: Mollaaghababa R. , Gottlieb C. A. , Vrtilek J. M. , Thaddeus P.
Published: July 2002

The millimeter-wave rotational spectrum of c-C3H2 in four vibrationally excited states was measured and the rotational and quartic centrifugal distortion constants determined. Of the vibrational states observed here, only ν3 has been previously detected in the infrared. Assignment of the three new states to ν6, ν5, and ν2 was based on relative intensities, comparison of calculated and measured inertial defects, and symmetry considerations. The spectroscopic constants determined will guide future infrared investigation of c-C3H2, needed for a complete elucidation of the vibrational structure of this molecule. The experimental values of the vibration-rotation coupling constants will allow comparison with theoretical calculations that have recently become feasible for molecules of similar size as c-C3H2. Detection in space of rotational lines from the lowest vibrationally excited states may be possible.
Distributed approximating function approach to time-dependent wave-packet propagation in more than one dimension: Inelastic collinear atom–diatom collisions
Authors: Huang Youhong , Kouri Donald J. , Arnold Mark , Marchioro Thomas L. , Hoffman David K.
Published: July 2002

The distributed approximating function (DAF) approach to time-dependent wave-packet propagation in the coordinate representation is utilized to treat collinear, inelastic atom–diatom collisions. The resulting discretized DAF (DDAF) 2D-propagator is highly banded, leading to an extremely efficient computational strategy. The approach is illustrated with application to the atom–harmonic oscillator collision system treated earlier by Secrest and Johnson [J. Chem. Phys. 45, 4556 (1966)].
Rate constants for the reaction of Ar+(2P3/2) with N2 as a function of N2 vibrational temperature and energy level
Authors: Viggiano A. A. , Morris Robert A.
Published: July 2002

Rate constants have been measured for the reaction of Ar+(2P3/2) with N2 as a function of N2 vibrational temperature for a rotational and translational temperature of 300 K. The rate constants increase from a value of 1.2×10−11 cm3 s−1 at a N2 vibrational temperature of 300 K to 2.0×10−10 cm3 s−1 at a N2 vibrational temperature of 4700 K. The data are used to derive rate constants for specific vibrational levels. The rates constants for v=0–3 are 1.2×10−11, 3.0×10−10, 7.6×10−10, and 7.6×10−10 cm3 s−1, respectively. Additionally, the data suggest that the rate constant for v≳3 drops from the v=3 value, although this is less certain. The increase in the rate constants with increasing v is postulated to arise from the N2(v)→N2+(v+1) channel becoming less endothermic with increasing v and therefore closer to being resonant. Vibrational energy is found to be more effective at promoting this reaction than are translational or rotational energy.
Resonance enhanced two photon ionization spectroscopy of RbCs in a very cold molecular beam
Authors: Kim Bongsoo , Yoshihara Keitaro
Published: July 2002

Resonance enhanced two photon ionization spectrum of RbCs is obtained in a very cold pulsed molecular beam. We observe three vibrational series in the range of 540–590 nm. The excited electronic state that shows a very long vibrational series is assigned to the (4) 1Σ+ state. The absolute vibrational quantum numbers are determined from the isotopic shift of the vibrational energy. Rotational constants of several vibrational levels are determined in comparison with the Fourier transform spectroscopic data reported previously. Another electronic state that shows a short and intense vibrational series is assigned to the (3) 1Π state and the vibrational quantum numbers are determined. The term values and vibrational frequencies of 85RbCs are determined as Te=16 628.6±1 cm−1 and ωe=24.51±0.2 cm−1 for the (4) 1Σ+ state and Te=17 418.9±1 cm−1 and ωe=22.53±0.2 cm−1 for the (3) 1Π state.
Intermolecular perturbation of a Jahn–Teller system: The triptycene⋅Nen (n=1–3) van der Waals clusters
Authors: Furlan Alan , Leutwyler Samuel , Riley Mark J.
Published: July 2002

The effect of rare gas complexation on the electronically excited S1(E′) state of triptycene (T), which is Jahn–Teller distorted, was investigated by two-color resonant two-photon ionization (2C-R2PI) spectroscopy of the supersonically cooled van der Waals complexes triptycene⋅Nen, n=1–3. These complexes afford unique possibilities to study the effects of weak intermolecular interactions on the intramolecular Jahn–Teller coupling. Since the atoms are adsorbed at high-symmetry positions, the system symmetry is lowered from D3h(n=0) to C2v for n=1 and 2, but reverts to D3h for n=3. A Jahn–Teller (A1⊕E)⊗e coupling model including a uniaxial external strain component was applied successfully to calculate the S1 state levels and S1←S0 electronic spectra of all three complexes. The spectrum of T⋅Ne3 was fully interpreted without inclusion of strain, implying a highly symmetric D3h structure in which each of the three V-shaped compartments of triptycene is occupied by a single Ne atom. In contrast, the vibronic spectra of T⋅Ne and T⋅Ne2 were fitted with a considerable uniaxial strain of −19.73 and 19.07 cm−1, respectively, confirming both the predicted C2v geometry of the complexes as well as the equal magnitude, but opposite sign of the intramolecular distortion induced by one or two Ne atoms. These distortions correspond to a small change of the angle of the V-shaped compartments by ∼0.8°. The spectra of T⋅Ne and T⋅Ne2 are much more complex compared to triptycene and T⋅Ne3 due to the splitting of the E vibronic levels in C2v symmetry, and the appearance of additional transitions to levels that are weak or symmetry forbidden in bare triptycene. The calculated Jahn–Teller potential energy surfaces and vibronic wave functions are discussed and classified in terms of their symmetry and localization/delocalization properties.
Characterization of the first excited 1Π1 and ground X 1Σ+ states of MgXe. II. Analysis of 1Π1 → X 1Σ + bound–free emission spectra
Authors: McCaffrey John G. , Funk David J. , Breckenridge W. H.
Published: July 2002

Laser-induced fluorescence (LIF) measurements of the Mg(3s3p 1P1)⋅Xe(1Π1) ←Mg(3s3s 1S0)⋅Xe(X 1Σ+) transition of the jet-cooled MgXe complex have revealed that the ground X 1Σ+ state is quite weakly bound (De″ ≊ 100 cm−1) compared to the 1Π1 excited state (De′ ≊ 1500 cm−1), and that the bond length in the 1Π1 excited state is some 1.5 Å shorter. Here we report the observation of dispersed fluorescence spectra from single vibrational levels of the 1Π1 state, consisting mainly of oscillatory bound–free continua which extend over some 7500 cm−1. These spectra are analyzed using the semiclassical method of Child and LeRoy to determine the repulsive portion of the Mg⋅Xe(X 1Σ+) interaction potential up to nearly 1 eV in energy. Attempts were made to fit the experimental potential curves to several simple analytical functions, with a two-term Buckingham-type function yielding the best representation. A comparison of the repulsive portion of the Mg⋅Xe(X 1Σ) potential curve with that of the analogous Na⋅Xe(X 2Σ) curve determined similarly by Zimmerman and co-workers shows that the Na⋅Xe curve is even less repulsive than the Mg⋅Xe curve, even though the Mg⋅Xe potential is itself quite ‘‘soft.’’ This can be rationalized by the ability of the more diffuse Na(3s) electron to ‘‘back-polarize’’ (spσ hybridize) away from the Xe atom compared to the two Mg(3s2) electrons, thus reducing repulsion.
Investigation of the ground vibrational state structure of H35Cl trimer based on the resolved K, J substructure of the ν5 vibrational band
Authors: Han Jun , Wang Zhongcheng , McIntosh Avery L. , Lucchese Robert R. , Bevan John W.
Published: July 2002

The high resolution rovibrational IR spectrum of the antisymmetric H35Cl stretching ν5 vibrational band in the (H35Cl)3 isotopomer of the trimer has been measured with a tunable infrared diode laser supersonic jet spectrometer. One of the most prominent features of the spectrum is a series of strong lines each formed with pP or rR transitions, with resolved K, J substructure developing between two adjacent pP or rR heads. Analyzing the spectrum of (H35Cl)3 as the perpendicular band of an oblate symmetric rovibrator, the following molecular parameters have been obtained: ν0−C′ζ=2809.776 98(6) cm−1, B″=6.804 14(55)×10−2 cm−1, B′=6.859 43(55) ×10−2 cm−1, C′–C″=2.737(78)×10−4 cm−1, DJK″=−8.40(38)×10−7 cm−1, DJK′=−8.14(38)×10−7 cm−1, DJ″=4.26(16)×10−7 cm−1, DJ′=4.16(16)×10−7 cm−1, DK″=4.32(22)×10−7 cm−1, DK′=4.16(22)×10−7 cm−1. A relationship among the centrifugal distortion constants establishes that the geometry of (H35Cl)3 is consistent with a dynamically averaged planar ground vibrational state. The centers of mass of the H35Cl components are separated by 3.693(1) Å in this structure. Ab initio methods were also used to estimate the splitting due to the tunneling motions between clockwise (cw) and counterclockwise (ccw) identical structures. Both theoretical and experimental evidence indicate that the (HCl)3 complex has a planar equilibrium structure with no observable tunneling probability between the cw and ccw forms, thus the molecular symmetry group of the complex is proposed to be C3h(M).
Anisotropic dissociation of CO around the C K and O K ionization edges
Authors: Bozek John D. , Saito Norio , Suzuki Isao H.
Published: July 2002

Asymmetry parameter values have been determined from the C+ and O+ signals in the time of flight mass spectra of CO excited and/or ionized with monochromatic synchrotron radiation around the C K and O K ionization edges using a rotatable time of flight mass spectrometer. Asymmetry parameter values of about −0.8 were obtained from the C+ and O+ ions at the positions of both the C1s−12π* and O1s−12π* resonances below the core edges. Positive values of about 0.5 were obtained at the positions of the σ shape resonances above the C and O K edges. The results are consistent with previous assignments of the symmetries of the final state orbitals for the pre-edge resonances and the symmetries of the outgoing channels which exhibit shape resonances above the C K and O K ionization edges. The results are also consistent with the recently reported symmetry resolved photoionization spectra of CO. Above the ionization edges, the experimental data shows good agreement with the molecular asymmetry parameter values calculated by the relaxed-core Hartree–Fock method. Effects of the relatively intense shake-up transitions accompanying C1s ionization on the asymmetry parameter values are considered.