bluuebunny/crossref_metadata_2025 huggingface dataset:
The derivation of a symmetry-adapted generalized spin Hamiltonian
Authors: Buckmaster H. A. , Chatterjee R. , Tuszynski J. A.
Published: July 2002

The formulation of a generalized spin Hamiltonian using the decomposition of double tensor operators into products of single tensor operators is reexamined critically using their transformation properties under parity, time reversal, charge and Hermitian conjugation, as well as the imposition of symmetry constraints. These properties depend on their polar or axial vector classification. It is concluded that limitations in the parameterization of experimental EPR and ENDOR spectral data are associated with an excessively restrictive interpretation of the constraints imposed in the derivation of phenomenological and generalized spin Hamiltonians. It is shown that the correct imposition of these constraints using the double tensor formulation leads to a symmetry adapted generalized spin Hamiltonian which includes additional zero field splitting terms characteristic of the various crystallographic groups and described by odd rank tensor operators whose matrix elements are nonzero provided account is taken of configuration interactions. It is noted that the conclusions derived are applicable to all physical systems that can be described by Hamiltonians formulated in terms of tensor operators.
Electronic structures of tetrahedral iron, cobalt, and nickel clusters. Partial quenching of magnetism in partially carbonylated derivatives
Authors: Holland Gary F. , Ellis Donald E. , Trogler William C.
Published: July 2002

Spin-polarized calculations of tetrahedral Fe4, Ni4, and Co4 clusters have been performed by the self-consistent field discrete variational Xα method. Partially carbonylated C3v clusters Co4(μ−CO)3 and (OC)3CoCo3(μ−CO)3 have also been examined to explore how magnetic moments vary with cluster coordination environment. The bare tetrahedral clusters are magnetic containing 12, 6, and 2 unpaired electrons for Fe4, Co4, and Ni4, respectively. Exchange splittings also decrease (2.68, 1.60, and 0.56 eV) along this series. Analysis of density of states plots for these clusters supports the notion that 4s and 4p mixing into the metal d band yields hybrid orbitals better suited for metal–metal bonding. Electronic transitions are predicted to occur in the near IR absorption spectra of the M4 clusters, a region yet to be studied experimentally. Calculations for C3v Co4(μ−Co)3 yield six unpaired electrons with a large spin density on the unligated apical cobalt. In (OC)3CoCo3(μ−CO)3, where the apical cobalt is coordinately saturated, all the spin density (four unpaired electrons) is localized in the basal plane. Bridging CO groups in the basal plan are antiferromagnetically coupled to cobalt.
Stability, kinetics, and magic numbers of Si+n (n=7–45) clusters
Authors: Phillips J. C.
Published: July 2002

Mass spectrum maxima of Si+n clusters recently measured by Bloomfield, Freeman, and Brown are discussed in terms of cluster geometries they inferred from photofragmentation branching ratios and additional geometries proposed here. Critical slowing of cluster formation kinetics for n near 10 is explained by a configuration coordinate model. The critical slowing arises from the same factors thought to be responsible for network glass formation in bulk. Specifically, Si+n can be described as the smallest ‘‘glass’’ cluster known.
Negative ion photoelectron spectroscopy of the negative cluster ion H−(NH3)1
Authors: Coe J. V. , Snodgrass J. T. , Freidhoff C. B. , McHugh K. M. , Bowen K. H.
Published: July 2002

The negative ion photoelectron spectrum of the negative cluster ion H− (NH3)1 has been recorded with 2.540 eV photons. This negative cluster ion was prepared in a supersonic nozzle-ion source involving the injection of electrons into an expanding jet. While the spectrum is dominated by a broadened peak centered at 1.430±0.019 eV, there is also a small feature centered at 0.997±0.031 eV. Our interpretation of this spectrum is that the main peak contains the origin of the photodetachment transition, and that the smaller one is due primarily to the excitation of a stretching mode in the ammonia solvent during photodetachment. An upper limit to the dissociation energy of H−(NH3)1 into H− and NH3 is found to be 0.36 eV. This result is in good agreement with calculations by Kalcher, Squires, and Schleyer. The separation between the main and the small peaks is 3490±130 cm−1, and the symmetric stretching frequency of free NH3 is 3506 cm−1. The small peak also shifts appropriately upon deuteration in support of this assignment.
Surface reactions of metal clusters. II. Reactivity surveys with D2, N2, and CO
Authors: Morse M. D. , Geusic M. E. , Heath J. R. , Smalley R. E.
Published: July 2002

Reactions on the surface of a variety of transition metal clusters have been studied in the gas phase at near room temperature using a newly developed fast-flow reaction device. Initial examples of the use of this device are provided by survey studies of the reactivity of iron, cobalt, nickel, copper, and niobium clusters in contact with low concentrations of D2, N2 and CO. Dissociative chemisorption of D2 is found to occur with dramatic sensitivity to cluster size in the cases of iron, cobalt, and niobium clusters, the detailed pattern of reactivity differing markedly for each metal. The corresponding reaction is also observed with nickel clusters, but here the reactivity shows only a slow, steady increase with cluster size. Copper clusters are found to be completely unreactive to H2 chemisorption under these conditions. Molecular nitrogen is found to chemisorb readily to clusters of cobalt and niobium, with a reactivity pattern very similar to that observed with D2. Iron clusters are found to show slight reactivity with N2; only a small amount of chemisorption is observed on the most reactive clusters at high N2 concentration, but the pattern of this reactivity with cluster size is consistent with that observed in D2 chemisorption. In contrast to these highly structured reactivity patterns of D2 and N2, carbon monoxide is found to show only a slow, monotonic increase in reactivity with cluster size. It is suggested that these dramatic reactivity patterns for chemisorption on metal clusters provide stringent tests for future theories as to the nature of chemisorption on metal surfaces at a detailed, molecular level.
The significance of imaginary time in quantal reactive scattering
Authors: Pollak Eli
Published: July 2002

The concept of time in quantal tunneling processes is reexamined. We find that the Wigner–Eisenbud definition of real time and the definition of imaginary time may be understood in terms of stationary phase analysis in complex time of the microcanonical flux–flux correlation function. This analysis explains why the real time should not be used to justify adiabatic approximations for perpendicular degrees of freedom at tunneling energies. A semiclassical analysis shows that, as suggested by Buettiker and Landauer, the imaginary time should be used to determine the validity of the adiabatic approximation for tunneling processes. Numerical examples for the hydrogen exchange reaction are provided. The implications on adiabatic and sudden approximations in reactive scattering are discussed. A theory unifying the two approaches is outlined.
State to state study of the dissociation of doubly charged carbon dioxide cations
Authors: Dujardin Gérald , Winkoun Dominique
Published: July 2002

Double photoionization of CO2 has been studied in the photon energy range 37–52 eV by the photoion–photoion coincidence (PIPICO) method. Three dissociative electronic states (α̃, β̃, and γ̃) of CO++2 have been observed with threshold energies at, respectively, 37.9, 40.5, and 45.0 eV. Comparison of these energies with those recently calculated by Flament et al. indicates that the lowest electronic state at 37.9 eV most probably corresponds to the 1∑+g state (1π−2g main configuration). CO++2 in its α̃, β̃, and γ̃ states dissociates into CO++O+ with a high exothermicity since the total kinetic energy released was found to be respectively 4.5, 6.5, and 9.4 eV. From these results we concluded that CO++2 in its α̃(1∑+g ) state dissociates into CO+(X 2∑+)+O+(4Su) via an indirect process (predissociation). Absolute values of the partial double photoionization cross sections σ++n  (E) (n≡α̃, β̃, and γ̃) were also determined as a function of the photon excitation energy E. A ratio of double to total photoionization cross sections as high as 3% at a 52 eV photon energy was observed.
Photoelectron study of the valence levels of CF4 and SiF4 from 20 to 100 eV
Authors: Yates B. W. , Tan K. H. , Bancroft G. M. , Coatsworth L. L. , Tse J. S.
Published: July 2002

Using monochromatized synchrotron radiation, the gas phase photoelectron spectra of the five valence orbitals of CF4 and SiF4 have been obtained from 21 to 100 eV photon energies. Branching ratios (BR) for both molecules, and partial cross sections (σ) for CF4, have been obtained from these spectra. The agreement between our σ values for CF4 and previous results is excellent. Theoretical branching ratios, partial cross sections, and β values have been obtained from MS Xα calculations, and compared with experiment. The generally good agreement between experiment and theory for SiF4 confirms the orbital assignment: 1t1<5t2<1e<4t2<5a1, in order of increasing binding energy. In contrast to the CF4 results which show little structure, five shape resonances are predicted for SiF4, at 3, 7, 13, 23, and 35 eV kinetic energies. The predicted resonance positions and intensities are in good agreement with experiment (e.g., for the 23 and 35 eV resonances), but the low energy resonance positions and intensities are usually not well reproduced experimentally. The radically different resonance features in CF4 and SiF4 are discussed.
The multiphoton ionization spectrum of complexes of benzene and acetylene
Authors: Carrasquillo M. Edwin , Zwier Timothy S. , Levy Donald H.
Published: July 2002

Weakly bound complexes of benzene and acetylene were formed in a supersonic molecular beam and were studied using resonantly enhanced multiphoton ionization spectroscopy. Spectral features produced by different complexes were overlapped but were resolved using a time-of-flight mass spectrometer detector. The 1B2u ← 1A1g transition of benzene is forbidden, but the 0–0 band of this transition was observed in the benzene–acetylene complex, the transition being weakly allowed under the lower symmetry of the complex. Features near the 610 band of benzene were observed and assigned to two different geometric isomers of the benzene–acetylene complex. Features assigned to the mixed trimer benzene–(C2H2)2 were also observed. Analysis of the rotational band contours of the benzene–acetylene origin transition indicated that in that isomer, the acetylene is located above the plane of the benzene off of the sixfold axis. Vibrational predissociation of the neutral complex was observed when one of the isomers of φ-C2H2 was excited to the 6111 vibrational level (1440 cm−1 of vibrational energy) but was not observed when this isomer was excited to the 61 level (520 cm−1 of vibrational energy). The second isomer did not vibrationally predissociate from 6111 but did predissociate from 6112 (2365 cm−1 vibrational energy).
Precise measurement of the ground state dipole moment of CaI
Authors: Ernst W. E. , Kändler J. , Lüdtke J. , Törring T.
Published: July 2002

High precision Stark effect measurements were performed on the X 2Σ+ state of CaI by using the molecular-beam laser-microwave double resonance method. The electric dipole moment was determined as μ=4.5968(50) D. With this result measured dipole moments are known for all calcium monohalides and are compared with the values predicted by two recently developed ionic bonding models.
Reactive scattering of Na(3 2P3/2)+HCl
Authors: Vernon M. F. , Schmidt H. , Weiss P. S. , Covinsky M. H. , Lee Y. T.
Published: July 2002

The reaction of electronically excited Na(3P) atoms with HCl has been studied in a crossed molecular beams experiment. At collision energies slightly above the endoergicity of the reaction, Na(3P) shows a dramatic enhancement of reactivity over ground state Na(3S). Detailed measurements of the laboratory angular and velocity distributions of the reactively scattered NaCl product at 5.4 kcal/mol collision energy have allowed determination of the product center-of-mass translational and angular distributions. These experimental results are compared to the DIPR model of electron transfer reactions. The broad translational energy distribution is in qualitative agreement with the DIPR model, but the angular distribution exhibits reduced intensity for scattering perpendicular to the relative velocity vector which cannot be reproduced by the DIPR model. The preferred transition state configuration, Na–Cl–H, is consistent with what would be predicted by a diffuse 3P orbital where the Na atom appears ion-like. This configuration is opposite to that given by the dominant term in the long range multipolar expansion of the neutral reactant potential.
A theory for the photodissociation of polyatomic molecules, with application to CF3I
Authors: Clary D. C.
Published: July 2002

A theory is described for calculating photodissociation spectra for polyatomic molecules larger than triatomics. The general method combines the vibrational close-coupling, rotational infinite-order-sudden approximation with the technique of Kulander and Light for calculating photodissociation integrals. The three-dimensional theory enables several vibrational states in the polyatomic photofragments to be coupled together and also allows for initial vibrational and rotational excitation in the parent molecule. The method has been applied to the CF3I→CF3+I (2P1/2) photodissociation process for the radiation frequency range 32 000–42 000 cm−1. Cross sections are reported for CF3I in the ground vibrational state, and also with C–I stretching and bending modes excited initially. Considerable vibrational excitation in the CF3 photofragments is obtained in the calculations at higher frequencies, a finding that is in agreement with experimental measurements. There is a marked preferential population of CF3 combination bands involving simultaneous excitation of both the v1 and v2 vibrational levels.
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o SCF calculation on LinHm molecules and cations with four or less atoms
Authors: Cardelino B. H. , Eberhardt W. H. , Borkman R. F.
Published: July 2002

All possible molecules and monopositive cations containing lithium and hydrogen, up to a total of four atoms, have been studied by ab initio calculations using SCF wave functions built from Gaussian-type orbitals. The triatomic molecules and ions Li2H, Li3, H+3 , LiH+2 , Li2H+, and Li+3 were all found to be stable, i.e., of lower energy than any possible dissociation product. The neutral molecules H3 and LiH2 were found to be unstable relative to H2+H and Li+H2, respectively. The ions H+3 and Li+3 were found to be equilateral triangles, Li2H+ was found to be a linear species, while all of the other stable triatomic species were found to be bent. The binding energies (relative to the most stable dissociation products) for the triatomic molecules and ions ranged from 4.53 eV for H+3 to 0.24 eV for LiH+2 . All of the ten possible tetra-atomic molecules and cations were found to be stable (except H4 neutral, for which no calculations were done). The lowest energy structures for LiH3 and LiH+3 consisted of planar structures with an H2 unit perpendicular to an LiH unit and having the H2 at the Li atom end. The lowest energy for Li2H2 was obtained for a planar rhombic structure, while for the corresponding cation it was obtained for a planar structure with an Li2 unit perpendicular to an H2 unit. Li3H and Li3H+ were found to have lowest energy for planar kite shaped structures with a lithium atom loosely bound to the base of a triangle formed by an Li–H–Li unit. The shapes of Li4 and Li+4 were respectively: a rhombic structure and a triangular Li+3 unit with the fourth lithium atom attached to a vertex to form a planar structure similar to H+4 .

The predicted binding energies for the tetra-atomic molecules and ions ranged from 0.08 eV for LiH3 (least stable) to 1.20 eV for Li2H2 (most stable). The present work predicted for the first time a stable LiH3 neutral molecule, and stable Li2H+2 and Li3H+ cations. It also predicted for the first time that the most stable Li+4 ion corresponded to a C2v structure analogous to that of H+4 .
Metal cluster ion photofragmentation
Authors: Brucat P. J. , Zheng L.-S. , Pettiette C. L. , Yang S. , Smalley R. E.
Published: July 2002

Photofragmentation studies are described for mass-selected transition metal cluster ions cooled in a supersonic expansion. Examples of the spectral utility of this technique are presented in the cases of Ni+2 and Nb+2 for which well resolved vibronic bands have been recorded by monitoring fragment yield as a function of dissociation laser wavelength. For larger, more complicated metal clusters the absorption spectrum is far too congested and perturbed for analysis, but photodissociation can reveal the thresholds and time scales of various fragmentation pathways. The order of the absorption event leading to dissociation is found from the extent of fragmentation as a function of laser fluence. Using this method, the one-photon dissociation threshold of Fe+2 is found to lie between 2.43 and 2.92 eV, which determines the binding energy of this transition metal dimer cation. Combined with previous ionization potential measurements this places the binding energy of the Fe2 neutral between 0.83 and 1.32 eV. Similarly, the threshold for dissociation of Fe+3 into Fe+2 +Fe was found to lie between 1.17 and 2.18 eV, placing the Fe3 neutral binding energy between 1.27 and 2.38 eV. For all transition metal clusters measured (Fex, Nix, and Nbx with x between 2 and 10) the primary one-photon fragmentation pathway was always found to be loss of a single metal atom from the cluster. Far more accurate measures of these dissociation thresholds should be possible with more extensive measurements of cold cluster ion photodissociation as a function of wavelength. Arguments are presented to the effect that these dissociation thresholds should generally provide valid measures of the true binding energies of transition metal clusters.
Electron attachment rate constants of SO2 and CS2 in Ar, N2, and CH4 at varied E/N
Authors: Wang W. C. , Lee L. C.
Published: July 2002

The electron attachment rate constants of SO2 and CS2 in the buffer gases of Ar, N2, and CH4 (150 to 530 Torr) at various E/N (1–16 Td) were measured by a parallel-plate drift-tube electron-swarm technique. The electrons were produced by irradiating the cathode with KrF laser photons. For the SO2–Ar mixture, the electron attachment rate constant of SO2 increases with increasing E/N and is independent of Ar pressure. For SO2 in N2 and CH4, the electron attachment rates decrease with increasing E/N and increase with increasing buffer gas pressure. For CS2 in N2 and CH4, the electron attachment rates increase with increasing CS2 and buffer gas pressures and decrease with increasing E/N. The electron attachment to SO2 and CS2 in the buffer gases of N2 and CH4 is a three-body process. The collisional-stabilized rates of ‘‘temporary’’ negative compound ions SO−*2 and CS−*2 by various gases are investigated.
The rotational spectra of NH3–CO and NH3–N2
Authors: Fraser G. T. , Nelson D. D. , Peterson K. I. , Klemperer W.
Published: July 2002

The rotational spectra of NH3–CO, ND3–CO, ND2H–CO, NDH2–CO, NH3–13CO, and NH3–N2 have been measured by molecular beam electric resonance. The K=0 ground vibrational state transitions for these species were fit to a linear molecule Hamiltonian and the following constants were obtained for NH3–CO; (B+C)/2 (MHz)=3485.757(2), DJ (kHz)=110.2(2), eQqNaa (MHz)=−1.890(7), μa (D)=1.2477(8). These constants were also determined for

ND3–CO [3078.440(7), 75.7(8), −2.028(15), 1.2845(9)], NHD2–CO [3202.303(4), 86.8(6), −1.972(11), 1.2686(8)], NH2D–CO [3338.235(4), 98.9(6), −1.916(12), 1.2546(8)], NH3–13CO [3451.684(5), 108.7(7), −1.870(15), 1.2452(8)]. For NH3–N2 (B+C)/2=3385.76(21), DJ =117.(10), and μa =1.069(14). For NH3–CO three ‖ΔJ‖=1, K=0 progressions were seen along with two ‖ΔJ‖=1, K=1 progressions, suggesting nonrigidity in the complex. The internal rotation of the NH3 subunit about its C3 axis is expected to be essentially free, but this motion, by itself, is not sufficient to explain

the observed spectra, thus, large amplitude dynamics are occurring in at least two degrees of freedom. The quadrupole coupling constants, eQqNaa indicate that in each of the isotopes of NH3–CO the NH3 subunit has its C3 axis relatively rigidly oriented at an angle of approximately 36° with respect to the line connecting the centers of mass of the two subunits. The structure is not hydrogen bonded; the N atom is closest to the CO subunit. The orientation of the CO subunit is not established. The distance between the N atom and the center of mass of the CO unit (RN–CO) is 3.54(3) Å. The spectroscopic constants suggest that the weak bond stretching force constant is quite small (0.01 mdyn/Å) but compatible with the long bond length.
Experimental and theoretical study of configuration interaction states of CO+2
Authors: Roy P. , Nenner I. , Millie P. , Morin P. , Roy D.
Published: July 2002

Configuration interaction states of CO+2 have been observed as satellites in a high resolution photoelectron spectrum of CO2 using an angle resolved electron spectrometer and far UV synchrotron radiation in the 30–55 eV photon energy range. Ten satellites have been identified in the 22–40 eV binding energy region and classified as the result of outer and inner valence ionization processes on the basis of their asymmetry parameter ( β) values. Theoretical calculations of configuration interaction states of CO+2 using an ab-initio SCF-CI method show that three-hole-two-particle excited configurations are necessary to explain the line positions and intensities. The calculated satellite spectrum, corrected for transition moments at 45 and 1254 eV, shows an excellent agreement with experiment and allows assignments of both outer and inner valence satellite lines, with Πg, Πu, Σ+g, and Σ+u symmetries.
The 4f states of He2: A new spectrum of He2 in the near infrared
Authors: Herzberg G. , Jungen Ch.
Published: July 2002

A new spectrum of the He2 molecule has been discovered in the region 5100–5900 cm−1. It is identified as the 4f–3d Rydberg transition of He2 with a weak contribution of 4d–3p in the same region. The uncoupling of l from the internuclear axis in both the triplet and the singlet system is almost complete in the 4f complex; the appropriate quantum number is N+ corresponding to the rotational angular momentum of the He+2 core. The electronic fine structure in each N+ level depends on the dipole polarizability α of the core and its anisotropy as well as on the quadrupole moment Q2 and higher moments Q4,... of the core. Conversely, from the observed structure some of the quantities characterizing the core have been determined, e.g., α=1/3(2α⊥+α∥) =2.82 ±0.02, Q2=1.39±0.13, Q4=0.99±0.14, all in atomic units. The detailed rotational analysis of the spectrum shows that widespread perturbations occur in the triplet 4f levels, the largest affecting the N=6,N+=3 and N=9,N+=7 levels which are shifted towards higher energies by 1 and 2 cm−1, respectively. The perturbations are clearly caused by Δl=2 interaction with corresponding levels of 4pπ for which Orth and Ginter have found the opposite perturbations. There are no similar perturbations in the corresponding singlet levels.
Photoionization and fragmentation of (N2O)n clusters
Authors: Kamke W. , Kamke B. , Kiefl H. U. , Hertel I. V.
Published: July 2002

Photoionization efficiency curves for (N2O)n, n=1,...,6, in the wavelength region λ=55–110 nm have been measured using a supersonic molecular beam and monochromatized synchrotron radiation from the electron storage ring BESSY at Berlin. The dependence of the photoionization cross section on λ in the threshold region varies significantly with cluster size. By measuring the energy loss of cluster ions which fragment in a field free drift region of the ion source, we were able to detect delayed unimolecular and collision induced fragmentations of the cluster ions (N2O)+n→(N2O)+n−m+(N2O)m with n up to 6 and m up to 3. From a study of the dependence of the ‘‘metastable’’ signals on background pressure it appears that only the removal of a single N2O at a time can occur by unimolecular decay. For n≥3 PIE curves for the metastable ions show no significant difference when compared with those of the ‘‘prompt’’ (zero energy loss) ions. For n=2 the metastable spectrum exhibits clear autoionization peaks, whereas in the prompt spectrum the peaks are washed out by (pre)dissociation and—at stagnation pressures of 1 bar and above—by fragments of heavier n clusters.
Observation of multimode vibronic interference effects in 2E1g benzene
Authors: Whetten Robert L. , Grant Edward R.
Published: July 2002

This paper presents the first experimental demonstration of multimode vibronic interference effects arising from the interaction of topologically inequivalent vibrational modes with a two-level electronic degeneracy. The system is that of the E1g molecular ion core of benzene Rydberg states. The interacting modes are the Jahn–Teller active e2g, ring bend, ν6, and the Renner, out-of-plane e2u ring bend, ν16. The nature of the interaction is seen as a coupling of the angular momenta of the two modes individually to the electronic angular momentum, producing new quantization conditions, within which interference interactions can be approximated perturbatively. Such a treatment explains qualitatively all splitting patterns observed in experimental spectra. Small quantitative deviations point the way to higher order couplings.
Orienting reactants using van der Waals precursors: OCO ⋅ HBr+hν → {OCO ← H}+Br → CO+OH+Br
Authors: Radhakrishnan G. , Buelow S. , Wittig C.
Published: July 2002

We report the results of an experimental study in which the reaction of atomic hydrogen with carbon dioxide is examined under conditions wherein the reagents are highly oriented relative to one another. Orientation is achieved using nozzle expansions to prepare a precursor of the form CO2 ⋅ HBr, which is best described as a weakly bound van der Waals or hydrogen bonded complex. The weak bond insures that the CO2 and HBr moieties are separable insofar as electronic excitations and HBr dissociation are concerned. Translationally hot H atoms are produced by the 193 nm photolysis of the HBr constituent of the nearly linear CO2 ⋅ HBr complex. Dissociation is direct, and the initial H-atom velocity is directed primarily along the HBr axis. Because the HBr is complexed with the CO2, the initial conditions for the ensuing reaction are quite restricted relative to cases in which there is no selective orientation, i.e., bulk or molecular beam conditions. OH(X2Π) is detected under experimental conditions which minimize effects due to uncomplexed bimolecular processes and/or contributions from higher clusters, and vibration, rotation, spin-orbit, and Λ-doublet populations are determined. The OH(X 2Π) deriving from the complexed reaction appears instantly on the time scale of the measurement, in contrast to the clear rise time observed under bulk conditions, and the rotational population distributions differ measurably from those obtained under bulk conditions. This difference can be due to the short lifetime of the HOCO intermediate, and/or interactions of the OH fragment with the nearby Br atom from the CO2 ⋅ HBr complex.
Correction of the basis set superposition error in SCF and MP2 interaction energies. The water dimer
Authors: Szczȩśniak M. M. , Scheiner Steve
Published: July 2002

There has been some discussion concerning whether basis set superposition error is more correctly evaluated using the full set of ghost orbitals of the partner molecule or some subset thereof. A formal treatment is presented, arguing that the full set is required at the Mo/ller–Plesset level. Numerical support for this position is provided by calculation of the interaction energy between a pair of water molecules, using a series of moderate sized basis sets ranging from 6-31G** to the [432/21] contraction suggested by Clementi and Habitz. These energies, at both the SCF and MP2 levels, behave erratically with respect to changes in details of the basis set, e.g., H p-function exponent. On the other hand, after counterpoise correction using the full set of partner ghost orbitals, the interaction energies are rather insensitive to basis set and behave in a manner consistent with calculated monomer properties. For long intersystem separations, the contribution of correlation to the interaction is repulsive despite the attractive influence of dispersion. This effect is attributed to partial account of intrasystem correlation and can be approximated at long distances via electrostatic terms linear in MP2-induced changes in the monomer moments.
Many-body calculations of hyperfine constants in diatomic molecules. II. First-row hydrides
Authors: Kristiansen P. , Veseth L.
Published: July 2002

Magnetic hyperfine parameters (Frosch and Foley parameters) have been computed for first-row diatomic hydrides by use of many-body perturbation theory. The computations are complete to third order in the many-body expansion, which means that core polarization corrections are included through second and thrid order, and correlation effects by their leading third-order corrections. Computed results are presented for the ground states, and in addition for the three excited states A 2Δ, A Π, and A 2Σ in CH, NH, and OH, respectively. The vibrational dependencies of the hyperfine parameters were also predicted, and even a hyperfine centrifugal distortion constant observed for CH was computed, in good agreement with experiment. Good agreements between computed and experimental parameters were generally obtained, in particular for the ground states, where the errors in the computed values are at most a few percent.
Structures and energetics of monovalent ion–water microclusters
Authors: Sung Shen-Shu , Jordan Peter C.
Published: July 2002

We have applied the polarizable water model to molecular dynamics simulations of water microclusters of alkali metal cations and halogen anions. We find that this model, with two adjustable parameters, yields stabilization energies in good agreement with Hartree–Fock calculations and with experimental data. Several new, low energy, less symmetric configurations of the ion–water microclusters are obtained. These are particularly favored by the larger, more polarizable ions.
van der Waals rovibrational states of atom–molecule complexes: Ar–benzene and Ar–tetrazine
Authors: Brocks G. , Huygen T.
Published: July 2002

Using an empirical intermolecular potential, we have calculated the van der Waals modes of the atom–molecule complexes Ar–benzene and Ar–tetrazine. In the lower states the atom appears as being localized on one side of the molecular plane. This spectrum can be interpreted qualitatively in terms of one- or two-dimensional oscillators. However, these a very anharmonic and especially the motions along the molecular plane have large amplitudes. At higher energies the increased density of states will lead to an increased coupling between the modes. Furthermore, we have calculated the rotational constants of the complex, which are found to be a good measure for the quality of the ground state potential.
Ar–O2 rotationally inelastic collisions
Authors: Faubel M. , Kraft G.
Published: July 2002

In a crossed molecular beams Ar–O2 time-of-flight scattering experiment individual rotational state-to-state transitions are observed up to values of Δj=18. For the collision energy Ec.m.=97 meV a maximum of excitation is found in the neighborhood of the 1→13 rotational transition. In the total differential cross section fast diffraction oscillations could experimentally be resolved. In addition, a new method is explored for extracting cross section information on specific rotational transitions from measurements of appropriate out-of-plane and in-plane total differential laboratory cross sections only. A preliminary comparison of the experiment with theory indicates a need for substantial corrections in the anisotropy of the most recent Ar–O2 potential by as much as 0.2 Å.
Zeeman quantum beats in the polyatomic molecule propynal
Authors: Dubs M. , Mühlbach J. , Huber J. Robert
Published: July 2002

Zeeman quantum beats have been detected in the fluorescence emission of propynal (HC≡CCHO) in a molecular beam after excitation of single rovibronic levels in the first excited singlet state. The observed beat modulation amplitudes and frequencies of Zeeman sublevels as well as their decay rates are explained by spin-orbit interaction of individual hyperfine components with neighboring triplet hyperfine states.
Measurement of hyperfine polarization quantum beats in a polyatomic molecule
Authors: Dubs M. , Schmidt P. , Huber J. Robert
Published: July 2002

Polarization quantum beats between hyperfine components of perturbed rotational states in the S1→S0 fluorescence of propynal (HC≡CCHO) have been measured. The quantum beats are explained by a periodic transfer of molecular alignment to nuclear spin polarization.
Inelastic scattering of Br2 from graphite surfaces: A Monte Carlo classical trajectory study
Authors: Nyman Gunnar , Holmlid Leif
Published: July 2002

The experimental results from Holmlid et al. (1982) for Br2 inelastic scattering from graphite surfaces were used as a reference point for the Monte Carlo classical trajectory calculations presented here. A one-dimensional potential energy model of the scattering system was constructed. No vibrational or rotational degrees of freedom for Br2 were included. The trajectories were generally found to be ‘‘long-lived.’’ Good agreement with experiments was found for six simulated C atoms moving in phase, in the case of one simulated C layer, and for two to three C atoms in each layer, in the case of two simulated vibrating layers. In these cases one-fourth of the Br2 molecules made large jumps on the surface and all molecules were hit more than once by the vibrating C atoms. The results found are in agreement with the one-phonon transfer model by Holmlid et al. Both models demonstrate agreement with experiments by simulating long interaction times and a more or less ‘‘statistical’’ energy transfer.
Theoretical studies of diatomic and triatomic systems containing the group IB atoms Cu, Ag, and Au
Authors: Walch Stephen P. , Bauschlicher Charles W. , Langhoff Stephen R.
Published: July 2002

Selected portions of the ground state potential energy surfaces of the Cu3, Ag3, AgCu2, and AuCu2 trimers are studied at the single-reference singles plus doubles configuration interaction and coupled pair functional levels correlating 33 electrons. The calculations use the effective core potentials (ECP) of Hay and Wadt to replace the deep core levels. The Cu3 and Ag3 molecules are found to have 2B2 obtuse-angled ground states, with low-lying 2A1 acute-angled excited states. The AgCu2 and AuCu2 molecules have 2A1 acute-angled ground states. The Cu3 molecule is found to have a smaller 3d population than Ag3, which is probably a consequence of the larger (n+1)s1nd10 → (n+1)s2nd9 excitation energy for Ag as compared to Cu. We also find that Cu3 has a smaller s electron density in the open-shell orbital than does Ag3, in agreement with recent ESR experiments. Calculations are also carried out for Ag2, AgCu, and Au2. The Ag2 calculations indicate that the estimated re value for Ag2, which is based on comparison to Cu2 and Au2, is too large.
Preparation and detection of alignment with high ‖m‖ selectivity by saturated laser optical pumping in molecular beams
Authors: Hefter U. , Ziegler G. , Mattheus A. , Fischer A. , Bergmann K.
Published: July 2002

We describe a technique for preparation of molecules in single ‖m‖ levels involving saturated laser optical pumping on molecular P, R, and Q transitions. This technique is not limited to small rotational quantum numbers j. It allows the determination of the populations of ‖m‖ levels for arbitrary distribution functions f ( j,m) or alternatively, the determination of all moments of f ( j,m). In principle, this method is able to completely determine the angular distribution of j vectors. Experimental verification of the high ‖m‖-state purity achieved in Na2 supersonic beams, as well as of inherent limitations due to hyperfine interaction is provided. For illustration, experimental data on laser-induced alignment, the dependence of the flow induced molecular alignment, and on Δm-propensity rules in differential rotationally inelastic scattering are presented.
Charge dependence of chemisorption patterns for transition metal clusters
Authors: Brucat P. J. , Pettiette C. L. , Yang S. , Zheng L.-S. , Craycraft M. J. , Smalley R. E.
Published: July 2002

A method is presented for the measurement of the chemisorptive reactivity of transition metal cluster ions at near room temperature. Similar to a technique introduced previously for neutral clusters [Rev. Sci. Instrum. 56, 2123 (1985)], this cluster ion method utilizes a fast-flow reactor attached to a supersonic, laser vaporization metal cluster source, followed by time-of-flight mass spectral analysis of the cluster ions as a function of reactant concentration. Results are presented for clusters of cobalt and niobium in the 1–22 atom size range for their chemisorptive reactions with CO, CO2, and N2. Both Nb+n and Co+n clusters displayed chemical reactivity that is remarkably similar to that of the corresponding neutral clusters. For both charge states of each metal, CO was found to chemisorb with a rate which varied in a slow, monotonically increasing fashion with cluster size. The chemisorption rate of N2 and CO2, on the other hand, was found to be significantly slower than that of CO and sharply dependent upon the cluster size, this dependency being roughly independent of whether the transition metal cluster had a net positive or neutral charge. Photodissociation measurements of the mass-selected positive ion chemisorption products showed that the desorption energy of these products parallels the relative reaction rate.
Multiphoton ionization in a reflectron time-of-flight mass spectrometer: Individual rates of competing dissociation channels in energy-selected benzene cations
Authors: Kühlewind H. , Kiermeier A. , Neusser H. J.
Published: July 2002

Decay rate measurements of internal energy selected ions were performed with a novel technique employing highly selective two-photon laser ionization in a reflectron time-of-flight mass spectrometer. Resonantly enhanced two-photon ionization leads to vibrational state selected C6H+6 cations. These are further excited with a second laser pulse of variable frequency beyond the dissociation threshold for the four decay channels of lowest energy. The excitation results in a metastable decay of C6H+6 with typical decay rate constants of some 106 s−1. Decay rate constants for two C-loss channels and for the first time also for the two H-loss channels are measured in the energy range from 5.1 to 5.5 eV. It is shown that H-loss and C-loss dissociation occurs from a common electronic state and is competing. From the total decay rate constants and the measured branching ratios individual decay rate constants for all decay channels are obtained and found to be in good agreement with RRKM calculations. This confirms the statistical character of the C6H+6 ion dissociation.
Reactive etching of positive and negative silicon cluster ions by nitrogen dioxide
Authors: Mandich Mary L. , Bondybey Vladimir E. , Reents William D.
Published: July 2002

Positively and negatively charged silicon cluster ions, Si+1–8 and Si−1–6, react exothermically with NO2. The predominant reaction is loss of a single silicon atom in the form of SiO. This reaction repeats sequentially such that the clusters are ‘‘etched’’ down to monatomic silicon ions. Charge transfer to form NO−2 also occurs for Si−1–4. Previous results have shown that all Si−1–6 exhibit electron transfer to WF6. Together, these observations bracket the electron affinities of the neutral silicon clusters: EA (Si2–4)≤2.6 eV and 2.6 eV≤EA (Si5–6)≤3.7 eV. The reaction rates for the silicon cluster cations decrease slowly from 7.3±0.8×10−10 to 0.4±0.2×10−10 cm3 molecule−1 s−1 with increasing cluster size for Si+1–8. Silicon cluster anion reaction rates show a slight decrease from 15±1×10−10 to 5.0±0.1×10−10 cm3 molecule−1 s−1 with increasing cluster size for Si−1–6. The ionic silicon cluster reactivity is explained by a radical–radical coupling mechanism involving the unpaired electron on NO2 and an unpaired electron on a silicon atom at a charged or trivalent center in the clusters. Involvement of a radical electron on the silicon cluster contrasts with ionic silicon cluster reactivity with CH3SiH3 which requires divalent, silylene reactive centers on the clusters. Previously identified ‘‘magic number’’ clusters, Si+4 and Si+6, do not exhibit atypical reaction rates or products in spite of a theoretically derived 1–2 eV lower reaction exothermicity relative to the other cluster sizes.
The structure of the carbon dioxide dimer from near infrared spectroscopy
Authors: Jucks K. W. , Huang Z. S. , Dayton D. , Miller R. E. , Lafferty W. J.
Published: July 2002

An F-center laser–molecular beam spectrometer has been used to obtain a sub-Doppler resolution infrared spectrum of the carbon dioxide dimer. The vibrational mode investigated in this study corresponds to the ν1+ν3 combination mode of the monomer located at 3716 cm−1. A qualitative assignment of the spectrum shows unambiguously that the equilibrium structure of the dimer is the slipped parallel, rather than the T-shaped, geometry. The observed spectrum cannot be fit to within experimental error using conventional asymmetric rotor formalism. This may be due to a number of factors such as Fermi resonance between the upper state levels of the band and nearby levels of the dimer, such as seen in the monomer, or it could arise from tunneling effects arising from the two large amplitude motions in the dimer.
Molecular dynamics simulation of water CCl4 mixtures
Authors: Evans M. W.
Published: July 2002

A computer simulation of the molecular dynamics of a mixture of water and CCl4 molecules has been carried out with site–site terms for the intermolecular potentials. These are made up of Lennard-Jones interactions for the atom–atom terms and partial charges for the electrostatics. In a subpicosecond interval of time the H bonding structure of the pure aqueous environment is disrupted by the more massive CCl4 molecules; both in a 50% and 11% n/n emulsion. In this time interval the fine emulsion of water molecules in carbon tetrachloride has not had time to separate into water and carbon tetrachloride layers as would be expected from the immiscibility of the two macroscopic liquids. Therefore, the simulation provides a means of studying in detail the dynamics of water molecules freed of hydrogen bonding in an environment of much more massive carbon tetrachloride molecules. This provides an opportunity of investigating the effect of H bonding on the atom–atom pair distribution functions and time correlation functions of the dynamical ensemble. The most pronounced effect of distributing the water molecules in this way is seen in single molecule cross correlation such as 〈v(t)ωT(0)〉 where v is the linear molecular center-of-mass velocity and ω the angular velocity of the same molecule.
One-electron properties of several small molecules using near Hartree–Fock limit basis sets
Authors: Feller David , Boyle Caroline M. , Davidson Ernest R.
Published: July 2002

Selected one-electron properties for eight small molecules (H2O, CO, N2, H2S, NH3, PH3, HCl, and HF) have been computed using extended Gaussian basis sets at the SCF and CI levels. The basis sets and CI methodologies were capable of recovering approximately 86% of the estimated total correlation energy (88% of the valence) and, to the best of our knowledge, yielded the lowest variational energies and closest overall agreement with experimentally determined properties to date. Compared to numerical Hartree–Fock properties the current values are generally within 1%, with the largest deviation being 2.6% for δ at the nucleus. The sensitivity of the computed properties to the basis set composition and type of CI are discussed.
Rotational rainbow scattering with large Δ j: Energy dependence of the anisotropy of the Na2–Ne, Ar interaction potential
Authors: Gottwald E. , Mattheus A. , Bergmann K.
Published: July 2002

Differential cross sections for rotational transitions in Na2–Ne, Ar collisions are measured up to very large Δj. It is shown that the energy dependent anisotropy ΔR(E) of the interaction potential can be determined from these data using simple classical relations of ΔR and the rainbow angle θR. Excellent agreement with ab initio data is demonstrated and underlines the usefulness of this concept.
Supernumerary rotational rainbows in Na2–He, Ne, Ar scattering
Authors: Gottwald E. , Bergmann K. , Schinke R.
Published: July 2002

We report the resolution of supernumerary rotational rainbows for rotationally inelastic ji  → jf=0 transitions in Na2–He, Ne, Ar scattering. The relation of the angular position of the supernumerary maxima and the anisotropy ΔR of the interaction potential is established. The analysis yields the energy dependent anisotropy ΔR(E) in very good agreement with data from other sources.
Effective core potential parameters for first- and second-row atoms
Authors: Pettersson Lars G. M. , Wahlgren Ulf , Gropen Odd
Published: July 2002

An improved effective core potential (ECP) technique is described and used to give ECP parameters for the atoms of the first two rows of the periodic table. A given basis set is parametrized which allows for a direct comparison with all-electron calculations. Extensive test calculations on first- and second-row molecules using the ECP have been performed, giving excellent agreement with the all-electron results at the SCF, CASSCF, and CI levels of treatment. Correlating and diffuse functions may be added without modifying the ECP parameters. The present ECP descriptions result in CPU time reductions of the order of 50% in addition to reduced disk storage.
Observation of the A 2Πi–X 2Πr band system of SO+ in a rotationally cold supersonic expansion
Authors: Milkman I. W. , Choi J. C. , Hardwick J. L. , Moseley J. T.
Published: July 2002

Rotationally cold emission has been recorded in the A 2Πi–X 2Πr band system of SO+ at a resolution of 0.3 cm−1 using a source consisting of a supersonic expansion excited by a corona discharge, and the observed spectra have been rotationally analyzed. The derived constants have been merged with those obtained from previously published spectra of SO+ as well as from new analyses of unpublished high resolution measurements. Derived constants for this band system are reported for 60 bands involving v″=0 through 10 and v′=0 through 11.