bluuebunny/crossref_metadata_2025 huggingface dataset:
Translational energy dependence of O+(4S)+N2→NO++N from thermal energies to 30 eV c.m.
Authors: Burley J. D. , Ervin Kent M. , Armentrout P. B.
Published: July 2002

Guided ion beam mass spectrometry is used to examine the kinetic energy dependence of the reaction of ground state atomic oxygen ion with molecular nitrogen. An O+(4S) source which produces less than 0.06% excited states is described. Cross sections for the NO++N product channel decrease with increasing energy below 0.25 eV but increase with energy at higher energies. Analysis of the region above 0.25 eV finds an effective barrier of 0.33±0.08 eV which previous theoretical work suggests is on the N2O+(1 4A″) hypersurface. Below this barrier, ground state products can only be formed via a spin-forbidden surface transition. The magnitude and energy dependence of the probability for this transition are in reasonable agreement with a Landau–Zener formalism. These results are compared to previous ion beam, flowing afterglow (FA), and flow/drift tube (FD) studies. Apparent disagreement between the present data and previous FA and FD measurement is shown to be caused primarily by differences in the ion energy distributions.
Laser photofragmentation and photodesorption of physisorbed CH3Br on lithium fluoride
Authors: Tabares Francisco L. , Marsh Eugene P. , Bach Gloria A. , Cowin James P.
Published: July 2002

Laser irradiation at 193 nm of multilayer of CH3Br physisorbed on LiF at 30 K leads to both photofragmentation and photodesorption of molecules. One-photon photofragmentation was observed leading to broad velocity distributions for ejected CH3 and Br. Fragments as fast as those found in gas-phase photofragmentation were seen as well as collisionally slowed fragments. High energy (2 eV, four times the gas-phase kinetic energy) Br fragments were also observed due to collisions with rebounding CH3. Collisionally ejected molecules were also seen. A small fraction of molecules desorbed via a thermal process, associated perhaps with small amounts of adsorbed particulate contaminants. This mechanism may be responsible for some of the molecular desorption seen by other groups. Thermal desorption studies show that CH3Br (or CO2, Xe, or H2O) does not wet LiF, indicating a general trend important to all alkali–halide overlayer studies.
A new basis set method for quantum scattering calculations
Authors: Miller William H. , Jansen op de Haar Bernadette M. D. D.
Published: July 2002

A new basis set approach for quantum scattering calculations is described and tested on model problems of elastic and inelastic collisions. The approach is essentially the Kohn variational method, but applied to the S or T matrix directly rather than to the K matrix as is normally done; it is seen that the result of the present approach is not equivalent to the usual Kohn method (i.e., for the K matrix) and is indeed preferable to it. The present approach is seen to have the same structure as the complex scaling/coordinate rotation expressions for the T matrix (but with some added features). Its potential advantage over the Schwinger variational method, another useful basis set technique, is that matrix elements of the Green’s function for some reference Hamiltonian are not required; the present method requires only matrix elements of the Hamiltonian itself between the basis functions. The essential reason for all of these desirable features is that the basis set which is used incorporates the correct scattering boundary conditions.
Moment analysis for absorption and magnetic circular dichroism bands of atomic P←S transitions: Application to matrix-isolated chromium
Authors: Vala M. , Pyzalski R. , Shakhsemampour J. , Eyring M. , Pyka J. , Tipton T. , Rivoal J. C.
Published: July 2002

Equations useful for the moment analysis of magnetic circular dichroism (MCD) and absorption bands of P←S electronic transitions of atoms or ions for any spin multiplicity have been derived. In particular, it is shown that the first through third MCD moments, together with the zeroth and second absorption moments may be employed to obtain such electronic and vibronic parameters as (1) the excited state spin–orbit coupling constant, (2) the excited state orbital g factor, and (3) the contributions of the cubic and noncubic metal/lattice cage modes to the spectral absorption bandwidths. The MCD and absorption spectra of Cr atoms isolated in krypton and xenon matrices are presented. Forbidden transitions between the two allowed 7P←7S bands are observed. Their appearance is ascribed to second-order spin–orbit coupling to the near-lying 7P states. The moment equations are applied in an appropriate manner to the observed allowed bands. Excited state spin–orbit splitting (−14 cm−1:Cr/Kr and −56 cm−1:Cr/Xe) are found to be significantly reduced from the gas phase value (+194 cm−1) for the z7P←7S transition. For the y 7P←S band, an enhancement (+275 cm−1:Cr/Kr) and a reduction (+171 cm−1:Cr/Xe) from the gas phase value of 206 cm−1 are observed. It is further shown that the Jahn–Teller effect is strongly operative in the z 7P←7S Cr band in both matrices.
Atomic energies from the large-dimension limit
Authors: Loeser J. G.
Published: July 2002

Analytic approximations to nonrelativistic atomic ground state energies are obtained from the first two terms of the 1/D expansion for the N-electron atom. These two terms describe the equilibrium structure (D→∞ limit) and normal mode oscillations (1/D term) of a completely symmetric N-dimensional configuration of localized particles. The connection between these large-D results and real atoms is established through the vibrational state, which is restricted by antisymmetry requirements at D=3. Convergence considerations lead us to consider three different approximations, depending on whether all, none, or part of the results obtained from the 1/D term are used (in addition to those obtained from the D→∞ limit); the maximum errors are respectively about 8%, 3%, and 1%. In all three approximations the dependence of neutral atom energies on the nuclear charge Z is roughly Z12/5 for physical Z (as observed for real atoms) and roughly Z7/3 for very large Z (in agreement with the known asymptotic result). The best approximation, which utilizes the 1/D term up to lowest nonvanishing order in 1/Z, is comparable in accuracy to single-ζ Hartree–Fock calculations.
Evidence for a secondary minimum in the ArHCl potential surface from far infrared laser spectroscopy of the lowest Σ bending vibration
Authors: Robinson Ruth L. , Gwo Dz-Hung , Ray Douglas , Saykally Richard J.
Published: July 2002

The lowest Σ bending vibration in the ArHCl van der Waals complex has been measured near 24 cm−1 by intracavity far infrared laser stark spectroscopy and FIR–microwave double resonance. In conjunction with the calculations of Hutson, the analysis of the molecular constants provides strong evidence that the intermolecular potential energy surface contains two minima at the ArHCl and ArClH linear geometries.
Electronic spectra and intramultiplet relaxation of jet-cooled Hg–CH4, –C2H6, –N2, and –CO complexes
Authors: Fuke Kiyokazu , Saito Takayuki , Nonose Shinji , Kaya Koji
Published: July 2002

The electronic spectra of jet-cooled Hg–CH4, –C2H6, –N2, and –CO complexes in the vicinity of Hg 3P1−1S0 were studied by means of the LIF, two-color MPI, and Hg(3P0) pump-and-probe techniques. From the observed spectra, the Ã and B̃ states of these complexes were characterized in analogy with those of Hg–rare gas complexes. These excited-state complexes were found to predissociate into the Hg (3P0) atom and the ground-state molecules. The intensity alternation of the predissociation rate with respect to the excitation of the bending vibration was newly observed for the CH4 and N2 complexes in addition to the rotational dependence of the rate. On the basis of these results and the fluorescence lifetimes, the mechanisms of the intramultiplet relaxation for these complexes were examined. It was concluded that, in the Ã state, the bending vibration and the rotational motion of the whole system about the symmetry axis play the crucial role in the relaxation. The relaxation from the B̃ state was interpreted by the two-step process via the Ã state.
The microwave spectrum of CuO, X 2Π, measured with optical detection
Authors: Gerry M. C. L. , Merer A. J. , Sassenberg U. , Steimle T. C.
Published: July 2002

The microwave pure rotational transitions J=3/2←1/2 and 5/2←3/2 have been measured for gas phase CuO in its X 2Π state, using microwave-optical double resonance. The copper hyperfine structure was observed, and has been analyzed for the first time for the ground state of an orbitally degenerate transition metal-containing molecule in the gas phase. Rotational, lambda-doubling, and magnetic and electric hyperfine parameters have been evaluated; the Fermi contact parameter bF has been found to be very large and negative. The problem of determining the signs of the ‘‘off-diagonal’’ hyperfine parameters in 2Π electronic states is discussed.
Elastic moduli of a nematic liquid–crystalline solution of polyelectrolytes
Authors: Vroege Gert Jan , Odijk Theo
Published: July 2002

The elastic moduli of polyelectrolyte liquid crystals are calculated within the second virial approximation. A quantitative numerical and analytical theory is set up for rod-like polyelectrolytes in excess salt, whereas qualitative relations are discussed for polyions under more general circumstances. Electrostatic interactions cause significant deviations from the usual hard rod interactions. The effective diameter increases, the persistence length increases as well, and a torsional or twisting effect manifests itself because two polyion rods want to adopt a perpendicular orientation with respect to each other. In consequence, for rod-like polyelectrolytes the twist modulus depends only weakly on the macromolecular concentration, whereas the bend modulus increases more strongly with concentration than it does for hard rods.
A
b i
n
i
t
i
o ground state properties of neutral X2Y and ionic X2Y+ (X,Y=Li, Na, K) alkali trimers
Authors: Pavolini D. , Spiegelmann F.
Published: July 2002

The electronic ground state properties of neutral and singly charged alkali trimers involving Li, Na, and K atoms (homogeneous and heterogeneous trimers) are studied using ab initio core pseudopotentials and configuration interaction. Equilibrium geometries, dissociation energies, vertical and adiabatic ionization potentials are given. All systems are found to be triangular. Neutral light–light–heavy trimers (Li2Na, Li2K, and Na2K) have Cs equilibrium geometry while neutral light–heavy–heavy trimers (LiNa2, LiK2, and NaK2) have C2v geometry. All ionic species are isosceles. The ionization potentials are in good agreement with available experimental results.
Theoretical study of the electron affinities of MF6 and MF−6 (M=Cr, Mo, and W) using a model potential method
Authors: Sakai Yoshiko , Miyoshi Eisaku
Published: July 2002

Electronic structures of MF6, MF−6, and MF2−6 (M=Cr, Mo, and W) were calculated using a model potential method in the Hartree–Fock–Roothaan scheme. Major relativistic effects were taken into account for the calculations on MoFq6 and WFq6 (q=0, −1, and −2). It is shown that the calculated electron affinities (EAs) are extremely high for all the MF6 molecules, and that the CrF−6 and MoF−6 anions also have positive EAs, whereas the WF−6 anion has a slightly negative EA. The behaviors of the EAs are interpreted with reference to the electronic structures of the MFq6 systems.
Internal state distributions of CO from HNCO photodissociation
Authors: Spiglanin Thomas A. , Perry Robert A. , Chandler David W.
Published: July 2002

The internal state distributions of CO produced by photodissociation of HNCO at 1930 (230.1 nm) and at 10 200 cm−1 (193.3 nm) in excess of the dissociation energy are determined from multiphoton ionization spectra of the CO fragment measured under collision-free conditions. The rotational state distribution of the CO produced at the lower photolysis energy is characterized by a temperature of (491±23) K. The rotational state distribution of CO produced by photodissociation at the higher photon energy in not well characterized by a temperature. This latter distribution has maximum population near J=37, extends beyond J=65, and accounts for ∼20% of the available energy in excess of that necessary to rupture the HN–CO bond. An impulsive dissociation model assuming that dissociation occurs from an excited HNCO complex containing a nonlinear N–C–O configuration accounts for the average CO rotational excitation while phase-space theory does not agree with the observed distributions. Fitting a semiclassical model to the data using a logically constructed potential energy surface and a ground-state-dependent function provides a useful parametrization for the impulsive dissociation. Although not absolute, this analysis suggests that the dissociation occurs directly on a repulsive excited state potential energy surface.
Collision induced dissociation of aluminum cluster ions with chemisorbed oxygen, AlnO+m (n=3–26, m=1,2): Influence of electronic structure on stability
Authors: Jarrold Martin F. , Bower J. Eric
Published: July 2002

The collision induced dissociation of size selected aluminum cluster ions with chemisorbed oxygen, AlnO+m (n=3–26, m=1,2) by argon has been studied using a low energy ion beam apparatus. The main product from AlnO+ is Al+n−2. Aln−2O+ and Al+n−4 are observed from AlnO+2. These products can be accounted for by loss of Al2O (which is very strongly bound) from the clusters. The experimental results were used to bracket the Aln/O interaction energies at 8.0±1.0 eV for Al+6 and 7.5±1.0 eV for Al+19. These large interaction energies indicate that oxygen is multiply bonded to the clusters. The collision induced dissociation cross sections show a strong cluster size dependence. Minima in the cross sections as a function of cluster size are ascribed to particularly stable clusters. The number of aluminum atoms in the particularly stable AlnO+m cluster increases with the addition of oxygen atoms. The stability of these clusters can be accounted for by the electronic shell model if it is assumed that binding oxygen atoms to the clusters removes electrons from the cluster orbitals so that more aluminum atoms are required to close the electronic shells.
Structure and properties of ionized and neutral condensed clusters of Si, Ge, Sn, and Pb
Authors: Phillips J. C.
Published: July 2002

The relative abundances of medium-size clusters X(+,−,0)n of X=Si, Ge, Sn, or Pb, with 7≤n≤30, as prepared in various ways after vapor-phase condensation, are determined by enthalpic, entropic, and dynamic factors. Valence electronic structure determines chemical trends in (magic number) peaks in these relative abundances. Theoretical models of the valence structure explain these trends in detail. New cylindrical structures are identified at n=15, 18, and 21 for X(−)n with X=Si and Ge. General trends in ionization potentials and electron affinities are discussed.
Semiempirical study of polyatomic rare gas halides: Application to the XenCl systems
Authors: Last Isidore , George Thomas F.
Published: July 2002

In order to calculate different electronic states in polyatomic rare gas–halogen systems, including ionic states with charge transfer, a semiempirical diatomics-in-ionic-systems (DIIS) method is presented. In this method the Hamiltonian matrix elements are expressed in terms of diatomic potentials, except for those which are responsible for coupling between the ionic and neutral states. The coupling matrix elements are determined by fitting the diatomic transition moments. Calculations are performed for Xe2Cl, Xe3Cl, and Xe12Cl. The results reveal at least three quasistable ionic Xe+2Cl− molecules. Such molecules can attract more Xe atoms, which are binded mainly by polarization forces.
Excess electrons in simple fluids. IV. Real time behavior
Authors: Nichols Albert L. , Chandler David
Published: July 2002

The polaron theory for an excess electron in liquids due to Chandler et al. [J. Chem. Phys. 81, 1975 (1984)] is extended to the real time domain by the method of analytic continuation. For the case of an adiabatic solvent, the theory predicts that the electron momentum correlations relax nonexponentially in time, and that this long time tail contributes to a diminuation of the electron mobility. However, for short ranged forces, a mean-field approximation employed in this application of the polaron theory leads to a decay that is one power of t−1 higher than the generally accepted result for the quantum Lorenz gas. Along with this analytical analysis, we present numerical solutions of the analytically continued equations for the case of an adiabatic hard sphere solvent. We find that at low solvent densities, the electronic states are relatively diffuse, and the absorption spectra is maximum at the zero frequency diffusive mode. In this density regime, the electron mobility is a decreasing function of temperature. At higher densities, the electron mobility drops precipitously and the spectra has its maxima at a nonzero frequency. Here, the mobility is an increasing function of temperature. Corresponding behaviors of the electron mean-square displacement correlation function are discussed. The high density behaviors are the dynamical consequences of ground state dominance or self-trapping where diffusion requires excitation to high energy extended states. These results augment our earlier work on the equilibrium or thermodynamic consequences of this theory.
Ammonia dimer: Further structural studies
Authors: Nelson D. D. , Klemperer W. , Fraser G. T. , Lovas F. J. , Suenram R. D.
Published: July 2002

New experimental results on the structural and dynamical properties of NH3 dimer are reported in this work. J=1–0, K=0 transitions of 14NH3–15NH3, 15NH3–14NH3, ND3 dimer, and ND3–ND2H have been measured at high resolution and 14N electric quadrupole coupling constants are reported for each of these species. The NH3 subunits comprising the dimer are inequivalent. The quadrupole coupling constant associated with the first ammonia subunit eqQ1aa, is measured in 14NH3–15NH3 [−627(8)kHz], in ND3 dimer [−531(15) kHz], and in ND3–ND2H [−991(18) kHz]. For the other subunit, eqQ2aa is reported in 15NH3–14NH3 [892(8)kHz], in ND3 dimer [745(13) kHz], and in NH3–ND2H [1013(18) kHz]. These numbers can be used to estimate the vibrationally averaged polar angles of these isotopomers of NH3 dimer. The result is (including the primary isotopomer) θ1 for 14NH3–14NH3 is 48.6°, for 14NH3–15NH3 is 48.7°, for ND3 dimer is 49.6° and for ND3–ND2H is 45.3°; while θ2 for 14NH3–14NH3 is 64.5°, for 15NH3–14NH3 is 64.3°, for ND3 dimer is 62.6°, and for ND3–ND2H is 65.8°. The remarkable invariance of these values rules out the possibility of large vibrational averaging or tunneling averaging in this system and establishes that the angles θ1=49° and θ2=65° are near equilibrium. The isotope effect in the

component of the electric dipole moment along the a inertial axis μa, is shown to correlate well with the trend in polar angles given by the quadrupole coupling constants. The absence of interchange tunneling effects in the observed states of NH3 dimer implies that these states are asymmetrically excited internal rotor states of the complex. These experimental structural results are in disagreement with all previous theoretically determined structures for NH3 dimer except one. A recent electronic structure calculation which incorporates correlation through the coupled pair functional approach (while systematically varying geometry) obtains a compact, asymmetric structure for the dimer in close accord to observations.
Potential energy surfaces and transition moments of a Cl atom in a Xe solid matrix
Authors: Last Isidore , George Thomas F. , Fajardo Mario E. , Apkarian V. A.
Published: July 2002

Xe solid containing a Cl atom is considered by the semiempirical diatomics-in-ionic-systems method which takes into account the charge delocalization in an ionic state and the coupling between the neutral and ionic states. The calculation shows that the Cl atom has motional freedom in substitutional trapping sites which leads to the broadening of absorption bands. The vertically accessed ionic states are treated as Xe+12 Cl− molecules with a nonuniform positive charge distribution. The most stable ionic complex has the Xe+2 Cl− molecular configuration. Cl atoms at interstitial sites are treated as Xe6Cl molecules with Xe+6 Cl− excited states. The results of the calculation are in general agreement with recent experiments.
Photodissociation of (CO)+2: Theoretical studies of ground 2B
u and excited 2B
g potential energy surfaces
Authors: Blair John T. , Weisshaar James C. , Carpenter John E. , Weinhold Frank
Published: July 2002

We have calculated ab initio ground and excited state potential energy surfaces for (CO)+2 and its constituent monomers at correlated, extended basis levels, in order to elucidate the anomalous photodissociation dynamics of this dimer cation. We obtained 160 points (UMP2//6-31G*level) on the ground 2Bu surface and 144 points (54-term MCSCF//6-31G level) on the excited 2Bg surface for stretch–bend interactions in symmetric trans planar (C2h) geometries. The calculated surfaces reveal remarkable low-energy ‘‘valleys’’ connecting regions of dissimilar electronic structure, leading to a surprisingly short CC approach distance, nonrigid geometry, and complex vibrational dynamics. Our results suggest that the experimental bond dissociation energy of Ng and co-workers significantly understates the true value. Natural bond orbital (NBO) analysis is used to relate the structure of (CO)+2 to that of CO, CO+ monomers, and to construct simple orbital diagrams that account for the principal topological features of the potential surfaces. From these studies there emerges a satisfactory qualitative interpretation of the observed (CO)+2 photodissociation spectrum, and of the electronic factors that distinguish this species from isoelectronic (N2)+2 and other (XY)+2 gas phase dimer cations.
Efficient and selective population of high vibrational levels by stimulated near resonance Raman scattering
Authors: Becker M. , Gaubatz U. , Bergmann K. , Jones P. L.
Published: July 2002

We report the successful application of an optically pumped molecular beam laser for efficient and selective population transfer from v″=0 to high lying vibrational levels of Na2 in the electronic ground state. More than 50% of the molecules in a thermally populated rotational level of v″=0 can be transferred to a state v″≫1 with typically ≥75% of the molecules excited out of the initial state found in the predetermined end vibrational level and approximately ≤25% distributed over other levels according to the Franck–Condon factors. The applicability of this technique to the study of collision processes involving vibrationally highly excited levels is discussed. This work also contributes in general to a better understanding of optically pumped dimer lasers.
The electronic states of Si2 and Si−2 as revealed by photoelectron spectroscopy
Authors: Nimlos Mark R. , Harding Lawrence B. , Ellison G. Barney
Published: July 2002

We have measured the photoelectron spectrum of Si−2 and find that the molecular electron affinity is EA(Si2)=2.199±0.012 eV. This spectrum apparently involves multiple electronic states of the Si−2 ion as well as several electronic states of the final neutral, Si2. In order to unravel our experimental findings, we have carried out ab initio MCSCF+1+2 CI calculations on both species. These calculations suggest that there are two nearly degenerate states for both Si2 and Si−2. We can fit our experimental data by assuming detachment from two states of Si−2 :X (2Π) [re=2.187 Å] and A (2Σ+g) [T0=0.117±0.016 eV, re=2.127 Å]. We observe two final states of Si2: X (3Σ−g) [re=2.246 Å] and A (3Πu) [T0=0.053±0.015 eV, re=2.171 Å]. These assignments are confirmed by an experimental study of the angular distributions of the photodetached electrons.
Gas phase studies of Zn+2, Ag+3, and Ag+5
Authors: Buckner Steven W. , Gord James R. , Freiser Ben S.
Published: July 2002

Laser desorption from ZnO and AgO produces small bare metal cluster ions. Laser desorption from a ZnO/AgO mixture produces an enhancement of the silver cluster ion signal with complete suppression of the zinc signal. The chemistry of Zn+2 indicates IP(Zn2)=9.0±0.2 eV and D0(Zn+–Zn)=0.56±0.2 eV. The reactivity of Zn+2 with alkenes and alcohols is characterized by displacement of a zinc atom and formation of Zn+–B (B=alcohol, alkene). The silver cluster ions are produced with excess kinetic energy; however, collisional cooling is achieved by trapping the cluster ions in a static pressure of argon. Charge transfer reactions indicate IP(Agn)<7.0 eV (n=3,5). Ag+3 and Ag+5 are unreactive with small alkanes, alkenes, and alcohols, but AgnL+2 (n=3,5; L=sec-butylamine) reacts with sec-butylamine via deamination and dehydrogenation indicating D0 (AgnL+2–butadiene) >1.73 eV.
Adiabatic unimolecular dissociation of heterogeneous alkali clusters
Authors: Bréchignac C. , Cahuzac Ph. , Pflaum R. , Roux J. Ph.
Published: July 2002

The unimolecular dissociation of metastable photoionized mass-selected heterogeneous alkali clusters is investigated using a tandem time-of-flight spectrometer. NaK+n and Nan K+ are found to dissociate by evaporation of either a single neutral atom or a neutral dimer of the most abundant constituant. This behavior differs from the evaporation of heterogeneous neutral clusters which might always evaporate potassium in order to explain the sodium enrichment in the NaxKy neutral cluster distributions. This difference in the dissociation patterns of neutral and ionic heterogeneous clusters puts into evidence the influence of the charge in unimolecular dissociation processes.
A simple interpretation of the Fe−2 photoelectron spectrum
Authors: Leopold Doreen G. , Almlöf Jan , Lineberger W. C. , Taylor Peter R.
Published: July 2002

The photoelectron spectrum of Fe−2 can be simply interpreted in terms of electron detachment from the mildly antibonding 4sσ*u orbital of a (4sσg )2 (4sσ*u )2 (3d)13 anion. This interpretation implies a (4sσg )2 (4sσ*u )1 (3d)13 configuration for the ground state of Fe2, correlating with one ground state (4s23d6) and one excited state (4s13d7) Fe atom. A comparison of the bond length and vibrational frequency of Fe2 to values for transition metal dimers containing single 4s–4s bonds is suggestive of 3d–3d bonding in this molecule. The results of preliminary full-valence configuration interaction calculations provide strong support for the proposed Fe2 and Fe−2 configurations.
Dynamics of surface-aligned photochemistry (theory). I. Trajectory study of H↘+BrH′(ad)
Authors: Polanyi J. C. , Williams R. J.
Published: July 2002

The displacement reaction H↘+BrH′(ad)→HBr(g)+H′ resulting from the photoinduced interaction of adsorbed HBr molecules on a LiF(001) substrate has been modeled by a quasiclassical trajectory (QCT) calculation. This process constitutes ‘‘surface-aligned photoreaction’’ (PRXN) in which atomic H↘ photoejected from HBr(ad) approaches an adjacent HBr(ad) with a restricted range of impact parameters b in either linear or bent (90°) configurations. The surface alignment (whether linear or bent) was found to eliminate the reaction pathway to yield H2 product, of major importance in the gas. Product HBr energy and angular distributions, as well as excitation functions Pr(ET), were obtained for each geometry at initial relative translational energies ET spanning 8–40 kcal mol−1. These distribution functions were compared to the gas. For both PRXN variants, linear or bent, reaction occurred with high probability, Pr(ET) rising sharply in each case from threshold to unity within a narrow range of ET. By contrast, Pr(ET) for the gas was broader and smaller. Product (HBr) vibrational excitation for PRXN exceeded that for gas-phase reaction; linear PRXN showed the greatest enhancement. Product rotational distributions were substantially narrower than for the gas-phase reaction, with P(J′) for bent PRXN exhibiting bimodality attributable to the occurrence of reaction with either positive or negative impact parameters. The computed HBr angular distributions from PRXN were markedly narrower than for the gas-phase reaction, and were specific to the reagent configuration (linear or bent). The existence of a reference plane in PRXN (the crystal surface) governing the plane of reaction, has the consequence that product angular distributions with respect to each polar angle Pr(θ′s) and Pr(φs), embody information concerning the reaction dynamics. It is evident that PRXN by restricting reagent parameters renders product attributes more informative.
Metallic vs chemical bonding: A valence bond analysis of small Lin clusters wave functions
Authors: Maynau D. , Malrieu J. P.
Published: July 2002

Simple molecular orbital (MO) arguments as well as valence bond analysis of accurate MO–CI wave functions discriminate between essentially neutral and essentially ionic eigenstates, which also appear clearly from careful studies of the Hubbard bands. While homoatomic molecules have a neutral ground state, Lin clusters for n=4–8 have ionic states in the sense of the VB theory, with large static or dynamic charge displacements. All these structures involve odd-membered rings. Secondary minima of neutral VB character appear to be built from four-membered rings and have large components on spin waves. This analysis supports a qualitative difference between chemical bonding in nonpolar systems with normal valency of the atoms and the metallic bonding (involving hypervalent atoms), which cannot be regarded as resonating covalent bonds. The role of the β/U ratio (i.e., delocalization/correlation) and of dimensionality is discussed (one-dimensional clusters would remain neutral).
State-to-state vibrational relaxation in 1B2u benzene-d6 induced by low energy collisions with He, H2, N2, and Ar
Authors: Rainbird Mark W. , Webb Brian S. , Knight Alan E. W.
Published: July 2002

Time-resolved dispersed fluorescence spectroscopy has been used to study very low energy collision-induced vibrational relaxation in 1B2u benzene-d6 expanded in a supersonic free jet. Several expansion gases have been used to enable rate coefficients for vibrational relaxation to be obtained as a function of collision partner in the very low energy collision regime. Benzene-d6 is found to undergo vibrational deactivation with each of the chosen collision partners He, H2, N2, and Ar. This study focuses on measuring state-to-state relaxation rate coefficients and branching ratios for vibrational relaxation from the S1 vibrational level 61 (εvib =498 cm−1). Relaxation occurs to all the known S1 vibrational levels lying lower in energy than 61, namely 101 (εvib =454 cm−1), 162 (εvib =414 cm−1), 111 (εvib =382 cm−1), 41 (εvib =306 cm−1), 161 (εvib =207 cm−1), and 00 (εvib =0 cm−1). Two other unidentified channels also participate. Nitrogen and argon are found to be the most efficient collision partners in inducing relaxation (σobs/σhs ∼1.1, 1.7, respectively), whereas hydrogen and helium are found to be an order of magnitude less efficient (σobs/σhs ∼0.08, 0.02, respectively). The relaxation channels 61→101 and 61→111 are the dominant deactivation pathways for all the gases studied. The relative importance of relaxation pathways changes with collision partner. There appears to be a light or small collision partner effect, whereby the propensity for transfer through the channel 61→101 (Δεvib =−44 cm−1) seems to be redistributed among other channels, involving a greater energy or momentum gap, as the collision partner becomes lighter.
Laser spectroscopic investigation of the van der Waals molecule NaKr84
Authors: Zanger Eckhard , Schmatloch Volker , Zimmermann Dieter
Published: July 2002

The absorption spectrum of the van der Waals molecule NaKr due to the transition X 2Σ→A2Π has been observed with high resolution using a tunable single-mode cw dye laser and producing the molecules by means of supersonic expansion. About 600 lines due to the most abundant Kr-isotope Kr84 have been analyzed. The spectroscopic parameters of molecular rotation, of fine structure splitting and of λ-type doubling have been obtained for the vibrational levels v′=10⋅⋅⋅14 of the A 2Π state and for the level v″=0 of the X 2Σ state. The experimental positions of the band origins of the vibrational transitions are in satisfactory agreement with the results of model potential calculations. The equilibrium distances of the X 2Σ and of the A 2Π state of NaKr84 have been determined to be 4.919(7) and 3.02(9) Å, respectively. For the well depth we obtain values of (69±4), (790±60), and (750±60) cm−1 for X 2Σ, A 2Π1/2, and A 2Π3/2, respectively.
Local mode and normal mode interpretations of the CH and CD stretching vibrational manifolds in C2H4 and C2D4
Authors: Duncan J. L. , Ferguson A. M.
Published: July 2002

The CH and CD stretching vibrational manifolds in C2H4 and C2D4 have been studied up to six and four quanta of excitation, respectively. They are interpreted in each case in terms of a minimum number of local mode parameters. In C2H4, perturbations arising from Fermi resonances are complex and involve both the C=C stretching and the CH2 scissoring vibrations, with the former itself being in close resonance with the in-phase CH2 rocking overtone. Nevertheless, once all major resonances are taken into account, a self-consistent set of vibrational parameters is determined, which reproduces observations well in both infrared and Raman spectra. In C2D4, no significant Fermi resonance complications are apparent, spectroscopic interpretation is straightforward, and approximations inherent in the simple local mode model may be tested. The local mode parameters determined for C2H4 and C2D4 obey closely their expected isotopic interdependences. Values determined for the interbond harmonic coupling parameters in each case are in excellent agreement with those predicted through the harmonic force constants of ethylene.
Electron-impact ionization cross sections of the SiF3 free radical
Authors: Hayes Todd R. , Shul Randy J. , Baiocchi Frank A. , Wetzel Robert C. , Freund Robert S.
Published: July 2002

Absolute cross sections for electron-impact ionization of the SiF3 free radical from threshold to 200 eV are presented for formation of the parent SiF+3 ion and the fragment SiF+2, SiF+, and Si+ ions. A 3 keV beam of SiF3 is prepared by near-resonant charge transfer of SiF+3 with 1,3,5-trimethylbenzene. The beam contains only ground electronic state neutral radicals, but with as much as 1.5 eV of vibrational energy. The absolute cross section for formation of the parent ion at 70 eV is 0.67±0.09 Å2. At 70 eV the formation of SiF+2 is the major process, having a cross section 2.51±0.02 times larger than that of the parent ion, while the SiF+ fragment has a cross section 1.47±0.08 times larger than the parent. Threshold measurements show that ion pair dissociation processes make a significant contribution to the formation of positively charged fragment ions.
Oxidation of small boron cluster ions (B+1–13) by oxygen
Authors: Hanley Luke , Anderson Scott L.
Published: July 2002

Absolute cross sections for all ionic products formed in reactions of B+1–13 with oxygen have been measured under single collision conditions, at collision energies from 0.25 to 10 eV. Three main reaction mechanisms appear to be important: oxidative fragmentation, collision induced dissociation, and boron atom abstraction. The dominant oxidation process are exoergic for all cluster sizes, but appear to have bottlenecks or activation barriers for the larger cluster ions. Clusters smaller than B+6 have similar chemistry, then there is a sharp transition in chemistry for clusters larger than B+6 . Correlations are explored between cluster reactivity and cluster stability, and the oxidation chemistry is compared to the similar results found for aluminum cluster ion oxidation.
Oriented molecule beams: Focusing and orientation of t-butyl iodide with analysis by polarized laser photofragmentation
Authors: Xu Qi-Xun , Jung Kyung-Hoon , Bernstein Richard B.
Published: July 2002

The tert-butyl iodide molecule is readily focused with the electrostatic hexapole, via its first-order Stark effect as a pseudo-symmetric top. The pulsed, seeded supersonic focused beam, characterized by 〈cos θ〉=Vth/ V0 (where θ is the angle between the molecular dipole axis μ and the electric field E; ±V0 the hexapole ‘‘rod voltage,’’ and Vth the so-called threshold voltage), passes into a small homogeneous electric field in which it is oriented. The degree of laboratory orientation achieved is measured using the method of linearly polarized laser-induced photofragmentation [S. R. Gandhi, T. J. Curtiss, and R. B. Bernstein, Phys. Rev. Lett. 59, 2951 (1987)], operating (at three laser wavelengths) on the I(2P3/2) and I(2P1/2) as well as the t-C4H9 radical photofragments. The results show that the oriented beam molecules of t-butyl iodide (at a rotational temperature near 15 K) have a higher degree of orientation than the prototype CH3I molecules (JKM state-selected and focused similarly), explainable by the greater importance of the so-called hyperfine disorientation effect for the prolate symmetric top (CH3I) than for the t-C4H9I. For the latter, orientations with photofragment up–down asymmetry ratios as large as a factor of 10 can be achieved, suggesting that t-C4H9I is an excellent candidate reagent for reactive asymmetry studies.
Infrared and microwave investigations of interconversion tunneling in the acetylene dimer
Authors: Fraser G. T. , Suenram R. D. , Lovas F. J. , Pine A. S. , Hougen J. T. , Lafferty W. J. , Muenter J. S.
Published: July 2002

A sub-Doppler infrared spectrum of (HCCH)2 has been obtained in the region of the acetylene C–H stretching fundamental using an optothermal molecular-beam color-center laser spectrometer. Microwave spectra were obtained for the ground vibrational state using a pulsed-nozzle Fourier transform microwave spectrometer. In the infrared spectrum, both a parallel and perpendicular band are observed with the parallel band being previously assigned to a T-shaped C2v complex by Prichard, Nandi, and Muenter and the perpendicular band to a C2h complex by Bryant, Eggers, and Watts. The parallel band exhibits three Ka=0 and three asymmetry-doubled Ka=1 series. The transitions show a clear intensity alternation with Kc with two of the Ka=0 series missing every other line. In addition, the perpendicular band has the same ground-state combination differences as the parallel band. To explain these apparent anomalies in the spectrum, we invoke a model consisting of a T-shaped complex with interconversion tunneling between four isoenergetic hydrogen-bonded minima. In this picture, the parallel and perpendicular bands arise from excitation of the acetylene units parallel and perpendicular to the hydrogen bond. The observation of rotation–inversion transitions in the microwave spectrum, in addition to the pure rotation transitions of Prichard, Nandi, and Muenter, verifies the model. The measured microwave splittings yield a tunneling frequency of 2.2 GHz which is consistent with a ∼33 cm−1 barrier separating the four minima.
A
b i
n
i
t
i
o configuration interaction study of mixed BeLik clusters (k=1–9)
Authors: Pewestorf Wolfgang , Bonačić-Koutecký Vlasta , Koutecký Jaroslav
Published: July 2002

Electronic and geometric structure of mixed neutral BeLik and cationic BeLi+k clusters (k=1–9) has been investigated employing ab initio configuration interaction quantum chemical methods. The values of atomization energies per atom for BeLik with k<7 and for BeLi+k with k<8 increase strongly with the cluster size. The binding energy per atom remains nearly unchanged for neutral BeLik with k=7–9 and cationic BeLi+k clusters with k=8 and 9. The quantities which measure cluster stability towards fragmentation processes predict high stabilities for BeLi6 and BeLi+7. The geometries of cluster, their stabilities as well as other properties can be easily interpreted as simple consequences of the nodal properties of the cluster MOs. The connection between the results obtained from this work and from the superatom model has been pointed out. The specific nature of the chemical bonding in mixed clusters obtained from quantum chemical investigations has been analyzed.
On the electron affinities of hexafluorides CrF6, MoF6, and WF6
Authors: Miyoshi Eisaku , Sakai Yoshiko , Murakami Akinori , Iwaki Hiroaki , Terashima Hidemi , Shoda Takayuki , Kawaguchi Takashi
Published: July 2002

The adiabatic electron affinities (EAs) of MF6 and MF−6 (M=Cr, Mo, and W) are calculated in a configuration interaction (CI) calculation using a model potential method. The calculated EA of 3.85 eV for WF6 agrees well with the observed values. The difference (1.52 eV) between the calculated EA of MoF6 and that of WF6 shows also a very good agreement with the experimental ones. CrF6 has a very high EA of 8.24 eV. The CrF−6 anion has positive EA and the CrF2−6 dianion is thus most stable in the CrFq−6 (q=0, 1, and 2) sequence, while WF−6 does not have a positive EA. The EAs of MF6 calculated by CI calculations are smaller than those by SCF calculations. This negative correlation effect on the AEs is also discussed.
A three dimensional reactive scattering study of the photodetachment spectrum of ClHCl−
Authors: Schatz George C.
Published: July 2002

In this paper, the photodetachment spectrum of ClHCl− is simulated theoretically by calculating the Franck–Condon overlap between the negative ion ground state vibration/rotation wave function, and a ClHCl scattering wave function that is determined accurately using a three dimensional hyperspherical reactive scattering method. The resulting spectrum shows a broad low energy peak that is due to direct scattering, and a narrow higher energy peak that is mainly due to a resonance associated with the transition state region of the Cl + HCl reaction. These two peaks are in good agreement in position and relative intensity with a spectrum recently measured by Neumark and co-workers.
Suppression of fragment contributions to mass-selected resonance enhanced multiphoton ionization spectra of van der Waals clusters
Authors: Bieske Evan J. , Rainbird Mark W. , Knight Alan E. W.
Published: July 2002

A technique is described for suppressing the contribution due to fragmentation of higher mass clusters in mass-selected resonance enhanced multiphoton ionization spectroscopy of weakly bound van der Waals (vdW) clusters formed in a skimmed supersonic expansion. Suppression is achieved by ionizing upstream from the extraction region of a time-of-flight mass spectrometer and by using a delayed voltage pulse to extract the ions. Normally, spectra associated with a particular cluster ion, e.g., AB+n will contain contributions from fragments formed by the dissociation of higher mass clusters, e.g., AB+n+m. However, with upstream ionization and delay extraction, AB+n fragments formed by AB+n+m dissociation that have different kinetic energy and altered trajectories from the directly ionized AB+n clusters, are not extracted efficiently and are hence discriminated against in detection. The technique is demonstrated with the aniline–argon vdW complex.
Nonadiabatic effects in the vicinity of multiple surface crossings. Evaluation of derivative couplings with respect to rotational and internal degrees of freedom. Application to the charge transfer reaction H++NO→H+NO+
Authors: Yarkony David R.
Published: July 2002

The nonadiabatic charge transfer reaction H++NO→H+NO+ is considered. Regions of significant nonadiabatic effects are located for specific C∞v nuclear configurations which correspond to the confluence of three potential energy surfaces, the 1,2 2A′ and 1 2A″ potential energy surfaces. The 2 2A′ and 1 2A″ states correlate asymptotically with the H++NO(2Π) system states while the 1 2A′ state correlates asymptotically with the H(2S)+NO+(1Σ+) system state. The three surface confluence consists of two seams corresponding to hydrogen approaching NO from either the nitrogen or oxygen end and is energetically accessible from the H++NO(2Π) asymptote for re(NO+)≤R(NO)≤re(NO). The region of the three surface confluence represents a C∞v symmetry allowed Σ–Π crossing and consequently evinces properties of both a conical intersection and a Renner–Teller surface touching. It is therefore necessary to treat nonadiabatic (derivative) couplings originating from both internal nuclear motion and overall nuclear rotation. The rotational coupling provides a mechanism for the direct (and indirect via the 2 2A′ state) coupling of the 1 2A″ state correlating with H++NO(2Π) to the 1 2A′ state correlating with H+NO+(1Σ+). No such coupling is possible if only internal modes are considered. Derivative couplings attributable to internal nuclear motion are evaluated using analytic gradient techniques introduced previously [Chem. Phys. Lett. 113, 159 (1985)] while derivative couplings attributable to overall nuclear rotation, which can also be evaluated using gradient methods, are evaluated, more efficiently, in terms of matrix elements of the total electronic angular momentum operator. The nonadiabatic interactions in the vicinity of the three surface confluence are compared with the analogous interactions along paths leading to the HNO+ and NOH+ equilibrium structures. These are regions of general Cs nuclear configurations for which avoided crossings are observed. An analysis of the wave functions in terms of the molecular dipole moment vector is presented.
Quantum Monte Carlo method for some model and realistic coupled anharmonic oscillators
Authors: Caffarel M. , Claverie P. , Mijoule C. , Andzelm J. , Salahub D. R.
Published: July 2002

A new quantum Monte Carlo (QMC) method of evaluating low lying vibrational levels for coupled modes is presented. We use a modified fixed-node (FN) approach in which an extremum principle for energy levels is invoked. In this way, the nodal hypersurfaces of the nuclear wave function are parametrized and then optimized for each excited state. The method is tested on the fundamental excitations of some two-dimensional model potentials and is applied to the case of realistic coupled modes of the CO molecule adsorbed on a palladium cluster. The effect of an external electric field is also examined. The quantum Monte Carlo results are compared with those obtained in the conventional variational treatment of the nuclear Schrödinger equation for coupled vibrations. The QMC results give the exact values with an error which is in general less than 1 cm−1 . In all cases (even in the case of strong coupling) the use of our procedure leads to ‘‘optimal’’ nodal lines (in the sense of the extremum principle used in this work) which are practically undistorted. A salient feature of the Monte Carlo method presented here is that it readily permits the evaluation of the fundamental excitations of an arbitrary number of coupled vibrations. Furthermore, the potential energy surface may be represented by any analytical form without practical difficulties.
Picosecond dynamics of surface electron transfer processes: Surface restricted transient grating studies of the n-TiO2/H2O interface
Authors: Kasinski J. J. , Gomez-Jahn L. A. , Faran K. J. , Gracewski S. M. , Miller R. J. Dwayne
Published: July 2002

The surface restricted transient grating is demonstrated as a sensitive probe of ultrafast surface reaction dynamics. Studies of doped single crystal n-TiO2 (001) surfaces in air demonstrate linear trapping processes, assigned to crystal defects within the surface deformation layer, that limit carrier lifetimes to 5 ns. Direct in situ grating studies at photochemically active n-TiO2/H2O interfaces demonstrate that the dominant mechanism of interfacial electron transfer in this system involves thermalized hole carriers at the atomic surface. The dynamics are consistent with adsorbed OH− as the initial hole acceptor. In addition, optical generation of coherent surface acoustic modes is demonstrated. A detailed theory is presented for the grating excitation of the surface acoustics. Acoustic propagation in the H2O half-space of the TiO2/H2O liquid interface gives evidence for a phase change of the water layer at the polar TiO2 (001) surface to a solid phase.