bluuebunny/crossref_metadata_2025 huggingface dataset:
Infrared photodissociation and cluster-specific detection of internally cold (C2H4)n van der Waals complexes
Authors: Huisken Friedrich , Pertsch Thomas
Published: July 2002

A steady supersonic molecular beam of internally cold (C2H4)n clusters is crossed by the IR radiation of a pulsed CO2 laser. Subsequent scattering by a secondary He beam disperses the cluster beam and allows unequivocal detection of selected cluster species which is not disturbed by fragmentation processes in an ionizer. The dependence of IR photon absorption on cluster size is investigated in the region of the ν7 vibration of C2H4; predissociation spectra are recorded for (C2H4)n complexes with n=2, 3, 4, and 5. The absorption profiles are structureless and all have, regardless of cluster size, the same width (FWHM=12 cm−1) and peak at the same position (ν0=953 cm−1). From the dependence of the dissociation yield on the laser fluence, transition probabilities are determined for n=2, 3, and 4.
Investigation of three exchange energy formulas, Hartree–Fock, LDA, and 10/9 LDA
Authors: Kemister G.
Published: July 2002

Three different formulas for the exchange energy are compared through comprehensive self-consistent calculations on atoms and diatomic molecules. The formulas considered are the Hartree–Fock exchange, the local density approximation (LDA) exchange, and the first order approximation to the new Ghosh–Parr exchange formula which is equivalent to 10/9 LDA. The 10/9 LDA results are vastly superior to the LDA results and comparable or better than the Hartree–Fock results. Possible reasons for this are discussed.
Mechanism of electron attachment to van der Waals clusters: Application to carbon dioxide clusters
Authors: Tsukada Masaru , Shima Nobuyuki , Tsuneyuki Shinji , Kageshima Hiroyuki , Kondow Tamotsu
Published: July 2002

A theory on the attachment of very slow electrons to van der Waals clusters was developed on the basis of the electronic structure theory, and was applied to clarify the mechanism of the collisional electron transfer from a high-Rydberg atom to a CO2 cluster. The strong coupled electron–phonon model is found to afford a reasonable mechanism of the attachment. The equilibrium geometry of (CO2)N (2≤N≤13) clusters are determined and their vertical affinity levels are obtained by the DV-X α-transition state method. Using this information, as well as some plausible assumptions on the values of the coupling constants, the attachment cross section σ is evaluated as a function of the energy of the incident electron. The theory predicts the existence of the threshold cluster size for the attachment and a sharp decrease of σ with the energy, which are consistent with the experimental results.
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o calculations on C2, Si2, and SiC
Authors: Bauschlicher Charles W. , Langhoff Stephen R.
Published: July 2002

Full configuration-interaction (FCI) calculations have been performed for the X 1Σ+g, a 3Πu, and b 3Σ−g states of C2 and the X 3Σ−g and A 3Πu states of Si2 in valence double-zeta plus polarization one-particle basis sets. Of the various approximate correlation treatments compared to the FCI, the CASSCF/MRCI method is found to be far superior in reproducing the FCI separations between the low-lying states. Hence this procedure was taken to near the one-particle basis set limit using [5s4p3d2f1g] carbon and [6s5p3d2f1g] silicon atom Gaussian basis sets contracted using the newly devised atomic natural orbital (ANO) method. The computed C2 separations in the large basis set are in excellent agreement with experiment. The ground state of Si2 is definitively determined to be X 3Σ−g, but the A 3Πu state is only 440±100 cm−1 higher in energy. For SiC, the A 3Σ− state is nearly 4000 cm−1 above the X 3Π ground state. Our best estimates of the spectroscopic parameters for the X 3Π state of SiC are re=1.719 Å, ωe=962 cm−1 and De=4.4 eV.
Small Al clusters. I. The effect of basis set and correlation on the geometry of small Al clusters
Authors: Bauschlicher Charles W. , Pettersson Lars G. M.
Published: July 2002

The effect of basis set and correlation on the bond length and atomization energy of Al2 and Al4 are studied. For Al13 the effect of basis set improvements is investigated only at the SCF level. A large valence basis set, such as a triple-zeta contraction of the Huzinaga (12s9p) primitive set, is needed to avoid significant superposition error and a poor description of the bonding. An ECP is developed from the Huzinaga basis, which accurately reproduces the all-electron results. It is also shown that an ECP developed from a (10s6p) primitive set reproduces the all-electron results in the same basis, but is in poor agreement with the ECP and all-electron results based upon the larger (12s9p) primitive set; this has implications on how ECP basis sets should be developed for the second row atoms. The d functions are found to be important for all clusters considered. At the SCF level, the addition of d polarization functions results in a sizable (about 0.2 a0) contraction of the Al–Al bond length and increases the dissociation energy per atom by about 0.25 eV. The inclusion of electron correlation results in a small bond contraction, but a large increase in the De per atom. The increase in De and reduction in re with the inclusion of electron correlation is almost independent of the number of d functions.
Determination of cluster binding energy from evaporative lifetime and average kinetic energy release: Application to (CO2)+n and Ar+n clusters
Authors: Engelking Paul C.
Published: July 2002

The binding of a molecule within a cluster may be determined within the context of a simple statistical model from the measurements of cluster evaporative lifetime and average kinetic energy release. If the final product density of states is approximated by a Kassel distribution, the expression for lifetime can be inverted to give the binding energy. Stringent demands are placed upon the accurate determination of kinetic energy release, but only moderate demands are placed upon the accuracy of the lifetime or model parameters. The method is illustrated with positively charged carbon dioxide and argon clusters. Binding energy determinations agree with previous results that CO2 molecules are bound within the cluster with an average of 156±26 meV. The binding energy for Ar atoms decreases, from 150 meV within clusters of size 6, to 60 meV for clusters of size 22. The ±16% average error for the argon clusters represents mostly the uncertainty in the kinetic energy measurements.
Spectroscopic identification of rovibronic levels lying above the potential barrier of the E
F 1Σ+g double-minimum state of the H2 molecule
Authors: Senn P. , Dressler K.
Published: July 2002

Spectroscopic data on the higher vibrational levels of the EF 1Σ+g state of the H2 molecule are scarce. Here we report new identifications of energy levels with J=0–5 which fall into the range between vEF=9 and 20, i.e., the energy region above the potential barrier of the double-minimum EF potential curve. Some previously published line assignments and term values are hereby corrected. The results are mostly based on new assignments of EF 1Σ+g–B 1Σ+u bands in Dieke’s hydrogen molecule wavelength tables published in 1972. The newly identified states are of interest in multiphoton spectroscopy and in comparisons of experimental properties with results of ab initio calculations.
Modulation technique for measuring diffusion coefficients of Ba in noble gases
Authors: Walker T. G. , Bonin K. D. , Happer W.
Published: July 2002

The diffusion coefficient of Ba (6s5d 3DJ) in noble gases was measured with a modulation/lock-in technique. This technique relies on a cylindrically symmetric source for the metastable state, and is independent of cell geometry. It is readily generalizable to other systems where the creation rate of a long-lived state can be modulated.
Photoionization of potassium clusters: Neutral and ionic cluster stabilities
Authors: Bréchignac C. , Cahuzac Ph. , Roux J. Ph.
Published: July 2002

Neutral potassium clusters of two different mass distributions generated either by a sonic or a conical nozzle were photoionized at several ionizing energies. By combining different nucleation conditions with several photoionization situations, variations of ion mass spectral patterns reflecting either neutral or ionic stabilities were obtained. The spectra show peaks or steps for those neutral and ionized clusters which contain 8, 20, and 40 electrons. These can be understood in terms of electronic shell structure for both neutral and ionized potassium clusters. However local irregularities appearing for the small cluster sizes reveal the influence of their geometrical structure. Moreover ionizing laser power dependencies taken at various photon energies were used to study fragmentation effects following ionization.
Collision-induced dissociation of aluminum cluster ions: Fragmentation patterns, bond energies, and structures for Al+2–Al+7
Authors: Hanley Luke , Ruatta Stephen A. , Anderson Scott L.
Published: July 2002

Collision-induced dissociation (CID) of cooled, mass selected aluminum cluster ions (Al+2–7) by xenon, has been studied over an energy range of 0–10 eV (center of mass). These experiments were carried out in a new apparatus which is described in detail. From the product branching ratios and cross section magnitudes we derive qualitative structural information about the cluster ions. The fragmentation thresholds are analyzed to yield dissociation energies, approximate ionization potentials, and further structural information about the cluster ions and their neutral counterparts. Cluster stabilities range from 0.85±0.40 eV for Al+4 to 2.25±0.70 eV for Al+7. The results provide a stringent test for recent calculations on Al2–6.
Classical, quantum mechanical, and semiclassical representations of resonant dynamics: A unified treatment
Authors: Martens Craig C. , Ezra Gregory S.
Published: July 2002

This paper addresses the general problem of zeroth order representation of resonant dynamics. We investigate the classical, quantum mechanical, and semiclassical transformation properties of two-dimensional isotropic and anisotropic uncoupled harmonic oscillators. The classical and quantal theories are presented in a manner that emphasizes the strong correspondence between the two, and in particular, the SU(2) symmetry exhibited by both the classical and quantum oscillators. The classical canonical transformations relating the action-angle variables appropriate for normal, local, and precessional motion of the isotropic oscillator are derived by explicit calculation of the generating functions. By employing a simple mapping relating the anisotropic and isotropic oscillators, expressions for action-angle variables appropriate for the topology of an arbitrary m:n resonance are determined. The resulting invariant tori are compared with the corresponding quantum mechanical wave functions and phase space densities. The relationship between the classical and quantum mechanical theories is illustrated by determining semiclassical approximations to the unitary transformation matrix elements, which are given in terms of the classical generating functions. Applications to problems of current interest, such as the adiabatic switching method for semiclassical quantization of nonseparable systems, are briefly discussed.
Photodissociation of rare gas cluster ions: Ar+3
Authors: Albertoni C. R. , Kuhn R. , Sarkas H. W. , Castleman A. W.
Published: July 2002

The photodissociation spectrum of the mass selected cluster ion Ar+3, produced by electron impact ionization of a supersonic cluster beam, has been measured for the first time. Measurements over the wavelength range 539 to 620 nm show the cross section to rise and peak near 545 nm and then decrease steadily throughout the longer wavelength range; the magnitude of the cross section is found to be smaller than the corresponding 2Σ+u→2Σ+g transition of Ar+2 in agreement with some theoretical predictions. Interestingly, Ar+ is the only detected photoproduct.
Evidence of structural isomerism in small niobium clusters
Authors: Hamrick Y. , Taylor S. , Lemire G. W. , Fu Z.-W. , Shui J.-C. , Morse M. D.
Published: July 2002

The reactivity of Nb clusters with nitrogen and deuterium was studied using a pulsed laser vporization source coupled to a fast flow reactor. The relative reactivity of the various clusters was determined by monitoring the depletion of the cluster ion signal as a function of reactant flow rate. A kinetic analysis of the chemisorption is given. The geometrical isomerism of the cluster ions and its relationship to reactivity is discussed.(AIP)
The interplay of structure and dynamics in the melting of small clusters
Authors: Beck Thomas L. , Berry R. Stephen
Published: July 2002

Extensive classical simulations of the melting–freezing transition of small clusters (N=7–33) of rare gas atoms have been performed in which quenching by steepest descent has been coupled to isoergic molecular dynamics. A mechanistic description of the phase change is given in terms of the local potential minima accessed in the transition region and the isomerization pathways for and the frequencies of interwell passages. All of the small clusters, at energies low in the transition range of energy, exhibit some separation (by factors of approximately 5 to 60) of the short time scale for motions about the various potential minima and the longer time scale separating interwell passages. The onset of diffusion is marked by passages over saddles linking the minima. Fully liquid-like behavior is observed for all the clusters when the time scale separation for the motions no longer exists. The coexistence and magic number phenomena observed in previous simulations are explained in terms of the kinds of potential minima on the surface and the accessibility of one from another. The existence of a potential energy minimum very low relative to the nearest accessible, high-lying minima as well as time scale separation are necessary conditions for the observation of the kind of coexistence of liquid-like and solid-like forms over a well-defined energy or temperature range predicted by a quantum statistical model and observed, e.g., in isoergic and isothermal simulations of the Ar13 cluster. Such conditions are met in some but not all clusters. Structures with underlying pentagonal, and especially icosahedral, symmetry are important for clusters in the size range N=7–33, not only in previously recognized cases; defective icosahedral structures occur among the lower-energy minima even for some clusters for which they had not been considered.
Polymer conformation in elongational flow
Authors: Rabin Y.
Published: July 2002

The elongational-flow-induced deformation of a polymer segment of arbitrary length and location along the contour of a macromolecule is calculated neglecting excluded-volume and nondraining effects. The resulting expression is valid for arbitrary flow strain rates in both steady-state and transient regimes of extension. For strain rates approaching a critical value at which a steady-state solution ceases to exist, the center of the macromolecule becomes much more stretched than its ends and it is argued that as a result of this nonuniform elongation, the stretching cannot be characterized by a single time scale corresponding to the uniform deformation of the polymer. The emerging picture of the process agrees with earlier conjectures made by the author and with Ryskin’s ‘‘yo-yo’’ model of polymer stretching in elongational flow.
Electron spin resonance of the C6, C8, and C1 molecules
Authors: Van Zee R. J. , Ferrante R. F. , Zeringue K. J. , Weltner W. , Ewing D. W.
Published: July 2002

Electron-spin-resonance (ESR) signals attributed to the linear C6, C8, and C10 molecules in their lowest 3Σ states, presumably their ground states, have been observed in solid neon and argon matrices at 4 K. There is evidence of two forms of the C10 molecule, perhaps indicating two slightly bent structural isomers. Laser vaporization of graphite and 13C-enriched graphite produced a high proportion of these larger molecules. Hyperfine interaction in the 13Cn molecules was small and resolved only for C6, indicating cumulene-type bonding with the unpaired spins in pπ orbitals, as in C4. The zero-field-splitting parameters ‖D‖ were found to be 0.363, 0.783, and 0.190 cm−1, respectively, in solid neon. The increase in ‖D‖ through C8 is attributed to a corresponding variation in the spin–orbit coupling with low-lying states, principally the 1Σ+g, as the chains lengthen. Gross orbital spin populations and 1Σ+g– X 3Σ−g energy differences were obtained from Hartree–Fock calculations in order to interpret the hfs and ‖D‖ data, respectively. Electron correlation was included via second and third order Mo/ller–Plesset perturbation theory. The possibility of quasilinear or nonlinear character in these chains is briefly considered. Relative concentrations of the linear and cyclic forms of these molecules in the vapor and in matrices were estimated from thermodynamics using their theoretically derived properties.
Stochastic trajectory studies of small argon cluster scattering from Pt(111)
Authors: Xu Guo-Qin , Bernasek Steven L. , Tully John C.
Published: July 2002

We have carried out stochastic trajectory calculations of the scattering of small argon clusters, n=1,2,3, and 5, from a Pt(111) surface at incident energies of 0.1 and 0.5 eV per atom. We employed a 6 by 6 by 2 layer slab of platinum atoms with periodic boundary conditions imposed in the x and y (surface plane) directions. We applied local friction and white random forces in the z (surface normal) direction to the bottom surface layer to maintain the proper temperature and to account for energy transfer with the bulk. We assumed Lennard-Jones interactions for Ar–Ar and Ar–Pt with realistic parameters. We have found that the scattering of small clusters from surfaces exhibits a very different behavior from the scattering of individual atoms or molecules. At the collision energies considered, most clusters fragment into atoms upon impact with the surface, but a surprising number survive either partially or totally intact. Angular distributions of the fragmented monomers are much broader than those of surviving clusters. The average energy of the fragmented monomers increases with scattering angle, the reverse of the ‘‘hard-cube’’ trend for atoms scattering from surfaces. In addition, cluster scattering is associated with an enhanced trapping probability and enhanced initial lateral mobility of the trapped species in comparison to individual atom scattering. A sequential binary collision model invoking gas–surface and gas–gas collisions is suggested to account for these results.
Corrected effective medium method. II. N-body formulation
Authors: Kress Joel D. , DePristo Andrew E.
Published: July 2002

A general corrected effective medium (CEM) theory is presented which yields the interaction energy of an N-atom system, in contrast to the previous version of the CEM theory which provides the energy of one atom interacting with the other (N−1) atoms acting as a host. The CEM method presented herein treats all N atoms on an equal basis without identifying all but one as a host, and is referred to by the acronym CEM-N. The basis for this theory involves expressing the interaction energy for the real system in terms of the sum of the interaction energies for each atom embedded into a homogeneous electron gas with compensating positive background (i.e., the effective medium is jellium). Minimization of the difference in kinetic-exchange-correlation energy between the real and effective system, evaluated using density functionals and the approximation of superposition of atomic densities for the system density, yields the prescription for choice of the electron densities of each jellium system. The full interaction energy then consists of three terms: the embedding energy, Coulombic energy, and kinetic-exchange-correlation difference energy. Applications and tests for the C2 , N2, and O2 molecules are presented using the SCF-LD embedding energies of Puska et al. The quality of these results illustrate the need for a new set of universal ‘‘covalent’’ embedding energies, which are constructed semiempirically in the present article.
The ground, excited, and negatively ionized states of Fe2
Authors: Tomonari Mutsumi , Tatewaki Hiroshi
Published: July 2002

The Fe2 molecule was studied by ab initio SCF and CI calculations. The basis sets as well as reference configuration state functions were carefully chosen. Results showed the ground state of the molecule to be 7Δu. The state could not be expressed as a single configuration state function. The equilibrium nuclear distance was 2.02 Å, which is close to the experimental value (1.87–2.02 Å). Resulting dissociation energy was 0.94 eV relative to 3d7 4s1 5F but −1.29 eV relative to 3d6 4s2 5D. In the anion state, an attached electron was found to be in an antibonding orbital (4pσu), and calculated electron affinity (0.45 eV) was favorably compared with the experimental value (0.90 eV). Calculations revealed the excited states to lie at 0.69 eV above the ground state, while that observed by negative ion photoelectron spectroscopy was 0.53 eV. A 20% reduction of vibrational frequency was also found in the excited and anion states. Theoretical results were in harmony with experimental ones, except for the dissociation energy value.
Chemical probes of metal cluster structure: Reactions of iron clusters with hydrogen, ammonia, and water
Authors: Parks E. K. , Weiller B. H. , Bechthold P. S. , Hoffman W. F. , Nieman G. C. , Pobo L. G. , Riley S. J.
Published: July 2002

Evidence is presented for structural changes in iron clusters in the Fe13 to Fe23 size range. Abrupt changes with cluster size are found for several chemical properties, including reactivity with hydrogen and binding energies of ammonia and water. These changes often come at the same cluster sizes, pointing to a common origin—fundamental changes in the structure of the bare iron clusters. In addition, changes in structure as a consequence of adsorbate binding are suggested. The experimental observations leading to these conclusions are detailed, and possible structures for clusters in this size range are proposed.
Absolute cross sections for electron-impact ionization and dissociative ionization of the SiF free radical
Authors: Hayes Todd R. , Wetzel Robert C. , Baiocchi Frank A. , Freund Robert S.
Published: July 2002

Absolute cross sections for electron-impact ionization of the SiF free radical from threshold to 200 eV are presented for formation of the parent SiF+ ion and the fragment Si+ and F+ ions. A fast beam of SiF is prepared by charge transfer neutralization of an SiF+ beam. The radicals form in the ground electronic state and predominantly in their ground vibrational state, as shown by agreement of the measured ionization threshold with the ionization potential. The absolute cross section for SiF→SiF+ at 70 eV is 3.90±0.32 Å2. The ratio of cross sections for formation of Si+ to that for SiF+ at 70 eV is 0.528±0.024; the ratio for formation of F+ to that of SiF+ is 0.060±0.008. The observed threshold energy for Si+ formation indicates the importance of ion pair formation SiF→Si++F−. Breaks in the cross section at 14.3 and 17 eV are assigned as dissociative ionization thresholds.
Effect of nuclear hyperfine structure on microwave spectral pressure broadening
Authors: Green Sheldon
Published: July 2002

The spectral pressure broadening formalism of Ben-Reuven [Phys. Rev. 145, 7 (1966)] is applied to rotational transitions of a closed-shell linear molecule with nuclear quadrupole hyperfine structure (hfs) due to a nucleus of spin I. If, as expected, nuclear spin does not affect molecular collision dynamics, generalized pressure broadening cross sections can be expressed in terms of the spin–free collisional S matrices. For the three hfs components of the lowest j=0–1 rotational transition, the line shape is a simple sum of three noninterfering Lorentzians each of which has the same width and shift as would be expected in the absence of nuclear spin. For higher rotational transitions, however, the line shape is no longer so simple; in general, each hfs component is described by a different width and shift, and collisions transfer intensity among them. Numerical results for HCN broadened by He atoms are presented using both the accurate close coupling (CC) collision formalism and also the infinite order sudden (IOS) approximation. For the case that broadening is very large compared with the hfs splittings it is shown (numerically, within the IOS approximation) that the line shape is nearly (but not exactly, except for j=0–1 as noted above) a Lorentzian with the same width as would be expected in the absence of nuclear spin.
Observation of ionic excimer states in the alkali halides
Authors: Steigerwald F. , Langhoff H. , Hammer W.
Published: July 2002

Ionic excimers, which are isoelectronic to the rare gas halides, are investigated. In a charge transfer reaction of He+ or Ne+ ions to CsF and RbF, continua in the VUV at 184.5 and 131 nm, respectively, have been observed, which show the same properties as the analogous transitions in the rare gas fluorides. The results are compared with predictions by a Rittner model. In KF a transition at 143 nm was observed which could be assigned to the transition of KF+→K+F.
Electron attachment and electron impact ionization of SF6 and SF6/Ar clusters
Authors: Stamatovic A. , Scheier P. , Märk T. D.
Published: July 2002

Electron attachment and electron impact ionization of SF6 and SF6/Ar clusters have been investigated in a molecular beam/electron impact ion source/mass spectrometer system. Positive ion mass spectra of neat SF6 clusters contain [(SF6)n⋅SFx]+ fragment ion series with 0≤x≤5 and in addition [(SF6)nS2F5]+ ions which have to be formed via international ion molecule reactions. Positive ion mass spectra of mixed SF6/Ar clusters include ion series of nonstoichiometric fragment ions (SF5⋅Arn)+, (SF6⋅SF5⋅Arn)+, [(SF6)2⋅SF5⋅Arn]+, and, in addition, stoichiometric ions comprised of (SF6⋅Arn)+ and [(SF6)2Arn]+. These stoichiometric ions are only present with n≥3. Electron attachment to neat SF6 clusters gives rise only to stoichiometric (SF6)−n ions, whereas attachment to mixed SF6/Ar clusters yields ion series consisting of (ArnSF6)−, [Arn(SF6)2]−, and [Arn⋅(SF6)3]−. This indicates that the dissociative channel into SF−5 present in the monomer is efficiently quenched in case of clusters. The negative ion mass spectra show in contrast to the positive ion mass spectra a strong dependence on electron energy due to a strong resonance of the attachment cross sections near zero electron energy.
Optothermal-infrared and pulsed-nozzle Fourier-transform microwave spectroscopy of rare gas–CO2 complexes
Authors: Fraser G. T. , Pine A. S. , Suenram R. D.
Published: July 2002

Sub-Doppler infrared spectra of Ne–CO2, Ar–CO2, and Kr–CO2 have been recorded near 3613 and 3715 cm−1, in the region of the 2ν02+ν3/ν1+ν3 Fermi diad of CO2, using an optothermal molecular-beam color-center laser spectrometer. In addition, pulsed-nozzle Fourier-transform microwave spectra are reported for the ground vibrational states of the complexes. The infrared and microwave spectra are consistent with T-shaped complexes as shown originally by Steed, Dixon, and Klemperer for Ar–CO2.1 The infrared band origins for the Ar and Kr complexes are red shifted, from that of free CO2, by 1.09 and 0.95 cm−1 for Ar–CO2 and by 1.97 and 1.76 cm−1 for 84Kr–CO2. For Ne–CO2, blue shifts of 0.15 and 0.19 cm−1 are observed. The lower Fermi components are free of perturbations, whereas the upper components of Ar–CO2 and Kr–CO2 are perturbed. For Ar–CO2 the perturbation is strong, shifting the positions of the observed Q-branch lines of the Ka =1←0 subband by as much as 500 MHz.
Resonances in molecular photoionization. III. Multichannel extension and application to polyatomic molecules
Authors: Sobolewski A. L. , Domcke W.
Published: July 2002

Starting from a model Hamiltonian comprising an arbitrary number of discrete electronic configurations and an arbitrary number of electronic ionization continua as well as the vibrational degrees of freedom, a comprehensive theoretical description of near-threshold autoionization structure in molecular photoionization is developed. The discrete–continuum interaction is treated to infinite order and the infinite Rydberg series converging to the ionization thresholds are included as a whole in the treatment of the nuclear dynamics. The equivalence of the Feshbach projection-operator formulation and the multichannel-quantum-defect description is explicitly established in this rather general context. We derive a simplified model from the general formalism which is shown to reproduce naturally some recently observed features of vibronic autoionization via nontotally symmetric modes in polyatomic molecules.
Diode laser spectroscopy of the ν2 band of CD3
Authors: Frye Joan M. , Sears Trevor J. , Leitner David
Published: July 2002

CD3 radicals were generated by excimer laser photolysis (248 nm) of CD3 I, and the transient populations detected by diode laser absorption using a gated integrator signal averager. The v2 =1←0, 2←1, and 3←2 bands of the CD3 radical were observed, and band center frequencies were determined to be 457.8133(11), 507.9311(19), and 542.4510(29) cm−1, respectively. Observed transitions were fitted, by least squares analysis, to yield rotational and centrifugal distortion constants and spin–rotation parameters, and results are compared to theory. Measurements were also carried out of the time dependence of the vibration–rotation level populations as a function of vibrational excitation and spin component.
The Stark effect and polarizabilities for a diatomic molecule
Authors: Bishop David M. , Lam Brenda , Epstein Saul T.
Published: July 2002

The influence of a uniform static electric field on both a single and a collection of diatomic molecules is analyzed in detail. Comparison is made with less rigorous and more ad hoc treatments and numerical results are given for 7LiH and 19FH which show the limitations of frequently invoked approximations.
Polarization-detected transient gain studies of relaxation processes in v4=1 Ã 1A2 formaldehyde-h2
Authors: Vaccaro P. H. , Temps F. , Halle S. , Kinsey J. L. , Field R. W.
Published: July 2002

Polarization-detected transient gain spectroscopy (PTGS), a time-resolved PUMP/PROBE scheme with single rotation–vibration level selectivity in both preparative and monitoring stages, has been used to study total depopulation and rotational state-to-state rates in the H2CO Ã 1A2 v4=1 vibrational level. For H2CO(Ã)/H2CO(X̃) collisions, single rovibronic level depopulation rates in the range 88±2 to 127±6 μs−1 Torr−1 (∼10 times faster than the hard sphere gas kinetic rate) were obtained from Stern–Volmer plots which were found to be linear over at least the 0.02–0.2 Torr pressure region. Rotational relaxation between the 50,5 and 60,6 eigenstates reflects a-dipole propensity rules with the sum of first order ΔJ=+1 and ΔJ=−1 processes accounting for well over 50% of the total removal cross section measured for a single rovibronic level. The agreement between collisional decay rates for the 132,12 eigenstate measured previously by the TGS technique and here by PTGS, 110±3 and 106±4 μs−1 Torr−1, respectively, suggests that elastic reorientation processes (ΔJ=ΔKa =ΔKc =0, ΔM≠0) occur with much less efficiency than inelastic energy transfer. Collisional depopulation from single rotational levels of H2CO Ã 1A2 v4=1 is a remarkably efficient process, but completely understandable in the small molecule, isolated binary collision limit. None of the multiexponential decay and nonlinear Stern–Volmer effects manifest in previous undispersed fluorescence studies remain when PTGS is used to populate and monitor a single rovibronic level.
The evolution of the wave function in a curve crossing problem computed by a fast Fourier transform method
Authors: Alvarellos José , Metiu Horia
Published: July 2002

We develop a unitary fast Fourier transform method for solving time dependent curve crossing problems. The procedure is described in detail and is illustrated by calculations for a two curve, one-dimensional example. The time evolution of the wave function and mean nuclear positions and energies for each curve are shown and discussed.
A picosecond bleaching study of quantum-confined cadmium sulfide microcrystallites in a polymer film
Authors: Hilinski Edwin F. , Lucas Patricia A. , Wang Ying
Published: July 2002

We report a picosecond pump–probe study of 55 Å cadmium sulfide microcrystallites embedded in polymer films. Large negative absorbance changes at wavelengths corresponding to energies near the band gap are observed. This absorption bleaching and the associated changes in refractive index are mainly responsible for the large nonlinearity observed in degenerate four-wave mixing experiments. Based on photoluminescence data, the known electron-trapping cross section of defects, and these pump–probe experiments, we show that the conventional carrier density-dependent band-filling mechanism cannot account for the data, and the absorption bleaching is due to the saturation of the excitonic transition. We further show that the phase-space filling and exchange effects from exciton–exciton and exciton-free carrier interactions fail to account for the observed data. Instead, we propose that the exciton-trapped carrier interaction is mainly responsible for the observed bleaching of the excitonic absorption. This interaction is unique for small semiconductor clusters since the presence of a high density of defects (most likely on the surfaces) causes the extremely rapid trapping of free carriers. According to this model, the recovery time of the absorption bleaching is determined by the trapped-carrier relaxation time, which is sensitive to the fabrication methods and can be controlled by surface chemistry. Our study also demonstrates that one needs to understand the effects of surfaces and control the surface chemistry before the important question of size effects on the nonlinear optical properties can be addressed.
Decay pathways of excited electronic states of Group IV tetrafluoro and tetrachloro molecular ions studied with synchrotron radiation
Authors: Lambert I. R. , Mason S. M. , Tuckett R. P. , Hopkirk A.
Published: July 2002

This paper describes experiments to probe the dynamics and decay pathways of the C̃ 2T2 and D̃ 2A1 excited electronic states of Group IV tetrahalide molecular ions MX+4 (M=C, Si, Ge; X=F, Cl) in the gas phase. Tunable vacuum UV radiation from a synchrotron source is used to ionize MX4 into these electronic states of MX+4. Flouorescence from ions initially produced or from fragments is monitored undispersed by a suitable pm tube. When the synchrotron is operated cw, such synchrotron-induced fluorescence spectra give energy thresholds for fluorescence, and in favorable cases an estimate of the fluorescence quantum yield (of MX+4 C̃ or D̃) can be made. When the synchrotron is operated pulsed, radiative lifetimes can be measured as a function of excitation energy. The fluorides and chlorides display very different decay properties. The dynamical behavior of these states is rationalized (a) with respect to their spectroscopic properties, and (b) with respect to the dissociation channels energetically ‘‘open’’ to them.
Variance and correlation length of energy estimators in Metropolis path integral Monte Carlo
Authors: Giansanti A. , Jacucci G.
Published: July 2002

Variance and error analysis of energy estimators for path integral Monte Carlo simulations is presented, taking the harmonic oscillator as an example and using the standard Metropolis sampling algorithm. A quantitative treatment of the persistence of correlation in the samples leads to conclusions that can be of general relevance for error estimates in this kind of computation.
The resonance Raman and visible absorbance spectra of matrix isolated Mn2 and Mn3
Authors: Bier K. D. , Haslett T. L. , Kirkwood A. D. , Moskovits M.
Published: July 2002

The resonance Raman spectrum obtained from manganese containing argon matrices previously assigned to the dimer [M.Moskovits, D.P.DiLella, and W.Limm, J. Chem. Phys. 80, 626 (1984)] is shown to belong to a zero order D3h trimer subject to Jahn–Teller distortions. The actual resonance Raman spectra of the dimer in both argon and krypton matrices are reported and discussed along with the visible absorption spectra. The resonance Raman of manganese dimer in krypton gives the 1Σ+g ground state constants of ωe =76.4 cm−1 and ωexe=0.53 cm−1. Trimer constants from the resonance Raman spectra in argon are (ωs−2xs)=196.79 cm−1, ωa=130.15 cm−1, Dωa =2.95 cm−1, xa=0.06 cm−1, and xas=0.79 cm−1. Independent values of ωs and xs could not be determined from our spectral data.
Vibrational predissociation in the CO2 dimer and trimer and rare gas–CO2 complexes
Authors: Pine A. S. , Fraser G. T.
Published: July 2002

Vibrational predissociation linewidths for the CO2 dimer and trimer and the Ne–CO2 and Ar–CO2 complexes have been resolved using a bolometer-detected (optothermal) molecular-beam color-center laser spectrometer. Observations were made on the pair of vibrations near 3715 and 3613 cm−1 corresponding to the ν1+ν3/2ν02+ν3 Fermi diad of CO2. Homogeneous linewidths of from ∼0.5 to ∼22 MHz (FWHM) were measured for these related complexes, with Ne–CO2 exhibiting both the broadest and the sharpest lines for the upper and lower bands, respectively. Ar–CO2 and (CO2)2 showed mode-independent intermediate predissociation rates while only the lower band of (CO2)3 could be found. The results indicate that V→V energy transfer processes are the dominant predissociation channels with symmetry selection or propensity rules and specific resonances playing a role.
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o calculations of the electronic structure of small Nan, Na+n, Kn,  and K+n clusters (n≤6) including core–valence interaction
Authors: Spiegelmann F. , Pavolini D.
Published: July 2002

The electronic structure of small Nan, Na+n, Kn, and K+n alkali clusters (n≤6) is investigated using nonempirical core pseudopotentials, configuration interaction within a 3s/1p/1d Gaussian basis set per atom and including the core–valence interaction through a perturbative treatment. Equilibrium geometries, stabilities, ionization potentials, and fragmentation channels are derived and the role of electronic correlation in small alkali clusters is examined.
Electron attachment to ammonia clusters: A study using path integral Monte Carlo calculations
Authors: Marchi Massimo , Sprik Michiel , Klein Michael L.
Published: July 2002

Path integral quantum Monte Carlo calculations have been used to study the nature of electron attachment to ammonia clusters composed of 16, 36, and 54 molecules. At 100 K, a negatively charged cluster of 16 molecules appears to be unstable in the sense that the electron binding energy is less than kBT. For both the 36 and 54 molecule clusters the electron binds to the cluster surface. The 54 molecule cluster also supports a (meta) stable interior solvated state. These findings are discussed in the light of experimental data on the same system.
Chemistry of semiconductor clusters: A study of the reactions of size selected Si+n (n=3–24) with C2H4 using selected ion drift tube techniques
Authors: Jarrold M. F. , Bower J. E. , Creegan K.
Published: July 2002

The chemical reactions of size selected Si+n (n=3–24) clusters with C2H4 have been studied at thermal energies using selected ion drift tube techniques. Except for Si+3 (which dehydrogenates C2H4 to yield Si3C2H+2) the dominant products arise from adsorption of C2H4 onto the silicon clusters. The reaction rates show large variations with cluster size. Si+13 and Si+14 were found to be particularly unreactive. The reactions were studied over a temperature range of 143–298 K. The reactivity increases as the temperature is lowered indicating that variation in the rates observed for the different cluster sizes is not due to an activation barrier, but reflects the stability of the SinC2H+4 adducts. Statistical phase space theory was used to model the reactions of Si+4–Si+10 and provide an estimate of the binding energy of the first C2H4 to these clusters. The binding energies vary between 0.8 and 2.0 eV. Binding energies of this magnitude are too small to be accounted for by strong di-σ bonding, suggesting that the C2H4 molecule is bound to the silicon clusters by a weaker π-bonding interaction. Kinetic evidence for the presence of structural isomers was found for several clusters. With Si+9 the relative abundance of an unreactive isomer could be changed from ∼1% up to 17% by changing the source conditions. More than one C2H4 molecule will adsorb onto the silicon clusters (up to eight adsorb on Si+4). Rate constants for the adsorption of the first few (up to seven) C2H4 molecules on Si+n (n=4–10) have been determined. They show large variations with the number of adsorbed C2H4 molecules.
Infrared multiphoton dissociation of SF6 at long wavelengths
Authors: Del Bello U. , Fuss W. , Kompa K. L. , Tablas F. M. G.
Published: July 2002

The dissociation probability Pd of 32 SF6 was investigated between 908 and 875 cm−1 at room temperature and at 140 K. In the region of the expected Pd maximum of 36SF6 (≊905 cm−1), we found a ten times higher Pd(32) (implying a ten times smaller selectivity for 36S) than expected from extrapolation of shorter wavelength data. The Pd spectrum in this and in other regions point to an important role of direct multiphoton transitions to levels containing quanta of the neighboring combination mode ν5+ν6 (870 cm−1) besides ν3 quanta. On the short wavelength side of ν3, the analogous transitions involve the ν2+ν6 mode (991 cm−1). The wings, and thus the spectral shape of Pd, in the ν3 region are also governed by them. They depend more strongly on temperature than stepwise transitions, because the intermediate near-resonances favor the high rotational states.
Calculations of the ionization thresholds and electron affinities of the neutral, positively and negatively charged C6—‘‘follene-60’’
Authors: Rosén Arne , Wästberg Bo
Published: July 2002

Ionization thresholds and electron affinities are calculated within the local density approximation for the neutral, positively and negatively charged clusters of C60. The evaluated energies are found to be in good agreement with available experimental data.
Time dependent calculations of the absorption spectrum of a photodissociating system with two interacting excited electronic states
Authors: Jiang Xue-Pei , Heather Robert , Metiu Horia
Published: July 2002

We use a time dependent method for solving the Schrödinger equation to calculate the photon absorption cross section for the photodissociation of a model H+3 system. The coupling V between the excited states is found to alter the absorption cross section if the time scale ℏ/V is less than the dissociation time. The influence of the relative orientation of the transition dipoles, on the absorption spectrum, is also investigated.
Examination of CD3 vibrational structure by resonance Raman spectroscopy
Authors: Westre S. G. , Kelly P. B.
Published: July 2002

The application of the resonance Raman spectroscopic method to the deuterated methyl radical is reported. Spectroscopic constants for CD3 are compared with previously reported values for CH3. The experimental and theoretical molecular physics of the methyl radical are discussed.
Dissociation energies of the intermetallic molecules CuIn, AgIn, and AuIn
Authors: Balducci G. , Di Nunzio P. E. , Gigli G. , Guido M.
Published: July 2002

The gaseous diatomic intermetallic molecules of the IA-IIIB groups, elements CuIn, AgIn, and AuIn, were investigated in a Knudsen cell-mass spectrometric study of the vapors over the respective alloys. From a number of all-gas equilibria their dissociation energies have been derived: Do0(CuIn) =183.7±7.9, Do0(AgIn) =162.8±4.9, Do0(AuIn) =282.3±5.7 kJ/mol. These values were compared to the values calculated from empirical models of bond. Of particular interest is the strikingly high dissociation energy of AuIn which was interpreted in terms of relativistic effects rather than due to multiple bond. The implications in the indium electronegativity value and the mode of formation of AuIn(g) are also discussed.
The effect of radiative transport on fluorescence emission
Authors: Martinho J. M. G. , Maçanita A. L. , Berberan-Santos M. N.
Published: July 2002

A model for radiative transport of electronic excitation energy in solution is presented and applied to the time-resolved and steady-state fluorescence of DPA (9, 10-diphenylanthracene) in benzene. The model predicts a nonexponential and wavelength-dependent decay at high concentrations, in agreement with experimental results. Recovered parameters, along with the time-resolved emission spectrum, are interpreted on the basis of a progression of the excitation with time farther into the cell, after the excitation pulse.
J dependence of χa(14N) for the Ar–HCN dimer: Coupling of stretching and bending in the potential function
Authors: Klots T. D. , Dykstra C. E. , Gutowsky H. S.
Published: July 2002

The previously reported rotational spectrum of Ar–HCN [J. Chem. Phys. 81, 4922 (1984)] has shown the weakly bound dimer to be highly nonrigid. Superficially linear, the dimer has several anomalies, including large centrifugal distortion and an unexpectedly large bending amplitude of the HCN. We here describe high-resolution rotational spectra which identify another anomaly. The 14N hyperfine interaction constant of the dimer increases linearly with J(J+1) for Ar–HCN, 36Ar–HCN, and Ar–DCN, indicating a decrease in the average HCN bending amplitude (θ). For Ar–HCN this is from 30.97° for J=0 to 30.17° for J=5. At the same time, there is an increase in the average Ar to HCN c.m. separation R from 4.3433 to 4.3496 A˚. The cause of this behavior and of the other anomalies is found to be the shape of the potential function as calculated with a largely classical electrical model employing low-order moments and multipole polarizabilities. The calculated potential surface exhibits strong coupling between radial and angular motions, with smaller angles favored at larger R. There is an axial hump in the potential curve generated by rotating the HCN through the linear structure, and the height of the hump is sensitive to R. In contrast, a hump is not found in the equilibrium region of the potential surface for Ar–HF. The difference between the two surfaces arises from the combination of their similar electrical interactions with different hard-wall effects. Basically, the differences in the shapes of HF (nearly spherical) and HCN (cigar shaped) give interaction surfaces that cause the properties of Ar–HCN to contrast with those of Ar–HF. An important part of the constrast is the fact that the hydrogen halides are unusual in being more spherical than most other highly polar molecules.
Rotation–vibration spectra of icosahedral molecules. II. Icosahedral symmetry, vibrational eigenfrequencies, and normal modes of buckminsterfullerene
Authors: Weeks David E. , Harter William G.
Published: July 2002

The icosahedral symmetry of molecules such as buckyball, B12H12−2, and C20H20, is analyzed using subgroup chain defined projection operators. The icosahedral analysis is used to determine the eigenvalues and eigenvectors of a classical spring mass model of buckyball. A spectrum of Raman and dipole active modes is given using the spring constants of benzene. Corresponding dipole active and Raman active normal modes are displayed stereographically. Several choices for springs constants are discussed and a comparison with spring mass systems of reduced symmetry is made.