bluuebunny/crossref_metadata_2025 huggingface dataset:
A condensed phase study of the benzene B2u←A1g three-photon transition
Authors: Cable J. R. , Albrecht A. C.
Published: July 2002

Although group theoretically allowed, the benzene B2u←A1g three-photon transition appears to derive intensity exclusively through a vibronic coupling route. The ν6 (e2g) vibration is dominant in providing e1u vibronic symmetry, just as it is in the one-photon transition. Several experiments are presented that provide further insight into the lack of allowed vibronic b2u activity in the three-photon spectrum. Pseudoparity, applicable to even alternate hydrocarbons in the PPP formalism, and offering selection rules beyond those based on symmetry alone, is examined as a possible explanation by looking for effects from the inductive perturbation in a fluorine substituted benzene. The possibility of nonresonant third harmonic generation in the condensed phase sample, which may lead to a viable one-photon absorption route to the excited B2u state, is also explored in both a three-photon photoselection study as well as in an experiment comparing the relative three-photon to one-photon cross sections for benzene and biphenyl. The results obtained for the benzene B2u transition and the corresponding A′2 transition in the symmetrically substituted trifluorobenzene appear to confirm the importance of pseudoparity selection rules and suggest that the third harmonic generation pathway is of minor importance.
Photoinduced evaporation of charged clusters
Authors: Engelking P. C.
Published: July 2002

The average cluster size remaining after photoinduced evaporation of a cluster of specific initial size can be predicted by an RRK/QET statistical model, provided that the correct average kinetic energy release is used. Theoretical justification for this correction, based upon detailed balance, is provided here. Agreement with experiments on (CO2)+n clusters at several wavelengths shows that for these aggregates the average bond strength above n=2 is approximately 3.6±0.6 kcal/mol.
Local space approximation for configuration interaction and coupled cluster wave functions
Authors: Kirtman Bernard , Dykstra Clifford E.
Published: July 2002

A local space approximation developed originally for the Hartree–Fock model of electronic structure is extended to configuration interaction and coupled cluster wave functions. In doing so many features employed by Saebo and Pulay in their local CI method are used to advantage. The computational effort is reduced to operations primarily on the local space (≡interaction complex) thereby making it feasible to study localized electronic structure changes in large systems. Charge transfer and polarization effects due to embedding the interaction complex in the surrounding medium are taken into account.
The electronic structure of small nickel clusters
Authors: Tomonari Mutsumi , Tatewaki Hiroshi , Nakamura Takashi
Published: July 2002

The electronic structure of small nickel clusters (Ni1–Ni6) has been investigated by ab initio SCF and CI calculations. Among others the symmetry broken SCF calculations have been performed for the ionization of the clusters, and it is found that an ionized state resulting from electron removal from a 3d-like orbital is the Fermi level or quite close to the Fermi level. The resulting d hole is shown to localize at one of the constituent atoms. The calculated band structure of the clusters is found to be similar to that of the solid Ni. The calculated first I.P. (Fermi energy with sign changed) is 4.5 eV for Ni1–Ni6, which is favorably compared with experimental results given by Rohlfing and co-workers.
Semiclassical quantization of a classical analog for the Jahn–Teller E×e system
Authors: Zwanziger Josef W. , Grant Edward R. , Ezra Gregory S.
Published: July 2002

We semiclassically quantize a classical analog Hamiltonian, derived using the classical electron formalism of Meyer and Miller, for the linear approximation to the Jahn–Teller E×e system. Quantization is achieved for realistic values of the nonadiabatic coupling parameter, by means of the method of adiabatic switching. The results are found to be in reasonable agreement with quantum mechanical eigenvalues, and a useful qualitative picture of the associated nonadiabatic dynamics is obtained.
Hydrogen bonded and non-hydrogen bonded van der Waals clusters: Comparison between clusters of pyrazine, pyrimidine, and benzene with various solvents
Authors: Wanna J. , Menapace J. A. , Bernstein E. R.
Published: July 2002

Solute–solvent clusters of pyrazine, pyrimidine, and benzene (solutes) and CnH2n+2 (n=1,2), NH3, and H2O (solvents) are studied by the techniques of supersonic molecular jet spectroscopy and two-color time-of-flight mass spectroscopy (two-color TOFMS). Spectral shifts, van der Waals (vdW) modes, dissociation energies, and vdW mode–solute mode vibronic couplings are characterized for most of the observed clusters. Based on these data and previous results for hydrocarbon systems, cluster geometries can be suggested. Lennard-Jones potential (6-12-1) calculations are also performed for these clusters and in all instances for which comparisons can be readily made, calculated and experimentally estimated geometries and binding energies agree completely. Clusters of N-heterocyclic solutes and H2O are not observed experimentally. Systematics and trends among the clusters reported herein and those previously reported are discussed and analyzed.
Electron binding and chemical inertness of specific Nbx clusters
Authors: Whetten R. L. , Zakin M. R. , Cox D. M. , Trevor D. J. , Kaldor A.
Published: July 2002

Photoionization threshold measurements on niobium clusters Nbx x=4–29, reveal an unprecedentedly strong dependence on x, the number of atoms in the cluster. Major maxima in threshold energies occurring at x=8, 10, and 16 correspond to those clusters recently shown to be unreactive toward molecular deuterium.
State-to-state collisional interelectronic and intraelectronic energy transfer involving CN A 2Π v=3 and X 2Σ+ v=7 rotational levels
Authors: Jihua Guo , Ali Ashraf , Dagdigian Paul J.
Published: July 2002

Collisional transfer within the CN A 2Π v=3 vibrational manifold and to the X 2Σ+ v=7 manifold has been studied with initial and final rotational state resolution by an optical–optical double resonance technique. Despite the large energy gap between these two manifolds, the interelectronic cross sections are significant for only a relatively small range of ΔJ, and there is no observable propensity for energy resonant, large ΔJ transitions. The even–odd alternation vs N, observed previously in vA=7 collisions [N. Furio, A. Ali, and P. J. Dagdigian, J. Chem. Phys. 85, 3860 (1986)] and indicative of the near homonuclear form of the CN–Ar interaction potentials, is even more pronounced here for vA=3. The relative rate of intraelectronic and interelectronic energy transfer for the vA=3 N=6 F1f initial level was found to be comparable to that for the corresponding vA=7 level, despite the smaller Franck–Condon factor and larger energy gap to the neighboring vX=vA−4 manifold for the former.
Clusters in solution: Growth and optical properties of layered semiconductors with hexagonal and honeycombed structures
Authors: Sandroff C. J. , Kelty S. P. , Hwang D. M.
Published: July 2002

Transmission electron microscopy and optical absorption were used to examine small clusters of the layered semiconductors, PbI2 and BiI3. In both systems, a layer of metal is sandwiched between two hexagonally closed-packed layers of iodine. We describe a simple solution preparation which gives rise to clusters corresponding to single layer sandwiches, roughly 7 Å thick, whose lateral dimensions vary from 12 to 90 Å depending on the solvent and the nature of the growing cluster interface. The cluster size distributions are markedly different for PbI2 and BiI3 reflecting the different structure in the metal planes of these systems. PbI2 cluster sizes are determined by hexagonal symmetry, with cluster growth achieved by placement of lead atoms symmetrically about a smaller cluster. In BiI3, whose metal plane has a honeycombed structure like graphite, clusters grow to be much larger with their sizes determined by the closure of six-membered rings. The optical absorption spectra of PbI2 and BiI3 can be quantitatively understood in terms of the measured cluster size distributions.
A theoretical study of the structure, bonding, and vibrational frequency shifts of the H2–HF complex
Authors: Bernholdt David E. , Liu Shi-yi , Dykstra Clifford E.
Published: July 2002

The complex formed from hydrogen fluoride and the hydrogen molecule has been investigated theoretically in two ways. Large basis, well-correlated ab initio electronic structure calculations have been used to map out regions of the potential energy surface and an electrical interaction model has been used to find the classical intermolecular interaction effects. From the ab initio potential surface, the fundamental vibrational transition frequencies of hydrogen and hydrogen fluoride are predicted to be red shifted by 20 and 15 cm−1, respectively. The Liu and Dykstra theory of vibrational frequency shifts that uses the intermolecular electrical interaction yields shifts of 31 and 19 cm−1, respectively. The equilibrium structure of the molecule is T shaped, a feature that is determined by electrical interaction, and the well depth is around 300 cm−1. A significant fraction of the well-depth results from electron correlation effects.
ESR spectrum of IO2
Authors: Byberg J. R.
Published: July 2002

Selective photolysis at room temperature of IO−4 embedded in KClO4 crystals and subsequent x irradiation at low temperature lead to formation of IO2, which may be detected by ESR. Detailed comparison of the spin Hamiltonians of ClO2, BrO2, and IO2, all trapped in KClO4, indicates that the distributions of electronic charge and spin in these molecules are very similar.
Considerations of the rates and lifetimes of intermediate complexes for the association of various ligands to metal ions: Ag+ and Cu+
Authors: Castleman A. W. , Weil K. G. , Sigsworth S. W. , Leuchtner R. E. , Keesee R. G.
Published: July 2002

Rates were measured for the association of CO, CH4, CH3F, NH3, ND3, CH3Cl, and CH3Br onto Ag+ and Cu+ at 298 K. In the order given above, the three-body association rate constants for Ag+ range from 2.5×10−30 to 3.9×10−27 cm6 s−1. The rate constants for Cu+ are about four to six times larger for a given neutral reactant. The rate constants display trends in the order expected considering the relative bond energies of the clusters, although the enormous range of reactivity is not reflected simply by differences in dipole moments and polarizabilities. There is a very large isotope effect, where the rate constant for ND3 association was found to be about three times greater than NH3 in the case of both ions. (Results for Na+ follow the same trend.) This suggests a coupling of ligand vibrations with orbiting motion which leads to enhanced lifetimes of the cluster intermediates. The trend found for the methyl halides also supports the involvement of this effect.
Intramolecular vibrational relaxation from CH stretching modes in dimethylnitramine
Authors: Sumpter Bobby G. , Thompson Donald L.
Published: July 2002

Intramolecular vibrational energy transfer in dimethylnitramine has been studied using classical trajectories. Model harmonic and anharmonic valence force-field potential-energy surfaces with force constants adjusted to give approximately the known normal-mode frequencies were used in the study. Both the rate and mechanism for energy flow from high CH stretch and methyl group excited states were investigated. The results show that the energy transfer is rapid (within approximately 0.25 ps) and irreversible. The energy from an excited CH stretch is initially transferred to the methyl bending motion and finally to the nitro and ‘‘second’’ methyl groups. The dominant energy transfer path involves flow from the stretch to the bend within the excited group followed by ‘‘methyl-to-methyl’’ transfer.
Electronic states of GaAs and GaAs+
Authors: Balasubramanian K.
Published: July 2002

Complete active space MCSCF (CASSCF) followed by first-order configuration interaction (FOCI) calculations are carried out on low-lying electronic states of GaAs and GaAs+. Among the 12 electronic states of GaAs considered here, six of them are bound and their spectroscopic properties are calculated. The ground state of GaAs is found to be 3∑− with a 3Π state only 1569 cm−1 above the 3∑− state. The ground state of GaAs+ is found to be 4∑− state. The vertical ionization potential of GaAs is calculated to be 5.91 eV. Our calculations predict the electronic spectra of GaAs and GaAs+ which are yet to be observed.
The association of ammonia with halide ions in the gas phase
Authors: Evans D. H. , Keesee R. G. , Castleman A. W.
Published: July 2002

Thermochemical properties of cluster ions, involving Cl−, Br− and I− solvated by ammonia, were measured by using high-pressure mass spectrometry. Standard enthalpy changes of −8.2, −7.7, and −7.4 kcal/mol and entropy changes of −15.4, −19.1, and −20.9 cal/K mol were obtained for the association of NH3 onto Cl−, Br−, and I−, respectively. Comparison with the halide affinities of SO2 and H2O along with alkali affinities of NH3, SO2, and H2O suggests the importance of quadrupole moments and polarizabilities in the qualitative agreement of electrostatics with the ordering of association affinities. Electrostatic and ab initio calculations were made which suggest a variation of halide orientation with respect to the ammonia where I− is closest to alignment with the ammonia dipole and F− is farthest removed from the dipole axis. A value of 11 kcal/mol is estimated for association energy of NH3 with F− by comparison of extrapolated and calculated values along with similar chemical systems.
The relationship between shape resonances and bond lengths
Authors: Piancastelli M. N. , Lindle D. W. , Ferrett T. A. , Shirley D. A.
Published: July 2002

A discussion is presented on the general nature of shape resonances in small molecules and how they may relate to molecular bond lengths. Criteria for assigning photoabsorption features as shape resonances are described, and the usefulness of photoemission experiments to such assignments is highlighted. Based on these criteria, all unambiguously identified K-shell shape-resonance features in molecules containing B, C, N, O, and F are examined in an attempt to identify empirically a shape-resonance energy/bond length relationship. Although the available data are insufficient to establish a quantitative correlation, they indicate the influence of other variables besides the bond length to the energy position of a shape resonance.
Effect of kinetic and electronic energy on the reactions of Cr+ with H2, HD, and D2
Authors: Elkind J. L. , Armentrout P. B.
Published: July 2002

The reactions of atomic chromium ions with H2, HD, and D2 are examined using guided ion beam tandem mass spectrometry. The ground electronic state (6S) is found to react inefficiently. The thresholds of these data are analyzed to yield a 0 K bond dissociation energy for CrH+ of 1.37±0.09 eV (31.6±2.0 kcal/mol). In the reaction of Cr+(6S) with HD, formation of the metal deuteride ion is strongly favored over that of the metal hydride ion in the threshold region but not at higher energies. These results indicate that Cr+(6S) reacts largely via an impulsive mechanism. No evidence for reaction of Cr+(6D), the first excited state, is found but higher lying excited states, 4D and 4G (produced by electron impact), are found to react at their thermodynamic threshold. In reaction with HD, these states produce CrH+ preferentially by a factor of 2–4 over CrD+. The differences in reactivity among these states can be explained by using simple molecular orbital concepts which have been developed for other atomic transition metal ions.
State-to-state photodissociation dynamics of t
r
a
n
s-glyoxal
Authors: Burak I. , Hepburn J. W. , Sivakumar N. , Hall G. E. , Chawla G. , Houston P. L.
Published: July 2002

The photodissociation of glyoxal has been investigated by monitoring the CO internal energy distribution using tunable vacuum ultraviolet laser-induced fluorescence on the A←X system. Appearance times for the CO are in excellent agreement with the glyoxal fluorescence decay times, indicating that there is no long-lived intermediate in the dissociation. The quantum yield for CO production is independent of the K quantum number describing the glyoxal rotation. The CO is formed almost entirely in v=0 but is spread over a broadly excited rotational distribution peaking at J≂42. Analysis of the CO Doppler profiles shows that the velocity of the CO increases with increasing rotational level and that the CO recoil velocity vector is oriented predominantly perpendicular to its angular momentum vector. These observations, which are in agreement with both previous time-of-flight data and molecular orbital calculations, are consistent with a model for the dissociation involving planar intermediates for the two channels leading to CO+H2CO or to 2 CO+H2. It appears that the highest rotational levels of CO are produced in coincidence with the H2CO channel.
Ion molecule reactions of carbon cluster ions with D2 and O2
Authors: McElvany Stephen W. , Dunlap Brett I. , O’Keefe Anthony
Published: July 2002

The ion/molecule chemistry of laser-generated carbon cluster ions (C+n; n=3–19) has been studied using Fourier transform mass spectrometry. The ion/molecule reactions of mass-selected carbon cluster ions with D2 and O2 have been studied and reaction rates measured. Reactions of the primary product ions with D2 and O2 were also studied. The change in reactivity observed as a function of cluster size suggests a structural change from linear to monocyclic rings in the cluster ions between n=9 and 10. Evidence for the presence of two structural isomers for the C+7 cluster ion has also been observed and has been attributed to the existence of both a linear and cyclic form of the ion. MNDO calculations have also been used to obtain structural and thermodynamic information on possible reactant and product ions in an attempt to explain the differences observed in the ion/molecule reactions. The results from collision induced dissociation studies of the carbon cluster ions are also discussed.
The uptake of ammonia by iron clusters: A new procedure for the study of metal cluster chemistry
Authors: Parks E. K. , Nieman G. C. , Pobo L. G. , Riley S. J.
Published: July 2002

A new procedure is described for the study of chemical reactions of metal clusters with simple gas phase molecules. It consists of determining the average number of reagent molecules taken up by the clusters as a function of reagent pressure over a very wide range of pressures. When the results of such experiments are appropriately analyzed, they provide information on cluster reaction kinetics, thermodynamics, and the composition of reaction products. An illustration of this procedure for the reaction of iron clusters with ammonia is presented.
Branching ratios for chemical reaction in collisional quenching of excited 5s1 2S1/2 gallium atoms
Authors: Mitchell S. A. , Hackett P. A. , Rayner D. M. , Flood M.
Published: July 2002

We report cross sections for fluorescence quenching and branching ratios for chemical reaction in collisional deactivation of Ga(5s1 2S1/2) by added gases at room temperature following resonance 4p1 2P○1/2 →5s1 2S1/2 excitation at 403.30 nm with a pulsed laser. Total quenching cross sections are obtained from Stern–Volmer analysis of fluorescence lifetimes. Chemical contributions to fluorescence quenching are investigated by a pump and probe technique involving saturation of the resonance transition by a pump laser pulse to produce a known fraction of gallium atoms in the excited state, and measurement of the depletion of the gallium atom concentration due to chemical reaction in the excited state. Relative concentration measurements in the presence and absence of the saturating pump pulse are by resonance fluorescence excitation by a probe laser pulse suitably delayed relative to the pump pulse. An analysis of the experiment in terms of rate equations shows how branching ratios for chemical reaction may be obtained from the depletion measurements. Fluorescence quenching cross sections (Å2) are large for N2O (99±10), CO2 (100±20), CH4 (55±6), C2H6 (77±20), C3H8 (100±20), n-C4H10 (130±20), and C2H4 (75±20), moderately large for CO (11±4) and N2 (8±2), and very small for CF4 (<0.3) and H2 (<0.05). Among the efficient quenchers only C2H4 showed no detectable contribution from chemical quenching. Branching ratios for chemical removal of Ga(5 2S1/2) are N2O (0.96+0.04−0.1), CO2 (0.55±0.1), CH4 (0.27±0.07), C2H6 (0.33±0.1), C3H8 (0.26±0.1), and C2H4 (0.0+0.05−0.0). Results for H2 and the alkane hydrocarbons are discussed with reference to simple concepts of orbital interactions in the entrance channel.
Variational treatment of a harmonic oscillator coupled to a dissipative heat bath
Authors: Parris P. E. , Silbey R.
Published: July 2002

We consider the problem of a single quantum oscillator coupled linearly to a heat bath of independent harmonic modes. An exact solution is presented for the system-oscillator observables of interest. The exact results are then used to evaluate the utility of a variational approach to the problem that has proven useful recently in elucidating the dynamics of dissipatively coupled systems. We find that the variational approach does provide a good description for most, but not all, observables of interest. Both the exact and the variational treatment demonstrate the important role played by the low-frequency bath modes in determining qualitative features of the dynamical behavior.
Observation of gas phase organometallic free radicals: Monomethyl derivatives of calcium and strontium
Authors: Brazier C. R. , Bernath P. F.
Published: July 2002

We have observed the organometallic free radicals CaCH3 and SrCH3 by the gas phase reaction of Ca or Sr metal vapor with Hg(CH3)2 and several other CH3 containing oxidants. The positions of the Ã 2E–X̃ 2A1 and B̃ 2A1–X̃ 2A1 transitions were determined, as well as several vibrational frequencies. The observation of predissociation in the Ã 2E state gives an upper limit of 46 kcal/mol (43 kcal/mol) to the bond dissociation energy for CaCH3 (SrCH3).
Kr+ laser excitation of NH2 in atmospheric pressure flames
Authors: Wong Koon Ng , Anderson William R. , Vanderhoff John A. , Kotlar Anthony J.
Published: July 2002

Fluorescence in the Ã 2A1−X̃ 2B1 system of the NH2 radical has been excited in NH3/N2O/N2, H2/N2O/N2, and CH4/N2O/N2 flames using the 6471 Å line of a krypton ion laser. Rotationally resolved fluorescence spectra indicate that the laser simultaneously pumps two rotational lines in the (0,11,0)–(0,2,0)∑ vibrational hot band of the radical. The placement of the laser line relative to the two molecular transitions has been inferred from intensities in the fluroescence spectrum. Rotational and vibrational energy transfer in the excited state were observed to be very slow in comparison to electronic quenching by the flame molecules. An NH2 density profile in a rich H2/N2O/N2 flame is given. Also, a method for the calculation of Einstein coefficients and oscillator strengths for nonperturbed, main-branch transitions is presented.
The microwave spectrum of the NF radical in the metastable electronic state (a 1Δ)
Authors: Yamamoto Satoshi , Saito Shuji
Published: July 2002

The microwave spectrum of the NF radical in the first electronically excited state a 1Δ has been observed in a dc glow discharge in a gaseous mixture of NF3 and H2. The transitions with J=3–2 and 5–4 were observed in the 220 and 360 GHz regions, respectively. The rotational constant, centrifugal-distortion constant, and hyperfine coupling constants were determined precisely. The nuclear-spin rotation coupling constants for the F and N nuclei were obtained to be 99.1(7.5) and 13.5(6.3) kHz, respectively, with three standard deviations in parentheses.
Asymptotic behavior of atomic momentals
Authors: Thakkar Ajit J.
Published: July 2002

Knowledge of the large and small momentum transfer behavior of the electron momentum distribution is an important ingredient in the analysis of experimental isotropic Compton profiles. This behavior ultimately rests upon the asymptotic behavior of atomic momentals (momentum space orbitals). The small momentum Maclaurin expansion and the large momentum asymptotic expansion of atomic momentals with arbitrary angular momentum quantum number are derived in this paper. Their implications for momentum densities and Compton profiles are derived and discussed.
Multiphoton ionization spectra of radical products in the F(2P)+ketene system: Spectral assignments and formation reaction for CH2F, observation of CF and CH
Authors: Hudgens Jeffrey W. , Dulcey C. S. , Long George R. , Bogan Denis J.
Published: July 2002

The reactions of F(2P)+ketene and F(2P)+ketene-d2 were studied in a flow reactor. Spectra of the radical products CH2F, CD2F, CH, CF, and atomic carbon were detected between 292–395 nm by resonance enhanced multiphoton ionization (REMPI) mass spectrometry. Fluoromethyl radicals were identified as a major product of the F+ketene elementary reaction. REMPI spectra of fluoromethyl radicals originated from two-photon preparation of 3p, 4p, and 5p Rydberg states (quantum defect ∼0.6). Absorption of a third laser photon ionized the radicals, a 2+1 REMPI mechanism. Rydberg band origins were observed in CH2F at ν00=52 863, 63 275, and 67 265 cm−1 and in CD2F at ν00=52 786, 63 195, and 67 186 cm−1. A normal mode analysis revealed the vibrational frequencies of the C–F stretch, CH2 scissors, and out-of-plane bending modes in the Rydberg states. The ground state out-of-plane bending frequency in CH2F is 260(30) cm−1 and in CD2F it is 170(30) cm−1. CH radicals were generated by the photolysis of ketene and observed at ∼311 nm by two-photon excitation through the D 2Π(v′=2) ←←X 2Πr band. The reaction mechanism that generated the CF radicals was not determined. The REMPI CF radical spectrum generated by a 2+1 photon mechanism appeared as a series of bandheads described by the constants ν00=∼52 572 cm−1, ωe =1820 cm−1, and ωexe =−18.2 cm−1.
Quantal treatment of the low energy collision of Ne*(3P0,2) with H2(1∑+g,v=0)
Authors: Stern B. , Baudon J.
Published: July 2002

A close-coupling channel computation of Ne*(3P0,2)–H2 (1∑+g, v=0) collisions, using the ab initio potentials given in a preceding paper, is reported. The molecule is considered as a rigid rotator. The autoionizing character of states dissociating into Ne* (2p53s)+H2 is ignored. Consequently Penning and chemi-ionizations are not considered. Combined fine structure and rotational transitions are the only inelastic channels expected. By means of a deflation procedure, collision channels leading to Ne*(1P1) are shown to be negligible in the present energy range (E≤175 meV). The ‘‘fragment’’ basis (i.e., Ne*+H2 at infinite R) is adapted to the collision treatment since all off-diagonal elements of the electronic Hamiltonian, including the spin–orbit interaction (static couplings) are small. The coupling of angular momenta ( j1=0,1,2 of Ne*, j2=0,2 of para-H2, 1 of ortho-H2, l for the relative motion) leads to 27 coupled equations for ortho-H2 and 54 equations for para-H2, for each value of the total angular momentum J. These equations are solved by using the algorithm of Gordon, and the 81×81 S matrix is derived. The rotational excitation ( j2=0→2) probability takes a maximum value of the order of 10−2 at J≊15.
Photoionization of the E
F excited state of H2: Calculation of vibrational branching ratios and photoelectron angular distributions
Authors: Cornaggia C. , Giusti-Suzor A. , Jungen Ch.
Published: July 2002

Vibrational branching ratios and photoelectron angular distributions have been calculated for photoionization from the lowest vibrational levels of the EF state in H2. The Rydberg-continuum dipole transition moments have been evaluated in the Coulomb approximation and the oscillator strengths have been calculated with the multichannel quantum defect method. Rovibronic coupling between continuum final-state channels has been included and rotational/vibrational preionization is also accounted for where it arises. Only minor deviations from the Franck–Condon principle are predicted, primarily related to the promotion of the pσu Rydberg (continuum) orbital. This result disagrees somewhat with the (2+1) REMPI experiment of Anderson, Kubiak, and Zare (1984) near 18.5 eV. On the other hand, the results obtained in a (4+1) REMPI experiment by Cornaggia et al. (1986) at 15.7 eV are correctly reproduced. The possible influence of doubly excited configurations on the photoelectron distributions in various REMPI experiments in H2 is discussed.
Topological phase in molecular bound states: Application to the E⊗e system
Authors: Zwanziger Josef W. , Grant Edward R.
Published: July 2002

We extend recent works of Berry, Simon, and others on the evolution of adiabatic wave functions in parameter spaces with nontrivial global geometry, to show the interesting ways in which wave functions can acquire nonintegrable phase (commonly termed Berry’s phase, geometric phase, or topological phase) upon transport along paths in the parameter space. We emphasize the case of arbitrary paths on the Born–Oppenheimer potential energy surfaces (the parameter space of the electronic states in an isolated molecule) of the linear plus quadratic E⊗e Jahn–Teller system. It is found that these surfaces are degenerate not only at the origin but also at three other, equivalent points, which lie on a radius ρ=2k/g. Here k and g are the linear and quadratic vibronic coupling constants, respectively. This radius is then shown to mark a sharp transition between Jahn–Teller behavior, characterized by half-odd-integral vibronic angular momentum, and Renner–Teller behavior, which has integral angular momentum. Finally, we examine the conditions necessary for adiabatic evolution in the E⊗e system, and the observable consequences of the geometric phase.
Structure of liquid equiatomic potassium–lead alloy: A neutron diffraction experiment
Authors: Saboungi Marie-Louise , Blomquist R. , Volin K. J. , Price D. L.
Published: July 2002

Neutron diffraction measurements have been performed on liquid equiatomic potassium–lead alloy at T=870, 900, and 930 K to determine the influence of temperature on the structure. A well-defined first sharp diffraction peak (FSDP) at Q=0.96 Å−1, similar to that found in glassy materials, was obtained in the total structure factors S(Q) at the three temperatures. While the position of the peak remained unaffected by temperature changes, its magnitude decreased with increasing temperature. The other features of S(Q) were almost unaltered by temperature, as expected. The FSDP can be related to the presence of chemical bonds or clusters whose stability decreases with increasing temperature. A molecular model based on the random packing of K4Pb4 structural units is shown to give a remarkable correspondence with the measured structure factors.
High resolution infrared spectra of ethylene clusters
Authors: Baldwin K. G. H. , Watts R. O.
Published: July 2002

Infrared predissociation spectra have been measured for ethylene clusters near the ν7 (950 cm−1) monomer band using CO2 lasers. One of the CO2 lasers was piezoelectrically scanned over a 240 MHz range around each rotational lasing transition, to obtain high resolution spectra of the ν7 band. The resulting spectra showed fine structure with widths of the order of 20 MHz, representing a lower limit on the predissociation lifetime of around 10 ns. The fine structure is shown to be due to ethylene dimers, and the broad background is attributed to a superposition of inhomogeneous contributions from larger clusters.
Ionization and fragmentation of Bi microclusters by electron impact
Authors: Walstedt R. E. , Bell R. F.
Published: July 2002

Results are presented for an electron impact study of ionization and fragmentation effects in Bi microclusters ranging in size from 1 to 63 atoms. Using results from both high-vacuum and gas condensation cluster sources, energy thresholds for the fragmentation of clusters larger than 20 atoms or so are identified and found to lie 5±1 eV above the corresponding ionization thresholds. The experimental distribution of net changes in intensity from fragmentation, for clusters from 3 to 63 atoms, has been fitted with a simple model in which it was necessary to assume (a) a strong peak in the fragmentation probability in the vicinity of ∼30 atoms and (b) a probability of ∼80% that the minor fragment retains the positive charge, contrary to expectations based on final energies. Ionization potentials are given for clusters up to n=38 atoms. These are found to show rough agreement with the metallic sphere model for n>5 atoms, i.e., behavior intermediate between that of simple metals and semiconductors. The great majority of clusters studied also exhibit second thresholds in their ionization curves, from 1.2 to 3.0 eV above the first onset. The interval between thresholds becomes nearly constant for n>10 atoms. We tentatively attribute this effect to structure in the filled electron states.
Application of the Hill determinant method to the vibrational motion of diatomic molecules
Authors: Estrín Darío A. , Fernández Francisco M. , Castro Eduardo A.
Published: July 2002

The Hill determinant method is shown to be useful in obtaining the eigenvalues of central-field systems with potentials that can be expanded in the Taylor series. Some model potentials for diatomic molecules are considered as illustrative examples.
Laboratory measurements of gas-phase reactions of polyatomic carbon ions C+n(n=1–6) and CnH+(n=2–5) with carbon monoxide
Authors: Bohme Diethard K. , Wl/odek Stanisl/aw , Williams Leslie , Forte Leonard , Fox Arnold
Published: July 2002

The selected-ion flow tube (SIFT) technique has been employed in the study of reactions of carbon monoxide with the polyatomic carbon cations C+n(n=1−6) and CnH+(n=2–5) at 296±2 K in helium buffer gas at ∼0.34 Torr and 1.1×1016 atoms cm−3. The polyatomic carbon cations were generated by electron impact on a suitable parent molecule. Carbon monoxide was observed to add rapidly to C+n(n=2–6) twice in succession to form polyatomic carbon monoxide and dioxide cations, and once to CnH+ to form polyatomic carbon monoxide cations. Further additions did not occur with measurable specific rates. This remarkable behavior is attributed to double bond formation at the terminal carbon atoms of the polyatomic carbon cations. The specific rate for the addition of CO was observed to vary with the size of the polyatomic carbon cation, increasing to a maximum for reactions with five atoms in the reacting ion. This trend is attributed to an increase in the lifetime of the intermediate addition complex.
Transition moment calculations on small peptides
Authors: Volosov Andrey
Published: July 2002

Electric transition moments of small peptides with known structure are calculated with a special version of the CNDO method. In this version called CNDO/OPTIC, a special procedure of configuration interaction (CI) is used which avoids strong oscillations in the results arising with changing the CI size. It is recommended that in the framework of the suggested method the Pariser approximation be used for two-center repulsion integrals when calculating optical properties of peptides near transitions. A description of electronic transition symmetry and transition moment directions is given. It is shown that in the case of myristamide the substitution of the hydrogen atom connected to the nitrogen atom by a methyl group changes the direction of the electric transition moment of the ππ* transition qualitatively in the same way as by the dimerization of this compound. The results are in good agreement with available experimental data.
Proton magnetic resonance study of the H2–Ar potential energy surface
Authors: Lemaire Claude , Armstrong Robin L. , McCourt Frederick R. W.
Published: July 2002

Values of T1/ρ)∞lin are deduced from previously reported measurements of the proton spin–lattice relaxation time (T1) in two H2–Ar mixtures at ten temperatures between 110 and 296 K. These data are compared with close coupling calculations using the BC3(6.8) potential energy surface of Le Roy and Carley, and the TT3(6,8) potential of Le Roy and Hutson, as well as two minor modifications of the latter potential. Qualitative agreement between theory and experiment is achieved.
Measurements of mobilities and longitudinal diffusion coefficients for Li+ ions in some molecular gases
Authors: Satoh Y. , Takebe M. , Iinuma K.
Published: July 2002

The transport coefficients for Li+ ions in some moleculars gases have been measured over a wide range of E/N with a conventional drift tube at temperatures close to 300 K. The zero-field reduced mobilities are found to be 3.91±0.11, 4.44±0.12, 3.64±0.10, 2.46±0.07, and 3.44±0.10 cm2/V s in N2, O2, CO, CO2, and CH4 gas, respectively, and these values except in O2 gas significantly deviate from the Langevin limit. In N2, CO, and CO2 gas, the resulting mobility curves show clear minima at intermediate E/N, but the depressions in O2 and CH4 gas are slight. The drop of the zero-field values in N2, CO, and CO2 gas is explained in terms of an effective ion–quadrupole interaction which provides the r−6 attractive behavior. From the mobility calculations with using n−4–6(γ) potential, it is suggested that the depression of the mobility curve is developed by the addition of a sufficient r−6 term and is partly attributed to inelastic collisions as expected even at intermediate E/N. The experimental diffusion data are compared with the values derived from the generalized Einstein relation. For all systems, the agreement is quite good at intermediate E/N, but there are large discrepancies at other E/N. The sources of the deviation are considered to be clustering reactions and inelastic collisions at low and high field, respectively.
Analytical solution of the mean spherical approximation for an arbitrary mixture of ions in a dipolar solvent
Authors: Blum L. , Wei D. Q.
Published: July 2002

We show that in the mean spherical approximation for a mixture of arbitrary size and charge hard ions in a dipolar solvent all the properties are expressible in terms of three parameters; a screening parameter Γ, a ion-dipole interaction parameter B10, and a dipole–dipole interaction parameter b2, which are given by the solution of a set of algebraic equations. In the case of equal ionic size, the solution is quite simple and very similar to previous results which the size of ion are equal to the size of dipole (σ=σn=1). In the low density and high coupling limits, explicit results are given for the three parameters and for the thermodynamics. The calculations show that the model becomes the primitive model for infinite dilution and vanishingly small solvent molecules.
Monolayer solids of tetrahedral molecules
Authors: Bruch L. W.
Published: July 2002

Monolayer solids of CH4, CD4, and CF4 adsorbed on the basal plane surface of graphite are modeled with atom–atom interaction models and quasiharmonic lattice dynamics. Single molecule adsorption, the stability of registry lattices at monolayer condensation, and the stability of triangular tripod-down lattices relative to a denser monolayer packing are treated. The Novaco–McTague perturbation theory of the modulation of an incommensurate monolayer by the substrate is extended to molecular adsorbates with orientational degrees of freedom. Experimental data for monolayers of tetrahedral molecules on graphite are summarized; the model calculations give a fair account of the data, with some discrepancies in the energies of monolayer condensation.
Nature, geometry, and binding strength of the ammonia–hydrogen chloride dimer determined from the rotational spectrum of ammonium chloride vapor
Authors: Howard N. W. , Legon A. C.
Published: July 2002

The rotational spectra of the seven isotopic species (14NH3,H35Cl), (14NH3,H37Cl), (15NH3,H35Cl), (15NH3,H37Cl), (14NH3,D35Cl), (14NH2D,H35Cl), and (14NH2D,D35Cl) of a dimer formed by ammonia and hydrogen chloride have been observed in the vapor above heated ammonium chloride by using pulsed-nozzle, Fourier-transform microwave spectroscopy. The ground-state spectroscopic constants determined for the symmetric-top species (14NH3,H35Cl) are B0=4243.2593(16)MHz, DJ=12.8(2) kHz, DJK=371.5(8) kHz, χ(14N)=−3.248(14) MHz, and χ(35Cl)=−47.607(9) MHz. Analysis of the rotational constants for the isotopomers investigated establishes that the observed dimer has C3v symmetry with the nuclei in the order H3NHCl and with r(N⋅⋅⋅Cl)=3.136 Å. The contribution χp to the change in the 35Cl–nuclear quadrupole coupling constant of HCl caused by the electric field due to NH3 in its equilibrium position has been determined to be 10.8 MHz from the χ(35Cl) values of (14NH3,H35Cl) and (14NH3,D35Cl). Simultaneously, the HCl subunit oscillation amplitude is established to be θav =cos−1〈cos2 θ〉=13.4(1)°. A simple hydrogen-bonded model H3N⋅⋅⋅HCl of the dimer, in which only polarization of the HCl by the NH3 subunit and a small extension of the HCl bond are invoked, adequately accounts for the value of χ(35Cl). No substantial proton transfer from HCl to NH3 need be considered. The value of the hydrogen-bond stretching force constant kσ=17.6(3) Nm−1 determined from DJ is also shown to be consistent with this conclusion.
Computer simulation of long polymers adsorbed on a surface. I. Corrections to scaling in an ideal chain
Authors: Livne Shelly , Meirovitch Hagai
Published: July 2002

This paper is the first in a series of papers in which polymer adsorption on a surface is studied by computer simulation using the ‘‘scanning method.’’ This method is especially efficient to handle chain systems with finite interactions and geometrical constraints. Here we test the method by applying it to models of a single random walk (without excluded volume) on a simple cubic lattice, which are solved analytically; in the immediately following paper a self-avoiding walk model is treated. The scanning method is found to be extremely efficient, where walks of up to N=105 steps can be simulated reliably, leading thereby to very precise estimates of transition temperatures and critical exponents. In particular we test carefully for a lattice model the range of validity of scaling functions developed by Eisenrigler, Kremer and Binder [J. Chem. Phys. 77, 6296 (1982)] for a continuous model. We pay a special attention to corrections to scaling and demonstrate that they are strong above the transition temperature for 〈R2〉⊥, the perpendicular part of the mean-square end-to-end distance and for ρ(z), the monomer concentration profile. We show that at T=∞, the asymptotic regime, in which these corrections become negligible, is obtained for N≊40 000 for 〈R2〉⊥ but a significantly larger N is required for ρ(z). This means that this regime corresponds to a real polymer length that is not realized experimentally.
A photoion–photoelectron coincidence study of (CO)2
Authors: Norwood K. , Guo J.-H. , Luo G. , Ng C. Y.
Published: July 2002

The photoion–photoelectron coincidence (PIPECO) spectrum for (CO)+2 has been measured in the wavelength region of ∼600–1 000 Å. The lower limits for the bond dissociation energies of CO+ (X̃)⋅CO, CO+ (Ã)⋅CO, and CO+ (B̃)⋅CO are determined to be 29.8, 17, and 16 kcal/mol, respectively. Evidence supporting the existence of long-lived (CO)+2 states with dissociation lifetimes >40 μs at energies above the CO+ (X̃,v=0)+CO asymptote is found. The dissociative lifetimes of CO+ (Ã)⋅CO and CO+(B̃)⋅CO are estimated to be longer than 4 μs and 50 ns, respectively.
Proton transfer in neutral gas-phase clusters: α-Naphthol⋅(NH3)n
Authors: Cheshnovsky Ori , Leutwyler Samuel
Published: July 2002

Efficient excited-state proton transfer in neutral acid–base clusters α-naphthol⋅Bn has been detected and studied by a combination of laser spectroscopic techniques (resonant two-photon ionization, fluorescence excitation, and emission spectroscopy). S1 state proton transfer was observed for B=NH3 and n≥4, as evidenced by several criteria: (a) large red shift and substantial broadening of the R2PI spectra of the n≥4 clusters relative to those of the bare α-naphthol and smaller clusters; (b) very large Stokes shift (∼8000 cm−1) of the emission spectra of the n≥4 clusters; (c) complete broadening of the fluorescence emission band for the n≥4 clusters; and (d) a striking similarity of the emission band position and width of the latter spectra to the emission spectrum of the α-naphtholate anion in basic aqueous solution. No proton-transfer reaction was observed for small solvent clusters with B=NH3 and n≤3, nor for any of the pair complexes studied, which involve a single base partner [B=triethylamine, 3-dimethylamino-1-aminopropane, 1,4-bis(dimethylamino)butane] which we have studied so far. This behavior illustrates the difficulty of achieving charge separation in neutral gas-phase complexes or clusters. A critical gas-phase proton affinity PAcrit =248±3 kcal/mol was determined for proton transfer to take place in the α-naphthol⋅Bn (or base B) system.
Spectroscopic studies of the jet-cooled aluminum trimer
Authors: Fu Zhenwen , Lemire George W. , Hamrick Yoon Mi , Taylor Scott , Shui Jin-Cheng , Morse Michael D.
Published: July 2002

Gas phase spectroscopic investigations of the jet-cooled aluminum trimer are reported using the technique of resonant two-photon ionization with mass spectrometric detection. A discrete band system in the 5200–6100 Å region is observed, consisting of an extended vibrational progression in a single vibrational mode. In addition, an apparent continuum absorption is observed which gradually grows in toward shorter wavelengths. The apparent continuum exhibits a long lifetime, 24–35 μs, which is most unusual and indicates that the continuum arises from spectral congestion and not lifetime broadening. At 19 378 cm−1 both the discrete and the continuum absorptions terminate abruptly, indicating the onset of dissociation above this energy. Although it is not certain that dissociation above this energy leads to ground electronic state Al2, this measurement nevertheless places an upper limit on D0(Al2–Al) of 2.40 eV.
Vibrationally hot emission and electronic relaxation of CO in Ne matrix
Authors: Bahrdt Johannes , Schwentner Nikolaus
Published: July 2002

Vibrational progressions due to radiative decay of A 1Π (v′=0, 2, 3 5, 6, 8), e 3∑ (v′=0, 3, 5, 12), a′ 3∑ (v′=5, 14), d 3Δ (v′=3, 5), a 3Π (v′=0, 1, 2) to the ground state and of e 3∑ (v′=3, 5, 7, 9, 11, 12) and d 3Δ (v′=3, 7, 11) to the a 3Π state have been observed for selective excitation of A 1Π (v′=0, . . . ,8). The intersystem crossing rate constants, the bottle necks, pathways and rate constants for internal conversion in the triplet levels and the triplet–singlet radiative rate constants are explained by an intramolecular mixing of electronic states and electron–phonon coupling with the matrix.
1.5 μm DFB semiconductor laser spectroscopy of HCN
Authors: Sasada Hiroyuki
Published: July 2002

Near-infrared DFB (distributed feedback) semiconductor lasers have been extensively applied to molecular spectroscopy for the first time. The overtone band of three isotopic species of HCN has been recorded from 6420 to 6590 cm−1 without any frequency gaps, and three hot bands of H12C14N have also been observed. The molecular constants of the observed bands and the dipole moment for the (0 00 2) state of H12C14N are obtained.
Computation of quadratic electric dipole moment functions
Authors: Stanton John F. , Magers David H.
Published: July 2002

A numerically reliable method for the calculation of molecular electric dipole moment functions is described. This scheme, which is an extension of a method due to Komornicki and McIver, is applied to the computation of the quadratic dipole moment function of water using many-body perturbation theory. A pronounced basis set dependence is observed for the stretch and stretch–stretch coefficients of the quadratic expansion of the dipole moment, while those involving the bending symmetry coordinate appear to be less sensitive. In addition, previously published ab initio dipole moment functions are discussed and compared to the present results.