bluuebunny/crossref_metadata_2025 huggingface dataset:
Intramolecular vibrational energy redistribution in the CD3H molecule
Authors: Iung Christophe , Leforestier Claude
Published: July 2002

We present a study of the intramolecular vibrational energy redistribution (IVR) from a highly excited C–H overtone of the CD3H molecule. The whole vibrational manifold has been explicitly considered in the calculations. Two different approaches have been used and compared. In a direct approach, we have kept all the states located below a given threshold energy, resulting in a basis set of 92 000 states. The second approach consisted in selecting the important states in order to define a dynamically relevant active space (AS) of much lower size (≊2000). The two approaches were first applied to the calculation of the n=6 C–H overtone spectrum, showing that the AS method is quite able to reproduce the exact results. More stringent test concerned the actual time evolution of the C–H stretch local mode ‖6ν1≳0. Explicit time propagation has been carried out in the larger basis set, over a 15 ps interval. Results show that the dynamics is mainly governed by a few resonant states involving the C–H bending and C–D stretching motions. Very slow relaxation out of the C–H chromophore is observed over this 15 ps period. The AS method, using a much smaller basis set, was shown to reproduce the correct behavior of the C–H chromophore dynamics during the first 2 ps.
Pseudoatoms of the electron density
Authors: Edgecombe Kenneth E. , Esquivel Rodolfo O. , Smith Vedene H. , Müller-Plathe Florian
Published: July 2002

The theory of molecular structure, developed by Bader and co-workers, has met with considerable success and is now being applied to many aspects of chemistry. One finding, however, that has complicated the theory is that of non-nuclear attractors, that is, pseudoatoms. In this study we examine basis set and to some extent electron correlation effects on the appearance and disappearance of non-nuclear attractors in Li2, Na2, Na4, and Na5. The appearance of a pseudoatom in a lithium molecule correlates remarkably well with the size of the region, in an atomic calculation, of −∇2ρ(r) for the valence shell of the atom. This and the fact that the pseudoatom is also present in the promolecule indicates that the pseudoatoms are remnants of or, in fact, are portions of atoms that are not perturbed enough in the molecule to remove an essentially atomic characteristic.
Perturbation analysis of the B 1Π state of 23Na85Rb molecule
Authors: Wang You-Chang , Matsubara Kensuke , Katô Hajime
Published: July 2002

A number of the rovibrational levels of the B 1Π state are found to be strongly perturbed. The energy shifts of the e and f levels of the B 1Π state are studied, and the perturbations between the B 1Π and (2) 3Σ+ states and between the B 1Π and (1) 3Π1 states are identified. Forty-seven centers of perturbation are observed, and the molecular constants of the (2) 3Σ+ and (1) 3Π1 states are estimated. The hyperfine splitting of the (2) 3Σ+ state is observed. Each rovibrational level splits primarily into four lines, and each of the four lines splits into six lines. The splitting is identified as originating from the magnetic dipole interaction primarily with the nuclear spin of 23Na (I=3/2) and secondarily with the nuclear spin of 85Rb (I=5/2).
Electronically diabatic atom–atom collisions: A self-consistent eikonal approximation
Authors: Cohen Joel M. , Micha David A.
Published: July 2002

A detailed theoretical and computational study of electronic transitions has been done in the short-wavelength (or eikonal) limit for the nuclear motions. Equations for nuclear trajectories in effective potentials are simultaneously solved with equations for electronic transition amplitudes. The treatment is done in adiabatic and diabatic electronic representations, and is applied to three model potentials: noncrossing, crossing in the repulsive region, and crossing in the attractive region. Results from the eikonal approach are compared with exact coupled-channel probabilities. It is found that a procedure based on the the diabatic representation and an average effective potential provides accurate results over wide ranges of collision energies, and is computationally efficient.
Multireference configuration-interaction potential surfaces for the collinear F+H2 reaction
Authors: Wright James S. , Kolbuszewski Marcin , Wyatt Robert E.
Published: July 2002

Potential-energy surfaces are reported for the collinear FHH system, using a multireference configuration interaction calculation combined with the rotated Morse curve-cubic spline technique to fit the ab initio points. Basis A contains only nuclear-centered atomic orbitals, while basis B adds bond functions at the midpoints of the FH and HH bonds. Results using basis B show significant improvement in the diatomic asymptotes, including the spectroscopic constants for H2 and HF, and the reaction exoergicity. The FHH surface data are fitted along each ray in the (θ,l) coordinate system using either three-parameter (GMF3) or five-parameter (GMF5) generalized Morse functions. The five-parameter fits are more accurate over a wider range, but the three-parameter fits are good enough for most purposes. Based on the five-parameter fits, surfaces A5 and B5 show the following properties (experimental values in parentheses): exoergicity 28.9 and 31.6 kcal/mol (31.7), barrier height 4.0 and 1.8 kcal/mol (1.5–2.3 kcal/mol), barrier location RFH, RHH=(1.47 and 0.77 Å) and (1.48 and 0.77 Å), and vibration frequencies (3550 and 720i cm−1) and (3500 and 830i cm−1). In addition, surface B5 contains reactant and product wells of depth 0.24 and 0.16 kcal/mol, respectively. The surface features obtained using basis B are consistent with the best current estimates obtained from either experimental or theoretical values, except for the location of the barrier which occurs somewhat later along the reaction coordinate.
Benzyl alcohol–water and benzyl alcohol–ammonia clusters: Ion fragmentation and chemistry
Authors: Li S. , Bernstein E. R.
Published: July 2002

Benzyl alcohol/ammonia, α,α-dimethylbenzyl alcohol/ammonia, and benzyl alcohol/water cluster ion fragmentation and chemistry are studied for isolated cold clusters by means of one and two-color mass resolved excitation spectroscopy, nozzle/laser timing delay, and deuteration experiments. Experiments lead to an identification of parent clusters for all fragment ion clusters observed. Three types of cluster ion fragmentation are observed for these systems: dissociation−solu+(solv)n→solu+(solv)k+m solv; acid-base chemistry−ArCH2OH+(B)n→ArCH2O(B)k+BmH+; and (benzyl) radical chemistry−ArCH2OH+(B)n→ArĊHOH(B)k+BmH+, ArCD2OH+(B)n→ArCDHOH+(B)k+BmB−d2 and ArCMe2OH+(B)n→ArĊMeOH+(B)k+BmṀe. Fragmentation reactions depend on cluster size, structure, and (weakly) on the vibrational energy deposited in the ion. Specifically, for benzyl alcohol+(NH3)1 only cluster radical chemistry and dissociation take place, while for higher order clusters, the acid-base reaction rate increases and this reaction becomes a major fragmentation pathway for benzyl alcohol+(NH3)4. For the benzyl alcohol(H2O)n system, cluster radical chemistry is not observed with n=1, only a weak α-hydrogen transfer reaction is observed with n=2, and acid base chemistry is not observed for clusters of any size. Cluster dissociative fragmentation is also a function of cluster size; large water and ammonia clusters dissociate much more easily than do n=1 clusters. The possible mechanisms for these fragmentation patterns are discussed.
A comparison of the coupled cluster and internally-contracted averaged coupled-pair-functional levels of theory for the calculation of the MCH2+ binding energies for M=Sc to Cu
Authors: Bauschlicher Charles W. , Partridge Harry , Scuseria Gustavo E.
Published: July 2002

The correlation contribution to the M–C binding energy for the MCH2+ systems can exceed 100 kcal/mol. At the self-consistent field (SCF) level, these systems can be more than 50 kcal/mol above the fragment energies. In spite of the poor zeroth-order reference, the CCSD(T), method is shown to provide an accurate description of these systems. The maximum difference between the CCSD(T) and internally contracted averaged coupled-pair function binding energies is 1.5 kcal/mol for CrCH2+, with the remaining systems agreeing to within 1.0 kcal/mol.
Observation of NO B 2Π(v=3)←X 2Π(v=0) absorptions with 1+1 multiphoton ionization: Precision line position measurements and parity assignment of the B 2Π state
Authors: Faris G. W. , Cosby P. C.
Published: July 2002

Absorptions are observed in the β(3,0) band of NO, B 2Π(v=3)←X 2Π(v=0) using 1+1 multiphoton ionization at photon energies in the range 47 801–48 542 cm−1. Line positions are reported in 10 of the 12 possible branches with an accuracy of 0.05 cm−1, directly traceable to the visible I2 absorption spectrum. Λ doubling in the β(3,0) band is explicitly resolved, allowing a definitive parity assignment of the B 2Π(v=3) rotational levels. Accurate term energies and molecular constants are presented for B 2Π(v=3). Comparison with previous measurements reveals significant discrepancies. Line positions are also reported in the γ(2,0) band, A 2Σ+(v=2)←X 2Π(v=0), of the 14N18O and 15N16O isotopes.
The van der Waals potential-energy surfaces and the structures of ArClF and ArCl2
Authors: Tao Fu-Ming , Klemperer William
Published: July 2002

The potential-energy surfaces of the ArClF and ArCl2 complexes are determined by the Hartree-Fock (HF) and Mo/ller–Plesset calculations (up to MP4) in an efficient basis set of 6-31+G(2df ) for the intermolecular energy. The interaction energies are calculated by the supermolecular approach with the full counterpoise corrections for the basis-set superposition error. Three local potential minima are found for ArClF corresponding to the linear Ar–Cl–F and Ar–F–Cl and the asymmetric T-shaped structures. For these the well depths and the distances are D(Ar–Cl–F)=233.5 (MP2) or 219.7 cm−1 (MP4), RArCl=3.38 Å; D(Ar–F–Cl)=119.2 (MP2) or 127.2 cm−1 (MP4), RArF=3.3 Å; and D(T-shaped)=130.4 (MP2) or 132.6 cm−1 (MP4), RArCl=3.83 Å. The results are in accord with the linear ArClF structure as the most-stable structure determined by experiment with the estimate of De=228 cm−1 at RArCl=3.33 Å. For the ArCl2 complex, minima are found corresponding to the linear and the T-shaped structures. At the MP2 level the well depths and distances are D(linear)=220.1 cm−1, RArCl=3.5 Å; D(T-shaped)=183.6 cm−1, RArCl=3.9 Å. Only a small change results at the MP4 level D(linear)=195.3 cm−1, D(T-shaped)=165.2 cm−1. The results for the T-shaped ArCl2 are in good agreement with the experimental results of De=185±1 cm−1 and RArCl=3.8±0.1 Å. Estimates for the effects of differences in zero-point energy show the two structures may be of similar stablity.
Reduction of the computational effort in quantum Monte Carlo calculations with pseudopotentials through a change of the projection operators
Authors: Flad Heinz-Jürgen , Savin Andreas , Preuss Heinzwerner
Published: July 2002

The usual form of nonlocal pseudopotentials which project the wave function on real spherical harmonics is inconvenient in quantum Monte Carlo calculations because the projection integrals are still functions of r. It is thus necessary to calculate the integral in every point of the simulation. We circumvent this problem by transforming the pseudopotential to a form where the projection occurs on Cartesian Gaussian-type functions which makes the projection integrals independent of the electron coordinates. We applied our method to two-valence electron systems using the pure diffusion method, where the approximation in the pseudopotential is the only one present.
Surface diffusion of Sb on Ge(111) monitored quantitatively with optical second harmonic microscopy
Authors: Schultz K. A. , Seebauer E. G.
Published: July 2002

Surface diffusion of Sb on Ge(111) has been measured with the newly developed technique of optical second harmonic microscopy. In this method, concentration profiles at submonolayer coverage are imaged directly by surface second harmonic generation with 5 μ spatial resolution. A Boltzmann–Matano analysis yields the coverage dependence of the diffusivity D without parametrization. Experiments were performed at roughly 70% of the bulk melting temperature Tm. In the coverage range 0≤θ≤0.6, the activation energy Ediff remains constant at 47.5±1.5 kcal/mol, but the pre-exponential factor D0 decreases from 8.7×103±0.4 to 1.6×102±0.4 cm2/s. Both Ediff and D0 are quite large, which is consistent with high-temperature measurements in other systems. The inadequacies of current theories for high-temperature surface diffusion are outlined, and a new vacancy model is proposed for low-coverage diffusion. The model accounts semiquantitatively for the large values of Ediff and D0, and suggests that these quantities may be manipulated using doping levels and photon illumination. An islanding mechanism is proposed to explain the decrease in D0 with θ.
Entropy, pressure, and chemical potential of multiple chain systems from computer simulation. II. Application of the Metropolis and the hypothetical scanning methods
Authors: Meirovitch Hagai
Published: July 2002

As in the preceding paper (Paper I) we study here a model of chains with excluded volume enclosed in a ‘‘box’’ on a square lattice. The system is simulated by the Metropolis Monte Carlo method and the entropy is extracted from the samples by using the ‘‘hypothetical scanning method.’’ With this method each system configuration is treated as if it has been generated step by step with the scanning method (studied in Paper I). The transition probabilities are reconstructed and three approximations of the entropy are obtained. Thus the pressure and the chemical potential are calculated directly from the results of the entropy as in Paper I using standard thermodynamic relations. These results are found to be in a very good agreement with those obtained in Paper I, which are considered to be exact within the statistical error.
High resolution laser spectroscopy of free radical-inert gas complexes: C5H5⋅He, C5H5⋅He2, C5H5⋅Ne, and CH3–C5H4⋅He2
Authors: Yu Lian , Williamson James , Foster Stephen C. , Miller Terry A.
Published: July 2002

Several inert-gas complexes of the free radical cyclopentadienyl and its methyl derivative (C5H5⋅He, C5H5⋅Ne, C5H5⋅He2, and CH3–C5H4⋅He2, and CH3–C5H4⋅He) have been studied at low temperature via their laser induced fluorescence spectra in a supersonic free jet expansion. The rotational structures of the near-uv electronic transitions of the radicals have been resolved and analyzed. These analyses lead to precisely defined geometries for the radicals. In contrast to inert-gas, closed-shell aromatic complexes, the inert-gas radical ring bond lengths are found to be strongly dependent upon electronic state and methyl substitution. Additional molecular parameters were measured in the complexes including the electronic angular momentum, the (2,2) interaction parameter for the C5H5 complexes, and for the methyl derivative, the barrier to methyl rotation. These are compared to their values in the uncomplexed radicals.
Pressure tuning of spectral holes in organic crystalline materials: Irreversible effects
Authors: Gradl G. , Feis A. , Friedrich J.
Published: July 2002

The behavior of spectral holes under pressure in a polycrystalline material, namely dimethyl-s-tetrazine (DMST) doped n-octane was investigated and compared with the behavior in a durene single crystal host and glasses. Application of pressure induces frequency shifts and line broadenings which are significantly larger than in single crystals and glasses. Part of the broadening is irreversible and is attributed to the creation of dipolar strain fields. The distribution of dislocation thresholds is continuous with no obvious lower cut off. The response of the material to pressure changes depends on its history.
A modified superposition approximation to the three-body distribution function
Authors: Hernando J. A. , Gamba Z.
Published: July 2002

Based on quite general arguments we propose a simple modification to the Kirkwood superposition approximation. It consists in the replacement of g2(r) by α(r)g2[α(r)r] with α(r)=1+γ/r. This modification is tested against numerical simulations of a Lennard-Jones fluid, and good results are obtained. The γ value seems to be quite independent of the thermodynamic parameters of the system and of the precise functional form of α. Based on short-range universality arguments, we propose γ=0.07 for all simple fluids.
The relation between intensity and dipole moment for bending modes in linear molecules
Authors: So/rensen Georg Ole , Jo/rgensen Uffe Gråe
Published: July 2002

By careful summation over the rotational fine structure, we examine the relation between dipole moment derivative and intensity of bending modes in linear molecules. The relation is derived without the usual assumption of the harmonic oscillator approximation and it is emphasized that the intensity of the degenerate mode cannot be viewed simply as a sum of two orthogonal one-dimensional oscillators, although the intensity is found to be close to two times the intensity of a nondegenerate mode with the same dipole moment derivative. The importance of using the Γ intensity rather than the S intensity for the total band intensity is demonstrated, and finally the effect of the Coriolis coupling is summarized. It is pointed out that most recent quantum mechanical computations predict band-intensities of the bending modes in linear molecules that are approximately a factor of 2 bigger than the observed intensities, and it is shown that the problem relates to the ab initio treatment.
Nonlinear polarization description of phase-locked pulse-pair spectroscopy
Authors: Ziegler L. D. , Scherer N. F.
Published: July 2002

The recently demonstrated technique of optically phase-locked pulse-pair (PLPP) excited spontaneous emission is described by a third-order perturbative density matrix approach. A nonlinear polarization description shows how PLPP spectroscopy depends on all the relevant material dephasing time scales. The time and frequency integrated resonance spontaneous emission consists entirely of resonance fluorescence, and is derived exclusively from excited-state population decay terms, i.e., diagonal second-order density-matrix elements. These third-order polarization results are proportional to the previously derived linear polarization expressions found to describe the observed PLPP I2 vapor emission. The nonlinear treatment allows a comparison of this technique to other forms of ultrafast pump–probe spectroscopies such as transient absorption and photon echo techniques. The role of impulsively prepared coherences is clearly described by this analysis. The effect of pulse duration, relative to material dephasing times, is explored for relaxation given by the optical Bloch equations. The most significant differences between a linear and nonlinear polarization treatment occur for pulse durations greater than the optical dephasing time or when excited state population and coherence decays are of the order of rovibrational periods. A fluorescence line narrowing effect, due to short pulse excitation, is predicted.
Quasiclassical trajectory simulation of the kinematically constrained reaction Ba+HI→BaI+H
Authors: Zhao Daqing , Zare Richard N.
Published: July 2002

We report a quasiclassical study of the kinematically constrained reaction Ba+HI→BaI+H. A London–Eyring–Polanyi–Sato (LEPS) potential is constructed based on spectroscopic data for BaI, BaH, and HI with its Sato parameters adjusted to reproduce the experimentally determined value of the maximum impact parameter for BaI(v=0). The main purpose of the study is to guide future experiments by testing how sensitive different features are to this mass combination. Under the conditions used in the previous Ba+HI crossed-beam experiment, we find that (1) the specific opacity function Pv(b) for a vibrational level v is narrow and overlaps those from neighboring v’s; (2) the most probable impact parameter for reaction producing a given vibrational level decreases as v increases (v≤12); (3) the initial orbital angular momentum is mapped onto BaI rotational angular momentum with a spread of ±19ℏ; (4) the recoil energy has a broad, featureless distribution that is nearly the same for different v levels, and shows no strong correlation with the initial relative velocity, vrel. Product internal-state distributions, recoil angular distributions, and isotope effects are also presented.
Examination of the product channels in the reactions of NH(a 1Δ) with H2 and D2
Authors: Tezaki Atsumu , Okada Satoru , Matsui Hiroyuki
Published: July 2002

A flash photolysis study (193 nm) of HNCO has been conducted and the mechanisms of the reactions NH(a 1Δ)+H2→NH2+H(1) and NH(a 1Δ)+D2→products (2) have been examined in detail at 295±3 K by monitoring NH(a 1Δ), H, D, NH2, and their D substituents via the laser induced fluorescence technique. From the pseudo-first-order analysis of the decay rate for NH(a 1Δ), rate constants have been determined as k1=(3.96±0.17)×10−12 and k2=(2.62±0.08)×10−12. (All the rate constants are expressed in units of cm3 molecule−1 s−1.) These rate constants are consistent with those determined from the time dependence of H and D atoms: they are k1=(3.76±0.70)×10−12 and k2=(2.78±0.17)×10−12. No pressure dependence has been observed for 10–100 Torr He. The branching fraction for H and D atoms as products for reaction (2) has been found to be [H]/[D] =0.24/0.76, where D production is more abundant than statistically predicted. This indicates that reaction (2) is dominated by insertion of NH(a 1Δ) into the D2 bond, but vibrational energy of the reaction intermediate NHD2 is still localized in newly formed N–D bonds before it passes through the exit barrier into NHD+D or ND2+H channels. NH2(X̃ 2B1) was observed in (0,0,0) and (0,1,0) vibrational states as a product of reaction (1), and the observed time dependence of both vibrational states could be satisfactorily simulated by solving the master equation for vibrational relaxation of NH2. This analysis has indicated that the vibrational energy partitioning in the product NH2 is nearly statistical.
Symmetry adaptation of correlated states in the valence bond basis
Authors: Ramasesha S. , Soos Z. G.
Published: July 2002

Symmetry-adapted linear combinations of valence-bond (VB) diagrams are constructed for arbitrary point groups and total spin S using diagrammatic VB methods. VB diagrams are related uniquely to invariant subspaces whose size reflects the number of group elements; their nonorthogonality leads to sparser matrices and is fully incorporated into a binary integer representation. Symmetry-adapated linear combinations of VB diagrams are constructed for the 1764 singlets of a half-filled cube of eight sites, the 2.8 million π-electron singlets of anthracene, and for illustrative S≥0 systems.
The nonresonant two-photon zero kinetic energy photoelectron spectrum from the electronic ground state of H2S
Authors: Fischer Ingo , Lochschmidt Andreas , Strobel Andreas , Niedner-Schatteburg Gereon , Müller-Dethlefs Klaus , Bondybey Vladimir E.
Published: July 2002

The nonresonant two-photon (1+1) ZEKE (zero kinetic energy) photoelectron spectrum from the X̃ 1A1 electronic ground state of H2S is reported. Complete rotational resolution of the X̃ 2B1 ground state of the ion was achieved for different temperatures in the jet. The ZEKE spectra show angular momentum transfer up to ΔN=2. The rotationalNKa+Kc++  ←  J″Ka″Kc″ branching ratios for the low-lying rotational levels were determined. The strongest transitions are those with ΔKa=Ka+−Ka″=±1 and ΔKc=Kc+−Kc″=0 (type c transitions) but transitions with ΔKa=0  and  ΔKc=±1 (type a transitions) also show a surprisingly high intensity. Changes in Ka or Kc up to ±3 are observed. The two-photon spectra are compared with those obtained via one-photon VUV photoionization by other authors.
Deuteron spin relaxation and molecular dynamics of a nematic liquid crystal (5CB) in cylindrical microcavities
Authors: Vrbančič N. , Vilfan M. , Blinc R. , Dolinšek J. , Crawford G. P. , Doane J. W.
Published: July 2002

Spin–lattice and spin–spin relaxation times of deuterium nuclei have been measured for a nematic liquid crystal confined in cylindrical channels of diameter 0.2 μm. In the isotropic phase the relaxation in the MHz regime is not affected by the confinement. The possibility of rotating the cylinders with respect to the magnetic field enables the first direct measurements of the anisotropy of deuteron relaxation of a monomeric liquid crystal in the nematic phase. The observed temperature dependence of the spin–spin relaxation rate in the isotropic phase roughly obeys the (T−T*)−1 law and allows the determination of the average time the molecules reside at the wall, which is 5×10−5 s for 5CB at the lecithin coated surfaces. In the nematic phase the spin–spin relaxation rate is affected by order director fluctuations and rotation induced by translational diffusion at orientations with a nonhomogeneous director distribution with respect to the magnetic field.
Light scattering observation of spinodal line in gelatin gels
Authors: Bohidar Himadri B. , Jena Sidhartha S.
Published: July 2002

Light scattering measurements have been performed on dilute gelatin gels to determine the spinodal curve through Tanaka’s mean-field approach of density fluctuations in gels. The spinodal zone has been found to be much below the sol–gel transition line determined through differential scanning calorimetry measurements.
Competition between hydrogen and deuterium abstraction by methyl radicals in isotopomerically mixed methanol glasses
Authors: Doba Takahisa , Ingold K. U. , Lusztyk Janusz , Siebrand Willem , Wildman Timothy A.
Published: July 2002

Rate parameters are reported for hydrogen and deuterium abstraction of methyl radicals embedded in glassy mixtures of CH3OH and CD3OD. The mole fraction of CH3OH in these isotopomeric mixtures is 0, 0.05, 0.075, 0.10, 0.15, or 1. The nonexponential time dependence of the radical concentration is analyzed in terms of distributions of first-order rate constants. For the isotopomerically pure matrices, lognormal distributions describe the decay satisfactorily. The large difference between characteristic H and D transfer rate constants indicates tunneling. In the mixtures, there is competition between H and D abstraction processes which depends on the local structure about a radical, so that the corresponding rate parameters contain information about this structure. On the basis of earlier work [J. Chem. Phys. 86, 6622 (1987)], the analysis begins with the assumption that the structure about a radical resembles one of the crystalline phases of methanol. The entire set of decay curves is described by a (disordered) β-phase structure in which the radical replaces a methanol molecule and is located near the position associated with a methyl group. However, this static picture is inadequate because the radical can diffuse through the glass on the time scale of the kinetic measurements. Diffusion allows the radical to encounter more CH3OH molecules than would be expected for the static structure on a statistical basis—the effective mole fraction of CH3OH in the mixtures is higher than the analytical concentration. For the xH=0.05 mixture, we estimate that on the average the radical encounters approximately 26 methanol molecules before abstraction occurs. This corresponds to diffusion over roughly 1100 pm through the lattice.
Nonlocal density functional calculations: Comparison of two implementation schemes
Authors: Lee Chengteh , Fitzgerald George , Yang Weitao
Published: July 2002

We have carried out nonlocal density-functional calculations of bond dissociation and isomerization energies of several polyatomic molecules in two schemes. In the first scheme, the nonlocal energy functional is incorporated into the optimization of both the electronic and nuclear degrees of freedom. In the second scheme, the nonlocal energy functional is only included in a non-self-consistent fashion in which we just use the molecular geometry and electron density determined by the corresponding local density calculations. Our study reveals that the differences of the energies are very small between these two schemes.
Analysis of SF6 quasicontinuum states. II
Authors: Angelié C.
Published: July 2002

The IR fluorescence data of SF6 excited in its quasicontinuum (QC), reported in paper I [C. Angelié, J. Chem. Phys. 96, 8072 (1992)], are analyzed in comparison with IR multiphoton absorption (IRMPA) data, either in the nanosecond or in the picosecond regimes. A general description of the QC transition matrix elements is first derived. All the following features must be taken into account: multiple rovibrational transitions, perturbative redistribution onto secondary resonances, dissipative redistribution onto the whole energy shell, and intramolecular mode inhomogeneity producing a frequency dispersion. The intramolecular couplings obey a hierarchy Vk versus the number k of quanta exchanged, with an effective density of couplings ρk. A model without arbitrary parameters settles that the parameter γ=πρ<V2≳ can be shared between a dissipative part γd∼1–1.5 cm−1 (for E∼10 000–15 000 cm−1), corresponding to the orders k≥5, and a perturbative part γp∼2–15 cm−1, corresponding to the lowest orders k≂3 and 4, i.e., the strongest couplings cannot produce the dissipative relaxation. The same model explains that the QC threshold is at E∼4000 cm−1, as demonstrated by Raman experiments, with a half-width γd∼0.21 cm−1, in excellent agreement with picosecond spectroscopy giving a T1 relaxation time of ∼11 ps. The parameter γ is also extracted from IRMPA cross sections. It is found from these data that γ(E) increases from 0.25 to 15 cm−1 when E increases from 4000 to 34000 cm−1. Finally, picosecond data, showing a spectacular enhancement of the number of photons absorbed for short pulses of duration τL∼30 ps, are fully explained by the previous QC description: Energy is mainly absorbed selectively in the ν3 mode, producing a dramatic enhancement of the Rabi width. Then, all known data on the SF6 QC can be incorporated in a unique framework, likely generalizable to other molecules.
First gas phase spectrum of the aluminum carbide molecule: The B 4Σ−–X 4Σ− system
Authors: Brazier C. R.
Published: July 2002

The aluminum carbide radical has been seen in the gas phase for the first time. The B 4Σ−–X 4Σ− system near 22 000 cm−1 was observed in emission from a hollow cathode source. Three vibrational sequences were detected: Δv=0, −1, and −2, and a full rotational analysis was performed on the five strongest bands, 0-0, 1-1, 0-1, 1-2, and 2-3.
Dipped adcluster model study for molecular and dissociative chemisorptions of O2 on Ag surface
Authors: Nakatsuji Hiroshi , Nakai Hiromi
Published: July 2002

Chemisorption of an O2 molecule on a Ag surface is studied theoretically with the use of the dipped adcluster model (DAM). Electron correlations in low-lying surface states and electron-transferred states from bulk metal, which are shown to be very important, are described by the symmetry adapted cluster (SAC)/SAC-configuration interaction (SAC-CI) method. Side-on geometries are used, different from the (bent) end-on geometries studied previously. Potential curves for the O2 approaching and dissociating processes are investigated with the use of Ag2O2 and Ag4O2 adclusters. For the occurrence of chemisorption, the electron transfer from bulk metal to the adcluster, and the electrostatic image force are important, which cannot be treated by the conventional cluster model. Two different molecular adsorption states are obtained from the calculations for the Ag2O2 adcluster, namely superoxide (O2−) and peroxide (O22−) species and the corresponding adsorption energies are calculated to be 5.5 and 17.8 kcal/mol, respectively, which compare well with the experimental value 9.2–9.3 kcal/mol. The O–O stretching frequencies of these species are in good agreement with the experimental values. From the calculations for the Ag4O2 adcluster, the potential minima corresponding not only to the molecular adsorption, but also to the dissociative adsorption are obtained. The dissociative adsorption is shown to be led from the peroxide. The geometry of the dissociative O− is at the bridge site on a silver surface and the calculated Ag–O bond distance of 2.16 Å agrees well with the experimental value 2.06–2.17 Å. The dissociative adsorption energy is estimated to be 44.0–61.4 kcal/mol, which is compared with the experimental value 40.8–44.0 kcal/mol.
The sequential exploration of phase space in selectively excited polyatomic molecules
Authors: Remacle F. , Levine R. D.
Published: July 2002

The squared Fourier transform of the optical absorption spectrum provides a very useful characterization of the intramolecular dynamics. In practice, most of the information content is in that portion of the transform whose magnitude is of the order of (1/N) of its value for time=0, where N is the number of eigenstates in the zero order nonstationary bright state which is optically accessed. If the highly resolved spectrum manifests inherent structures (‘‘clumps’’) at lower levels of resolution then each clump can be regarded, for the purpose of the analysis, as a bright state with its own survival probability. This offers a significant advantage. We discuss theoretically and provide computational examples how this can be implemented within a maximum entropy formalism. We determine both the density of the region in phase space sampled up to time t and its entropy. Analytically and computationally it is shown that the evolution in phase space is sequential. Also discussed is the structure of the Hamiltonian matrix which can give rise to a nested inherent spectra. It is argued that each time scale is characterized by its set of good constants of motion which decrease in number upon the transition to the next time regime.
Incoherent cross-correlation spectroscopy
Authors: Tong P. , Xia K.-Q. , Ackerson B. J.
Published: July 2002

A new light scattering method is developed to measure scattering amplitude fluctuations resulting from either internal motions of flexible macromolecules or rotations of rigid particles. With a single-beam two-color cross-correlation scheme, the technique becomes insensitive to rapid phase fluctuations of the scattered light produced by translational motions of the scattering particles. We frame the scattering theory so as to encompass rotations and number fluctuations of small particles in a steady laminar flow. Experiment verifies the theory and demonstrates its applications. The technique can be used to measure the magnitudes of the local velocity and the flow vorticity. It can also be used to measure internal motions and shape fluctuations of flexible macromolecules. The advantages of the technique are its high spatial resolution, fast temporal response, and ease of use.
A theoretical study of the electronic structure and spectroscopic properties of a new diatomic molecule, BeC
Authors: Borin Antonio Carlos , Ornellas Fernando R.
Published: July 2002

The 13 lowest-lying electronic states of a new diatomic molecule, BeC, are described theoretically for the first time using the multireference single and double excitations configuration interaction approach. Potential energy curves are presented for all the states as well as a description of the electronic structure characterizing each of them. Dissociation and vertical excitation energies, and dipole moment functions complete the electronic structure description. Vibrational and rotational levels spacings, on the other hand, allowed the calculation of a whole set of spectroscopic constants characterizing each state. In its ground state, a 3Σ−, this molecule is more strongly bound (De=2.39 eV) than the recently described species BeN (De=1.34 eV) but still less strongly bound than BeO (De=4.69 eV) and BeF (De=5.82 eV). The first excited triplet state (3Π) shows an avoided crossing at short internuclear distances which should certainly reflect in perturbative effects in its spectrum. A distinctive feature is also the existence of a very low-lying strongly bound quintet state.
Experimental and theoretical characterization of the BAr van der Waals complex: The X 2Π, A 2Σ+, and B 2Σ+ electronic states
Authors: Hwang Eunsook , Huang Yu-Lin , Dagdigian Paul J. , Alexander Millard H.
Published: July 2002

The BAr van der Waals complex and its electronic transition correlating with the B atom 3s 2S–2p 2P transition have been characterized in a combined experimental and theoretical investigation. The experimental portion of the study consisted of the observation by laser fluorescence excitation of rotationally resolved bands of this molecule in a supersonic jet. Specifically, four bands of the (v′,0) progression of the B 2Σ+–X 2Π1/2 band system of each of the 11,10BAr isotopomers were observed and analyzed, where the upper state vibrational quantum numbers v′=4–7 were determined from the isotope splittings. Vibrational and rotational constants were obtained through fits to the observed transition wave numbers. These experimental results were compared with ab initio calculations of the X 2Π, A 2Σ+, and B 2Σ+ electronic states of BAr. In order to obtain the interaction energies of the excited states of this weakly bound system accurately, multireference, internally contracted, configuration-interaction calculations were carried out, with additional provision for the effect of higher order excitations. The information derived about these states from the experiment and calculations agree reasonably well. An interesting feature of the BAr B 2Σ+ state is the presence of a barrier in the potential energy curve.
Comment on: Measurement of density for nematic–isotropic phase transition of a micellar system and a different data interpretation
Authors: Barbosa Alberto A. , Pinto Abio V. A.
Published: July 2002

Measurements on the density as a function of temperature are presented for the micellar systems: decylammonium chloride/ammonium chloride/water. The measurements cover the nematic–isotropic phase transition. The actual data, density vs temperature, show that the early pointed out steplike feature, at transition, is a circumstantial occurrence of phase coexistence, slow rate of temperature at the transition and an unconventional use of the cell densimeter position.
Phenomenological many-body potentials from the interstitial electron model. I. Dynamic properties of metals
Authors: Li Mo , Goddard William A.
Published: July 2002

Inspired by the ab initio generalized-valence-bond calculations of small metal clusters, we propose a phenomenological many-body interaction model, the interstitial electron model (IEM), for interactions of ions and electrons in metals. In this model, the valence electrons are treated as classical particles situated at the crystal lattice interstitial positions. Simple pair potentials are used for ions and interstitial electrons, allowing the inhomogeneity and anisotropy of electron density distributions to be taken into account phenomenologically. To test the efficacy and applicability of this approach, the IEM is applied to lattice dynamics in fcc metals: Cu, Ni, Ag, Au, Pd, Pt, Al, Ca, Sr, and γ-Fe. The phonon dispersion relations, densities of states, and Debye temperature are calculated and found to be in good agreement with experiments. Extension of the IEM to the construction of a new many-body potential in metals and alloys is discussed.
Calculation of the excitation transfer matrix elements between the S2 or S1 state of carotenoid and the S2 or S1 state of bacteriochlorophyll
Authors: Nagae Hiroyoshi , Kakitani Toshiaki , Katoh Tetzuya , Mimuro Mamoru
Published: July 2002

Formalism of the excitation transfer matrix element applicable for any multiconfigurational wave functions is made. On the basis of the resultant formulas, the excitation transfer matrix elements between the S2 or S1 state of a carotenoid, neurosporene, and the S2 or S1 state of bacteriochlorophyll a are calculated at various stacked configurations of the two molecules. The results show that the excitation transfer from the carotenoid S1 state to the bacteriochlorophyll S1 state via the Coulomb mechanism including multipole–multipole interactions takes place very efficiently in a speed more rapid than that via the electron-exchange mechanism. The results also show that the excitation transfer from carotenoid to bacteriochlorophyll occurs directly from the carotenoid S2 state, as well as from the carotenoid S1 state. Furthermore, it is shown that the excitation transfer matrix element due to the electron-exchange interaction has an oscillatory dependence on the displacement of one molecule from the other when the distance between the planes of the π systems is kept constant. Based on these results, a possible mechanism of the excitation transfer from carotenoid to bacteriochlorophyll in vivo is discussed.
C
3v versus C2v Cd(1S,3P,1P)–CH4 van der Waals complexes: A variational and perturbational multireference configuration interaction study
Authors: Ramírez-Solís Alejandro , Castillo A. Sidonio
Published: July 2002

The Cd(1S,3,1P)–CH4 interaction is studied for the C3v vertex-on Cd(1S,3,1P)–HCH3 and face-on Cd(1S,3,1P)–H3CH approaches as well as the C2v Cd(1S,3,1P)–H2CH2 edge-on approach of the metal atom to methane using extensive ab initio multireference configuration interaction plus second order multireference Mo/ller–Plesset calculations. The ground state Cd(1S)–CH4 is totally repulsive for the C3v vertex-on and C2v edge-on approaches. Only a stable face-on Cd(1S)–H3CH complex was found. Although vertex-on and edge-on minima were found for the Cd(3P)–CH4 excited complexes, they are less attractive than the corresponding Cd(3P)–H3CH face-on exciplexes. The optimal orientation of the face-on occupied 5p orbital of Cd(5s5p) is that which minimizes the overlap with C–H bonds leading to a ‘‘3Πy’’ exciplex. For the Cd(1P)–CH4 interaction, two face-on and two edge-on very stable complexes (all with De≥1300 cm−1) were found. The Cd(1S)–H3CH (1A1) complex was found to have a 185 cm−1 well at 8.03 a.u. Both excited P states present shorter equilibrium distances—the Cd(3P)–H3CH (3E) exciplex has a 799 cm−1 well at 6.08 a.u., while the Cd(1P)–H3CH exciplex (1E) has a 1337 cm−1 well at 5.57 a.u. These results are in good agreement with the recent experimental values X 1O+(1S)—(De=121 cm−1, Re=7.9 a.u.); A 3O+(3P)—(De=677 cm−1, Re=5.80 a.u.); and 1P—(De≂1300 cm−1, Re=5.59 a.u.) obtained by Wallace and Breckenridge [J. Chem. Phys. 97, 2318 (1992)]. The present work confirms the C3v facial nature of the observed (1S) ground and (3P) excited complexes. The present results are inconclusive as to the facial or edge-on nature of the Cd(1P)–CH4 exciplex, but they preclude the C3v vertex-on possibility.
Shear-induced homogenization of semidilute solution of polymer mixture. II. Composition dependence
Authors: Fujioka Kiyotoshi , Takebe Tomoaki , Hashimoto Takeji
Published: July 2002

Shear-induced homogenization of semidilute solutions of polybutadiene (PB) and polystyrene (PS) with dioctylphthalate as a solvent was studied as a function of temperature T, polymer compositions at a fixed total polymer concentration of 3.3 wt%. The drop of homogenization temperature ΔTc(S) as measured in the increasing and decreasing cycles of shear rate S (s−1) was found to be given by ΔTc(S)/Tc(0)=(1.3±0.1)×10−3 S0.5±0.02 for all the mixtures with PS/PB compositions 30/70, 40/60, 50/50, 60/40, and 70/30 where ΔTc(S)=Tc(0)−Tc(S), Tc(S) is the homogenization temperature at a given S, and Tc(0) is the cloud point at S=0. A remarkable hysteresis effect was discovered on the shear-induced homogenization for off-critical mixtures with 20/80 and 80/20 compositions: the critical shear rates Sc,i and Sc,d for the homogenizations observed, respectively, in the increasing and decreasing cycles of S satisfy Sc,i≥Sc,d. The difference δSc≡Sc,i−Sc,d is greater at a lower ΔT(0)≡Tc(0)−T for a given mixture and at more biased compositions for a given ΔT(0). δSc is essentially zero for the 50/50 mixture of PS/PB at all ΔT(0) covered in this experiment. The hysteresis on Sc also gives the hysteresis on Tc(S) or ΔTc(S), viz., ΔTc(S)/Tc(0)=P×10−3 S0.5±0.1 for the data obtained in the decreasing cycles of S, and ΔTc,(S)/Tc(0)=Q×10−4 S1.0±0.1 for the data obtained in the increasing cycles of S for 20/80 and 80/20 mixtures, where P and Q are the constants of order unity. The hysteresis effect is none or small for the mixtures with compositions between 30/70 and 70/30.
The spectroscopy and relaxation dynamics of three-phonon bound states in crystal CO2
Authors: Bini Roberto , Remigio Salvi Pier , Schettino Vincenzo , Jodl Hans J.
Published: July 2002

The high-resolution Fourier-transform infrared spectrum of a single CO2 crystal has been studied in the (ω1+ω2);3ω2 Fermi resonance region. The spectrum shows three types of vibrational excitation, one where the three ω2 vibrons propagate freely (P+P+P) into the crystal, a second with two (out of three) coupled on the same molecule (BP+P), and the last with all three ω2 modes linked on one molecule and travelling as a single excitation (TP). Fermi resonance may split off the three-phonon continuum (ω2+ω2+ω2) both the biphonon+phonon (BP+P) band and the sharp triphonon (TP) peak. A theoretical analysis of triphonons following a Green-function approach is proposed. The temperature dependence of triphonon bandwidths has been measured in the temperature range 20–200 K. This gives information on the relaxation mechanisms of bound crystal states. Our results emphasize the role of dephasing through bending mode in the vibrational relaxation of the upper triphonon. The lower triphonon has a behavior with temperature which may be explained as due only to dephasing by lattice phonons. Triphonon depopulation is treated theoretically in a highly simplified form to enlighten the main channels available for decay.
Sub-Doppler optical double-resonance spectroscopy and rotational analysis of Na3
Authors: Eckel H.-A. , Gress J.-M. , Biele J. , Demtröder W.
Published: July 2002

Based on rotationally resolved sub-Doppler spectroscopy and optical–optical double resonance (OODR) experiments on cold Na3 molecules in a collimated supersonic argon beam seeded with sodium, 51 rotational transitions selectively excited by OODR in the complex electronic A 2A2←X 2B2 system of Na3 could be assigned unambigiously. Accurate values of the rotational constants and the molecular geometrical parameters have been derived from these measurements.