bluuebunny/crossref_metadata_2025 huggingface dataset:
A study of the D 1Σ+u–X 1Σ+g band system of Cs2
Authors: Yokoyama Kazushige , Baba Masaaki , Katô Hajime
Published: July 2002

The transition lines to the low v′ levels of the D 1Σ+u state are overlapped with the lines of the C 1Πu –X 1Σ+g transition. By choosing an appropriate wave number through a monochromator to observe a selected series of the D 1Σ+u (v′,J±1)–X 1Σ+g (v″,J) transition of Cs2, we measured the fluorescence intensity as a function of wave number of the laser line. 1237 spectral lines have been assigned to this band system of J′=4–108, v′=0–5. The Dunham coefficients of the D 1Σ+u state, which are useful to reproduce the measured line positions and also to construct a reasonable RKR potential curve, were determined by an iterative procedure. The molecular constants reproduced the observed 983 nonoverlapping line positions with a standard deviation of 0.0106 cm−1. The electronic term energy Te, the dissociation energy De, and the equilibrium internuclear distance Re of the D 1Σ+u state were determined to be 16 698.997 cm−1, 1547.6±0.8 cm−1, and 5.712 Å, respectively. The potential curves of the D 1Σ+u and C 1Πu states were estimated to cross at R=5.85 Å and E=16 700 cm−1.
Coriolis and anharmonicity couplings in the intramolecular vibrational energy flow: H+3 potential
Authors: Lin C. S.
Published: July 2002

A classical study was conducted on the effects of Coriolis coupling on the internal energy flow between the two degenerate normal mode oscillations of H+3 . Strong correlations between the energy flow and the types of trajectories with the direction of the molecular rotation are revealed. The angle at which a trajectory, near its onset, approaches the isoenergetic contour on the potential surface is shown to determine the type of the trajectory and general characteristics of the system.
On the origin of hydrogen clusters produced by particle induced desorption
Authors: da Silveira E. F. , Schweikert E. A.
Published: July 2002

Positive and negative hydrogen ion clusters were studied using particle desorption mass spectrometry (PDMS). The only species found were H−, H+, H+2 , and H+3. Their relative desorption yields were determined for a variety of samples and substrate temperatures. It was found that the H+ , H+2 , and H+3 yields obey roughly an exponential law with respect to the square root of their masses. The mechanism of the desorption of hydrogen clusters is discussed on the basis of the gas phase and Coulomb repulsion models.
3
1P spectral spin diffusion in crystalline solids
Authors: Kubo Atsushi , McDowell Charles A.
Published: July 2002

We give a derivation for the line shape function g(ω) involving the spin diffusion constant TSD, which had earlier been given by Suter and Ernst, and by Henrichs et al. using different methods. There are three approximations which lead to different ways to evaluate the zero quantum line shape function g(ω). The first used by Suter and Ernst, another by Henrich et al., and the third which we show leads to the same equations as the latter authors. The angular dependencies of TSD for 31P spectral spin diffusion in single crystals of dipotassium α-D-glucopyranose-1-phosphate dihydrate (GLP) and triphenylphosphine (TPP) were determined. We conclude that the 31P spin diffusion under the influence of extraneous protons is well described by a Gaussian zero quantum line shape [Eq. (20)] but not by a Lorentzian function. This distinguishes the present situation from the one treated by Suter and Ernst. It is also confirmed by theory and experiment that the equation for TSD given by Henrich et al. is satisfactory when the S1 and S2 spins involved in the flip–flop process are quite distant from each other.
Photoelectron spectroscopic studies of polyatomic molecules: Angular distributions for ionization in oriented T
d systems
Authors: Chandra N.
Published: July 2002

A recently developed theory for angular distribution of electrons ejected by interaction of light with nonlinear molecules held fixed in space has been used to study ionization in a1 orbital of those systems which transform like the Td point symmetry group. Expressions for photoelectron angular distributions in the dipole approximation with the electric vector in the radiation beam both perpendicular and parallel to the molecular axis are derived. The properties of the two formulas, which are found to have completely different structures, have been studied. These expressions are shown to depend not only on all those quantities [i.e., the partial integrated cross-section σ̄, the asymmetry parameter β̄, and the polar angle θ of the propagation vector k (k,θ,φ) of the ejected electron] which are present in photoionization of an unoriented molecule, but involve in certain cases also the azimuthal angle φ, phase shifts of the continuum waves representing the outgoing electron, and the phase of the dipole transition amplitudes. Such fixed-molecule photoelectron angular distributions will therefore provide more stringent tests of theoretical models and probes of photoionization dynamics than the hitherto performed gas phase experiments on randomly oriented targets. They can also be used to identify the orientation of a molecule and/or geometry of a chemisorption site. The formulation presented here sets a frame work for the analysis of measurements and the calculations of spectra in those Td molecules which are fixed in space. We have applied it, as an example, to ionization in 4a1, 6a1, and 7a1 orbitals of oriented CF4, CCl4, and SiCl4, respectively. Without doing any dynamical calculations, using instead the experimentally measured values of σ̄ and β̄ as a function of the photon wavelength, the variations in the angular distributions with respect to the energy of the ejected electron as well as to the angles (θ,φ) and to the phases (which are treated as parameters) involved have been studied in detail. These distributions are found to have very rich and complicated structures arising from the spectral, angular, and/or phase variations of the angular momentum composition

of the photocurrent.
Photoelectron spectroscopy of the halocarbene anions HCF−, HCCl−, HCBr−, HCI−, CF−2, and CCl−2
Authors: Murray Kermit K. , Leopold Doreen G. , Miller Thomas M. , Lineberger W. C.
Published: July 2002

The 488 nm photoelectron spectra are reported for the HCX(X̃1A′)+e−←HCX−(X̃2A″) and HCX(ã3A″)+e−←HCX−(X̃2A″) transitions in HCF−, DCF−, HCCl−, HCBr−, and HCI− and for the CX2(X̃1A1)+e−←CX−2(X̃2B1) transitions in CF−2 and CCl−2 . Adiabatic electron affinities are found to be 0.557±0.005 eV (HCF), 0.552±0.005 eV (DCF), 1.213±0.005 eV (HCCl), 1.556±0.008 eV (HCBr), 1.683±0.012 eV (HCI), 0.179±0.005 eV (CF2), and 1.603 ± 0.008 eV (CCl2). Bounds for the triplet excitation energies are determined for all the halocarbenes for which photoelectron spectra were observed, with the exception of CCl2. For the HCX halocarbenes, upper bounds for the triplet excitation energies are 14.7±0.2 kcal/mol (HCF, DCF), 11.4±0.3 kcal/mol (HCCl), and 9±2 kcal/mol (HCBr). A more detailed analysis of HCF suggests the actual triplet excitation energy to be 11.4±0.3 kcal/mol, 14.7±0.2 kcal/mol, or 8.1±0.4 kcal/mol, with the first value the most likely. Since the multiplicity of the ground state of HCl is not known, we report the energy of its first excited state to be less than 9±2 kcal/mol. The absence of an observed triplet state in the CF−2 photoelectron spectrum allows us to assign a lower bound on the triplet excitation energy of CF2 of 50 ± 2 kcal/mol.
Autoionizing Rydberg states of Li2: Analysis of electronic–rotational interactions
Authors: Schwarz M. , Duchowicz R. , Demtröder W. , Jungen Ch.
Published: July 2002

Highly excited Rydberg states of 7Li2 have been measured by two-step excitation with two narrow band pulsed dye lasers in a collimated lithium beam. A complex many-line two-photon ionization spectrum has been observed which consists of sharp lines whose natural widths are smaller than the laser line width (0.03 cm−1). The spectrum has been analyzed with the help of multichannel quantum defect theory. Radial electronic dipole transition moments for the second excitation step were calculated and provided the key to the analysis. The spectra correspond to the excitation of s and d Rydberg series with n∼18–40. The series are affected by strong rotational l uncoupling as well as electronic s–d mixing. The analysis yields accurate values for the vibrational and rotational constants for the X 2Σ+g state of Li+2 and an improved value of the ionization potential of Li2.
Alignment effects in Ca–He(5 1P1–5 3P
J) energy transfer collisions by far wing laser scattering
Authors: Lin K. C. , Kleiber P. D. , Wang J. X. , Stwalley W. C. , Leone Stephen R.
Published: July 2002

The far wing absorption profiles for excitation on the Ca(4s2 1S0–4s5p 1P01) atomic transition, broadened in collisions with He are measured. We observe strong absorption in both wings and a blue wing satellite near Δ∼125 cm−1. We tentatively identify this satellite as due to a maximum in the CaHe(4s2 1∑+–4s5p 1∑+) difference potential. These line-broadening techniques are used to study electronic energy transfer in the spin-changing collisions of Ca with He: Ca(5p 1P01) +He→Ca(5p 3P0J)+He+ΔE. Measurements of the ‘‘single collision’’ triplet–singlet branching ratio as a function of laser detuning from the atomic resonance frequency indicate a clear red wing/blue wing asymmetry. We interpret this asymmetry in terms of a preferential orbital alignment effect in the energy transfer process [Phys. Rev. Lett. 53, 2296 (1984)]. No clear structure is observed in the range of detunings probed that might indicate the curve crossing responsible for the energy transfer.
Broadening and shifting of the Raman Q branch of HD
Authors: Rosasco G. J. , May A. D. , Hurst W. S. , Petway L. B. , Smyth K. C.
Published: July 2002

The line broadening and shifting of the vibrational Q branch in pure HD has been measured for transitions J=0 to 3 at room temperature over the density range 0.8 to 10.6 amagat. The shifting and broadening coefficients have been determined with an uncertainty of ±0.2×10−3 cm−1 /amagat, which now provides a discriminating test for various semiclassical and quantal theoretical calculations. The line broadening coefficients are compared with linewidth data from other spectroscopic branches and with measurements of the rates of state-to-state rotational energy transfer. Use of an exponential gap law for the rates of rotational energy transfer allows estimates to be made of the contributions to the linewidths from rotationally inelastic, elastic vibrational dephasing, and elastic reorientation processes. This analysis suggests that rotational energy transfer occurs approximately 30% faster in v=1 than in v=0.
Mixed-mode oscillations in an electrochemical system. II. A periodic–chaotic sequence
Authors: Schell Mark , Albahadily F. N.
Published: July 2002

We describe a sequence of alternating periodic and chaotic behaviors that occur in the anodic dissolution of copper from a rotating disk in phosphoric acid. Each dynamical state of the sequence possesses a different combination of large and small amplitude oscillations (mixed-mode oscillations). The sequence is generated by incrementing either the potential or rotation speed of the copper-disk electrode. We find that chaotic regimes are linked to periodic regimes through tangent and period-doubling bifurcations. Measured waveforms of the chaotic mixed-mode oscillations can appear as mixtures of the waveforms of the adjacent periodic states. Phase portraits and one-dimensional maps are used to characterize a transition to chaotic mixed-mode oscillations from small amplitude chaotic oscillations. A brief comparison is made with the sequences of mixed-mode oscillations found in the Belousov–Zhabotinski reaction.
Intermolecular potentials for liquid argon
Authors: Ross Marvin
Published: July 2002

Properties of liquid argon calculated using an accurate variational theory, show that the intermolecular potentials of Barker and Aziz give equivalent results and that the three-body Axilrod–Teller term is essential for quantitative agreement with experiment.
The structures and dipole moments of Ar–PF3 and Kr–PF3
Authors: Taleb-Bendiab Amine , LaBarge Marabeth S. , Lohr Lawrence L. , Taylor Robert C. , Hillig Kurt W. , Kuczkowski Robert L. , Bohn Robert K.
Published: July 2002

The complexes of PF3 with Ar and Kr, were studied by Fourier transform microwave spectroscopy. The force constants and amplitudes of vibration for the van der Waals modes of the complexes and the average moments of inertia and structural parameters were estimated from the centrifugal distortion constants. The distance (Rc.m. )ave between the rare-gas atom and the center of mass of PF3 is 3.959 Å for the Ar complex and 4.077 Å for Kr while the angle (θc.m. )ave between the Rc.m. vector and the C3 axis of the PF3 is 69.30° and 67.25°, respectively. The dipole moments of both complexes and of free PF3 were determined. The induced dipole components estimated for the rare gas using electric fields from ab initio calculations of PF3 agree with the experimental values for a conformation with the rare gas over a PF2 face. The PF2 face conformation is also consistent with the observed and ab initio estimates of the 83 Kr nuclear quadrupole coupling constant for the 83 Kr–PF3 species.
Nondipole light scattering by partially oriented ensembles. II. Analytic algorithm
Authors: Tian Duan , McClain W. M.
Published: July 2002

We consider the elastic single scattering of light from an ensemble of identical particles with nonrandom orientational distribution. The particle is modeled as an arbitrary rigid array of N dipole polarizabilities, and the polarizabilities within each particle interact through the retarded dipole–dipole tensor. We consider the scattering ensemble to be an ‘‘optical element’’ in the Müller formalism; thus its polarized scattering properties, both dipole and nondipole, are specified completely by a four-by-four Müller matrix M(ψ), where ψ is the scattering angle. In a previous work on orientationally random ensembles, the slow convergence of the nondipole elements suggested that they might be particularly sensitive to small orienting forces. This was confirmed by numerical calculations, which, however, also demonstrated the need for a faster analytic averaging algorithm. In this paper we extend the uniform distribution analytic algorithm to cases in which the orientation of the particles is specified by a general nonuniform distribution. We then specialize the general formula to the case of axially nonrandom orientation, such as might be caused by an external static electric field E operating on a permanent electric dipole of the model particle. We present an explicit algorithm for M(ψ,E) for this special case, together with an estimate of its computability. For models with just a few subunits, it should be computable by a desktop machine with four megabytes of memory. For more realistic models using 1000 subunits, the job can easily rise into the supercomputer range.
Dynamics of conformational transitions to isomeric states favoring intramolecular excimer formation in aromatic polyesters with methylene or oxyethylene spacers
Authors: Bahar Ivet , Mattice Wayne L.
Published: July 2002

The dynamic rotational isomeric state formalism described in the preceding paper is used to calculate the rate of first passage from non-excimer-forming conformations to excimer-forming conformations in seven aromatic polyesters with different flexible spacers between the aromatic rings. The excimer-forming conformations in these polymers were identified recently by Mendicuti et al. The time dependence of the probability that a flexible spacer will at least once have passed through an excimer-forming conformation can be written as the product of two factors, one of which is time dependent, the other of which is determined by the equilibrium conformational statistics. In the polyesters considered here, the static factor is much more sensitive than the dynamic factor to the identity of the flexible spacers. Consequently, the equilibrium chain statistics provides a good description of the relative excimer population for these polyesters, even at times where the dynamic contribution is significant. The chromophores in these polyesters have such short fluorescence lifetimes that it is only in the most mobile flexible segments that there is an important dynamic contribution to the excimer formation.
Weak infrared bands in solid CD4
Authors: Calvani Paolo , Lupi Stefano
Published: July 2002

The spectrum of solid CD4 has been studied between 900 and 5500 cm−1 with improved sensitivity. The sideband associated to the normal mode ν3 has been resolved: two librational peaks are observed in CD4 II, and six in CD4 III. The latter result indicates the existence of three inequivalent sites in phase III of methane, as predicted by the model of Maki et al. In CD4 II, the A–T tunneling splitting of the librational ground state has been determined by scaling the observed librational frequency. The librations associated to the mode ν4, and to its overtones 2ν4 and 3ν4 have also been observed: in CD4 II, the dependence of the librational energy on the vibrational final state is much stronger than in CH4 II.
Production and trapping of gaseous dimeric ClO: The infrared spectrum of chlorine peroxide (ClOOCl) in solid argon
Authors: Cheng Bing-Ming , Lee Yuan-Pern
Published: July 2002

The dimeric ClO has been generated in a gaseous discharge–flow system at 220 K and 10–30 Torr by reacting atomic Cl with O3, ClOCl or OClO, and reacting atomic O with ClOCl or OClO, in a stream of Ar. The gas mixture in the flow tube was sampled through a pinhole and condensed on a CsI window maintained at 12 K. The infrared absorption spectrum of the matrix formed after 30–120 min deposition was then recorded. Absorption lines at 752.6, 649.8, and 647.6 cm−1 have been assigned to ClOOCl, and previous assignments for dimeric ClO seem to be in error. The results are in excellent agreement with recent ab initio calculations. Under our experimental conditions, no other isomer of Cl2O2 was observed.
High lying gerade Rydberg states of molecular iodine
Authors: Wu Ming , Johnson Philip M.
Published: July 2002

Two new (2+1) multiphoton ionization spectra of gerade Rydberg states of molecular iodine are observed in the 310.0–322.5 and the 285.5–295.5 nm ranges. These spectra were obtained in both a conventional MPI cell and in a TOF molecular beam apparatus. A long progression in the 310.0–322.5 nm range is suggested to be 2g(2Π3/2g)ndσg, having a band origin at 62 440 cm−1. Three progressions in the 285.5–295.5 nm range are suggested to be 2g, 1g, 0g(2Π3/2g)(n+1)dσg, having band origins at 68 335, 68 407, and 68 434 cm−1, respectively. The photodissociation behavior of molecular iodine in a MPI process is discussed.
Statistical mechanics of small chain molecular liquids. I. Conformational properties of modeled n-butane
Authors: Enciso E. , Alonso J. , Almarza N. G. , Bermejo F. J.
Published: July 2002

The density functional formalism can also be profitably applied to the statistical mechanics of molecular fluids with internal degrees of freedom. The change of the intrinsic chemical potential for different conformers of nonpolar molecules is given by a zeroth order perturbation approach. This approach is applied to study the isomerization of n-butane in liquids, which models the neat liquid and CCl4 solutions. We find that a nonpolar solvent medium produces a shift in the conformational equilibria of model liquid n-butane from that found in the gas phase. The theoretical predictions show a good agreement with recent Monte Carlo and molecular dynamics simulations results. Discrepancies with Jorgensen et al.’s data are explained in terms of the attractive intermolecular forces used in their MC codes.
Computer simulations of photoionization in ionically bonded clusters
Authors: Martin T. P. , Wassermann B.
Published: July 2002

Photoionization spectra of clusters record a continuous distribution of ionization energies because each cluster has a different atomic arrangement at the moment of ionization. Using the technique of molecular dynamics we have generated photoionization spectra for (Cs2O)n clusters.
Thermal diffusion at 300 K in binary systems of helium with CH4, C2H4, C2H6, and CHF3
Authors: Dunlop Peter J. , Bignell C. M.
Published: July 2002

Thermal diffusion factors are reported at 300 K for binary mixtures of helium with CH4, C2H4, C2H6, and CHF3. Limiting experimental values of the data when the mole fraction of the heavy component is zero are compared with the corresponding values calculated by means of the Chapman–Cowling theory for spherical potentials using the spherical and asymmetrical intermolecular potentials which have been reported in the literature.
An ultraviolet photoelectron spectroscopic study of BF3–donor complexes
Authors: Pradeep T. , Sreekanth C. S. , Rao C. N. R.
Published: July 2002

He i spectra of strong n–v type adducts of BF3 with H2O, CH3OH, (C2H5)2O, and CH3CN as well as of weak complexes of BF3 with NO and H2S are reported along with assignments based on MO calculations. The energy of the fluorine orbitals of BF3 is shown to be shifted in proportion to the strength of the donor–acceptor interaction. BF3 seems to form a contact pair with CS2.
Statistical thermodynamics of flexible-chain surfactants in monolayer films. II. Calculations for a modified cubic lattice model
Authors: Cantor Robert S. , McIlroy Peter M.
Published: July 2002

A modified cubic lattice model of chain conformations is incorporated within the general theory developed in the preceding article, and used to predict pressure-area isotherms for monolayers at the interface between water and a hydrophobic ‘‘solvent.’’ The statistical weight of each cubic lattice conformation is determined by enumerating all rotational isomeric states, and then performing a Boltzmann sum over the set of RIS conformers best described by that cubic lattice state. The isotherms are predicted to depend strongly on both chain length and on the energy of chain-solvent mixing, but only weakly on the energy difference between chain–water and solvent–water contact. In particular, with only a slight positive energy of chain–solvent mixing, two phase transitions are predicted. Both transitions are characterized by a critical point and involve only fluid phases. One extends to very low surface densities and resembles a liquid/gas transition. The other appears at high densities, and, by comparison with predictions of structural properties, is shown to result from the independence, in part, of conformational and translational degrees of freedom of the surfactant molecules.
Symmetry breaking in HCl and DCl dimers: A direct near-infrared measurement of interconversion tunneling rates
Authors: Schuder M. D. , Lovejoy C. M. , Nelson D. D. , Nesbitt D. J.
Published: July 2002

The interconversion tunneling frequencies for (HCl)2 and (DCl)2 are obtained from near-infrared absorption spectra of the H(D)Cl stretching transitions, to spectroscopic precision for the mixed 35Cl–37Cl dimers. A phenomenological model of the interconversion process explains several experimental observations, and provides good estimates of the splittings expected for the 35Cl–35Cl and 37Cl–37Cl species.
Molecular integrals with Slater basis. II. Fast computational algorithms
Authors: Fernández Rico J. , López R. , Ramírez G.
Published: July 2002

A new algorithm for the calculation of molecular integrals involving STOs is reported. The algorithm enables us to obtain every two-center one-electron integral and the long-range many-center one- and two-electron integrals. The efficient implementation of the algorithm is discussed and its performance is thoroughly tested. The analysis on the stability of the relations employed in the calculation of multipolar moments is included. Futhermore, the computer time required to carry out each step (construction of basic matrices, calculation of multipolar moments, and calculation of two-electron integrals) has also been analyzed. The range of validity of this approach is shown in several molecular integrals.
Cage effect for the abstraction of H from H2O in Ar matrices
Authors: Schriever R. , Chergui M. , Kunz H. , Stepanenko V. , Schwentner N.
Published: July 2002

A red wing in the absorption of H2O in the first continuum which extends beyond 200 nm is induced by the matrix. A threshold energy of 6.95 eV is found for the quantum efficiency of permanent dissociation of H2O into OH and H. This corresponds to an additional barrier of 1.8 eV due to the matrix cage. The barrier is attributed to the sum of repulsive H–Ar pair potentials in an interstitial site of D3 symmetry, which has to be surmounted by the H atom on the way from the initial substitutional H2O site to the final stable Oh site of the abstracted H atom.
N(2D) production from predissociation of n
s and n
d Rydberg levels in NO
Authors: Gadd G. E. , Jusinski L. E. , Slanger T. G.
Published: July 2002

N(2D) can be generated by two-photon dissociation of NO above the threshold wavelength of 279 nm. A comparison of the three-photon NO+ ionization signal with the N(2D) excitation spectrum resulting from its detection by 2+1 resonance-enhanced multiphoton ionization (REMPI) shows both similarities and differences. The NO Rydberg states that are generated in the initial two-photon process either ionize or predissociate. The NO+ and N+ signals are indicative of predissociative lifetimes and channels, since an NO molecule that does not predissociate to N(2D) and O(3P) can separate to two lower atomic limits. In the 268–279 nm region there are three two-photon absorption bands, 5sσ-X 3-0, 4dπ−-X 2-0, and 5dπ−-X1-0, which show very high N(2D) predissociative yields from the upper states. A fourth band, 4dδ-X2-0, behaves in a distinctly different manner from its 4dπ− complex partner. The presence of the valence G2Σ− state, which lies within the experimental energy range, is not in evidence from either the three-photon NO+ spectrum, or from predissociation in the N(2D) channel. It appears that this state predissociates rapidly to the lower N(4S)+O(1D) or N(4S)+O(3P) limits, even above the N(2D)+O(3P) threshold.
State-to-state integral cross sections for the inelastic scattering of CH(X 2Π)+He: Rotational rainbow and orbital alignment
Authors: Macdonald R. Glen , Liu Kopin
Published: July 2002

The state-to-state integral cross sections for the inelastic scattering of CH(X 2Π) with He were measured in a newly constructed crossed molecular beam machine. Use of laser-induced fluorescence in an unconventional flux mode of detection provided single fine-structure state specific detection of the products. Two types of measurements were performed to further our understanding of the collision dynamics of open shell systems: (1) the product state distribution at a fixed and well-defined collision energy and (2) the dependence on collision energy of product state-resolved cross sections. A qualitative understanding of the collision dynamics can be obtained by properly factoring out features dependent on the fine-structure states, i.e., effects involving individual Λ-doublet states and features dependent on the rotational level alone, i.e., effects remaining after summing over all four fine-structure states associated with a given rotational quantum number. As for the fine-structure effects, a preferential population of product Λ-doublet states with reflection symmetry Π(A″) was observed. The physical origin of this observed electronic orbital alignment can be attributed to a quantum interference phenomenon, as detailed in the accompanying paper. At the rotational level, the dominance of rotational rainbow scattering is unambiguously identified from both the existence of dynamical thresholds and a strong correlation between rotational level distributions at fixed translational energy and level specific excitation functions. These effects combined with other experimental observations lead us to visualize the CH+He scattering dynamics in a novel fashion. The collision can be regarded as a series of approximately independent sequential events each mediated by different regions of the interaction potential during the course of the whole encounter.
Convergence of the hard soft sphere potential to the hard sphere potential. A molecular dynamics study of transport coefficients
Authors: Borgelt P. , Hoheisel C.
Published: July 2002

The transport coefficients of the soft sphere (SS) dense fluid are computed by molecular dynamics for increasing powers of the usual SS potential expression. It is shown that the considered transport coefficients, namely the self-diffusion, the shear and bulk viscosity coefficients, and the thermal conductivity coefficient, exhibit an asymptotic behavior for n>30, where n denotes the exponent of the SS potential inserted. A direct comparison of the velocity autocorrelation function of the hard sphere and the n=42 SS system proves the excellent agreement of both systems with respect to the dynamics. Variation of density and temperature in certain limits do not change these findings, as expected.
X-ray structural characterization of larger CdSe semiconductor clusters
Authors: Bawendi M. G. , Kortan A. R. , Steigerwald M. L. , Brus L. E.
Published: July 2002

X-ray powder patterns obtained from three different types of 35–40 Å CdSe nanoclusters are analyzed. We simulate the effects of thermal fluctuations, stacking faults, surface reconstructions, and bond compressions on powder patterns of CdSe clusters using the Debye formula. We find that the spectra of capped and annealed CdSe particles grown in inverse micelles are best fit by a mixture of crystalline structures intermediate between zinc-blende and wurtzite. We describe a new preparation for CdSe clusters, the structure of which appears to be well-defined wurtzite with, on average, less than one stacking fault per cluster. Thermal effects are found to be important and to mask the subtle effects of likely surface and core reconstructions.
New variational principles for photodissociation: L2 amplitude density and scattered wave methods
Authors: Kouri Donald J. , Truhlar Donald G.
Published: July 2002

In this paper we present two new formulations for the time-independent quantum mechanical calculation of photodissociation amplitudes. The first is based on a variational L2 amplitude density approach, and the second is based on a new scattered wave variational principle, both approaches having been developed previously for the treatment of general reactive scattering problems. It is shown that, apart from an inhomogeneity term, the algebraic equations which must be solved are identical in form to those already successfully treated in recent three-dimensional, converged quantum reactive scattering studies. The new variational principles should provide a practical method for carrying out converged, three-dimensional quantal calculations for photodissociation processes in which any number of fragmentation pathways are possible.
Line shifts and broadenings in polarizable liquids
Authors: Chen Yong-Cong , Lebowitz Joel L. , Nielaba Peter
Published: July 2002

We present a new dynamical derivation of the approximation used by Thompson, Schweizer, and Chandler and by Ho/ye and Stell for the frequency dependent polarizability of a quantum fluid with harmonically bound dipole moments; the Drude model. The derivation is the same for classical and quantum liquids—as is of course the result which agrees with that of these authors. We then refine the theory by taking account of the limited number of energy levels available, i.e., we replace the harmonic approximation by a two level approximation, for the target atom. This leads to a prefactor ω0I/ω0 in the line shift of an impurity atom in a fluid computed by Chandler, Schweitzer, and Wolynes: ω0 and ω0I being the characteristic frequencies of the fluid and impurity atoms, respectively. This factor improves the agreement between theory and experiment, especially those of Nowak and Bernstein on benzene in argon and some other liquids. We also compute the line shapes of the impurity atom by Monte Carlo simulations and obtain reasonable agreement with experimental observations.
Theoretical state-to-state cross sections for collisions of N+2(v)+Ar. II. Results at higher energies
Authors: Parlant Gérard , Gislason Eric A.
Published: July 2002

State-to-state charge–transfer cross sections have been computed for N+2(X;v=0,1,2) +Ar at 12 collision energies between 1.2 and 320 eV. A classical path method is used, whereby the vibronic degrees of freedom are treated quantum mechanically as the system moves along a classical trajectory. The calculations use the potential energy surfaces computed by Archirel and Levy. Comparison is made with experimental results for this system, including the recent work from Ng’s laboratory. In most cases the agreement is quite good. There is, however, a significant difference in the charge–transfer branching ratios to produce Ar+(2P3/2) or Ar+(2P1/2) products. Possible explanations of the discrepancy are discussed. As expected, the cross sections obey the Franck–Condon principle at energies above 200 eV.
Deactivation rate constants and product branching in collisions of the Xe(6p) states with Kr and Ar
Authors: Xu J. , Setser D. W.
Published: July 2002

The quenching kinetics of the Xe(6p[1/2]0 ), Xe(6p[3/2]2 ), and Xe(6p[5/2]2 ) states have been studied in Kr and Ar buffer gas at room temperature using the two-photon, laser-excitation technique. The total quenching rate constants and the primary product distributions were measured to obtain state-to-state rate constants. Collisions between Xe[1/2]0 and Kr mainly gave energy transfer to Kr(5s,3P2 ) rather than relaxation to the Xe(6p or 5d) levels. The transfer of energy from Kr(5s,3P2 ) back to the Xe(6p) manifold also was observed. The collisional coupling between Xe[1/2]0 and Xe(3d5 ) in Ar, reported previously, was confirmed. The collisions of Kr and Ar with Xe[3/2]2 and Xe[5/2]2 atoms gave intramultiplet relaxation; observation of the time dependence of the primary products, the Xe[3/2]1, [5/2]3, and [1/2]1 states, permitted assignment of some state-to-state rate constants for these states. The flow of energy through the Xe(6p) manifold is discussed.
Binary nucleation of ethanol and water
Authors: Schmitt J. L. , Whitten J. , Adams G. W. , Zalabsky R. A.
Published: July 2002

The authors have used a fast expansion cloud chamber to measure binary homogeneous nucleation rates in several ethanol–water mixtures as a function of temperature, ethanol and water activities and nucleation rate. Data (ethanol and water activities) are presented for a range in nucleation rate from 103 to 105 drops/cm3 s from 263 to 293 K for mixtures having mole ratios (ethanol/water) of 10, 4, 0.1, 0.01, and 0.001. A comparison of the extensive data set to other data in the literature shows good agreement. We find current theory, as expected, is unable to accurately predict the data at low ethanol concentrations.
Simplified antisymmetrized product of geminals wave function for extended systems. I. Delocalized geminals
Authors: Karadakov P. , Castan̄o O. , Calais J.-L.
Published: July 2002

A simple antisymmetrized product of geminals wave function is proposed for the treatment of electron correlation in extended systems. The individual geminals are constructed using the pairing concept introduced with the alternant molecular orbital method. Explicit expressions are reported for the ground-state energy and for its optimization conditions. The model is applied to the π-electronic system of cyclic polyenes employing delocalized closed-shell Hartree–Fock orbitals to build the geminals.
The thermodynamics of solvophobic effects: A molecular-dynamics study of n-butane in carbon tetrachloride and water
Authors: Tobias Douglas J. , Brooks Charles L.
Published: July 2002

We have carried out molecular-dynamics simulations with holonomic dihedral angle constraints on two models of n-butane in CCl4 and in water to study the effects of apolar and polar solvents on the gauche-trans equilibrium. We calculated distributions of conformers from the torsional free-energy surfaces for each model of butane in both solvents. For a four-atom model of butane, the gauche-trans equilibrium constant in either solvent is unchanged relative to its gas-phase value. For an all-atom model of butane, the equilibrium population of gauche conformers is increased relative to its gas-phase value by 14% and 31% in CCl4 and water, respectively. The all-atom results are consistent with the idea of solvophobic stabilization of the gauche conformation. We also computed finite-difference temperature derivatives of the free energy to determine its energetic and entropic components. The gauche conformer of the four-atom model is stabilized by entropy and destabilized by energy in CCl4. We find the opposite thermodynamic driving forces for the all-atom model in CCl4. The gauche conformer is favored entropically and opposed energetically for both models in water. This result supports the idea that the butane hydrophobic effect is a manifestation of the hydrophobic interaction. Average interaction energies show that changes in solute–solvent interactions contribute significantly to the trans-gauche internal energy differences in CCl4, while the internal energy differences in water are dominated by changes in the solvent–solvent interactions. Solvent–solute radial distribution functions show that CCl4 packing around the butane molecular is similar to that around other CCl4 molecules, and it is not sensitive to the butane conformation. The water distributions around the butane molecule are very flat, but they show that the water is more disordered around the gauche conformer than the trans.
Polarization resolved two-photon resonant ionization spectroscopy of indole and 3-methylindole
Authors: Cable J. R.
Published: July 2002

Two-photon resonant, four-photon ionization spectra of vapor phase indole and 3-methylindole have been taken using both linearly and circularly polarized excitation. From these measurements, the wavelength dependent polarization ratio Ω has been determined. The value of Ω has been used to identify the onset of absorption to the second electronically excited state La amidst overlapping vibronic transitions to the lower lying Lb excited state. This onset occurs 1000 cm−1 above the Lb origin in indole and 100 cm−1 above the Lb origin in 3-methylindole. Several resolved transitions are also seen in the indole spectrum, roughly 1400 cm−1 above the Lb origin, which are tentatively assigned to the La origin. An analysis of the indole Lb origin rotational contour suggests that the two-photon absorption tensor for this transition is traceless and is dominated by the diagonal elements. The value of Ω for the La transition implies that its two-photon absorption tensor must have a nonzero trace and hence should exhibit a very different rotational contour.
Molecular theory of vapor phase nucleation: The physically consistent cluster
Authors: Reiss H. , Tabazadeh A. , Talbot J.
Published: July 2002

A statistical mechanical theory is developed to demonstrate that the appropriate (natural and consistent) physical cluster (to be used in a molecular theory of the nucleation of drops in supersaturated vapor far from the critical temperature) is that introduced by Reiss, Katz, and Cohen as well as by Lee, Barker, and Abraham—modified to account for the effects of the surrounding vapor. This modification allows the constraining spherical shell centered on the cluster’s center of mass to not be a constraint at all, but only a procedural device for organizing the derivation. Indeed, the shell can have variable volumes, natural to the system. The properties of the nucleus can then be established in a truly consistent molecular manner by Monte Carlo simulation. It appears as though the new development represents a very substantial advance in the molecular theory of vapor phase nucleation.
State-to-state rotational energy transfer measurements in silane by infrared double resonance with a tunable diode laser
Authors: Hetzler J. R. , Steinfeld J. I.
Published: July 2002

Infrared double resonance spectroscopy has been used to study state-resolved rotational and vibrational energy transfer in vibrationally excited SiH4. Completely specified rotational levels (v,J,Cn) are populated by CO2 laser radiation. Subsequent energy transfer is followed by diode laser transient absorption. The total relaxation efficiencies of the initially populated levels for self-collisions and collisions with Ar and CH4 follow the ordering σ(F2)>σ(A2)>σ(E) and are slightly larger than the Lennard-Jones cross sections. State-to-state rotational energy transfer in the ν4 vibration of SiH4 is extremely state specific. In addition to a differentiation between the A, E, and F symmetry levels, there is a selectivity with respect to the fine-structure levels within each rotational state. A preference for transfer to other levels of the same Coriolis sublevel of ν4 was found. This can be phrased as a Δ(J−R)=0 propensity rule. Principal pathways, only one per J per symmetry, are identified. Within each rotational level, the principal-pathway final states are closely spaced; this effect is related to the clustering of the rovibrational levels of the dyad. Large changes in J are possible in a single collision between silane molecules. A kinetic master equation has been used to model energy flow among rotational levels in silane, from which state-to-state energy transfer parameters could be extracted. Collision-assisted absorption of two CO2 photons into the triad has also been detected. A simple modification of the kinetic analysis allows us to obtain an estimate for the relaxation rate out of the triad levels. These laser pumping and relaxation processes determine the efficiency with which high vibrational levels of silane may be populated by infrared multiple photon excitation.
Multiple oscillations observed in the rotational state population of I2(B) formed in the photodissociation of (I2)2
Authors: Philippoz J.-M. , Monot R. , van den Bergh H.
Published: July 2002

Several oscillations are observed in the rotational state population of I2B 3Π(0+u) produced in the photodissociation (I2)2+hν→I2(Bv′,J′)+I2(X). The initial excitation is above the dissociation limit of the I2 B state. These oscillations are pronounced on some v′ states, but not on others. For a given state v′, the rotational state population oscillations essentially do not depend on the photolysis wavelength over the range of applied conditions.
Dispersive excitation transport at elevated temperatures (50–298 K): Experiments and theory
Authors: Stein Alan D. , Peterson Kristen A. , Fayer M. D.
Published: July 2002

Time-resolved fluorescence depolarization has been used to measure electronic excitation transport among naphthyl chromophores in polymeric glasses. 2-ethylnaphthalene randomly distributed in PMMA and 2-vinylnaphthalene/methyl methacrylate copolymer in PMMA were studied. It was found that excitation transport is dispersive at all temperatures studied, from 50 K to room temperature, i.e., the extent of transfer depends on the excitation wavelength within the S0–S1 absorption band. A theory based on the nondispersive, Förster mechanism for excitation transfer has been developed to describe dispersive transport. Good agreement between the theoretical and experimental results are achieved without resorting to adjustable parameters. Both the theory and experiment show that, for the observable used here, excitation at a certain wavelength, called the ‘‘magic wavelength,’’ results in a time dependence that is identical to the Förster nondispersive result, i.e., dispersive transport appears to vanish.
Recombination of Xe+ with F− ions in ambient helium
Authors: Lee H. S. , Johnsen R.
Published: July 2002

Rate coefficients for the ion–ion recombination process Xe++F−+He→XeF*+He have been determined in ambient helium gas in the density range from 0.2 to 0.7 NL (Loschmidt’s number NL=2.687×1019 cm−3). The experimental methods consisted of observing the conductivity decay during the afterglow of a photoionized plasma, in conjunction with mass spectrometry of plasma ions and optical spectroscopy of the XeF* excimer radiation. The measured rate coefficients agree well with theoretical results by Bates and Mendaš and Monte Carlo simulations by Morgan et al.
Rotationally and translationally resolved hot atom collisional excitation of the CO2 Fermi mixed bend/stretch vibrational levels by time-dependent diode laser spectroscopy
Authors: Hewitt Scott A. , Hershberger John F. , Chou James Z. , Flynn George W. , Weston Ralph E.
Published: July 2002

High resolution IR diode laser absorption spectroscopy is employed to monitor the nascent rotational population distributions and transient linewidths in the Fermi mixed symmetric stretch/overtone bend vibrational mode of CO2 (1000/0200) following collisions with translationally hot hydrogen and deuterium atoms, produced from the 193 nm excimer laser photolysis of H2S or D2S. The nascent 1000/0200 rotational distribution produced by H* atom collisional excitation peaks at J∼26 and is well fit by a 747 K Boltzmann distribution. The transient linewidths are 1.5–3 times the ambient, room temperature CO2 Doppler linewidths, are∼0.002 cm−1 larger for D* atom collisions than H* atom collisions, and increase with increasing rotational quantum number. The experimentally determined relative cross sections for H* atom collisional excitation of CO2 vibrational states are as follows: 0001 antisymmetric stretch: 1000 Fermi mixed upper level: 0200 Fermi mixed lower level: 0220 bend≊1.0 : 0.6 : 0.6 : 0.4. The absolute cross section for inelastic collisional scattering of CO2 by H* atoms into 1000 J=38 is (1.4±0.8)×10−2 Å2, and the total excitation cross section for the 1000 vibrational state is 0.37±0.21 Å2. A statistical model and a simple quantized Landau–Teller model are unable to explain qualitatively the observed data; however, a breathing ellipsoid model, coupled with an IOS quantum scaling relation, reproduces the major features in the experimental data for both the 1000/0200 and 0001 states. The differences in the experimental data for distinct vibrational motions can be attributed to hot atoms sampling different regions of the potential surface.
Ab initio studies on the dimer of sulfur dioxide and hydrogen cyanide
Authors: Chattopadhyay Soumitra , Moore Plummer Patricia L.
Published: July 2002

Ab initio electronic structure calculations have been performed for the van der Waals dimer HCN⋅SO2 and the associated monomers, HCN and SO2. The results of both Hartree–Fock and Mo/ller–Plesset calculations are reported for energy, optimized geometry, rotational constants, and vibrational harmonic frequencies for each system. An anti-hydrogen-bonded structure is predicted to be the most stable structure for the dimer in agreement with a previous experimental analysis.
On excess electron transport in liquid long chain n-alkanes
Authors: Jay-Gerin J. -P. , Ferradini C.
Published: July 2002

We compare the mobility (μe)–viscosity (η) relationship found in n-alkane (n-CxH2x+2) liquids at 295 K (2≤x≤14, except 13) with that obtained in pure polar solvents at room temperature. The results seem to be consistent with the existence of a transition in the mechanism of electron transport in liquid n-alkanes from the familiar ‘‘two-state’’ regime in the low-η liquids to a hydrodynamic-like behavior at sufficiently high η. In fact, for n-alkanes longer than C6, the excess electron appears to be localized in trap states quasi-permanently and to migrate in very much the same way as if it were ‘‘solvated.’’