bluuebunny/crossref_metadata_2025 huggingface dataset:
Ion condensation on solid particles: Theory and simulations
Authors: Groot Robert D.
Published: July 2002

For polyelectrolytes the Manning theory predicts an effective charge density, which increases with the bare charge density up to the point where the thermal energy of an ion balances the reversible work necessary to remove the ion from the polymer. From that point on, the effective charge remains constant, as no more ions will desorb. For spherical colloidal particles, a similar maximum effective charge can be expected, which indeed is predicted by the Poisson–Boltzmann theory. To check these predictions quantitatively, Monte Carlo simulations have been performed on the ionic distribution of a salt-free colloid at a volume fraction of 36%, both in the spherical cell model and in a periodic system of cubic symmetry. In the latter system the interactions were evaluated using the Ewald summation method. These simulations show that the effective charge of a colloid at this volume fraction does not reach a plateau value for large bare charges, but instead the effective charge passes through a maximum, and decreases again as the number of ions present increases. To extrapolate the simulation results to large particles, a weighted density-functional theory was developed that not only reproduces this maximum, but even predicts the simulation results within approximately 1%. According to this theory, the maximum is always present, and for large particles it appears at a given surface charge density which is nearly independent of the colloid volume fraction.
Time-of-flight spectra and angular observables for ultraviolet photodesorption of molecules from surfaces
Authors: Messina Michael , Coalson Rob D.
Published: July 2002

We show how to efficiently compute time-of-flight spectra and angular distributions associated with ultraviolet photodesorption phenomena using quantum wave packet simulations. Our method can be applied to adsorbates with internal vibrational and rotational degrees of freedom, and includes effects of (finite temperature) lattice vibrations on the photodesorption dynamics. Numerical applications to photodesorption of (a) a vibrating diatom from a collinear chain of harmonic oscillators and (b) a rigid rotor from the (100) face of LiF demonstrate the utility of our approach.
Kinetic and diffusional control of geminate recombination
Authors: Burshtein A. I. , Zharikov A. A. , Shokhirev N. V. , Spirina O. B. , Krissinel E. B.
Published: July 2002

A study is presented of geminate recombination and separation kinetics and their rates in liquid solutions. Applicability limits are proposed for the ‘‘exponential model’’ of this process, which is widely used for interpreting experimental data on back-electron transfer following photoionization. It is proved that this model qualitatively contradicts an analytical description of the process in polar media. A numerical experiment shows that the exponential model may be used only for nonpolar media of moderate viscosity. In this case the separation rate is linearly connected with the recombination constant and may be used to check the energy gap law characteristic for electron transfer.
Monte Carlo results for binary multi-Yukawa mixtures. Evaluation of the accuracy of the mean spherical approximation for realistic hard-core potentials
Authors: Arrieta E. , Jedrzejek C. , Marsh K. N.
Published: July 2002

Monte Carlo (MC) simulations are used to determine the properties of two different binary multi-Yukawa mixtures studied previously through a quasianalytic Yukawa-MSA (mean spherical approximation) algorithm [Arrieta, Jedrzejek, and Marsh, J. Chem. Phys. 95 XXXX (1991)]. These mixtures are composed of spherical hard-core molecules with multi-Yukawa interactions fitted (beyond the core diameters σLJij) to Lennard-Jones potentials, including thus a significant repulsion (negative slope) interval. The characteristics of the first mixture were chosen to produce a nearly ideal solution, while those of the second mixture (large size difference between components, weak unlike-particle attractions) favored nonideal behavior. For a variety of compositions, densities (in the liquid range), and temperatures, the following properties are determined: configurational energy, pressure, and chemical potentials. The latter were obtained through a new implementation of Widom’s particle insertion method. This simple implementation allowed the calculation of chemical potentials at high densities, where the usual procedures tend to fail. An analysis of the standard deviations and of the internal consistency of the MC data was used to confirm the general reliability of the simulation results. The good general agreement found between MC and MSA leads to the conclusion that the Yukawa-MSA quasianalytic algorithm provides not only a convenient but an accurate description of dense fluid mixtures, both ideal and nonideal.
Effects of Cooper minima in resonance enhanced multiphoton ionization-photoelectron spectroscopy of NO via the D 2Σ+ and C 2Π Rydberg states
Authors: Wang Kwanghsi , Stephens J. A. , McKoy V.
Published: July 2002

Cooper minima are predicted to occur in the 3pσ→kσ(l=2) and 3pσ→kπ(l=2) channels in the resonance enhanced multiphoton ionization of NO via the D 2Σ+(3pσ) Rydberg state. The low energy kσ(l=2) Cooper minimum leads to the observed ΔN=N+−N′=0 photoelectron peak, in addition to the ΔN=±1, ±3 peaks seen in the rotational spectra. The Cooper minima are accompanied by significant l mixing in the continuum due to the nonspherical molecular potential and result in a strong dependence of rotational branching ratios and angular distributions on photoelectron kinetic energy. A Cooper minimum is also predicted in the 3pπ→kδ(l=2) channel for photoionization of the C 2Π(3pπ) Rydberg state. The effect of this Cooper minimum on photoelectron spectra and photoelectron angular distributions is also investigated.
An adjusted global potential surface for HCN based on rigorous vibrational calculations
Authors: Gazdy Bela , Bowman Joel M.
Published: July 2002

We report extensive trial and error modifications of the Murrell–Carter–Halonen potential surface for HCN to improve agreement with experiments on highly excited stretching and bending states. The vibrational calculations make use of an exact Hamiltonian for nonrotating HCN and use an exact formalism to obtain energies. Two experimental data bases are used to compare against the calculations. One is for highly excited stretch states, but with no bend excitation, and the other is for highly excited bend and CN stretch states, but with no CH stretch excitation. The combined data base consists of 58 vibrational energies for nonrotating HCN. The modifications applied are angular and stretch coordinate scaling and an angular-dependent potential scaling. In addition, the saddle point position is adjusted to agree with the results of a recent ab initio calculation.
Λ-doublet substate specific investigation of rotational and fine structure transitions in collisions of OH with H2 and D2
Authors: Andresen P. , Aristov N. , Beushausen V. , Häusler D. , Lülf H. W.
Published: July 2002

The selective population of rotational, spin, and Λ-doublet states of OH(2Π,v=0,1) by inelastic collisions with H2 and D2 is investigated in two experiments. In the first experiment OH radicals are generated by photolysis inside a pulsed nozzle beam source and prepared in the ground state j=1.5, v=0, 2Π3/2 with equal amounts in both Λ-doublets by rotational cooling in the subsequent expansion. The collisional excitation of OH by the secondary beam is probed via laser-induced fluorescence selectively for the Λ-doublet states for the different rotational levels in both spin manifolds of v=0. Integral cross sections σ(j=1.5,Ω̄=1.5→j ′,Ω̄′,ε′) averaged over the Λ-doublet substates in the input channel, but Λ-doublet substate resolved in the output channel are obtained in this case. In contrast to some previous predictions the Π(A′) Λ-doublet substate is preferentially populated. This implies a population inversion in the Λ-doublets for 2Π1/2 and an anti-inversion for 2Π3/2, which rules out previously proposed pump mechanisms for astronomical OH masers. In the second experiment OH is generated also by photolysis, in this case however in a flow system. Single Λ-doublet states are prepared by infrared excitation of the thermally relaxed OH in 2Π3/2, v=1 for two rotational states (j=1.5,4.5). The redistribution in v=1 induced by collisions with H2 is probed by laser-induced fluorescence. Rate constants are obtained for transitions from the initially prepared Λ-doublet states to the other Λ-doublet of the same j and also for transitions to other rotational states.
The range dependence of fluorescence anisotropy in molecular energy transfer
Authors: Andrews David L. , Juzeliūnas Gediminas
Published: July 2002

In the theory of excitation energy transfer it is generally considered that species initially excited by photoabsorption transfer their energy to other molecules by two distinct mechanisms, known as radiative and radiationless energy transfer. Recently it has been shown that the two mechanisms for energy transfer are in fact indistinguishable, each being the asymptotic limit of a unified mechanism involving virtual photon coupling. The familiar R−6 dependence associated with Förster radiationless transfer is the short-range limit, while over longer distances retardation effects modify the radial dependence to R−2, and the result is the classical radiative transfer law. For radiationless energy transfer, wide use is made of Galanin’s result concerning fluorescence depolarization losses due to single-step transfer. Here Galanin’s work is extended to obtain a general formula for the residual fluorescence anisotropy following energy transfer over arbitrary intermolecular distances. Hence a connection is established with the depolarization associated with reabsorption. In particular, it is shown that the anisotropy increases significantly from its initially low value over distances considerably less than those normally associated with radiative energy transfer. The necessary criteria for experimental identification of the transition from radiationless to radiative character are described, and model systems for their realization are considered.
Concerning the role of dipolar disorder on charge transport in molecularly doped polymers
Authors: Borsenberger P. M. , Bässler H.
Published: July 2002

The field and temperature dependencies of the hole mobility of 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) doped polystyrene have been measured and compared to results obtained for the TAPC doped polycarbonate and pure TAPC. The results are described by the disorder formalism, due to Bässler and co-workers. The mobility of TAPC doped polystyrene is approximately 100-fold greater than that observed for TAPC doped polycarbonate. This effect is interpreted in terms of (1) the elimination of random dipolar fields due to static dipole moments of the polycarbonate that affect the energetic disorder, and (2) improved electronic intermolecular coupling with a concomitant reduction of positional disorder.
Adsorbate-assisted adsorption: Trapping dynamics of Xe on Pt(111) at nonzero coverages
Authors: Arumainayagam Christopher R. , Stinnett James A. , McMaster Mark C. , Madix Robert J.
Published: July 2002

The trapping dynamics of Xe on Pt(111) has been probed as a function of Xe coverage with supersonic molecular-beam techniques. Adsorption probabilities were directly measured at a surface temperature of 95 K at coverages ranging from zero to monolayer saturation at incident translational energies between 6 and 63 kJ/mol and incident angles between 0° and 60°. In apparent agreement with the predictions of the original Kisliuk model, the adsorption probability at the lowest incident translational energy (6 kJ/mol) remains almost constant with coverage up to near monolayer saturation. However, in contradiction to the original Kisliuk model, at higher incident translational energies, the trapping probability increases nearly linearly with xenon coverage up to near monolayer coverage. For example, the trapping probability increases from 0.06 to 0.42 for an incident translational energy of 63 kJ/mol at normal incidence as the coverage is increased from zero to saturation monolayer coverage. This behavior can be explained adequately by a model that incorporates enhanced trapping onto the monolayer compared to the clean surface, a property of the model that is confirmed directly by experiments presented herein. The angular dependence of the adsorption probability shows progressive deviation from normal energy scaling with increasing Xe surface coverage, proving that the degree to which parallel momentum participates in the adsorption process increases with adsorbate coverage. The initial trapping probability of Xe onto the monolayer is independent of incident angle indicating total-energy scaling. The above findings are qualitatively identical to our previous results for the molecular adsorption of ethane on the same surface, suggesting that these phenomena occur, in general, for weak molecular adsorption regardless of molecular shape and internal degrees of freedom, at least for small molecules.
Variation of the polarization ratio for rotationally inelastic collisions with laser selected velocity
Authors: Fell Christopher P. , McCaffery Anthony J. , Reid Katharine L. , Ticktin Anton
Published: July 2002

We extend the technique of velocity selection by narrow linewidth laser excitation by measuring the polarization of emission following energy transfer as a function of selected velocity. We apply this to rotationally inelastic collisions in A1ΣuLi2–Xe and find that for most transitions, there is a noticeable decrease in the circular polarization ratio at the center of the Doppler profile. We speculate as to whether the reason for this is dynamical, geometrical, or a combination of the two. We are able to deconvolute cross sections for the transfer of orientation as a function of relative speed which can then be compared with cross sections for the transfer of population.
Polarization ratio velocity dependence dips: A collision frame model relating spectroscopic observables to the differential cross section
Authors: Reid Katharine L. , McCaffery Anthony J.
Published: July 2002

We present a model that accounts for the observation described in the first paper of this series (I) of a dip in the polarization ratio profile observed in certain rotationally inelastic processes as a narrow line laser is tuned across the Doppler profile. The model is based on conservation of the projection of j along the kinematic apse [Khare, Kouri, and Hoffmann, J. Chem. Phys. 74, 2275 (1981)], a consequence of rotational angular momentum transfer in the impulsive limit. As an illustration, we present calculations of lab frame observations expected for several examples of scattering behavior in the collision frame and conclude that information on the state-to-state differential scattering cross section is contained in the polarization profile. We display a number of figures that make explicit the relationship between the relative momentum and rotational angular momentum vectors in the experiment and illustrate the ‘‘stereochemical dynamics’’ responsible for the polarization dip.
A model for salt and water transport in polyelectrolyte solutions
Authors: Phillips Christopher G.
Published: July 2002

As a model for salt and water movement in polyelectrolyte solutions, we present theoretical calculations of small-ion and water transport through regular arrays of parallel, charged circular cylinders. We formulate the problem for steady fluxes of water and univalent co- and counterions in response to gradients of salt concentration, pressure, and electric potential. The fluxes are related to these gradients by effective transport coefficients, such as the flow resistance and the effective ionic diffusion coefficients. These coefficients, and the equilibrium ionic partition coefficients, are calculated numerically in two approximations. First a linear approximation is used, appropriate when the cylinders are weakly charged, and second a nonlinear calculation is performed, in which the geometry is simplified using a cylindrical-cell model. The physical significance of these numerical results is discussed.
Gaussian density functional calculations on the allyl and polyene radicals: C3H5 to C11H13
Authors: Sim Fiona , Salahub Dennis R. , Chin Steven , Dupuis Michel
Published: July 2002

The electronic structure of the allyl radical C3H5 and the polyene radicals C5H7, C7H9, C9H11, and C11H13 have been calculated using the linear combination of Gaussian-type orbitals-local spin density method. In contrast to the results obtained using the Hartree–Fock model, which show large errors, the geometries are in excellent agreement with multiconfiguration self-consistent-field calculations and with experiment. LSD yields a C2v symmetry for the allyl radical, while the polyenes C5H7 to C11H13 have C–C bonds alternating between single and double bonds. The harmonic vibrational frequencies were calculated for the allyl radical and C5H7 (the 1,4-pentadienyl radical). The unscaled vibrational frequencies calculated for the allyl radical are in excellent agreement with experiment.
Ostwald ripening in open systems
Authors: Nakahara Akio , Kawakatsu Toshihiro , Kawasaki Kyozi
Published: July 2002

We consider coarsening processes of precipitates in open systems where homogeneous cells are coupled together by the diffusion of the solute. Using dynamical scaling assumptions and performing numerical simulations on a two-cell model, we have obtained the following results: First, the supersaturation in each cell asymptotically obeys the same Lifshitz–Slyozov–Wagner (LSW) scaling law for a closed system. Second, other physical quantities, such as the size distribution function of precipitates, obey new scaling laws, when the total amount of both the solute and the precipitates in the cell flows out into the other cell. The asymptotic long time behavior of such physical quantities are highly dependent on the initial conditions.
Characterization of asymmetry transients in rotational coherence spectroscopy
Authors: Joireman Paul W. , Connell Leslie L. , Ohline Shane M. , Felker Peter M.
Published: July 2002

The characteristics and origin of a new type of rotational coherence effect—asymmetry transients—are considered. Asymmetry transients are of two types, which we call C-type and A-type, respectively. C-type features are spaced by time intervals equal to 1/(4C). A-type features are spaced by time intervals equal to 1/(4A). Both types of transients can provide information on rotational constants that is not available from other types of rotational coherence effects. We show that the C-type transients arise from coherences between rotational eigenstates of the form Jτ and (J+2)τ−2 (in the Jτ notation) and that the A-type ones arise from coherences between eigenstates Jτ and (J+2)τ+2. The fact that such coherences produce asymmetry transients is shown to be a consequence of the limiting behavior of asymmetric top energy levels at high values of J. Experimental results in which asymmetry transients are present are reported for jet-cooled perylene, tryptamine (D conformer), 1-naphthol–water, and fluorene–benzene.
Luminescence and excitation spectroscopy of ruthenocene
Authors: Riesen Hans , Krausz Elmars , Luginbühl Werner , Biner Margret , Güdel Hans U. , Ludi Andreas
Published: July 2002

The luminescence and excitation spectra of the perdeuterated and the protonated ruthenocene have been carefully investigated. The electronic origins are at 21 459 cm−1 and at 21 390 cm−1 for the perdeuterated and protonated crystals, respectively. The two observed luminescence progressions in the metal-ring stretch a′1 (D5h notation) in the perdeuterated crystal are separated by 155 cm−1 compared to the 165–170 cm−1 reported for the protonated compound. The metal-ring stretch frequency is reduced to 318 cm−1 in the perdeuterated crystal from the 333 cm−1 found for the protonated compound. In both crystals the observed doubling of the vibrational progression is due to a false origin based on a skeletal bending mode δ(ring-Ru-ring) which transforms as e1 in D5h. This mode is split by 7–8 cm−1 via the low site symmetry (Cs) in the crystal. The excitation spectrum of both compounds shows only a single progression in the metal-ring stretch. This frequency is reduced by 16% in the excited state for both the protonated and the perdeuterated crystal. Deuteration increases the lifetime from 127 μs to 990 μs at 1.5 K.
Theoretical study of CF3: Absorption and emission spectra
Authors: Larrieu C. , Chaillet M. , Dargelos A.
Published: July 2002

A b Initio CI calculations for the fundamental and excited electronic states of CF3 are reported. The two first vertical ionization potentials are calculated at 11.55 and 15.85 eV. The uv spectrum is analyzed and the nature of Rydberg transitions is discussed. In order to discuss the vibrational structure observed from the cross section photoionization of CF3, as well as from the electronic absorption and emission spectrum, the Franck–Condon terms were calculated. The emission spectrum of CF3 is discussed and the particular role of one valence excited state is underlined in the uv region.
Local versus hyperspherical modes of water and formaldehyde: Effect of molecular complexity on mode-selective structures and dynamics
Authors: Hartke B. , Janza A. E. , Karrlein W. , Manz J. , Mohan V. , Schreier H.-J.
Published: July 2002

Effects of molecular complexity on mode-selective phenomena are studied for models of water and formaldehyde. Here, complexity is measured by the numbers of vibrational degrees of freedom which interact in the model systems, including both OH stretches and the bending motion for H2O and both the CH and the CO stretches for CH2O. Neglect (i.e., decoupling or ‘‘freezing’’) of the bending vibration in H2O, or the CO stretch in CH2O, yields simpler model systems which serve as references for the more complex original ones. The mode-selective phenomena that are compared for these systems include structural and dynamical effects of highly excited local and hyperspherical modes. The methods employed include expansions of vibrational states in terms of simple, i.e., Morse or harmonic-oscillaton basis functions for the individual stretches and bends, as well as fast-Fourier-transform propagations of the representative wave packets: The validity of these techniques is discussed in detail, depending on the properties of the selective states considered. The most important result is that increasing molecular complexity does not necessarily destroy all mode selectivity. However, the conservation of mode selectivity depends on the system, and on the property considered. Thus, for H2O, the structures of local modes are conserved, whereas very highly excited hyperspherical ones are modified when the bend is switched on. In contrast, for CH2O both local and hyperspherical structures are conserved, and the ratio of rates for fast local mode vs slow hyperspherical mode decay remains very large (≫100:1) when the CO stretch is coupled to the CH2 fragment. In addition, the lifetimes of local modes decrease as the complexity of the model system increases from CH2 to CH2O, indicating inverse intramolecular relaxation of vibrational energy. Extrapolation of these results suggests that mode selectivity may extend from small to larger systems.
Triplet state spectroscopy and photofragment dynamics of N2+2
Authors: Mullin Amy S. , Szaflarski Diane M. , Yokoyama Kazushige , Gerber G. , Lineberger W. C.
Published: July 2002

The photofragment spectrum of N2+2→N+(3Pg)+N+(3Pg) has been studied using a coaxial laser–ion beam spectrometer. Transitions from the excited 3Σ+u(v=0,1) state of N2+2 to the predissociative 3Πg(v=0) state result in two bands with dense rotational structure at 15 300 and 13 100 cm−1. A complete analysis of the 27 rotational branches associated with the 3Πg(v=0)←3Σ+u(v=0) transition provides bond lengths and spectroscopic constants for both states. A perturbation is observed in the (0,0) band, caused by the interaction of 3Σ+u(v=0) with 3Πu(v≊10). From a deperturbation analysis, the coupling constants and the energy difference between the 3Πu(v≊10) and 3Σ+u(v=0) states are determined—ξ=1.5±0.1 cm−1, η=0.220±0.003 cm−1, and Tpert=365±25 cm−1. A broad unstructured band at 16 400 cm−1 arises from a transition out of v=0 in the 3Σ+u state into a very short-lived v=1 level of the 3Πg state. Vibrational spacings between v=0 and v=1 in both electronic states are determined—3Σ+uω0=2210±20 cm−1 and 3Πg ω0≊1100 cm−1. The fragmentation of N2+2 is explored by measuring the dependence of predissociation lifetimes on the rotational quantum number of the 3Πg(v=0) state. Experimentally determined lifetimes of 50–70 ps for N′=2–17 are almost independent of N′ and indicate that the predissociation occurs by the interaction of 3Πg(v=0) with the 3Σ−g continuum rather than by tunneling through the 3Πg barrier.
Spin–orbit coupling constants in a multiconfiguration linear response approach
Authors: Vahtras Olav , Ågren Hans , Jo/rgensen Poul , Jo/rgen Aa. Jensen Hans , Helgaker Trygve , Olsen Jeppe
Published: July 2002

Spin–orbit coupling constants between singlet and triplet states are evaluated as residues of multiconfiguration linear response functions. In this approach, the spin–orbit coupling constants are automatically determined between orthogonal and noninteracting states. Sample calculations are presented for the X 3Σ−g–b 1Σ+g transition in O2 and the 1A1–3B1 transition in CH2. The convergence of the coupling constants is examined as a function of basis set and level of correlation. An exotic behavior is observed in the correlation of the 1A1 state for CH2 when increasing the active space, demonstrating an intricate coupling between the dynamic and static correlation. In general, the results indicate that reliable spin–orbit coupling constants between valence states may be obtained with a 4s3p2d1f basis set for first row atoms and a modest active orbital space.
Time-of-flight measurements of single rovibrational states of carbon monoxide
Authors: Price J. M. , Ludviksson A. , Nooney M. , Xu M. , Martin R. M. , Wodtke A. M.
Published: July 2002

A new technique has been developed for the measurement of the translational energy of molecules in single rovibrational quantum states. Molecules from a given rotation–vibration level are excited to a long-lived electronic state by a pulsed, tunable ultraviolet (UV) laser and are allowed to collide with the surface of a low work function metal. Since the energy of the metastable state exceeds that of the metal’s work function, collisions result in the ejection of electrons from the metal surface, which may be detected with high efficiency. This technique has been applied successfully to the carbon monoxide system, where measurements of molecular beam velocities and extremely weak forbidden electronic transitions have been made. The detection efficiency of this technique is estimated to be 2.0×10−4, comparable with electron impact ionization and mass-selected detection. Possible future applications of the technique in spectroscopy, photodissociation, and photon stimulated desorption experiments are discussed.
Correlation lengths and chain sizes in PS/PVME blends: Influence of compressibility, interactions, and monomer structures
Authors: Dudowicz Jacek , Freed Karl F.
Published: July 2002

The lattice cluster theory is applied to explain the observed strong concentration dependence of the apparent radius of gyration and the correlation length in polystyrene/poly(vinylmethylether) blends. The analysis emphasizes the importance of using a theory that properly describes blend compressibility, monomer molecular structures, interactions, and local correlations.
Hydrogen atom transfer reactions of N+2 with H2, HD, and D2 from thermal to 10 eV center of mass
Authors: Schultz Richard H. , Armentrout P. B.
Published: July 2002

Reactions of N+2 ions with H2, HD, and D2 are studied under single-collision conditions in a guided-ion beam mass spectrometer over a much broader range of interaction energies than in any previous study, including the low energy region of thermal to 0.1 eV. Reactant ions are formed in a flow tube source to ensure thermalization. Possible reaction mechanism are discussed, and the present results are compared to previous measurements. We find that the reaction proceeds at the rate predicted by the classical ion–molecule capture collision theory at thermal energy, but exceeds this prediction at energies above 0.1 eV. This behavior is discussed and attributed to details of the interaction between the N+2+H2 surface and the N2+H+2 surface along which the hydrogen atom transfer reaction proceeds. Intramolecular isotope effects and product ion dissociation behavior suggest that the reaction occurs via a direct mechanism with no long-lived intermediate at elevated energies.
Infrared multiphoton excitation dynamics of CF3I. II. Collisional effects on vibrational and rotational state populations
Authors: Abel B. , Hippler H. , Troe J.
Published: July 2002

Time-resolved resonantly enhanced multiphoton ionization spectroscopy of I(2P3/2) and I(2P1/2) dissociation products of CF3I can be used to characterize populations of highly-excited CF3I*. This technique has been applied to monitor infrared multiphoton excitation dynamics of CF3I in the presence of collisions. Compared to collision-free situations, collisions tend to destroy bimodal vibrational energy distributions and to enhance the overall pumping rate. Collisional energy transfer could be monitored for molecules above the dissociation threshold. Collisional relaxation of highly-excited molecules has been followed and analyzed with respect to average energies transferred per collision.
Resonance ionization spectroscopy of HCO and DCO. I. The 3p 2Π Rydberg state
Authors: Song Xiao-Mei , Cool Terrill A.
Published: July 2002

Resonance two-photon ionization (R2PI) spectroscopy has been used for medium resolution studies of HCO and DCO at wavelengths from 208 to 222 nm. Vibronic analysis of subband positions originating from both A′ and A″ symmetry components of the 3p 2Π Rydberg states of HCO and DCO yields values for the Renner–Teller parameter Ε, dipolar vibronic parameter gk, quartic anharmonicity constant g22, harmonic bending vibrational frequency ω2, anharmonicity constant x22, and spin–orbit splitting constant A. Values of the harmonic frequencies ω2 and anharmonicity constants x22 for the bending vibrations of the HCO and DCO X̃ 2A′ ground states are also determined. Previously unidentified two-photon subbands, thought to be excitations of the 3s 2Σ+ Rydberg state, are assigned to the symmetric A′ component of the 3p 2Π state.
Resonance ionization spectroscopy of HCO and DCO. II. The B̃ 2A′ state
Authors: Cool Terrill A. , Song Xiao-Mei
Published: July 2002

Resonance two-photon ionization (R2PI) spectroscopy of the HCO B̃←X̃ ‘‘hydrocarbon flame’’ band system performed with a jet-cooled molecular beam at 5 K has permitted the assignment of subband origins for 17 vibronic bands of HCO and 20 of DCO. Term values, the three harmonic vibrational frequencies and six anharmonicity constants are measured for the B̃ 2A′ states of both molecules. The observation of an increase in the effective asymmetric top rotational constant, A–(B+C)/2, with increasing vibrational quantum number identifies the B̃ state bending vibration with fundamental frequencies of 1382 and 1213 cm−1 for HCO and DCO, respectively. The highest fundamental frequencies of 2597 cm−1 for HCO and 1944 cm−1 for DCO correspond to CH(CD) stretching, while the lowest fundamental frequencies of 1066 cm−1 for HCO and 922 cm−1 for DCO correspond to the vibration of singly bonded C–O.
Relativistic effects in physics and chemistry of element 105. I. Periodicities in properties of group 5 elements. Electronic structure of the pentachlorides
Authors: Pershina V. , Sepp W.-D. , Fricke B. , Rosén A.
Published: July 2002

A detailed study of the electronic structure and bonding of the pentahalides of group 5 elements V, Nb, Ta, and element 105, hahnium (and Pa) has been carried out using relativistic molecular cluster Dirac–Slater discrete-variational method. A number of calculations have been performed for different geometries and molecular bond distances. The character of the bonding has been analyzed using the Mulliken population analysis of the molecular orbitals. It is shown that hahnium is a typical group 5 element. In a great number of properties it continues trends in the group. Some peculiarities in the electronic structure of HaCl5 result from relativistic effects.
Laser-induced emission of SO in matrices: The c̃ 1Σ−→ã 1Δ and the Ã′ 3Δ→X̃ 3Σ− transitions
Authors: Zen Ching-Chi , Tang Fa-Tai , Lee Yuan-Pern
Published: July 2002

The laser-induced emission was observed during irradiation of samples containing Cl2SO, SO2, or OCS/NO2 in a Ne matrix by a KrF excimer laser at 248 nm. Two progressions are assigned to SO. The progression in the range 353–601 nm is assigned to the Ã′ 3Δ→X̃ 3Σ− transition with ν00=29 494±5 cm−1, ω″e=1151±2 cm−1, and ω″ex″e=6.2±0.2 cm−1, whereas the progression in the range 491–822 nm is assigned to the c̃ 1Σ−→ã 1Δ transition with ν00=22564±5 cm−1, ω″e=1115±3 cm−1, and ω″ex″e=6.8±0.2 cm−1. In an Ar matrix, the corresponding progressions of SO in various matrix sites were also observed. The thermoluminescence lines observed previously from solid Ar correspond to the phonon wings of an unstable site before annealing. A third progression that was also observed is assigned to the emission from the v′=1 level of the c̃ 1Σ− state; its separation by 670 cm−1 from the main c̃→ã progression agrees with the predicted ω′e of the c̃ 1Σ− state.
The electronic states of lithium atoms in ammonia clusters and solution
Authors: Martyna Glenn J. , Klein Michael L.
Published: July 2002

A new model electron–ammonia pseudopotential parameterized to ab initio quantum chemistry calculations on small lithium ammonia clusters, Li(NH3)n(n=1,4), is studied in a variety of environments. For Li(NH3)n clusters, n=16,32,64,128 the valence electron of the lithium is found to exist in a surface state far from the cation which is localized near the center of mass of the cluster. No bulk states were stabilized. Cluster anions (NH3)−n were also studied and the electron centroid-cluster center of mass probability distribution for (NH3)−64 calculated using umbrella sampling. In the present model, there is apparently no barrier to the dissociation of the surface states. No bulk states were found. This set of results appears to disagree with experiments which have been interpreted to indicate bulk states for cluster anions and the clusters containing lithium. Bulk properties of both a single excess electron and the lithium atom in solution are also reported. The solvation energy agrees well with experiment but the spectrum of the excess electron remains somewhat blue shifted as in earlier calculations. However, the valence electron of the lithium atom is found to spontaneously dissociate; a property not reproduced in previous work.
193 nm photodissociation of KI: Branching ratio and collisional mixing rate of K(5 2P
J) doublets
Authors: Wang Kung-Chung , Lin King-Chuen , Luh Wei-Tzou
Published: July 2002

Through a three-level kinetic model, the branching ratio of the nascent photofragment K in the 5 2PJ fine-structure states following photodissociation of KI by a 193 nm excimer laser has been experimentally determined to be K(5 2P3/2)=0.791 and K(5 2P1/2)=0.209 with ±1% accuracy. The model has taken into account the rapid energy transfer between the 5 2PJ doublets and the result appears to be more accurate than those fluorescence intensity ratio measurements under low pressure condition. The cross section of fine-structure mixing induced by H2 collisions has also been measured to be 134±6 Å2 for the transition 5 2P3/2←5 2P1/2 and 72±5 Å2 for its reverse process. The ratio 1.86 is consistent with the value 1.89 predicted by principle of detailed balance. Using the Stern–Volmer equation, we have also obtained the radiative lifetime 137±4 ns for the K(5 2PJ) state and its quenching cross section 10.4±1.8 Å2 by collision with H2 molecule. The latter appears much smaller than those of fine-structure energy transfer processes by an order of magnitude.
Photoelectron spectroscopy of SF−
Authors: Polak Mark L. , Gilles Mary K. , Lineberger W. C.
Published: July 2002

The 351 nm photoelectron spectrum of SF− has been recorded with 9 meV resolution. Transitions from the 1Σ+ SF− ground electronic state to the two spin–orbit components of the 2Π SF ground state are observed. The adiabatic electron affinity of SF is determined to be 2.285(6) eV. From Franck–Condon simulation of the spectrum we obtain re(SF−)=1.717(15) Å, and observation of vibrational hot bands enables measurement of ωe(SF−)=635(15) cm−1. The spin–orbit constant of the 2Π ground state of SF is determined to be A=−417(10) cm−1.
Accurate specific molecular state densities by phase space integration. I. Computational method
Authors: Berblinger Michael , Schlier Christoph
Published: July 2002

The semiclassical determination of the specific density of quantum states, ρ(E;J), at energy E with fixed total angular momentum J is discussed for small molecules. Monte Carlo integration allows the accurate numerical determination of the phase space volume of systems with J>0 and arbitrary anharmonicity. The corresponding semiclassical number of states can be corrected for the effects of zero point motion in analogy to the well-known Whitten–Rabinovitch procedure. In this paper, the procedures are tested by comparison with rigid rotor harmonic oscillator models, while a comparison with recent exact quantum calculations on H+3 and HD+2 is described in the following paper. We conclude that, if the intramolecular potential is known or assumed, this numerical semiclassical procedure is a viable and simple way to get state densities of a much improved accuracy.
Semiclassical collision theory in the initial value representation: Efficient numerics and reactive formalism
Authors: Campolieti G. , Brumer Paul
Published: July 2002

Two contributions to semiclassical mechanics, within the initial value representation, are presented. The first is the introduction of an efficient integration scheme, based upon number theoretic lattices, which allows the effective evaluation of S matrix elements despite highly oscillatory integrands. Applications to collinear H+H2 and three-dimensional He+H2 inelastic scattering show good agreement with quantum results for both classically allowed and tunneling transitions with relatively few trajectories. The second contribution is the derivation of an initial value integral representation for reactive scattering using the idea of asymptotic equivalence with the classical-limit formulas. The result is a generally useful formula, requiring only real valued trajectories, for reactive scattering above and below the classical reaction threshold.
Coupled-cluster calculations on the C2 molecule and the C+2 and C−2 molecular ions
Authors: Watts John D. , Bartlett Rodney J.
Published: July 2002

Coupled-cluster methods including effects of triple excitations have been used with large basis sets to study several electronic states of C2, C−2, and C+2. re, ωe, De, and Te have been computed for each state considered. For those states for which experimental data are available, the errors are typically less than 0.002 Å, 20 cm−1, and 0.1 eV for re, ωe, and De, respectively. These errors are comparable with the best previous results on C2 and significantly smaller than those for previous calculations on C−2 and C+2. For Te the results are also satisfactory and comparable with previous work, but some difficulties arise when one or both of the states have considerable multireference character. For two states of C+2 not experimentally characterized, the present data should be sufficiently accurate to be of some assistance to experimental studies.

The electron affinity of the ground state of C2 and the ionization potentials of the ground and first excited states of C2 have been computed, and particular attention has been given to correlation errors in these quantities. It has been shown that triple excitations significantly decrease the electron affinity. By neglecting or taking insufficient account of triple excitations, some previous calculations were biased towards the anion and obtained fortuitously good agreement with experiment. The correlation error of the present calculations is such that the calculations are slightly biased towards the neutral molecule, but the error is only 0.2 eV. For the ionization potentials, too, triple excitations play an important role. Reasonable agreement with experiment has been achieved by the present calculations. Overall, the present study: (1) underscores the importance of triple excitations; (2) shows the success of single-reference coupled-cluster theory in calculating properties of a single state, even when that state has considerable multireference character; and (3) highlights the care which must be taken when comparing different electronic states with these methods when one or both states have considerable multireference character.
Translational and internal energy distributions of CO photochemically desorbed from oxidized Ni(111)
Authors: Asscher M. , Zimmermann F. M. , Springsteen L. L. , Houston P. L. , Ho W.
Published: July 2002

The first translational, rotational and vibrational energy resolved study of CO desorption from epitaxically grown NiO(111) at 70 K in photochemistry study is reporeted. (AIP)
Time-dependent Hartree–Fock simulations of the dynamics of polyatomic molecules
Authors: Field Martin J.
Published: July 2002

The use of the time-dependent Hartree–Fock method for performing molecular-dynamics simulations of polyatomic systems is explored. The theory is outlined for closed-shell restricted Hartree–Fock wave functions which employ the neglect of diatomic differential overlap approximation. The motions of the nuclei are treated classically. If the starting electronic wave function for a simulation obeys the time-independent variational principle then trajectories calculated with the algorithm stay very close to those obtained with a method in which the wave function obeys the time-independent variational principle at all times.
Dependence on collisional energy of negative ion formation in collisions of He(14 1P) Rydberg atoms with SF6
Authors: Pesnelle A. , Perdrix M. , Watel G.
Published: July 2002

The formation of negative ions by electron attachment in collisions of He atoms in the well-defined low-Rydberg state (n=14)1P with SF6 is studied as a function of the relative velocity of the reactants in a crossed-beam experiment at thermal energies. The data show that the cross section does not follow the behavior predicted by the free electron model; it presents a maximum and then decreases at low kinetic energy, thus providing a direct observation of the postattachment Coulomb interaction of the He+ and SF−6 product ions. It is assumed that, at low kinetic energy, rovibrational energy of SF−6 and SF6 which coexist inside a temporary collision complex (He+SF6)−, is transferred into translational energy of the collision products, allowing SF−6 and He+ to escape from the collision complex with reduced vibrational energy in SF−6 and increased kinetic energy of He+. Both can thus be detected even when the ion pair He+–SF−6 is expected to stay bound. This explains qualitatively the nonzero value of the cross section in this low kinetic energy range. At high kinetic energy, transfer of translational energy into rovibrational energy is assumed to occur inside the collision complex, leading to SF−6 ions with increased vibrational energy and thus shorter lifetime against autodetachment. This explains qualitatively the steeper than 1/vr decrease of the cross section in this high kinetic energy range.
Unimolecular fragmentation rate theory revisited: An improved classical theory
Authors: Zhao Meishan , Rice Stuart A.
Published: July 2002

We propose a modification of the approximate classical theory of unimolecular fragmentation developed by Gray, Rice, and Davis; the important difference is the use a vibrational state dependent separatrix as the bottleneck to reaction. This version of the theory is well adapted to the description of the predissociation of van der Waals molecules, the rates of fragmentation of which are poorly described by the conventional statistical theory of unimolecular reaction. A test of the modified theory against molecular dynamics simulations of the predissociation of HeI2 reveals considerable improvement relative to the predictions of the Gray–Rice–Davis treatment. For large vibrational excitation, when intramolecular energy transfer is fast enough to not influence the predissociation rate, there is also very good agreement with the experimental data of Johnson, Wharton, and Levy. For small vibrational excitation it is necessary to explicitly account for the slow intramolecular transfer of energy in order to reproduce the experimental data.
A quantum mechanical, time-dependent wave packet interpretation of the diffuse structures in the S→S1 absorption spectrum of FNO: Coexistence of direct and indirect dissociation
Authors: Suter Hans Ulrich , Huber J. Robert , von Dirke Michael , Untch Agathe , Schinke Reinhard
Published: July 2002

We have investigated the photodissociation of FNO in the first absorption band (S0→S1) by a two-dimensional wave packet study based on an ab initio potential energy surface. The quantum chemical calculations were performed in the multiconfiguration self-consistent field (MCSCF) approach including the N–O and the F–NO bond distances with the FNO bond angle being fixed. The most striking feature of the time-dependent dynamical analysis is a bifurcation of the wave packet near the Franck–Condon point: while one part of the wave packet leaves the inner region of the potential energy surface very rapidly, a second part remains trapped for several periods in an extremely shallow well at short F–NO distances. The direct part leads to a broad background in the absorption spectrum while the trapped portion of the wave packet gives rise to relatively narrow resonances, i.e., well resolved diffuse vibrational structures. The bandwidth decreases with the degree of internal excitation. The calculated spectrum agrees well with the measured one.
Internal-state distributions of H2 desorbed from mono- and dihydride species on Si(100)
Authors: Shane Stacey F. , Kolasinski Kurt W. , Zare Richard N.
Published: July 2002

Following adsorption of atomic hydrogen on Si(100)–(2×1), the surface is heated and the desorbed H2 is detected via (2+1) resonance-enhanced multiphoton ionization (REMPI). H2 desorption correlated with the decomposition of dihydride groups on the surface (SiH2) is detected at a surface temperature Ts near 660 K, and with the monohydride species (SiH) near Ts=780 K. Although the H2 rotational distributions are nearly identical for the mono- and dihydride species, the vibrational distributions differ with roughly 0.2% and 1% of the population in H2(v=1) for the monohydride and dihydride, respectively. The enhancement in the [H2(v=1)]/[H2(v=0)] population ratio over that of a thermal distribution at Ts is, however, roughly 20 times for both mono- and dihydride species. The results are interpreted within a model that assumes desorption proceeds through a common intermediate, which is identified as the dihydride.