bluuebunny/crossref_metadata_2025 huggingface dataset:
The rotational relaxation of NH(c 1Π) in collisions with Ar: A combined theoretical and experimental investigation
Authors: Yang Moonbong , Alexander Millard H. , Werner Hans-Joachim , Hohmann J. , Neitsch L. , Stuhl F. , Dagdigian Paul J.
Published: July 2002

We report full quantum close-coupling and coupled-states calculations of cross sections and rate constants for the rotational relaxation of NH(c 1Π) in collisions with Ar, based on a new ab initio description of the potential energy surface for this system. Single-collision, state-to-state thermal rate constants were extracted from measured fluorescence intensities of relaxing NH(c 1Π,v=0,J) distributions in the presence of Ar starting with singly populated rotational states. Overall, the calculated and experimental rate constants agree well, although the dependence on the initial J is somewhat different. At high J the relaxation is dominated by J→J−1e/f changing transitions. This process is facilitated by approach on the more attractive ArNH(c)A′ PES followed by a curve crossing to the more repulsive A″ PES which correlates asymptotically to the next lower rotational level. For approach in a ‘‘helicopterlike’’ orientation, this process results in Jf→J−1,e transitions, while for approach in a ‘‘pinwheellike’’ orientation, this process results in Je→J−1,f transitions. Thus, similar to what we found earlier for the relaxation of high rotational levels of the CH(X) radical [M. H. Alexander and P. J. Dagdigian, J. Chem. Phys. 101, 7468 (1994)], a strong v, J correlation in the reactant channel results in a strong Λ, J correlation in the product channel. A full kinetic simulation, based on the calculated cross sections, reproduces nearly quantitatively the experimental observations of the pressure dependence of the equilibration of NH(c) Λ doublet populations monitored some years ago by Quinton and Simons.
Symmetry induced kinetic isotope effects in the formation of (CO2)+2
Authors: Yoo R. K. , Gellene Gregory I.
Published: July 2002

Symmetry induced kinetic isotope effects (SIKIE) have been observed mass spectrometrically in the termolecular association reaction of CO+2 and CO2 to produce (CO2)+2, where CO+2 is produced by electron ionization. Distinct and independent kinetic effects are determined for 13C and 18O isotopic substitution making this reaction system the first one for which SIKIE is observed for two different elements. The results can be understood generally in terms of a recently proposed diatom/diatom symmetry correlation scheme extended to treat the present case of linear triatomic molecules. The 18O SIKIE shows a pronounced dependence on the ionizing electron energy decreasing smoothly from a factor of 27 at 25 eV to a factor of 8 at 100 eV. Alternatively, the 13C SIKIE was about a factor of 5 independent of the ionizing electron energy. In addition, the efficiency of CO+2 ions in producing (CO2)+2 increases with increasing energy of the ionizing electrons. These dependencies on ionizing electron energy are interpreted in terms of propensities for e/f parity label state production in electron ionization in qualitative agreement with similar conclusions reached in a recent study of SIKIE in O+4 formation.
Time dependent Monte Carlo simulations of H reactions on the diamond {001}(2×1) surface under chemical vapor deposition conditions
Authors: Dawnkaski E. J. , Srivastava D. , Garrison B. J.
Published: July 2002

Time dependent Monte Carlo (TDMC) simulations are performed to determine the effects of a variety of H reactions at a diamond {001}(2×1) surface exposed to gaseous atomic and molecular hydrogen under chemical vapor deposition conditions. The simulation time in the TDMC method is the same as the real time measured in experiments because all of the considered reactions are allowed to occur with probabilities which are the product of the TDMC time step and the corresponding reaction rates. The reaction rates are either explicitly calculated via molecular dynamics or transition state theory methods, or taken from experimental measurements. The simulation takes into account H adsorption, H abstraction, H2 desorption, H diffusion, and the reverse of these reactions. The relative values of the calculated rates and how they affect the surface radical density and distribution, as well as the effect of CH3 on radical site diffusion are discussed.
Study on the hydration structure of L-xylo and D-arabo ascorbic acid solutions by time domain reflectometry
Authors: Umehara Toshihiro , Tominaga Yasunori , Hikida Akiko , Mashimo Satoru
Published: July 2002

The hydration structure of L-xylo and D-arabo ascorbic acids in aqueous solutions were investigated by a dielectric relaxation measurement over a wide frequency range from 10 MHz to 10 GHz from a standpoint on the difference of biological activity at 25 °C. In order to clarify the hydration structure the concentration dependence of dielectric relaxation was investigated not only in aqueous solution but in water–ethanol mixtures. Two kinds of dielectric relaxation processes were observed in each isomerism solution. The low frequency process is assigned to cooperative motions of ascorbic acid molecules and hydrated water. The high frequency process is assigned to reorientational motions of bulk water. From the results of the dehydration process out of the ascorbic acid surface by ethanol it is concluded that the amount of hydrated water of the L-xylo ascorbic acid is more than that of the D-arabo ascorbic acid.
Study of bimetallic Pd–Pt clusters in both free and supported phases
Authors: Rousset J. L. , Cadrot A. M. , Cadete Santos Aires F. J. , Renouprez A. , Mélinon P. , Perez A. , Pellarin M. , Vialle J. L. , Broyer M.
Published: July 2002

We study PdPt bimetallic clusters in both free and supported phases. These clusters have been produced with a laser vaporization source. Free clusters directly produced by the source are studied by time of flight mass spectrometry and photofragmentation technique. We observed a sequential evaporation of Pd atoms in the mixed clusters consistent with a palladium segregation process. This tendency has been also observed on supported particles from which the structure and the composition are determined by high resolution transmission electron microscopy and energy dispersive x-ray analysis. A main result is that each particle has the composition of the massic rod vaporized in the source. The supported particles are well crystallized and exhibit truncated octahedron shapes. Experimental observations are well explained using a modified tight binding model. Indeed, within this model, we found that the equilibrium shape is strongly related to the variation of the cohesive energy with atomic coordination number. Also, some preliminary results on the specific reactivity of these bimetallic clusters are presented.
Predissociative linewidths of (4pσ) M 2Σ+ (v=1) and (3dσ,π) H 2Σ+, H′ 2Π (v=2) Rydberg states of NO studied by the two-color laser-induced grating technique
Authors: Ishii Juntarou , Uehara Kiyoji , Tsukiyama Koichi
Published: July 2002

The two-color laser-induced grating (TC-LIG) technique has been employed to probe the predissociative Rydberg states (4pσ) M 2Σ+ (v=1) and (3dσ,π) H 2Σ+, H′ 2Π± (v=2) and the non-Rydberg B 2Π (v=26) state of nitric oxide. The ultraviolet pump laser frequency is fixed to a specific rotational line of the A 2Σ+ (v′=1 and 2)←X 2Π3/2 (v″=0) transition of NO. Interference of two pump laser beams crossing at a small angle in the gas sample forms a population grating. Then, the visible probe laser beam is diffracted off the grating as the signal beam when the probe laser frequency is resonant to a transition from the rotational level excited by the pump laser. Term values and rotational assignments of the H 2Σ+, H′ 2Π± (v=2)–B 2Π3/2 (v=26) states have been established for the first time, resulting in the value of ∼1.77 cm−1 for the l-uncoupling constant. Line broadenings due to predissociation are observed in the TC-LIG spectra for the M 2Σ+ (v=1) and H 2Σ+, H′ 2Π+ (v=2) states. The linewidths of the M 2Σ+ (v=1) state show no marked rotational dependence except for the N=4 level which is locally perturbed. On the other hand, the predissociative lifetimes of the H 2Σ+ and H′ 2Π+ (v=2) states exhibit a significant rotational dependence: The linewidth of the former state decreases with N, while the reverse tendency is seen for the latter. The mixing between the H 2Σ+ and H′ 2Π+ states caused by l-uncoupling explains the observed rotational dependence successfully.
Phonon dynamics and relaxation processes in isotopically pure 35Cl2 and natural crystalline chlorine
Authors: Bussotti Laura , Becucci Maurizio , Califano Salvatore , Castellucci Emilio , Dows David A.
Published: July 2002

We have measured the Raman spectrum of natural and 99% isotope 35 chlorine crystal, in the lattice and in the internal mode regions, using a high resolution Raman spectrometer. We report the temperature dependence of the linewidth for both the lattice and the internal vibrational modes. The data were analyzed in terms of anharmonic interactions and contributions from the isotopic disorder. The lattice phonons are little affected by the isotopic disorder, while the internal vibrons are extremely sensitive to the presence of isotopic impurities. In the latter case the linewidths of the Ag and B3g internal vibrons are about one and two orders of magnitude, respectively, larger in the natural than in the isotopically pure sample. The Raman spectrum of natural chlorine in the internal region is correctly reproduced by an harmonic lattice dynamics calculation weighted over the statistical distribution of the isotopic species in the lattice.
On the ‘‘direct’’ calculation of thermal rate constants
Authors: Thompson Ward H. , Miller William H.
Published: July 2002

We present a new approach for the direct (and correct) calculation of thermal rate constants k(T) (‘‘direct’’ meaning that one avoids having to solve the state-to-state reactive scattering problem, and ‘‘correct’’ meaning that the method contains no inherent approximations). The rate constant is obtained from the long time limit of the flux-position correlation function, Cf,s(t), whose calculation is made efficient by taking advantage of the low rank of the flux operator. Specifically, the trace required to obtain Cf,s(t) is evaluated by a Lanczos iteration procedure which calculates only the nonzero eigenvalues. The propagation in complex time, tc=t−iℏβ/2, is carried out using a Chebychev expansion. This method is seen to be both accurate and efficient by application to the Eckart barrier, the collinear H+H2 reaction, and the three-dimensional D+H2 (J=0) reaction.
Effects of fluctuations on the wetting transition in amphiphilic systems
Authors: Schmid F. , Schick M.
Published: July 2002

The wetting of the oil–water interface by the middle phase in an amphiphilic system is considered for both balanced and unbalanced systems within the context of a one-component Ginzburg–Landau theory. Whereas mean field theory predicts a continuous transition in the balanced system at the disorder line, the effect of capillary fluctuations is to cause the transition to be first order, and to occur on the microemulsion side of the disorder line. With reasonable parameters, the location is shifted beyond the Lifshitz line, as is found in experiment. When the wetting transition is brought about in the unbalanced system, we find that the middle phase is more structured at the transition than in the balanced system. This occurs because the fluctuations become greater as the critical end point is approached, and are able to bring about the unbinding of interfaces in the presence of even stronger binding potentials. Experiments are consistent with this prediction.
Near-edge core photoabsorption in polyenes
Authors: Carravetta Vincenzo , Ågren Hans , Pettersson Lars G. M. , Vahtras Olav
Published: July 2002

We present calculations on near-edge x-ray absorption fine structures (NEXAFS) in polyenes based on a newly derived direct, atomic orbital, static exchange method. The trends of different NEXAFS features with respect to the site of ionization and with respect to the number of ethylene subunits in the oligomer chain are studied. The trends for energies and intensities are found to be regular and alternant. A substantial reduction of the π to σ intensity ratios with the number of ethylene subunits is predicted, indicating delocalization of the screening π electrons. In contrast to NEXAFS spectra of a heterogeneous group attached to hydrocarbon chains, the NEXAFS spectra of oligomer sequences, here the polyenes, converge slowly. The building block principle is less appropriate than the final state rule for rationalizing the present findings.
Velocity dependent state-to-state differential cross sections for rotational transfer in Li2–Xe using velocity selected double resonance
Authors: Collins Timothy L. D. , McCaffery Anthony J. , Richardson Jonathan P. , Wilson Ruth J. , Wynn Michael J.
Published: July 2002

We describe a new and wholly spectroscopic technique in which the state-to-state differential scattering cross section (dcs) is determined for rotationally inelastic atom–molecule collisions. The method uses two single frequency tunable dye lasers in a sub-Doppler double resonance experiment which has the added advantage that dependence on collision velocity may readily be determined. The method is illustrated by a determination of the dcs for rotational transfer (RT) in Li2 A 1Σ+u–Xe collisions. The dcs is obtained from the shape of the double resonance line and rotationally inelastic transitions Δj=−4 to +10 were studied. For each a range of initial relative velocities was selected and the scattering angles so obtained are differential in both angle and velocity. These are the first such measurements in atom–molecule scattering. The trends observed in scattering angle with Δj and with velocity are successfully interpreted using a hard ellipse model. We have observed significant differences in the dcs between upwards and downwards Δj transitions. The origins of these differences are discussed and emphasize the importance of the threshold velocity for a particular Δj channel.
Multiexponential approximations to the torsional time correlation function for one-dimensional systems with many barriers
Authors: Pratolongo Roberto , Perico Angelo , Freed Karl F. , Szabo Attila
Published: July 2002

A multiexponential approximation is derived for the torsional time correlation function of a one-dimensional system with many barriers. This approximation couples a jump model, governed by a Master equation describing transitions between wells, to a model of diffusional fluctuations within individual wells. The rate constants defining the jump model are calculated using the Kramers approximation or from a more accurate number time correlation approach. These approximations compare very favorably to the exact correlation times for torsional diffusion in a periodic potential with multiple barriers, especially when the more accurate rate constants are used. The importance of the multiexponential fluctuation-jump model approximation lies in the possibility of extending it to multidimensional systems (of polymers or proteins) where exact solutions to the Smoluchowski dynamics are no longer available.
Structure and electron localization of anionic NaCl clusters with excess electrons
Authors: Yu Naichang , Xia P. , Bloomfield L. A. , Fowler Michael
Published: July 2002

The structure and electron localization mode of (NaCl)−n clusters with a single excess electron are studied with simulated annealing. We find that the attachment of the excess electron induces a dramatic reorganization of the ionic configuration. The electron is localized in most clusters, either in an F-center state or partially neutralizing a sodium ion sticking out of the cluster. In rare cases the electron is found to be delocalized, spreading over the surface of the cluster. The calculated electron binding energies agree very well with recent photoelectron measurements.
Fragmentation dynamics of the vibrationally excited ammonia–argon van der Waals complex
Authors: Millan J. , Halberstadt N. , van der Sanden G. C. M. , van der Avoird A.
Published: July 2002

Quantum mechanical calculations have been performed to study the fragmentation dynamics of the Ar–NH3 van der Waals complex excited in the ν2 (‘‘umbrella’’) vibrational level with v2=1. Vibrational predissociation has been investigated for different initial quasibound states, corresponding to excitation in the stretching or bending van der Waals levels or in the tunneling motion of the NH3 umbrella inversion, for both ortho and para varieties of NH3, and for three values of the total angular momentum J and its projection Ω onto the intermolecular axis, (J,Ω)=(0,0), (1,0), and (1,±1). The calculations were performed for two different intermolecular potential energy surfaces, one determined ab initio by Bulski et al. [J. Chem. Phys. 94, 491 (1991)], and the other where the ab initio potential has been scaled to fit spectroscopic data of Ar–NH3, proposed by van Bladel et al. [J. Phys. Chem. 95, 5414 (1991)]. The lifetime obtained from the ab initio potential lies within the limits set by experiment for the lowest ortho Π (i.e., ‖Ω‖=1) level with J=1, while the scaled potential gives too short a lifetime. The results also show that excitation of the van der Waals stretching mode accelerates the predissociation but does not affect the NH3 final state rotational distribution very much. On the other hand, bending or tunneling excitation does lead to a very different final rotational state distribution. Ortho states have very similar final state distributions for Σ(‖Ω‖=0) and Π(‖Ω‖=1) states, but this is not true for para states. Finally, comparing the ab initio and scaled potentials, no systematic trend emerges for the amount of even (+) and odd (−) symmetry of the final states, but the scaling of the potential shortens the lifetimes, gives a wider final state rotational distribution, and provides more excitation in the k quantum number (the projection of the NH3 rotational angular momentum on its C3 axis).
Electronic and vibrational excitation of acrylonitrile by low and intermediate energy electrons
Authors: Motte-Tollet F. , Messina D. , Hubin-Franskin M.-J.
Published: July 2002

Electronic and vibrational excitation of acrylonitrile induced by 3–50 eV energy electrons has been investigated by the electron energy loss spectroscopy. Electronic excitation spectra have been recorded for 30 and 50 eV impact energies at a 10° scattering angle in the energy loss range from 5.5 to 11.5 eV, corresponding to the excitation of electrons belonging to the outermost-valence-shell molecular orbitals. We have reviewed the assignment of the valence excited states occurring in the 5.5–9 eV energy loss region. The vibrational patterns associated with the two lowest-energy singlet valence excited states have also been re-examined. Moreover, we have proceeded for the first time to the analysis and attribution of several Rydberg series converging to the ionic ground state and to its two lowest-energy electronic excited states. The study of the excitation function of the C—H stretching modes of acrylonitrile in the 3–11 eV electron impact energy range has shown evidence of a broad shape resonance built on the electronic ground state of the molecule and centered at 5.85 eV. This resonance contributes to a preferential excitation of the C—H stretching modes suggesting that the charge distribution of the additional electron is very likely that of a σ*(C—H) valence molecular orbital. A comparison has been made between the resonances observed in C2H4 and CH2CHCN, in order to discuss the symmetry of the resonant state and also to analyze the substitution effect of the cyanogen group.
Isomer dependence of HF vibrational frequency shift for ArnHF (n=4–14) van der Waals clusters: Quantum five-dimensional bound state calculations
Authors: Liu Suyan , Bačić Zlatko , Moskowitz Jules W. , Schmidt Kevin E.
Published: July 2002

The HF vibrational frequency shifts for ArnHF van der Waals (vdW) clusters with n=4–14 are predicted to be strongly isomer-specific, providing distinct spectroscopic signatures for different cluster isomers. This represents an extension of our recent studies of the size dependence of the vibrational frequency shift for ArnHF clusters [J. Chem. Phys. 101, 6359, 10 181 (1994)]. The HF vibrational frequency shifts calculated for the two or three lowest-energy isomers of each cluster size considered differ by at least a couple of wave numbers. Their relative magnitudes directly reflect the number of Ar atoms that each ArnHF isomer has in the first solvation shell around HF. The calculations are performed on pairwise additive intermolecular potential energy surfaces constructed from spectroscopically accurate Ar–Ar and anisotropic Ar–HF potentials. In the frequency shift calculations, the Arn subunit is treated as rigid, frozen in the geometry of one of the global or local ArnHF minima found previously by simulated annealing [J. Chem. Phys. 100, 7166 (1994)]. The 5D coupled intermolecular vibrational levels of what is now effectively a floppy Arn–HF dimer, are calculated highly accurately by the quantum 5D bound state methodology which is described in detail. The 5D vdW vibrational zero-point energy of the ArnHF cluster affects significantly the energy gap between various isomers.
High pressure crystal phases of solid CH4 probed by Fourier transform infrared spectroscopy
Authors: Bini Roberto , Ulivi Lorenzo , Jodl Hans J. , Salvi Pier R.
Published: July 2002

High pressure infrared spectra of solid CH4 are reported in the range 0.8–30 GPa at room temperature, coupling a Fourier transform infrared spectrometer to a membrane diamond–anvil cell by means of a high efficiency beam condensing optical system. Two crystal phases, A and B, have been investigated. The phase transition is affected by hysteresis and occurs at 9±0.5 GPa during compression and at 7±0.5 GPa during expansion. Due to hysteresis, the transition has been studied as a function of time at higher pressures and found to undergo a first-order kinetics, with rate constant increasing with pressure. Since our experimental apparatus allows us to perform high pressure Raman measurements too, structural properties of both A and B phases have been proposed from the analysis of the infrared and Raman data. Within the framework of the widely used three-site model, the A phase structure is consistent with a D4h unit cell symmetry. On the contrary, the analysis of the ω1 infrared and Raman multiplets in phase B as a function of pressure suggests quite plausibly a single site, well-ordered crystal structure. By means of group-theoretical arguments it is concluded that CH4 molecules occupy sites of Cs symmetry, while the unit cell symmetry must be chosen among D4h, D6h, Th and Oh groups. Qualitative considerations point to D6h as the more favored unit cell symmetry for phase B.
Crossed-beam reaction of C(3P
j) with C2H2(1∑+g): Observation of tricarbon-hydride C3H
Authors: Kaiser R. I. , Lee Yuan T. , Suits Arthur G.
Published: July 2002

The reaction between ground state carbon atoms, C(3Pj), and acetylene, C2H2(1∑+g), was studied at an average collision energy of (8.4±0.3) kJ mol−1 using the crossed molecular beam technique. The product angular distribution and time-of-flight spectra of m/z=37, i.e., C3H, were recorded. Only m/z=37 was detected, but no signal from the thermodynamically accessible C3(1∑+g)+H2(1∑+g) channel. Forward-convolution fitting of the results yielded a center-of-mass angular flux-distribution forward scattered in respect to the carbon beam, whereas the translational energy flux distribution peaked at only (5.4±1.2) kJ mol−1, suggesting a simple C–H-bond-rupture to H+C3H. The reaction likely proceeds on the triplet surface with an entrance barrier to the C3H2–PES of <(8.4±0.3) kJ mol−1 via addition of the carbon atom to two bonding π-orbitals located both at C1 or at C1 and C2 of the acetylene molecule. The explicit identification of C3H product under single collision conditions strongly demands incorporation of atom-neutral reactions in reaction networks simulating chemistry in the interstellar medium, in interstellar shock waves, and in outflows of carbon stars.
Bound states and resonances of the hydroperoxyl radical HO2: An accurate quantum mechanical calculation using filter diagonalization
Authors: Mandelshtam Vladimir A. , Grozdanov Tasko P. , Taylor Howard S.
Published: July 2002

An accurate calculation of bound and resonance spectra of the non-rotating odd O2 exchange symmetry HO2 radical is presented. The calculation has been carried out by a recently developed iterative technique which uses filter diagonalization of a sparse matrix of the system Hamiltonian with absorbing boundary conditions. We were able to obtain 361 bound states and some 232 isolatable resonances (Γ<0.01 eV) in a wide energy range corresponding to the HO2→H+O2 unimolecular decomposition reaction. It is shown that all resonances found have the same nature as the bound states in that they all are localized in the same region of space over the deep potential well, and moreover the extrapolated smoothed density of the bound states merges easily with the smoothed density of the resonance states. The level statistics for both bound and resonance states indicates a highly chaotic regime consistent with the random matrix theory. Strong mode mixing makes assignments of most bound and resonance states impossible because the corresponding wave functions do not show any simple pattern. Interestingly, the randomly fluctuating high resolution density of states after smoothing shows a structure resembling two basic frequencies corresponding to the O2 stretch and HOO bend motion of the HO2 molecule.
Monte Carlo investigations of hexadecane films on a metal substrate
Authors: Balasubramanian Sundaram , Klein Michael L. , Siepmann J. Ilja
Published: July 2002

The structural properties of liquid n-hexadecane films, ranging in thickness from 1 to 4 nm and adsorbed on a flat metal (Au) surface, have been studied by the configurational-bias Monte Carlo method. Over the temperature range 350 K to 650 K, the substrate was found to induce a well-defined layering of the molecules for the portion of the film closest to the surface. Even at the highest temperature, this layering extends beyond a monolayer. In the surface layer, molecules are predominantly oriented with their long axes and their backbone zig–zag planes parallel to the substrate. Molecules close to the surface are also characterized by fewer gauche defects and are more densely packed than those present in the bulk region of the film. At a given temperature, the density and the characteristics of the molecules in the first adsorbed layer are substantially independent of the total thickness of the film. With increasing temperature, the layering diminishes and molecules in the first layer exhibit properties closer to those of the bulk liquid at a lower temperature. Comparisons are made with experiments and previous simulation studies on related systems.
Theoretical investigation of the Kerr effect for CH4
Authors: Bishop David M. , Pipin Janusz
Published: July 2002

The vibrational contributions to the Kerr effect and to electric-field-induced second-harmonic generation (ESHG) are calculated for methane for a number of optical frequencies. The latter results, together with the experimental ESHG values of the total mean second hyperpolarizability, allow for the determination of the ω2L-dispersion curve for the mean electronic hyperpolarizability. Since this curve is identical, to fourth order, for both processes, we are able to combine it with the calculated Kerr vibrational hyperpolarizabilities and predict the total Kerr hyperpolarizabilities for CH4 for several laser frequencies.
Investigation of the reliability of density functional methods: Reaction and activation energies for Si–Si bond cleavage and H2 elimination from silanes
Authors: Nachtigall P. , Jordan K. D. , Smith A. , Jónsson H.
Published: July 2002

In order to test the reliability of plane-wave and Gaussian-orbital based DFT methods for calculating reaction energies and activation barriers, detailed calculations are performed for several reactions involving gas phase silanes and a simple model of H2 desorption from the Si(100)2×1 surface. This study is motivated in particular by apparent discrepancies between the results of cluster-model and slab-model calculations of the activation energy for H2 desorption from the Si(100)2×1 surface. The DFT results obtained with several different exchange-correlation functionals are compared with the results of calculations with the generally reliable QCISD(T) method and, where possible, with experiment. It is found that the functionals usually employed in plane-wave DFT calculations significantly underestimate the activation energies. The Becke3LYP functional, on the other hand, is found to give reaction and activation energies close to experiment and to those from QCISD(T) calculations.
Spin dynamics under the Hamiltonian varying with time in discrete steps: Molecular dynamics-based simulation of electron and nuclear spin relaxation in aqueous nickel(II)
Authors: Odelius Michael , Ribbing Carl , Kowalewski Jozef
Published: July 2002

A method of calculating the time correlation functions for electron spin is proposed, based on solving the time-dependent Schrödinger equation for a spin Hamiltonian that contains a term varying randomly in discrete time steps. It is applied to the study of electron spin relaxation in aqueous solution of nickel(II) ions with S=1. The random term in the spin Hamiltonian in this case is the zero-field splitting (ZFS) interaction. The method is evaluated by an application to a model system (the pseudorotation model) for which an analytical solution to the electron spin relaxation problem is known. The same method is then employed to study the electron and nuclear spin dynamics in a system where the time variation of the zero-field splitting is obtained by a combination of ab initio quantum chemistry and molecular dynamics simulations.
Photoelectron spectroscopy and zero electron kinetic energy spectroscopy of germanium cluster anions
Authors: Burton Gordon R. , Xu Cangshan , Arnold Caroline C. , Neumark Daniel M.
Published: July 2002

Anion photoelectron spectra of Ge−n, n=2–15, have been measured using an incident photon energy of 4.66 eV. In addition, the spectra of Ge−2, Ge−3, and Ge−4 have been measured at photon energies of 3.49 and 2.98 eV. From these spectra the electron affinity of the corresponding neutral cluster has been determined. Vibrational frequencies and term values for several electronic states of Ge−2 and Ge−3 have been determined. Vibrational structure in the 3B3u excited state of Ge4 has been resolved using zero electron kinetic energy (ZEKE) photoelectron spectroscopy. The assignment of the spectra of Ge−3 and Ge−4 is facilitated by a comparison to the similar spectra of Si−3 and Si−4, respectively. The spectra of the larger clusters, Ge−n, n=5–15, are characterized by many broad structureless features which indicate the presence of multiple electronic transitions. Several of these were assigned based on comparison with previous ab initio calculations on germanium and silicon clusters.
Spin–orbit branching ratios for photoionization of the 3dπ gerade states of O2: Evidence for preferential ionization of the Ωc=3/2 core states
Authors: Glab W. L. , Dehmer P. M. , Dehmer J. L.
Published: July 2002

We have obtained high-resolution photoelectron spectra for eight of the ten 3dπ electronic states of O2, using a rotationally resolved, two-color 2+1′ excitation and ionization scheme. A magnetic bottle photoelectron spectrometer, used to acquire the spectra, provided a resolution sufficient to observe the yields of the individual spin–orbit states of the ground electronic state of the ion. Branching ratios were obtained from the spectra for three different photoionization wavelengths. The overall behavior of the branching ratios for different electronic states can be explained by calculating the core state composition of the intermediate states, taking into account the spin–orbit mixing of Hund’s case (a) basis states, and assuming that electronic states having the core state Ωc=3/2 have a larger photoionization probability than those having an Ωc=1/2 core.
Molecular-orbital decomposition of the ionization continuum for a diatomic molecule by angle- and energy-resolved photoelectron spectroscopy. II. Ionization continuum of NO
Authors: Park Hongkun , Zare Richard N.
Published: July 2002

The quantum-state-specific photoelectron angular distributions (PADs) from the NO A 2Σ+(ν=0, N) and D 2Σ+(ν=0, N) states are analyzed based on the theoretical formalism presented in the preceding companion article. The dynamical parameters in this analysis can be divided into two distinct types, one that directly pertains to the dynamics in the ionization continuum of NO that yields the NO+ X 1Σ+(ν+=0, N+) ion and the other that depends both on the ionizing state and on the ionization continuum. The continuum parameters obtained in this study determine various molecule-frame scattering matrices that describe the short-range collision dynamics between the photoelectron and the NO+ X 1Σ+(ν+=0, N+) core and agree very well with the corresponding quantum-defect quantities determined for high-lying Rydberg states converging to the NO+ X 1Σ+(ν+=0, N+) ion. Specifically, it is found that sσ- and dσ-partial waves mix almost completely because of the anisotropic interactions between the photoelectron and the other electrons in the ion core whereas the orbital angular momentum of the other partial waves are relatively unperturbed by scattering with the ion core. The dynamical parameters determined in the analysis also constitute complete descriptions of the photoionization events of the NO A 2Σ+(ν=0, N) and D 2Σ+(ν=0, N) states and provide detailed quantitative information about the Cooper minimum in the 3pσ→dσ ionization channel that appears in the photoionization of the NO D 2Σ+(ν=0) state. The present study represents the first direct experimental determination of the scattering dynamics between the photoelectron and the ion core in a molecular system.
Evanescent-wave forced Rayleigh scattering: A novel method for near-surface diffusion investigations
Authors: Sainov S.
Published: July 2002

Holographic grating recording with evanescent waves is shown to increase the sensitivity and lateral resolution of the conventional forced Rayleigh scattering technique. This new method can be easily applied in the experimental practice for investigations on thin film and surface diffusion and photochemical reactions.
The high resolution absorption spectrum of jet-cooled CS2 between 50 500 and 65 500 cm−1
Authors: Cossart-Magos Claudina , Horani Marcel , Jungen Martin , Launay Françoise
Published: July 2002

The absorption spectrum of jet-cooled CS2 has been photographed between 50 500 and 65 500 cm−1 at a resolution limit of 0.008 Å. Ab initio calculations are carried out to estimate the energy of the lower Rydberg states of ionic cores ...2π3g, ...2π3u, and ...5σu, as well as those belonging to the doubly excited configuration ...2π2g3π2u. Assigned electronic states include the ...2π3g4s 1Πg (with vibronic bands 201 and 2n0, n=1,3,5,7, observed thanks to a concurrent of Herzberg–Teller intensity borrowing and Renner–Teller intensity redistribution) and ...2π3g4p 1,3Πu, and 3Σu− (observed bands being the electronic origin, 110 and 220 bands). Model calculations of Renner–Teller interaction in the 1Π states are performed leading to ε and ω2 parameter values. In the case of the 4p 1Πu state, Fermi resonance between the v1′=1 level and the v2′=2 RT-split levels of Πu vibronic symmetry, is also taken into account. The rotational band contours of the origin bands of the whole set of the ...2π3g4p Rydberg transitions excited by one or three photons are calculated using a model based on Hund’s case (e) representation. The calculated band contours, wave numbers, and relative intensities are in excellent agreement with the observations of the present vuv absorption spectrum, and the multiphoton (3+1) ionization spectrum of Baker et al. [J. Chem. Phys. 103, 2436 (1995)] provided that in the latter, the 1Σ+u state is reassigned as 3Σu−. Fitted values of the 4p quantum defects and of the 4pπ(1Σ+u)/4pσ(1Πu) transition moment ratio are found to be perfectly consistent with the ab initio calculation results. Possible assignments of 3d transitions in the present and previous vuv absorption spectra as well as in recent multiphoton ((1+1′)+1) ionization spectra of CS2 below 65 500 cm−1 are also discussed in the light of the present calculation results.
Early-time photodissociation dynamics of chloroiodomethane in the A-band absorption from resonance Raman intensity analysis
Authors: Kwok Wai Ming , Phillips David Lee
Published: July 2002

We have obtained resonance Raman spectra and absolute Raman cross sections for h2-chloroiodomethane (fourteen excitation wavelengths between 200 nm and 355 nm) and d2-chloroiodomethane (for 282.4 nm excitation) in cyclohexane solution. Most of the intensity in the A-band resonance Raman spectra appears in the nominal C–I stretch overtones progression and combination bands of the nominal C–I stretch overtones with the fundamentals of the CH2 wag, CH2 scissor, and the Cl–C–I bend or C–Cl stretch fundamentals. The A-band absorption and absolute resonance Raman intensities were simulated using a simple model which included preresonant contributions to the fundamental Raman peaks and time-dependent wave packet calculations. The motion of the wave packet on the excited state surface was converted from dimensionless normal coordinates into internal coordinates using the results of normal coordinate calculations. The A-band short-time photodissociation dynamics of chloroiodomethane shows that the C–I bond lengthens, the I–C–Cl and H–C–I angles become smaller, and the H–C–Cl angles become larger. These internal coordinate motions which are associated with relatively low frequency modes are consistent with a simple impulsive ‘‘soft’’ radical model of the photodissociation and the CH2Cl group changing to a more planar structure. However, the C–H bond length does not change much and the H–C–H angle (associated with higher frequency modes) becomes slightly smaller which is inconsistent with the ‘‘soft’’ radical model and the CH2Cl group changing to a more planar structure. This suggests that an impulsive ‘‘semirigid’’ radical model may be more appropriate than the ‘‘soft’’ radical model to qualitatively describe the chloroiodomethane photodissociation. An ambiguity in the assignment of the 724 cm−1 Raman peak and its associated combination bands to combination bands of the nominal C–I stretch overtones with the fundamentals of the Cl–C–I bend or C–Cl stretch fundamentals limits what we are able to determine about the C–Cl bond length changes during the initial stages of the photodissociation.
Partitioning of the nonfixed excess energy and the reverse critical energy in CH2OH+→CHO++H2: A classical trajectory study
Authors: Lee Tae Geol , Kim Myung Soo , Park Seung C.
Published: July 2002

Dynamics of the four-centered elimination reaction CH2OH+→CHO++H2 has been investigated over the internal energy range 4.6–5.9 eV using the classical trajectory method. A realistic semiempirical potential reported previously [J. Chem. Phys. (in press, 1996)] has been used for the calculation. It has been found that the disposal of the nonfixed excess energy at the transition state and of the reverse critical energy can be considered independently as manifest in the sum rule analysis. The former is determined statistically while the latter dynamically. Based on the above idea, a method to determine the kinetic energy release distribution originating only from the reverse critical energy has been developed.
Dependence of the vapor–liquid equilibrium on the attractive intermolecular forces
Authors: Cuadros F. , Okrasinski W. , Sanfeld A.
Published: July 2002

By means of fitting computer simulation data of 3D Lennard-Jones fluids, we present very simple analytical equations for the pressure and chemical potential as functions of the temperature and density. The standard thermodynamic requirement of liquid–vapor equilibrium, i.e., at a given temperature, the pressure and chemical potential in the gas phase are equal to the corresponding values in the liquid phase, leads to the determination of the vapor–liquid coexistence curve. By then using the Weeks–Chandler–Andersen separation of the intermolecular potential, we were able to determine the differing location of the coexistence curve in the phase plane as the intensity of the attractive forces is changed. This curve varies linearly with respect to the perturbative parameter of the Weeks–Chandler–Andersen theory. These results could be very useful in the study of vapour pressure curves of fluids composed of noninert molecules—ionic fluids, ferrofluids, polar fluids, etc.—which have a different intensity of the attractive intermolecular forces relative to the repulsive forces than the relation between the two forces given by the Lennard-Jones model. Finally, the variation in the location of the critical point with respect to differing intensities of the attractive forces is also dealt with.
A (1+1′)+1 multiphoton ionization study of CS2 in the 68 500–73 000 cm−1 energy region. The 3d and 5s Rydberg states
Authors: Baker Jacob , Couris Stelios
Published: July 2002

The two color (1+1′)+1 resonance enhanced multiphoton ionization spectrum of CS2 has been recorded in the 68 500–73 000 cm−1 excitation energy range. Strong bands were recorded in the 68 800–70 000 cm−1 region and the upper states assigned to 3d and 5s gerade Rydberg states with quantum defects of 0.0 and 2.0, respectively. The two color spectra were recorded in both parallel and crossed polarization configurations to assist in state symmetry assignments. Some weaker features in the 70 500–71 500 cm−1 region are most likely associated with vibronically induced transitions to a 5p Rydberg state.
Correlation functions of hard sphere fluids adsorbed in porous media
Authors: Meroni A. , Levesque D. , Weis J. J.
Published: July 2002

We report Monte Carlo (MC) simulations for the pair correlation functions of a hard sphere fluid in a hard sphere matrix. In particular, we evaluate the blocking function hb(r) which is that part of the fluid pair correlation function which accounts for the correlation between fluid particles mediated through at least one matrix particle. The results are compared with integral equation theory. A scheme for determining the bridge functions is presented which predicts excellent agreement of the pair structure with MC results.
An effective fragment method for modeling solvent effects in quantum mechanical calculations
Authors: Day Paul N. , Jensen Jan H. , Gordon Mark S. , Webb Simon P. , Stevens Walter J. , Krauss Morris , Garmer David , Basch Harold , Cohen Drora
Published: July 2002

An effective fragment model is developed to treat solvent effects on chemical properties and reactions. The solvent, which might consist of discrete water molecules, protein, or other material, is treated explicitly using a model potential that incorporates electrostatics, polarization, and exchange repulsion effects. The solute, which one can most generally envision as including some number of solvent molecules as well, is treated in a fully ab initio manner, using an appropriate level of electronic structure theory. In addition to the fragment model itself, formulae are presented that permit the determination of analytic energy gradients and, therefore, numerically determined energy second derivatives (hessians) for the complete system. Initial tests of the model for the water dimer and water-formamide are in good agreement with fully ab initio calculations.
Isotropic and anisotropic Raman scattering from molten LiCl–CsCl mixtures: Composition and temperature effects
Authors: Papatheodorou G. N. , Kalogrianitis S. G. , Mihopoulos T. G. , Pavlatou E. A.
Published: July 2002

Molten mixtures of lithium chloride and cesium chloride have been studied using Raman spectroscopy at temperatures up to 850 °C. Reduced isotropic and anisotropic Raman relative intensities have been measured at different compositions and temperatures. The spectra of the mixtures show two bands with ‘‘characteristic’’ frequencies ωCs and ωLi close to the frequencies of the bands of the pure components. Both ωCs and ωLi shift to higher energies with increasing mole fraction of CsCl and LiCl, respectively. The scattering intensity spectra of the mixtures are compared with the simulated additive spectra of the component salts. Increasing temperature increases drastically the isotropic intensity of the ωLi band but has minor effects on the anisotropic intensities. The data are discussed and interpreted in terms of interaction-induced polarizability fluctuations. Short range overlap interactions, mainly between Li+ and Cl−, and the ‘‘symmetry’’ of the local structure around the anion determine the breathing-like fluctuations which contribute to isotropic scattering and account for the drastic temperature and composition dependence of the Raman intensities. The main contribution to anisotropic scattering arises from near-neighbor dipole-induced-dipole interactions between the highly polarized Cs+ and Cl− ions.
The impact of discrete wall structure on stratification-induced structural phase transitions in confined films
Authors: Schoen Martin
Published: July 2002

Molecular mechanisms by which a fluid closely confined between plane-parallel solid walls tends to order itself in layers parallel with the walls (i.e., stratifies) are investigated by a grand canonical ensemble Monte Carlo method. The walls are composed of individual atoms distributed across each wall according to the (100) plane of a face-centered cubic (fcc) crystal. Wall atoms are either rigidly fixed (model A) or thermally coupled (model B) to the film, that is the walls are ‘‘soft’’ on account of intermolecular interactions. As for a film between unstructured (i.e., molecularly smooth) walls [Schoen et al. J. Chem. Phys. 101, 6865 (1994)] stratification is accompanied by a subtle phase transition manifested as a maximum in density fluctuations at the transition point where packing characteristics of film molecules change in transverse directions. Thus, the transition involves phases with different degrees of transverse fcc (100)-like order induced by the walls which act like templates. If the transition involves films comprising only one and two layers, the mechanism of stratification is qualitatively similar to the one previously reported (see above): Well off the transition point the degree of fcc (100)-like order is nearly identical for one- and two-layer films and higher than at the transition point where the film is least ordered. The mechanism of stratification-induced phase transitions is different if it involves thicker films which tend to gain fcc (100)-like order more or less abruptly at the transition point. If wall atoms are not thermally coupled the film may solidify under geometrically favorable conditions. Solidification is not observed in model B under identical thermodynamic conditions.