bluuebunny/crossref_metadata_2025 huggingface dataset:
A molecular dynamics study of the collective correlation functions of a hard-core fluid with a Yukawa tail
Authors: Alemany M. M. G. , Rey C. , Gallego L. J.
Published: July 2002

We present a molecular dynamics study of the collective correlation functions of a hard-core system with an attractive Yukawa tail, for various thermodynamic states in the fluid and liquid regions of the phase diagram. The results are compared with available information for hard spheres. The small-q behavior of the intermediate scattering functions indicates the propagation of sound waves, i.e., phononlike collective excitations, in the hard-core Yukawa system. The upper limit of q for these collective modes is practically independent of the thermodynamic state. The computed transverse current correlation functions show that at liquid densities the hard-core Yukawa system is able to sustain shear wave propagation above a critical q; the upper limit of q for sound waves and the lower limit for shear waves nearly coincide. All of these features are qualitatively similar to those found for hard spheres. However, there are significant quantitative differences, which reflect the influence of the attractive Yukawa tail on the dynamical behavior of the system.
Direct calculation of radial distribution function for hard-sphere chains
Authors: Tang Yiping , Lu Benjamin C.-Y.
Published: July 2002

The Laplace transform of the average radial distribution function of hard-sphere chains is obtained following the approximation suggested by Chiew [Mol. Phys. 73, 359 (1991)]. The transform expression is of a simple analytical form. The inverse Laplace transform is made analytically. The resulting expression is capable of calculating directly the radial distribution function of hard-sphere chains for any value of r.
Early stage spinodal decomposition in viscoelastic fluids
Authors: Kumaran V. , Fredrickson G. H.
Published: July 2002

The effect of viscoelasticity on the early stages of spinodal decomposition is examined. In addition to the concentration and momentum equations for the fluid, the effect of viscoelasticity is included using a linear Maxwell equation for the stress tensor. The growth in the amplitude of the fluctuations depends on the transport coefficient, the viscosity of the fluid, and the relaxation time in the Maxwell model. For simplicity, the nonlinearity due to the quartic term in the expression for the Landau–Ginzburg expression for the free energy is neglected, as are the inertial terms in the momentum conservation equation. The momentum and Maxwell equations are solved exactly to obtain the velocity as a function of concentration, which is then inserted into the concentration equation. There are two types of nonlinearities in the conservation equation—one proportional to the cube of the concentration which leads to a four point vertex, and one proportional to the product of the concentration and the random noise in the stress equation which leads to a three point vertex. In the leading approximation, the renormalization of the transport coefficient due to these vertices is determined using the Hartree approximation, and the renormalization of the noise correlation due to the three point vertex is determined using a one-loop expansion. The renormalized transport coefficient and noise correlation are inserted into the concentration equation to determine the effect of the nonlinearities on the growth of the structure factor. It is found that an increase in the relaxation time tends to increase the rate of growth of the structure factor, and tends to decrease the wave number of the peak in the structure factor.
Observation of strong hidden lines in the infrared spectrum of the CO–He complex
Authors: Chan M.-C. , McKellar A. R. W.
Published: July 2002

In the only previous observation of spectra of the weakly bound complex CO–He [C. E. Chuaqui, R. J. Le Roy, and A. R. W. McKellar, J. Chem. Phys. 101, 39 (1994)], only 6 out of the 21 strongest transitions were observed, and much of the analysis therefore relied on relatively weak transitions. These strong ‘‘hidden’’ transitions are located very close to, and were obscured by, pressure-broadened transitions of the CO monomer. In the present paper, the measurement of all but one of the 15 hidden transitions has been achieved by using a sensitive tunable diode laser probe which allows much lower sample pressures. The results fully confirm the previous analysis, and provide additional precise data on the bound energy levels of this fundamental molecular system, especially for the CO–3He isotope.
Hyperfine quantum beats in the 209 nm fluorescence excitation spectrum of cyanogen
Authors: Hemmi Naoki , Cool Terrill A.
Published: July 2002

Singlet–singlet, singlet–triplet, and polarization hyperfine fluorescence quantum beats are observed following 209 nm excitation of the 1141 vibronic level of the Ã(1Σ−u) S1 singlet state. Singlet–singlet quantum beats arise from S1–S0 vibronic coupling to states of the S0 manifold lying above the dissociation limit, but isolated from the dissociative continuum by a high barrier to predissociation. Measurements of fluorescence lifetimes and quantum beat spectral widths set an upper bound on this predissociation rate of approximately 106 s−1. The singlet–singlet quantum beats for the N=9 rotational state provide the first example of quantum beats arising from superposition states from two separate (I=0 and I=2) nuclear spin states. Singlet–triplet quantum beats for N=12 exhibit magnetically induced modulations that provide further support for a theoretical model previously used to describe similar modulations for the N=8, 12, and 18 rotational states of the 41 vibronic level excited at 219 nm. Analysis of hyperfine polarization beats, associated with the reversible interchange of molecular polarization and nuclear spin polarization, yields results in satisfactory agreement with the predictions of the perturbation theory formulation of Fano and Macek.
Microwave spectroscopy of the HCCS and DCCS radicals (X̃ 2Πi) in excited vibronic states: A study of the Renner–Teller effect
Authors: Tang Jian , Saito Shuji
Published: July 2002

The microwave spectra of the HCCS and DCCS radicals are studied in the frequency range of 160–400 GHz and the rotational transition series are assigned to several low-lying vibronic states in the CCS or H(D)CC bending vibration. Analysis is carried out to obtain effective constants for respective vibronic states. The γeff constants for the vibronic μ/κ2Σ states are found to be anomalous, in that the variation of the γeff constants in the same bending mode is large up to 3 GHz and the γeff value can reach to nearly twice the rotational constants Bv. This behavior cannot be understood by the current Renner–Teller theory. We have developed a theory to include cross vibronic interaction between two vibronic 2Σ(vt=1) states in different bending modes. Since the difference of the vibrational quantum numbers for these states is Δ(v4+v5)=0, the interaction has a much larger effect than the one considered by Petelin and Kiselev [Int. J. Quantum Chem. 6, 701 (1972)] for the vibronic states with Δ(v4+v5)=±2. Calculation with the newly derived expressions for γeff reproduces the anomaly in HCCS when the Renner parameters are fixed at ε4=−0.37 and ε5=+0.10 from the ab initio calculation, and the parameter |ε45| for the cross vibronic interaction is varied to be 0.4, a value which is obtained for the first time. The relative sign of the above ε4 and ε5 values is explicitly judged to be correct. In addition, the Beff and the P-doubling constants in the 2Πi and 2Δi states are found to be effected by a higher-order perturbation of the cross vibronic interaction.
Unimolecular decomposition of chemically activated deutero-substituted ethanol molecules studied by infrared chemiluminescence from H2O, HOD, and D2O
Authors: Butkovskaya N. I. , Setser D. W.
Published: July 2002

Vibrationally excited H2O, HOD, and D2O molecules formed by unimolecular elimination from deutero-substituted ethanol molecules C2H5OH*, C2H5OD*, CH2DCH2OH*, and CH2DCH2OD* with an excitation energy of about 100 kcal mol−1 were observed by infrared chemiluminescence in the 2400–3900 cm−1 range. The activated ethanol molecules were produced via the successive reactions H+CH2ICH2OH→HI+CH2CH2OH and H+CH2CH2OH→CH3CH2OH* in a fast flow reactor that was observed with a Fourier transform spectrometer. The vibrational distributions of the H2O, HOD, and D2O molecules were determined by computer simulation of the experimental spectra; the distributions decline with increasing vibrational energy giving 〈fv〉=0.15 and 〈fv〉=0.14 for H2O and HOD from the decomposition of C2H5OH* and C2H5OD*, respectively. The vibrational energy in the bending mode of H2O is comparable to the energy in the stretching modes. Comparison with the statistical vibrational distributions shows a substantial overpopulation of the bending levels and a preferential excitation of one O–H or O–D stretching quantum in HOD from C2H5OD or CH2DCH2OH, respectively, i.e., in the newly formed bond. Kinetic isotope effects of [H2O]/[HOD]=3.6±0.8 and [HOD]/[D2O]=3.1±0.8 were found for the two elimination pathways of CH2DCH2OH* and CH2DCH2OD*, respectively, which agree with calculated RRKM values of kH2O/kHOD=3.2 and kHOD/kD2O=2.7.
Full quantum state determination via time dependent spectrum data
Authors: Trippenbach Marek , Band Y. B.
Published: July 2002

We develop a method of quantum state imaging (determination of the full initial density matrix of an atomic or molecular state) using time dependent spectrum data. The method determines the wave function (or more generally, the density matrix) created via an arbitrary excitation process even when degenerate rotational degrees of freedom are present. It requires application of external electric and magnetic fields, without which determination of the full density matrix is in general impossible. In the presence of external fields, quantum state imaging from angle and polarization resolved (or unresolved) time dependent spectrum data is feasible.
Optical activity of electronically delocalized molecular aggregates: Nonlocal response formulation
Authors: Wagersreiter Thomas , Mukamel Shaul
Published: July 2002

A unified description of circular dichroism and optical rotation in small optically active molecules, larger conjugated molecules, and molecular aggregates is developed using spatially nonlocal electric and magnetic optical response tensors χ(r,r′,ω). Making use of the time dependent Hartree Fock equations, we express these tensors in terms of delocalized electronic oscillators. We avoid the commonly-used long wavelength (dipole) approximation k⋅r≪1 and include the full multipolar form of the molecule–field interaction. The response of molecular aggregates is expressed in terms of monomer response functions. Intermolecular Coulomb interactions are rigorously taken into account thus eliminating the necessity to resort to the local field approximation or to a perturbative calculation of the aggregate wave functions. Applications to naphthalene dimers and trimers show significant corrections to the standard interacting point dipoles treatment.
Spectroscopy of argon fluoride and krypton fluoride exciplexes in rare gas matrices
Authors: Bressler C. , Lawrence W. G. , Schwentner N.
Published: July 2002

The spectroscopy of argon fluoride and krypton fluoride exciplexes in rare gas matrices is investigated using synchrotron radiation over the range of 50 to 600 nm. The diatomic Ar+F− and Kr+F− species are observed in excitation and emission spectra. Extensive vibrational progressions are observed in the excitation spectra and are used to determine the spectroscopic parameters of the ArF B state (Te=50233 cm−1,ωe=415.5 cm−1, ωexe=3.1 cm−1) and D state (Te=51738 cm−1,ωe=476.7 cm−1,ωexe=3.8 cm−1) and of the KrF B state (Te=39024 cm−1,ωe=342.4 cm−1,ωexe=2.0 cm−1) and D state (Te=44479 cm−1,ωe=331.6 cm−1,ωexe=1.4 cm−1). Lifetimes of 4 to 8 ns are measured for the diatomic emissions. A perturbed KrF species is observed which is identified in analogy to a similarly perturbed XeF species. Excitation spectra of the triatomic exciplexes Kr+2F− and Ar+2F− in neon are presented and described in terms of previous ab initio calculations. Emission and excitation of argon fluoride and krypton fluoride species in other matrices are presented. Large stimulated emission cross sections on the order of 10−16 cm2 are determined for the diatomic D→X and B→X emissions, which, together with nonradiative relaxation processes within the B and D states of ArF that efficiently populate the B(v=0) regardless of initial excitation, make ArF in neon a viable solid-state laser candidate.
Low-field expressions for reversing-pulse electric birefringence of ionized polyions with permanent, ionic, and electronic dipole moments: A further extension of the ion-fluctuation theory and the application to poly(α,L-glutamic acid)
Authors: Yamaoka Kiwamu , Sasai Ryo , Kohno Kazuhiro
Published: July 2002

A rigorous theory was further extended for the reversing-pulse electric birefringence (RPEB) of the ionized polyion of cylindrical symmetry with the permanent dipole moment μ3, in addition to two previously considered electric dipole moments, i.e., the root-mean-square-average moment 〈m23〉1/2 resulting from the fluctuation of ion–atmosphere along the longitudinal (3)axis of the polyion with a single relaxation time τI, as originally proposed by Szabo et al. [J. Chem. Phys. 85, 7472 (1986)], and the electronic moment from the intrinsic covalent polarizability anisotropy Δα between the longitudinal and transverse axes of the polyion by Yamaoka et al. [J. Chem. Phys. 101, 1625 (1994)]. The extended RPEB expressions were derived with three electric and hydrodynamic parameters (p=μ23/kTΔα, q=〈m23〉1/2/kTΔα, and τ*=τI/τθ) in the Kerr-law region. Calculated with appropriate values to these parameters, the theoretical curves show such new features that either maxima or minima appear in the buildup and reverse processes. The present theory was used to analyze a set of experimental RPEB signals of poly(α,L-glutamic acid) in helical conformation in methanol and in methanol–water containing sodium hydroxide. By fitting the observed data to theoretical curves, the contribution of 〈m23〉1/2 was shown to surpass that of μ3 for the same helical sample that was partially ionized by neutralization with sodium hydroxide.
Instantaneous normal mode analysis of liquid Na
Authors: Wu Ten-Ming , Tsay Shiow-Fon
Published: July 2002

The instantaneous-normal-mode (INM) densities of states for liquid Na in four realistic thermodynamic states are obtained by molecular-dynamics simulation. The exponent functions derived from the INM densities of states are found to fit well with a binomial of frequency for the imaginary-frequency lobe and a three-term polynomial for the real-frequency lobe. Through the study of the size dependence for the participation ratios of INMs, we show that for both the real-frequency and the imaginary-frequency lobes, INMs of low frequencies are extended modes and INMs in the high-frequency end are localized modes. However, each lobe has a broad transition region in frequency from the extended to localized INMs.
Mixed quantum/classical simulation of the photolysis of HCl on MgO(001)
Authors: Hintenender M. , Rebentrost F. , Kosloff R. , Gerber R. B.
Published: July 2002

Quantum and mixed quantum/classical calculations of the photolysis of a HCl adsorbate on a MgO surface are reported. In the quantum calculation of the hydrogen dynamics (with rigid surface and chlorine atoms) a strong oscillatory structure is found in the angular distribution of the photofragmented hydrogen as well as in the absorption spectrum. These resonances are caused by temporary trapping of the hydrogen atom between the chlorine atom and the surface and reflect the initial perpendicular adsorption geometry. Corrugation of the surface potential leads to a significant modification of these interference patterns, which exist even for a flat surface. Within a mixed quantum/classical time-dependent self-consistent field (Q/C TDSCF) propagation the influence of surface degrees of freedom on the interference patterns is investigated. The thermal motion of the surface and inelastic collisions of the hydrogen atom with the surface and the chlorine atom washes out most of the oscillatory structure. In the fully angular and energy resolved spectra nevertheless clearly distinguishable peaks are seen. They can be used in practice to extract information about adsorption geometry and surface potential parameters.
London equation of state for quantum hard disks
Authors: Solís M. A.
Published: July 2002

For a two-dimensional hard-disk boson system we propose an analytical London-type expression for the ground-state energy as a function of density ranging from zero to close packing. Such an equation interpolates between the leading term of the exact low-density series expansion and the behavior expected at close-packing based on uncertainty-principle arguments and determined by the correct value of the residue which is a dimensionless constant whose value depends on the space dimension as well as on the kind of close packing configuration involved. As for any reasonably accurate equation of state, variational Monte Carlo calculations for both fluid and crystalline energy branches turn out to be close upper bounds to the resulting so-called London–Hubbard energy which includes pair-correlation effects.
Rayleigh–Schrödinger perturbation theory for resonance tunneling
Authors: Fernández Francisco M.
Published: July 2002

We calculate accurate tunnel resonances by means of standard Rayleigh–Schrödinger perturbation theory, exactly as if they were ordinary bound states. We choose one-dimensional barriers with a single maximum to test the convergence properties of the perturbation series. The method consists of an expansion of the potential-energy function about its maximum and a dilatation of the coordinate by means of a complex scaling parameter completely determined by the curvature of the barrier.
Intracavity laser absorption spectroscopy of HOCl. II. High overtones, perturbations, and intramolecular dynamics
Authors: Hamann Hilmar H. , Charvát Aleš , Abel Bernd , Kovalenko Sergey A. , Kachanov Alexander A.
Published: July 2002

High resolution spectra of the 4ν1 and 5ν1 OH-stretch overtone and the weak 4ν1+ν2 combination band of transient HO35(37)Cl in the energy range 12 500–16 500 cm−1 have been recorded using ultrasensitive intracavity laser absorption spectroscopy (ICLAS). For this investigation, two different spectrometers, a titanium:sapphire and a dye-jet ICLA spectrometer have been employed. We report line assignments for both isotopomers, refined Dunham parameters, and the spectroscopic constants for the excited rovibrational states. Strong and weak Fermi-type resonances as well as Coriolis interactions at ≈65%–85% of the dissociation energy E0 have been found and the role of dark perturbers is discussed. The dark 2ν1+3ν2+3ν3 state has been analyzed and identified to be the perturber of 3ν1+2ν2. From the detailed analysis of the experimental data the anharmonicity parameters of the asymmetric stretch ν3 are refined. The 5ν1 band at about 85% of E0 has been found to be strongly mixed with the 4ν1+2ν2+ν3 zero order vibrational state, and both states are coupled to other background states by additional (weaker) interactions. The extent and the magnitude of perturbations in all Ka bands of the 5ν1/4ν1+2ν2+ν3 system and additional interactions with other background states may be regarded as the signature of the onset of efficient but restricted intramolecular vibrational energy redistribution (IVR) in the overtone spectrum of HOCl. The intramolecular dynamics of this small molecule after coherent short pulse excitation of the 5ν1 zero order state is discussed in terms of a simple tiers model.
On the scattering phenomena for various kinds of polarized light in a nonpolar binary liquid mixture
Authors: Lee Dong J. , Kim Kyoung-Ran
Published: July 2002

The explicit result for the Mueller matrix for the sum of single and double scatterings is obtained in a nonpolar binary liquid mixture composed of a chiral solute and an achiral solvent are obtained with the aid of the Ornstein–Zernike form for the correlation function of concentration fluctuations. The explicit result of the matrix shows that the law of reciprocity does not hold in the mixture. The scattering phenomena for six kinds of completely polarized light in a nonpolar binary liquid mixture are different from those in a pure chiral fluid because of the failure of the law and the large effect of concentration fluctuations. The scattering phenomena are in detail discussed in the region far from and near the critical point.
Photoemission cross sections for CH radicals produced by collisions of He(23S) atoms with CH3X (X=H, Cl, Br, I)
Authors: Tokue Ikuo , Sakai Yuko , Yamasaki Katsuyoshi
Published: July 2002

Photoemission cross sections (σem) for the A–X, B–X, and C–X bands of CH resulting from the He(23S)+CH3X (X=H, Cl, Br, I) reaction have been studied in the relative collision energy (ER) of 120–210 meV. Formation cross sections (σ) for CH(A, B, C) were evaluated from the σem’s taking account of predissociation. A good correlation was found between the sum of the σ’s for CH and the dipole-induced dipole interaction of He(2 3S) with targets. The σem’s from methane increase with ER, while those from methyl halides decrease with increasing ER. The positive energy dependence for methane implies that effective potentials leading to CH are repulsive, whereas attractive potentials play a dominant role in the reaction of He(2 3S) with methyl halides. Model potentials between CH3Cl and He*(Li) calculated using ab initio molecular orbital methods indicate that He(2 3S) approaches CH3Cl not from CH3 side but from Cl side.
Polar CdSe nanocrystals: Implications for electronic structure
Authors: Schmidt Mark E. , Blanton Sean A. , Hines Margaret A. , Guyot-Sionnest Philippe
Published: July 2002

We report direct spectroscopic evidence for parity-forbidden electronic transitions in CdSe nanocrystals and relate it through perturbation theory to an internal electric field. This field is consistent with an estimated spontaneous polarization of Ps=−0.6 μC/cm2 for CdSe. The observed mixing of odd and even band edge hole states by this symmetry breaking internal electric field indicates that current theories of nanocrystal electronic structure, which assume inversion symmetry, require substantial revision.
Low energy collision-induced dissociation and photodissociation studies of the (N2O,H2O)+ cluster ion
Authors: Bastian Michael J. , Dressler Rainer A. , Levandier Dale J. , Murad Edmond , Muntean Felician , Armentrout P. B.
Published: July 2002

Low energy collision-induced dissociation (CID) and photodissociation measurements of monohydrated nitrous oxide cluster ions are presented. The CID measurements have been conducted with ions produced in both thermal and supersonic jet sources, and with both Ne and Ar as collision gases. In all experiments, H2O+, N2O+, and N2OH+ fragments are observed, for which CID thresholds (0 K) of 1.04±0.06, 1.43±0.12 and 1.32±0.10 eV are determined, respectively. The thermal source experimental thresholds are consistent with all fragment ions originating from a single isomeric precursor ion, [N2O⋅H2O]+. Whereas both N2O+ and N2OH+ CID curves are comparable in the thermal source and supersonic jet source experiments, considerable differences are observed in the H2O+ CID measurements. The differences are attributed to loosely bound cluster-ion isomeric forms produced in the jet source experiment. In the photodissociation experiments, branching ratios measured with the present jet source are very similar to those observed in previously reported thermal experiments [S. T. Graul, H-S. Kim, and M. T. Bowers, Int. J. Mass Spectrom. Ion Proc. 117, 507 (1992)]. All of the fragment ions can be accounted for by invoking an optical transition from ground state [N2O⋅H2O]+ to an excited state associated with the N2O+H2O+ (Ã) dissociation limit. The H2O+ and N2O+ branching ratios are primarily governed by predissociation of the upper state followed by charge-transfer dynamics along a repulsive dissociation coordinate. Formation of N2OH+ is postulated to be controlled by a competing internal conversion process that leads to a longer-lived complex that decays to proton transfer products.
The 1 and 2 Π1 twin states of KRb revisited
Authors: Leininger T. , Stoll H. , Jeung G.-H.
Published: July 2002

We performed new ab initio calculations with a core-polarization-potential treatment of the core-valence correlation for the ground and the two first Π1 excited states of the KRb dimer. Contrary to a previous result [Phys. Rev. A 51, 1929 (1995)], the potential curves obtained are now in perfect agreement with recent experimental studies. However, it is shown that the qualitative discussion given previously remains valid.
Density functional crystal orbital study on the normal vibrations of polyacetylene and polymethineimine
Authors: Hirata So , Iwata Suehiro
Published: July 2002

Optimized molecular structures and k=0 (Brillouin zone center) vibrational frequencies are obtained for trans- and cis-transoid polyacetylene and for anti- and syn-transoid polymethineimine by the density functional crystal orbital method with the Becke3–Lee–Yang–Parr functional. The analytical energy gradient scheme is implemented in the density functional crystal orbital method, and the force constants of the infinite polymers are evaluated by numerical differentiation of the analytical energy gradients. For the trans- and cis-transoid isomers of polyacetylene, the vibrational frequencies calculated and then uniformly scaled by a single scale factor are in reasonable agreement with the observed frequencies. For polymethineimine, it is found that the calculated frequencies of the anti-transoid isomer completely disagree with the observed frequencies. In contrast, the calculated frequencies of syn-transoid polymethineimine are in agreement with the observed, although there are only three observed frequencies available. The total energy of syn-transoid polymethineimine is found to be lower than that of the anti-transoid isomer by 15.6 kJ mol−1.
Phase equilibria of model ternary mixtures: Theory and computer simulation
Authors: de Miguel Enrique , Telo da Gama Margarida M.
Published: July 2002

We report the study of the phase diagram of a three-dimensional continuum model of symmetrical ternary amphiphilic mixtures, representing water, oil, and surfactant, using mean-field approximations as well as Monte Carlo simulations. In line with the results of various lattice models, the continuum model exhibits a region of three-(isotropic) liquid-phase coexistence consisting of water-rich, oil-rich, and surfactant-rich phases. The dependence of the phase diagram on the strength of the anisotropic water–(oil–)surfactant interactions is investigated using a modified mean-field approximation that takes into account, at the lowest level of approximation, the contribution of the water–(oil–)surfactant correlations. The phase behavior of the model ternary mixture is further examined using Monte Carlo simulation techniques in the semigrand canonical ensemble. The results of the simulations for symmetrical mixtures are consistent with the existence of a region of three-(isotropic) liquid-phase coexistence below a tricritical point. This region is analyzed in more detail using the Gibbs Monte Carlo simulation technique. It is shown that the simulation results are in qualitative agreement with the theoretical predictions.
The role of the attractive potential of a droplet in unary and binary steady state nucleation
Authors: Arstila Hanna , Vasil’ev Oleg V. , Kulmala Markku
Published: July 2002

The role of the attractive potential of a droplet in steady state unary and binary nucleation under normal pressures is considered. We determined the nucleation rate by direct numerical solution of the birth and death equations. The correction due to the attractive potential is found to be unessential for three distinct systems considered (pure water, nearly ideal mixture of CCl4 and SiCl4 and highly nonideal mixture of H2O and NH3).
The role of three dimensional structure in electron transmission through thin organic layers
Authors: Kadyshevitch A. , Ananthavel S. P. , Naaman R.
Published: July 2002

In this work we probe the effect of the three dimensional structure of the medium on the efficiency of electron transmission (ET) through it, and demonstrate that all three dimensions are playing a crucial role in the ET through thin films. By producing Langmuir–Blodgett layers from two type of amphiphiles we could vary the order in the plane perpendicular to the direction of electron propagation. It was found that the order in this plane affects the low energy electron transmission efficiency. The results are explained by the long wavelength associated with the low energy electrons.
Metal-insulator segregation in lithium rich LinHm + clusters
Authors: Antoine R. , Dugourd Ph. , Rayane D. , Benichou E. , Broyer M.
Published: July 2002

The metallicity of lithium rich LinHm + cluster ions [1⩽m⩽6, n⩽22, and (n–m)>3] is studied by measuring unimolecular dissociation rates. These clusters are found to decompose by evaporation of a Li atom or a Li2 molecule. The evaporative rates of mixed clusters display features characteristic of metallic clusters. This confirms and extends to a larger size range the previous results obtained by photoionization and absorption cross-section measurements. The simulation of the evaporative rates, with a statistical Rice–Ramsperger–Kassel (RRK) model which has been adapted to mixed clusters, confirms that there is a clear separation between a metallic Lin–m  + part and an insulating (LiH)m part.
Ionization and dissociation mechanism of superexcited ketene using time-of-flight mass spectrometer
Authors: Pan Ching-Chwan , Chou Chih-Chiang , Lu Cheng-Hwa , Tai Yian , Lin King-Chuen
Published: July 2002

The interrelation between ionization and fragmentation of ketene is studied. A jet-cooled ketene is first excited to a 3p Rydberg state prior to ionization with the same laser irradiation; the resultant parent and fragment ions are then detected by a time-of-flight (TOF) mass spectrometer. From the dependence measurements of laser power, ketene pressure, and intensity and duration of a pulsed electric field, we find that the ketene ions are obtained predominantly by an autoionization process from a superexcited state. The superexcited ketene lies in the energy level of ∼5.6 eV above the first ionization potential by absorbing four photons energy. The CH2+ ion is fragmented by a dissociative ionization of the superexcited state in competition with the autoionization. The factors of pulse field and collisions may enhance the autoionization rate significantly, leading to an opposite effect on the branching ratio of ketene ion and CH2+ fragment. In the time-resolved ketene ion measurement, we may differentiate two types of ion sources. The first type, resulting from autoionization, relaxes in the 440–550 ns range, depending on various Rydberg states excited; the second, resulting from direct photoionization, is ten times slower. The consequence may be caused by a distinctly different temporal behavior between the first excited and the ground state ketene ion, as evidenced previously in photoelectron experiments. Finally, we examined an arrival time distribution of the CH2+ fragment at the TOF detector as a function of polarization direction of the photolysis laser. The CH2+ mass spectra for both parallel and perpendicular positions appear to be identical when the 3p Rydberg state is excited. The independence of molecular alignment indicates that the dissociation lifetime of the superexcited ketene is longer than its rotational period, which is estimated to be >20 ps.
One-dimensional surface diffusion: Density dependence in a smooth potential
Authors: Beenakker J. J. M. , Krylov S. Yu.
Published: July 2002

A kinetic treatment of the transport of mobile molecules in nanometer channels is presented, that explores ideas of the Enskog theory of dense gases combined with molecular relaxation to the substrate. Transport diffusivity is shown to increase strongly with density, as an effect of effective transfer of mass in the nonlocal intermolecular collisions. The effect is partially compensated by the additional collisional relaxation, that would be absent in the truly one-dimensional system of hard rods, but is shown to occur in the realistic cases of quasi-one-dimensional channels and/or “soft” particles. A resulting behavior of diffusivity as a function of density close to 1/(1−θ), where θ is the degree of occupancy, is expected. This suggests an explanation for the puzzling universal behavior observed earlier in zeolites.
Control of photofragment angular distribution by laser phase variation
Authors: Kim Hong Lae , Bersohn Richard
Published: July 2002

When a molecule is dissociated through separate excitations by one or two polarized photons the fragment distribution is forward–backward symmetric with respect to the polarization axis. However,if the molecule is simultaneously excited by optical electric fields, E(ω) and E(2ω), the amplitude for excitation is a sum of an amplitude for absorbing one photon of frequency 2ω and an amplitude for absorbing two photons of frequency ω. When the sum is squared to obtain the fragment angular distribution, there is a cross product which depends on the phase difference Δφ=φ2ω−2φω. With suitable adjustment of the relative intensities at the two frequencies large unidirectionality can be obtained. The left–right anisotropy on the polarization axis depends on the symmetry of the transitions, real and virtual. In the most favorable case 97% of a particular fragment is ejected in one hemisphere and only 3% in the other. In a less favorable case 60% of the fragment is ejected in one hemisphere.
Resonances in He + H2+→HeH+ + H reaction in three-dimensions: Energy resolved total reaction probabilities by the time-dependent wave packet method
Authors: Mahapatra S. , Sathyamurthy N.
Published: July 2002

Vib-rotational state-selected and energy resolved total reaction probabilities for the title reaction in three dimensions are calculated on the McLaughlin–Thompson–Joseph–Sathyamurthy potential energy surface, by the time-dependent wave packet (WP) method for the total angular momentum J=0. The probabilities as obtained by the time-energy mapping of the reactive flux of the WP across a dividing surface in the asymptotic product channel reveal a large number of narrow resonances, in agreement with the time-independent quantum mechanical results available in the literature. While initial vibrational excitation of H2+ is shown, in general, to enhance the reaction probability considerably, in agreement with the experimental observations, rotational excitation has only a marginal influence.
The excited electronic states of H2CSi
Authors: Hilliard Randall K. , Grev Roger S.
Published: July 2002

We have studied the excited electronic states of silylidene(1-silavinylidene), H2CSi, by ab initio quantum-mechanical methods to determine their equilibrium geometries, relative energies, and harmonic vibrational frequencies. Besides the ground A11 state, we have also studied the A21 and A23 states, obtained by exciting an electron from the π orbital of silylidene to a py lone pair orbital, and the B21 and B23 states obtained by exciting an electron from an a1 symmetry silicon lone pair orbital to the py orbital. We have also investigated the lowest-lying cation states. Our predictions of the geometries of the A11 ground state and second excited singlet state, B21, are in good agreement with the experimental results of Harper, Waddell, and Clouthier [J. Chem. Phys. 107, 8829 (1997)]. Our predicted excitation energies and excited state vibrational frequencies also agree well with experimental values. The geometry of the B̃ 1B2 state differs markedly, and unexpectedly, from that of the X̃ 1A1 and b̃ 3B2 states. We explain this as resulting from the large electron repulsion between the unpaired electrons in the B̃ state, which essentially reside on the same atom.
Optical spectroscopy of jet-cooled FeC between 12 000 and 18 100 cm−1
Authors: Brugh Dale J. , Morse Michael D.
Published: July 2002

Iron monocarbide has been investigated between 12 000 and 18 100 cm−1 in a supersonic expansion by resonant two-photon ionization spectroscopy. Six new electronic states have been identified for which origins relative to the ground state have been determined. Three of these possess Ω′=3, one possesses Ω′=4, and two possess Ω′=2. The Ω′=3 state with an origin near 13 168 cm−1 is likely a Δ33 state and has been assigned as the analog of the [14.0]2Σ+←X2Σ+ charge transfer transition in CoC. The Ω′=4 state is most likely a Φ43 state. Additionally, seven bands with Ω′=2 have been observed that have proven impossible to systematically group by electronic state. Because every transition rotationally resolved in this study possesses a lower state with Ω=3, the ground state has been confirmed as arising from an Ω=3 state that is most likely the Ω=3 spin orbit component of a Δi3 term derived from a 1δ39σ1 configuration. The ionization energy (IE) of FeC has been determined as 7.74±0.09 eV by varying the wavelength of the ionization photon. When combined with the known IE of Fe and the bond energy of FeC+, the bond energy of FeC is calculated to be 3.9±0.3 eV. Presentation of the results is accompanied by an analysis of the bonding in FeC from a molecular orbital standpoint.
Deuterium isotope effects in collision-induced dissociation and photodissociation of the (N2O,H2O)+ cluster ion
Authors: Williams S. , Chiu Y.-H. , Levandier D. J. , Dressler R. A.
Published: July 2002

We present low energy collision-induced dissociation (CID) and visible photodissociation (PD) data of jet-cooled (N2O,H2O/D2O)+ cluster ions. The same three fragment ions are observed in both CID and PD, namely, H2O+/D2O+, N2O+, and N2OH+/N2OD+. The H2O+ and N2O+ product channels exhibit a small deuterium isotope effect in both CID (10%) and PD (15%). The N2OH+ product channel, on the other hand, exhibits a much larger deuterium isotope effect for CID (30%) and PD (230%). The large difference in deuterium isotope effects observed in the N2OH+ product channel for CID and PD suggests that this product is formed via collision-induced and photoinduced proton-transfer reactions. These results are consistent with the interpretation that a single isomeric form, namely a (N2O⋅H2O)+ association complex, predominates. No evidence is found supporting the existence of a proton-transfer complex, i.e., N2OH+⋅OH. Evidence for the formation of minor amounts of a weakly bound isomeric form with the charge localized on the H2O moiety, i.e., N2O⋅H2O+, is found.
Spectroscopic determination of ΔH° for axial/equatorial and ethyl rotor conformations in 4-methyl and 4-ethyl cyclohexanone cooled in a supersonic jet
Authors: Potts Alan R. , Baer Tomas
Published: July 2002

Two types of conformational equilibria and the ΔH for their interconversion have been investigated by 2+1 resonance enhanced multiphoton ionization (REMPI) of jet cooled 4-methyl and 4-ethyl cyclohexanone. The axial-equatorial interconversion equilibria have been investigated for both molecules, while the additional interconversion of the ethyl rotor conformations have been studied for ethylcyclohexanone. van’t Hoff plots (ln Keq vs 1/T) obtained by varying the pulsed valve temperature have yielded the ΔHax/eq and ΔHrotor for both equilibria. The measured enthalpy differences for axial/equatorial conformers were 2.1±0.20 kcal/mol for 4-methyl cyclohexanone, and 2.2±0.20 kcal/mol for 4-ethyl cyclohexanone. The ΔH for the ethyl rotor in 4-ethyl cyclohexanone was measured to be 1.1±0.10 kcal/mol. In the case of the axial-equatorial equilibrium, the van’t Hoff plot was linear up to the highest valve temperatures investigated (200 °C). This demonstrates that in the supersonic expansion the vibrational cooling is much more rapid than axial-equatorial interconversion. On the other hand, the van’t Hoff plot for the ethyl rotor equilibrium leveled off at 80 °C. This demonstrates that above this temperature, the ethyl rotor equilibration rate is faster than the vibrational cooling rate. In all cases, the final vibrational temperature, as measured from the absence of spectral hot bands is below 50 K.
The desorption of molecular hydrogen from Si(100)-2×1 and Si(111)-7×7 surfaces at low coverages
Authors: Flowers Michael C. , Jonathan Neville B. H. , Morris Alan , Wright Steven
Published: July 2002

The mechanisms leading to desorption of molecular hydrogen from Si(100)-2×1 and Si(111)-7×7 surfaces have been elucidated and refined by detailed examination of the thermal desorption kinetics with particular emphasis on low and very low coverages. In the case of hydrogen desorption from Si(100)-2×1, a lattice-gas model incorporating the interactions that are responsible for pairing and clustering of adsorbed hydrogen atoms has been employed to fit temperature programmed desorption (TPD) peaks resulting from initial coverages between 0.01 and 1.0 monolayer (ML). From analysis of our low coverage data, we find that the pairing and clustering energies are (3.2±0.3) kcal mol−1 and (3.4±0.5) kcal mol−1, respectively. A subtle shift of the TPD peak maximum position as the initial coverage increases from 0.2 to 1.0 ML indicates that the pre-exponential factor and activation energy are weakly coverage dependent. We discuss how this is consistent with coupling of a dihydridelike transition state to its neighbors. The rate of molecular hydrogen desorption from Si(111)-7×7 is found to be very nearly second order in total hydrogen coverage when the initial coverage is low. This result is consistent with a two site model involving preferential adsorption of hydrogen atoms at rest atom sites rather than adatom sites.
Microwave spectrum of the furan–CO van der Waals complex
Authors: Brupbacher Th. , Makarewicz J. , Bauder A.
Published: July 2002

The microwave spectrum of the furan–CO van der Waals complex has been measured between 6–19 GHz with a pulsed nozzle Fourier transform microwave spectrometer. Due to a large amplitude motion of the CO subunit between two equivalent minima, the observed rotational transitions were split into two components. Their center frequencies were compatible with an asymmetric rotor with CO located above the plane of furan. The rotational spectrum has been analyzed using an effective model Hamiltonian that accounts for an internal rotation between furan and CO hindered by a low barrier.
Molecular dynamics simulations of the MgO(001) surface hydroxylation
Authors: Oviedo Jaime , Calzado Carmen J. , Sanz Javier Fernández
Published: July 2002

A first principles study of the MgO(001) surface hydroxylation is reported. Using ab initio Hartree–Fock embedded cluster calculations, pair potentials for the species H+-Osurf, H+-Mgsurf, HO−-Osurf, and HO−-Mgsurf pairs have been obtained. These potentials are used to simulate, by means of molecular dynamics, the hydroxylation of the MgO(001) surface. The hydroxyl groups are found to remain at the surface, bound to Mg cations, inducing a noticeable relaxation. The protons are found to leave the surface, stabilizing beyond the third layer. This proton penetration agrees with recent elastic recoil detection analysis in which the protonation is observed to at least 5000 Å. The structure of the surface, as well as the coordination environment of inner protons, is discussed in light of radial distribution functions and spectra of density of states.
The photodepletion action spectrum of the (BaF)2 cluster
Authors: Pereira R. , Skowronek S. , González Ureña A.
Published: July 2002

The (BaF)2 cluster has been produced in a supersonic expansion of a mixture of helium and CH3F which contained laser desorbed barium. Its photodepletion action spectrum measured by time-of-flight mass spectrometry displays low depletion cross sections (≈3 Å2) and a clear vibrational structure. This structure has been analyzed to obtain spectroscopic constants of the excited state involved in the photofragmentation of the cluster. Furthermore, its ionization potential (I.P.) has been determined to be 4.03 eV, and a discussion on the spectroscopy, structure, and dynamics of the cluster is also presented.
Nearside–farside analysis of differential cross sections: Ar+N2 rotationally inelastic scattering using associated Legendre functions of the first and second kinds
Authors: McCabe P. , Connor J. N. L. , Sokolovski D.
Published: July 2002

We have carried out the first nearside–farside (NF) analysis of angular scattering for molecular collisions in which the partial wave series for the scattering amplitude is expanded in a basis set of associated Legendre functions (of the first kind.) The practical implementation of the NF theory is described, which exploits in an essential way properties of associated Legendre functions of the second kind. The new concept of a restricted nearside–farside (resNF) decomposition of the scattering amplitude is introduced, which takes into account the caustic structure of the associated Legendre functions. The resNF theory is used to analyze polarization and degeneracy averaged differential cross sections for the Ar+N2 collision system, treated as an atom+rigid rotor. The resNF analysis always provides a clear physical interpretation of the scattering (except sometimes for scattering angles≈0,180°) for phenomena such as diffraction oscillations, potential rainbows, and rotational rainbows, as well as more complicated interference effects.
Nonstatistical bond breaking in the multiphoton ionization/dissociation of [Fe(CO)5]mArn clusters
Authors: Bililign S. , Feigerle C. S. , Miller John C. , Velegrakis M.
Published: July 2002

Photoionization of iron pentacarbonyl/argon clusters with 30 ps, 266 nm laser pulses results in the ultimate detection of iron ions solvated with argon atoms. Clusters such as Fem+Arn (m=1,2; n=1–26) are readily observed following the laser-induced decarbonylation of the [Fe(CO)5]mArn species formed in the supersonic jet. An interesting intensity alternation of the Fe+Arn ions is observed up to a magic number at n=6, after which a monotonically decreasing intensity pattern is noted. The pattern is similar to a more extensive spectrum (with additional magic numbers) observed in separate experiments where metallic iron is laser ablated into a supersonic flow of argon, krypton, or xenon, and the resulting cluster ions are detected in a reflectron mass spectrometer. In another experiment in the latter apparatus, the sputtering of iron into a mixture of carbon monoxide and argon shows the relative reactivity of Fe+ to form Fe+Arn and Fe+(CO)n ions. Mechanisms for the energy disposal during the ionization/dissociation process are discussed. Additionally, the intensity pattern of iron–rare gas clusters, Fe+(RG)n, is discussed in the context of structural arguments which have previously been applied successfully to a large number of metal–rare gas systems.