bluuebunny/crossref_metadata_2025 huggingface dataset:
The search for direct vibrational excitation in gas–surface collisions of CO with Au(111)
Authors: Rettner C. T.
Published: July 2002

I have examined the dynamics of energy transfer in the CO/Au(111) system to determine the probability of direct vibrational excitation. In contrast to earlier studies of NO/Ag(111), NH3/Au(111), and H2/Cu(111), no direct vibrational excitation is observed. Measurements have been made using molecular beam techniques to control the collision energy Ei and angle θi and using laser ionization detection to probe the internal state distributions of the scattered molecules. The probability of direct vibrational excitation is found to remain below 10−3 for Ei up to 1.4 eV at surface temperatures Ts of 300 and 800 K and θi=10°. I have also determined the probability of deexcitation for CO(v=1) colliding with Au(111). Within the uncertainties (∼20%), no deexcitation is apparent for Ei in the range 0.2–1.1 eV with Ts=300 K. With Ts=800 K and Ei=1.1 eV, the survival probability was also indistinguishable from unity. As part of this study, I have determined the variation of the trapping probability of CO on Au(111) as a function of kinetic energy for different surface temperatures, and I report rotational distributions for scattered CO for Ei of 0.5, 0.9, and 1.4 eV with θi=10°. In contrast to the weak coupling to molecular vibration, a high degree of rotational excitation is observed, yielding pronounced rotational rainbows. Using beams with ∼1% v=1 populations, I find that the rotational distributions of scattered v=1 molecules are the same as those for scattered v=0 within the uncertainties of the measurements.
On the vibronic structure of the S↔S1 transitions in azulene
Authors: Negri Fabrizia , Zgierski Marek Z.
Published: July 2002

The semiempirical (quantum chemical consistent force field/π-electron and complete neglect of differential overlap/spectroscopic parametrization) and ab initio methods are used to study the vibronic structure of the optical transitions between the S0 and S1 states of azulene. It is shown that although the excited state is described quite well as arising from the promotion of one electron from the highest occupied molecular orbital to the lowest unoccupied one, the inclusion of doubly excited configurations in description of the wave functions of the two states in question is essential for a proper characterization of the geometry change upon electronic excitation and, therefore, for the resulting Franck–Condon structure of the absorption and emission spectra. The vibronically induced intensities of the b2 fundamentals in the two spectra are calculated and compared with the experimental data, and the problem of correlation between the b2 modes in the two states is solved.
Kinetics and product branching ratios of the CN+NO2 reaction
Authors: Park Joonbum , Hershberger John F.
Published: July 2002

The CN+NO2 reaction was studied using time-resolved infrared diode laser absorption spectroscopy. The total rate constant for this reaction over the temperature range 298–650 K was fit to the expression k1=10−10.30±0.03 exp[(171±32)/T], in reasonable agreement with previous measurements. Branching ratios at room temperature were also measured. The dominant product channel is NCO+NO, which accounts for 86.8±6.0% of the total reaction rate. The CO2+N2 and CO+N2O channels account for 5.6±6.0 and 7.6±3.2%, respectively. The dominant mechanism for this reaction is formation of an NCONO intermediate followed by O–N bond cleavage.
Adsorption and photodecomposition of Mo(CO)6 on Si(111) 7×7: An infrared reflection absorption spectroscopy study
Authors: Richter Lee J. , Buntin Steven A. , Chu Pamela M. , Cavanagh Richard R.
Published: July 2002

The adsorption and photodecomposition of Mo(CO)6 adsorbed on Si(111) 7×7 surfaces has been studied with Auger electron spectroscopy, temperature programmed desorption, low energy electron diffraction and infrared reflection absorption spectroscopy in a single external reflection configuration. The external-reflection technique is demonstrated to have adequate sensitivity to characterize submonolayer coverages of photogenerated Mo(CO)x fragments. It is proposed that the first layer of Mo(CO)6 adsorbs in ordered islands with a Mo(CO)6 atop each adatom of the 7×7 reconstructed Si surface. UV irradiation of these islands produces a carbonyl fragment, identified as chemisorbed Mo(CO)5. The Mo(CO)5 thermally decarbonylates via two subcarbonyl intermediates with little CO dissociation. Photolysis of thicker layers results in the formation of Mox(CO)y dimers/polymers, as evidenced by the appearance of bridging CO, which is attributed to a facile association reaction. The dimer/polymer species correlate with deposition of C and O on the surface.
Crystallization kinetics of water below 150 K
Authors: Hage Wolfgang , Hallbrucker Andreas , Mayer Erwin , Johari G. P.
Published: July 2002

Metastable liquid water, obtained by heating its hyperquenched glassy state above its glass→liquid transition temperature, crystallizes to cubic ice. Kinetics of this crystallization has been studied by Fourier transform infrared spectroscopy by determining the change in the spectra of stretching vibrations of the decoupled OD oscillator in 3.6 mole % HOD in H2O. The crystallization kinetics follows the equation x=[1−exp(−ktn)] and is diffusion controlled. Annealing at a temperature below its glass→liquid transition temperature alters this kinetics as the grain–growth process begins to control the early stages of crystallization.
Crossed-beam (full collision) versus van der Waals (half collision) studies. Application to the determination of the D(Ca...HCl)
Authors: Menéndez M. , Garay M. , Verdasco J. E. , González Ureña A.
Published: July 2002

The dissociation energy of the van der Waals molecule Ca...HCl in its ground state, D00(Ca...HCl), has been determined using energy balance arguments. By combining collisional information from the Ca(3P)+HCl→CaCl(A 2Π)+H reaction with spectroscopic information from the Ca...HCl+hν→CaCl(A 2Π)+H reaction we found D00(Ca...HCl)≤150 meV.
Wavelet analysis of time series for the weakly forced and weakly damped Morse oscillator
Authors: Permann Delmar , Hamilton Ian
Published: July 2002

We consider a wavelet analysis of various time series of the position for a Morse oscillator which is weakly forced and weakly damped. We focus on the highly anharmonic regime (close to the dissociation energy) where the time series are highly aperiodic and chaotic. In this regime, there are many significant frequency components in a Fourier expansion of the time series. Consequently, the usual Fourier analysis is problematic since it is assumed that the time series are stationary. Our aim is to show that a wavelet analysis can be used to determine quantitatively multiple forcing frequencies. The utility of a wavelet analysis is due primarily to the localization of the orthogonal wavelet basis functions in both frequency and time.
Adiabatic potential curves and nonadiabatic coupling functions for the first five excited 1∑+g states of the hydrogen molecule
Authors: Wolniewicz L. , Dressler K.
Published: July 2002

On closer examination, some of the spectroscopic assignments for the fourth excited 1∑+g state of H2 (4dσ,P) and all of the ones for the fifth state (4s,O) prove to be wrong. To aid the re-evaluation of these experimental data we have performed (i) the first accurate ab initio calculation, over a wide range of internuclear distances, of the fifth and sixth electronic 1∑+g states, i.e., of their Born–Oppenheimer electronic energy curves and adiabatic corrections, and of their nonadiabatic couplings with the lower excited electronic 1∑+g states, (ii) an improved calculation of the first three excited electronic 1∑+g states and of their mutual couplings, and (iii) the first calculation of relativistic corrections for the first two of these excited states, EF and GK, at the minima of their double-well potential energy functions. We have found that the electronic energy curves of the pairs of singlet ndσ and ns states (n=3, 4), unlike the triplet 3dσ and 3s states, cross each other at internuclear distances which are much smaller than their equilibrium R values and with off-diagonal electronic energies that are at least 2 orders of magnitude smaller than the diabatic coupling energies at the avoided crossings of the corresponding triplet states.
Double modulation-high resolution infrared spectroscopic technique: The ν3 band of the CH3 radical and excited states of CH4 in a hollow cathode discharge
Authors: Tanarro I. , Sanz M. M. , Bermejo D. , Domingo C. , Santos J.
Published: July 2002

In this work several lines of the ν3 fundamental band of the CH3 radical have been detected in a refrigerated hollow cathode discharge multipass cell filled with CH4, by using a discharge and laser amplitude double modulation technique, with a difference frequency laser spectrometer and a double phase sensitive detection. Bands of the precursor CH4 and of C2H4 and C2H6 produced in the discharge have been observed too. By Doppler broadening and absorption variations, the increase of temperature with the establishment of the discharge has been tested. It is only ∼15 °C. Nevertheless, a very high sensitivity in transmittance variations with temperature is achieved for transitions from highly excited levels of stable species. The method proposed can be very useful for the assignment of very weak bands.
Lattice-gas model mimicking the NO+CO reaction on Pt(100)
Authors: Meng Baoqi , Weinberg W. Henry , Evans J. W.
Published: July 2002

Various NO-reduction reactions on Pt(100) exhibit similar dynamical behavior, presumably due to an empty site requirement for NO dissociation. This motivates analysis of a lattice-gas model which incorporates this feature, and which here is chosen to mimic the NO+CO reaction on Pt(100): both reactants adsorb at single empty sites, NO instantaneously dissociates given an adjacent empty site (and nitrogen adatoms thus formed are immediately removed), and adjacent CO and O instantaneously react. We also include desorption of adsorbed NO and CO, but no adspecies diffusion. At lower temperatures where desorption is absent, we show that poisoning occurs with the reaction rate decreasing as RCO2∼ e−kt, where k≳0 except for equal reactant adsorption rates. The introduction of desorption produces reactive steady states, and (in different regimes) nonequilibrium poisoning transitions, critical points, and transitions to bistability.
Effect of adsorbed potassium on the electrostatic potential on Rh clusters in relation with photoemission of adsorbed noble gases
Authors: Janssens T. V. W. , Niemantsverdriet J. W. , van Santen R. A.
Published: July 2002

A locally lowered surface potential develops around potassium atoms on a transition metal surface. These potentials are accessible experimentally by measuring the ultraviolet photoemission spectra of adsorbed noble gases. We determine the potassium-induced change in electrostatic potential on K/Rh clusters on the basis of calculated electronic structures using density functional theory. It appears that the calculated potentials depend critically on the geometry of the cluster and may differ substantially from the experimentally observed values. Clusters that allow a substantial charge redistribution around the potassium atoms give the best results. Another improvement is obtained from adding the contribution of potassium atoms further away from the potential. We also present theoretical evidence that the potassium-induced change in ionization energy of Xe, Kr, and Ar on a Rh15K3 cluster is mainly determined by the change in electrostatic potential. This provides support for the use of photoemission of adsorbed noble gases as a probe for changes in local surface potential induced by adsorbed species.
Adiabatic capture models for fast chemical reactions
Authors: Ramillon M. , McCarroll R.
Published: July 2002

It is shown that the adiabatic capture centrifugal sudden approximation and the statistical adiabatic channel model for the determination of state selected reactive rate constants in atom-molecule collisions are identical.
Nuclear spin-lattice relaxation dispersion and segment diffusion in entangled polymers. Renormalized Rouse formalism
Authors: Fatkullin N. , Kimmich R.
Published: July 2002

A formalism for polymer melts was derived linking the spin-lattice relaxation time T1, the correlation function of chain tangent vectors and the mean-square segment displacement with memory functions. Potential normal-mode number dependences are included. In the limit of infinitely fast decaying memory functions the theory reproduces known expressions characteristic for Rouse dynamics. Interchain excluded-volume forces were taken into account in the frame of the renormalized Rouse approach [K. S. Schweizer, J. Chem. Phys. 91, 5802 (1989)]. The power law limits predicted on this basis are T1∝ω1/2, T1∝ω1/4, and T1∝ω1/5 for the T1 dispersion in a sequence of regimes from high to low frequencies. The mean-square segment displacement obeys 〈r2〉∝t1/4, 〈r2〉∝t3/8, and 〈r2〉∝t2/5 in a sequence of limits for increasing times. The spin-lattice relaxation dispersion of different polymers was studied mainly by the aid of the field-cycling NMR technique. The covered proton frequency range is less than 103 Hz to more than 108 Hz. The frequency dependence can be described by a series of power laws arising from chain dynamics. Two of these, namely T1∝ω0.5 and T1∝ω0.25 tending to appear at high and low frequencies, respectively, can be perfectly explained on the basis of the derived renormalized Rouse limits. The third power law, T1∝ω0.44, which was observed only at rather low frequencies, has no theoretical counterpart in the frame of the renormalized Rouse theory. Some hints that farther reaching polymer theories such as the mode–mode coupling approach [K. S. Schweizer, J. Chem. Phys. 91, 5822 (1989)] can help to understand this finding are discussed.
An approach for improved variational quantum Monte Carlo
Authors: Sun Zhiwei , Soto M. M. , Barnett Robert N. , Lester William A.
Published: July 2002

Sampling of core electrons in Monte Carlo approaches to electronic structure is a major bottleneck to efficient studies of large molecules. To overcome this problem, we propose an improved Metropolis algorithm for variational Monte Carlo which includes the second derivatives (Hessian matrix) of the pseudopotential 𝒫=−ln‖ΨT‖2 in its transition probability in addition to the commonly used first derivatives (or quantum force). To minimize computational effort, we use only the diagonal elements of the Hessian matrix, which are readily obtained from information already available in the Monte Carlo computation. We analyze the effect of these diagonal terms on the transition probability and core-electron sampling. The approach automatically reduces the step sizes of the innermost electrons and does not require further considerations such as choice of coordinate system or assignment of electrons to specific shells. In addition, heteronuclear molecules pose no difficulty for the present algorithm. Application of the method to representative systems, Ne, Ar, and KCl, has shown that it increases the acceptance ratio of the innermost core electrons by a factor of 5 over previous algorithms.
Method for the calculation of the vibrational frequency shift of physisorbed molecules. Application to H2 adsorbed in NaA zeolite
Authors: Larin A. V. , Cohen De Lara E.
Published: July 2002

The vibrational frequency shift of physisorbed diatomic molecules is related to the interaction with the adsorbent expressed in terms of the internuclear distance ρ. It is calculated by the Schrödinger equation, the perturbation theory, and a simplified method. We show that it is sufficient to calculate the interaction potential for the values of ρ in the ground and first vibrational states in order to get a precision of 10% on the frequency shift. The comparison between the theoretical and experimental frequency shift of H2 adsorbed in NaA zeolite is used to adjust the interaction potential, especially in terms of the ionicity of the crystal.
‘‘Confined’’ water and hydrophobic attraction as a result of metastable coordination, stabilized by hydrophobic surfaces
Authors: Gompper G. , Hauser M. , Kornyshev A. A.
Published: July 2002

The hydrophobic hydration and interaction between hydrophobic surfaces are treated as a ‘‘wetting phenomenon’’ in terms of a phenomenological Landau–Ginzburg approach. The model is based on the assumption that the breakdown of hydrogen bonds at a hydrophobic wall can stabilize a layer of four-coordinated water near the surface. The theory predicts the formation of more structured, four-coordinated, confined water between two hydrophobic surfaces, when the two layers overlap. A peculiar shape of the disjoining pressure isotherm follows from this picture, including exponential attraction at short and long distances (with longer decay length at short distances), a plateau in between, ended by a jump (first order transition) to the exponential decay at large distances.
Alignment probing of Rydberg states by stimulated emission
Authors: Spain Eileen M. , Dalberth Mark J. , Kleiber Paul D. , Leone Stephen R. , Op de Beek Stefan S. , Driessen Jan P. J.
Published: July 2002

The possibility of probing the collisions of aligned Rydberg atoms by stimulated emission is assessed with studies of a polarized state and a new measurement of a collisional alignment effect in atomic Ca. The stimulated emission method uses a laser to dump the desired state to a lower level which subsequently fluoresces. The technique can be used to obtain populations and polarization dependent information. First, the method is tested by applying it to an aligned Ca(4s17d 1D2) state. Alignment curves are measured when the initial state is prepared with both parallel and perpendicular relative polarizations. The experimentally observed alignment compares well with that derived from theoretical considerations of a saturated stimulated transition. Second, a two-vector collisional alignment experiment (initial state and relative velocity vector) is performed to study the energy transfer process Ca(4s7d 1D2)+He→Ca(4s6f 1F3)+He+ΔE=17.7 cm−1, and alignment effects are measured by both stimulated emission and conventional direct fluorescence detection. A preference for the ‖m‖=1 and 2 initial states is observed in the relative cross sections. Essentially identical data are obtained with the two detection methods when elliptically polarized light is used for the stimulated emission detection method. The stimulated emission technique can provide alignment and population information of the final states, making it an excellent new tool for both three-vector correlation experiments and state-to-state Rydberg transitions.
Vibrational numbering and potential of the c 3Σ+ state of NaK determined from the c 3Σ+→a 3Σ+ transition
Authors: Kowalczyk P. , Sadeghi N.
Published: July 2002

We report the observation of the c 3Σ+→a 3Σ+ emission in NaK following excitation of single rovibronic levels in the c 3Σ+ state. The dispersed fluorescence displays both diffuse and discrete features. The characteristic reflection structure of the bound–free spectra permits a direct, unambiguous assignment of the vibrational numbering in the c 3Σ+ state: The v=20 level is the first vibrational level of c 3Σ+ lying above v=0 in the B 1Π state. The c 3Σ+ state potential curve is determined from bound–free parts of the observed spectra with the inversion procedure of LeRoy et al. [J. Chem. Phys. 89, 4564 (1988)]; its most important parameters are Te=15 857±15 cm−1 and Re=0.445±0.001 nm.
Application of pair correlation theory to positronium compounds
Authors: Saito Shiro L. , Sasaki Fukashi
Published: July 2002

Extension of weak orthogonality (WO) techniques developed by Szalewicz et al. and coupled pair equations derived by Jeziorski et al. to positronium compounds are presented. These methods enable us to calculate many-electronic positronium compounds with explicitly correlated functions. These methods were applied to positronium hydride (PsH) with Hylleraas-type functions (HTF’s), and the total energies, the positron-electron two-photon annihilation rates, and the positronium binding energies were calculated. Extended coupled pair equations were solved by CEPA(2)-type, complete coupled pair (CCP) procedures, and independent pair approximation (IPA) of CCP. For the total energies, IPA, CEPA(2), and CCP procedures gave −0.78899, −0.78238, and −0.77471 au, respectively. For the two-photon annihilation rates, the respective procedure gave 2.088, 2.064, and 1.972 ns−1, respectively.
Avoided crossings in potential curves of BF2+: A study of models for bonding in diatomic dications
Authors: Kolbuszewski Marcin , Wright James S. , Buenker Robert J.
Published: July 2002

An ab initio study of the potential energy curves for BF2+ is reported. The 1Σ+ state is metastable and is characterized by an unusually strong bond with a well depth of 6.06 eV. The origin of this bond is clearly traced to the interaction between the strongly bonding configuration B2+ + F and the repulsive configurations B++F+. Bound metastable states are also present for 1Π and 3Π manifolds. Dipole moments are used to assign types of bonding in each region of internuclear distance for the most important electronic states. An approximate diabatic representation is given for the two low-lying states derived from the 1Σ+ adiabatic curves. This study confirms the utility of an avoided crossing-diabatic coupling plus polarization model for systems of intermediate polarity.
Vibrational relaxation in the NO+–He collision system: Implication of the Gislason–Ferguson model
Authors: Zenevich V. A. , Lindinger W. , Pogrebnya S. K. , Cacciatore M. , Billing G. D.
Published: July 2002

The rate constants for vibrational relaxation of NO+ in collisions with He are calculated using the semiclassical (classical path) approach of Billing and the interaction potentials, constructed by us on the base of the model of Gislason and Ferguson. An encouraging agreement of the theoretical results with near thermal selected ion flow drift tube data is achieved. The role of the anisotropy of the interaction potential in the vibrational relaxation process is also investigated.
Validation of self-consistent hybrid density functionals for the study of structural and electronic characteristics of organic π radicals
Authors: Adamo Carlo , Barone Vincenzo , Fortunelli Alessandro
Published: July 2002

Extensive density functional calculations are reported for the geometrical structures, thermochemistry, infrared, and hyperfine parameters of representative carbon-centered π radicals. Local functionals can be considered sufficient for geometrical and vibrational parameters, but seriously fail in the computation of thermochemical data and of spin-dependent properties. Gradient corrections (especially Becke exchange and Lee–Yang–Parr correlation functionals) sensibly improve matters. Inclusion of some Hartree–Fock exchange in a fully self-consistent density functional implementation delivers a further significant improvement, approaching the accuracy of the most refined post Hartree–Fock computations. Purposely tailored basis sets are also introduced which are small enough to be used in molecular computations, but still give high quality geometries and hyperfine coupling constants.
Temperature programmed desorption of molecular hydrogen from a Si(111) surface: Theory and experiment
Authors: Flowers Michael C. , Jonathan Neville B. H. , Liu Yong , Morris Alan
Published: July 2002

New experimental temperature programmed desorption (TPD) data have been obtained under carefully controlled conditions for atomic deuterium on single crystal Si(111). A wide range of initial coverages from Θ=1.25 to 0.05 ML was used. It was found that the results could only be satisfactorily interpreted in terms of a two-site adsorption model in which it is suggested that two formally second-order reactions involving the monohydride (deuteride) contribute to the well-known β1 desorption peak at ≊810 K with the relative importance of these two reactions changing with initial coverage. The pre-exponential factors for these reactions were found to be 2 cm2 s−1 and 1 cm2 s−1 with corresponding activation energies of 57.5±2 kcal mol−1 and 56.5±2 kcal mol−1, respectively, for deuterium desorption when the energy difference between the two sites was taken to be 2.5 kcal mol−1. The other desorption channel (β2) was also found to exhibit second-order kinetic behavior involving the dihydride (deuteride). In this case the pre-exponential factor was determined as 0.5 cm2 s−1 and the activation energy as 46.2±2 kcal mol−1. Some suggestions are made concerning the reaction mechanisms and transition states for these desorption processes.
Grain size control in polycrystalline colloidal solids
Authors: Palberg Thomas , Mönch Wolfgang , Schwarz Jürgen , Leiderer Paul
Published: July 2002

Recent experiments on the static and dynamic properties of polycrystalline colloidal solids show a pronounced influence of morphological details. Here we investigate several possibilities to vary systematically one key morphological parameter, namely the average crystallite radius rc of polycrystalline solids. We report measurements of rc as observed by microscopy in well-characterized Yukawa model suspensions. The pair energy of interaction is systematically varied through precise experimental adjustment of the suspension parameters packing fraction Φ, number of ionic surface groups N, and concentration of screening ions c. The average size is found to systematically decrease with increasing interaction. At fixed suspension parameters we performed solidification under shear, i.e., in the presence of alternating electric fields. We report preliminary results in dependence on both the electric field strength and frequency. The grain size increases with increasing shear rates. It shows a complex behavior as a function of the frequency and the wave form of the applied field. Qualitative explanations are discussed and a first application is presented.
The stress tensor in colloidal suspensions
Authors: Strating P.
Published: July 2002

This paper presents a new derivation of an expression for the average stress tensor in colloidal suspensions of interacting particles, based on methods from statistical mechanics. In conventional methods all contributions to the stress tensor are added one by one to compose the final result, and each contribution requires a separate justification. Especially hard sphere interaction potentials between the colloidal particles in combination with Brownian motion present conceptual difficulties. The method presented here is advantageous in two ways. First, the hard sphere interaction potential is treated the same as any other potential, yielding direct hard sphere contributions in a very simple way and second, all contributions to the stress tensor are present from the start, eliminating the need of superposition of contributions to the stress tensor. Thus familiar contributions to the stress tensor are recovered, which for hard spheres coincide with the well-known results of Batchelor [G.K. Batchelor, J. Fluid Mech. 83, 97 (1977)], and less obvious, those of Brady [J.F. Brady, J. Chem. Phys. 99, 567 (1993)], obtained with different methods. Furthermore, a calculation is presented that produces the direct hard sphere contribution for arbitrary densities within the framework of the conventional theories of the stress tensor. The two methods of calculating the same direct hard sphere contribution are clarifying when discussing the interpretation of the hard sphere property of the colloidal particles as Brownian motion in a restricted configuration space, or as a normal short range repulsive pair potential.
Electronically diabatic quantum dynamics of molecular desorption
Authors: Bekšić Dario , Micha David A.
Published: July 2002

The photodesorption of a diatomic from a metal surface, following absorption of visible or UV light, involves electronic transitions of the desorbing species coupled to the lattice vibrations and electron–hole excitations of the substrate. We present a general treatment of these phenomena, based on the Liouville–Von Neuman equation for the density operator, and a stochastic theory of localized perturbations in an extended system. The Hamiltonian of the extended molecular system is divided into a term for the localized primary degrees of freedom (DFs) affected by the desorption, coupled to secondary DFs that acts as a time-evolving bath. A self-consistent field treatment gives an effective (non-Hermitian) Hamiltonian for the primary DFs that accounts for energy fluctuation and dissipation in terms of the properties of adsorbate and substrate. A diabatic electronic representation is used to eliminate momentum couplings between adsorbate electronic states. The bath dynamics is studied for lattice vibrations and for electronic excitations. Electron–hole excitations of the substrate are considered for intraband and interband transitions. The assumption of Brownian motion leads to expressions for the dissipative potentials in terms of the time-correlation functions of lattice displacements and of electron density fluctuations. The dissipation depends on time, allowing for time-dependent substrate temperatures and generalizing the Langevin description. Dissipation contributes to the time evolution of both ground and excited electronic states of the desorbing species. The model is discussed for the special case of Ni(001)(ads)CO.
Accurate ab initio potential energy functions for doubly charged diatomics
Authors: Bennett Frederick R.
Published: July 2002

Potential energy functions for a set of experimentally characterized electronic states of NO2+ and N2+2 have been calculated using a series of correlation consistent basis sets ranging from cc-pVDZ through to cc-pV5Z. By exploiting the convergent properties of the correlation consistent basis sets, complete basis set limit approximations of each calculated point on the potential energy surfaces can be extrapolated in a simple fitting procedure. The resultant complete basis set approximated potential energy functions are able to yield very accurate spectroscopic constants outperforming the pure ab initio potentials in reproducing experimentally determined data.
A reduced model of short range interactions in polypeptide chains
Authors: Kolinski Andrzej , Milik Mariusz , Rycombel Jakub , Skolnick Jeffrey
Published: July 2002

A simple model of short range interactions is proposed for a reduced lattice representation of polypeptide conformation. The potential is derived on the basis of statistical regularities seen in the known crystal structures of globular proteins. This potential accounts for the generic stiffness of polypeptides, the correlation between peptide bond plates, and the sequence dependent correlations between consecutive segments of the Cα-trace. This model is used for simulation of the equilibrium and dynamic properties of polypeptides in the denatured state. It is shown that the proposed factorization of the local conformational propensities reproduces secondary structure tendencies encoded in the protein sequence. Possible applications for modeling of protein folding are briefly discussed.
Xen clusters in the alpha cages of zeolite KA
Authors: Jameson Cynthia J. , Jameson A. Keith , Gerald Rex E. , Lim Hyung-Mi
Published: July 2002

We have observed the individual signals of the Xen clusters (n=1–5) trapped in the alpha cages of zeolite KA. The 129Xe NMR chemical shift of each cluster in zeolite KA is larger than that of the corresponding Xen cluster in zeolite NaA. The temperature dependence of the chemical shifts of the clusters vary systematically with cluster size as they do in NaA, but the change of the temperature coefficients with n is somewhat more pronounced for Xen in the cages of KA than in NaA. The Xen chemical shifts and their variation with temperature are reproduced by the grand canonical Monte Carlo (GCMC) simulations. GCMC simulations of the distribution of the Xe atoms among the alpha cages in KA provide the fractions of cages containing n Xe atoms which agree reasonably well with the observed equilibrium distributions. The characteristics of Xe distribution and chemical shifts in KA are compared with that in NaA.
Using laser pulse dynamics to probe the relaxation of an anisotropic velocity distribution of excited iodine
Authors: Nicholson J. W. , Rudolph W. , Hager G.
Published: July 2002

The pulse characteristics of a photolytic, gain-switched iodine laser are a sensitive measure for relaxation processes that occur in the active medium. On a time scale shorter than a few microseconds this concerns the relaxation of the speed and the angular parts of the anisotropic velocity distribution which is produced by the photodissociation of the parent molecule CF3I. A comprehensive rate equation model of the laser dynamics is fit to experimental data, to obtain time constants for these collision-controlled relaxation processes for various buffer gases and pressures.
A Monte Carlo simulation study on the collapse transition of model polymers: Possible solvent effect and relevance to protein folding
Authors: Sen Srikanta
Published: July 2002

A clear theoretical understanding of the protein folding problem both in kinetic as well as in thermodynamic aspects, has not yet been achieved. In this context, we have studied here the possible role of solvent on the equilibrium collapse transition characteristics of a polymer that serves as a simple model protein chain. We have assumed the standard bead–stick representation of the polymer where each bead possesess physical properties similar to that of the amino acids of the real proteins. The equilibrium transition was studied by Metropolis Monte Carlo simulation of the chain on a simple cubic lattice. We have presented tentative arguments in favor of considering the stability factor of a nearest neighbor bead–bead pair to be temperature dependent, on the basis of a change of the order of water molecules in the immediate vicinity of the protein molecule. We have considered two different stability schemes; (i) temperature dependent and (ii) temperature independent and have shown that only in the case of temperature dependent stability scheme, there may be a cooperative collapse transition in the case of a model chain which is very similar to that observed in real experiments, while, in the other case, there is practically no cooperative transition over the same temperature range. The transition has been described by the evolution of two different physical quantities (i) the average number of bead contacts (N̄c) and (ii) the average rms radius (R̄rms). In both the cases cooperative transitions have been observed for the temperature dependent stability factor. The results show that the cooperativity in such transitions might be, at least partially, a consequence of the temperature dependent property change of the aqueous solvent. We have also studied the chemical denaturant induced transition of model chain in a similar framework. The results presented here are mainly on homopolymers.
Resonant vibrational excitation in high-resolution electron energy-loss spectroscopy studies of trimethylamine chemisorbed on GaAs(100)
Authors: Mulcahy C. P. A. , Aquino A. A. , Rogers J. J. , Jones T. S.
Published: July 2002

High-resolution electron energy-loss spectroscopy has been used to study the vibrational properties of trimethylamine [(CH3)3N] chemisorbed on GaAs(100) at room temperature. A number of vibrational modes of the adsorbate, involving both the C–H and C–N bonds, are preferentially excited via a resonance scattering mechanism involving the formation of a temporary negative ion due to capture of the incident electrons. The resonance, which is centered around 6 eV, is characterized by a large enhancement in the intensity of several fundamental vibrations and the appearance of a number of combination and overtone bands. Detailed energy-dependent studies and comparison with previous electron scattering studies of related gas phase molecules suggest that electron capture can occur in three molecular orbitals of the adsorbed molecule. By considering the electron density distributions obtained from semiempirical calculations of the isolated molecule, an assignment of the vibrational modes which are selectively excited due to occupation of a specific molecular orbital has been attempted.
Electronic absorption spectra of molecules and aggregates with interatomic charge transfer using a normal mode treatment of the atomic monopole–dipole interaction model
Authors: Shanker Balasubramaniam , Applequist Jon
Published: July 2002

The theory of the atom monopole–dipole interaction model for electronic polarizability is extended to the case of complex frequency-dependent atom polarizabilities, allowing the prediction of absorption spectra in terms of a set of electronic normal modes. The charge constraints on the system are taken into account using Lagrangian and penalty function methods. Expressions are obtained for the complex polarizability of an aggregate of molecules in which intermolecular charge transfer is precluded. The results are illustrated by calculations for naphthalene using atom polarizability parameters which give an approximate fit to the experimentally observed absorption spectrum. We also treat helical stacked aggregates of naphthalene molecules and predict intensity shifts analogous to those observed in the stacked aromatic bases of helical nucleic acids.
Formation of hydrogen bonded aggregates of aminomethylidyne on Pt(111)
Authors: Jentz D. , Mills P. , Celio H. , Belyansky M. , Trenary M.
Published: July 2002

Reflection absorption infrared spectroscopy (RAIRS) and x-ray photoelectron spectroscopy (XPS) were used to study the aggregation of aminomethylidyne (CNH2) on Pt(111). The molecule exists in three distinct environments: an isolated molecule at low coverages, large aggregates at saturation coverage, and small aggregates in coexistence with both large aggregates and isolated molecules at intermediate coverages. The molecules in each environment have distinctly different ir spectra. Correlation of the ir spectra with a particular environment was supported by Xe coadsorption experiments. The XPS results suggest that the interactions between CN and CNH2 are not responsible for the effects observed in the ir spectra, rather they are due to CNH2–CNH2 interactions. The ir spectra show the characteristic features of hydrogen bonding, which is proposed to be the origin of the attractive adsorbate–adsorbate interaction. Interpretation of the ir spectra was supported through normal mode calculations of both the CNH2 monomer and a hydrogen bonded CNH2 dimer.
Two-photon resonance enhanced multiphoton ionization photoelectron spectroscopy of the SH (SD) radical below and above the lowest ionization threshold
Authors: Milan J. B. , Buma W. J. , de Lange C. A.
Published: July 2002

A two-photon resonance enhanced multiphoton ionization spectroscopic study on the mercapto radical is carried out in the one-photon energy region between 258 and 208 nm. Thirteen previously unobserved Rydberg states converging upon the a 1Δ or b 1Σ+ excited ionic states are reported. Identification and characterization of several states with excitation energies below the lowest ionization limit are performed by analyses of the rotationally resolved two-photon excitation spectra, polarization studies, and photoelectron spectroscopy after excitation of the states. Detailed comparisons between the experimental and simulated excitation spectra, which show for three states clear indications of intensity anomalies, as well as the observation of multiple ionization channels demonstrate that for all states Rydberg–Rydberg and/or Rydberg–valence interactions determine the finer details of their electronic wave functions. Above the lowest ionization limit two-photon resonance enhancement is observed for six Rydberg states. These states are found to ionize by autoionization, rather than by absorption of a third photon. Remarkably, rotationally well-resolved excitation spectra can still be obtained for two of these states. Comparison of the dominant electronic configuration of the six Rydberg states allows us to explain these differences in terms of the possible autoionization channels.
Identification of two 3Σu−←X3Σg− transitions of 16O2 near 88930 and 90780 cm−1
Authors: England J. P. , Lewis B. R. , Ginter M. L.
Published: July 2002

New 16O2 photoabsorption cross-section measurements are presented for two bands near 88930 cm−1 and 90780 cm−1. We have assigned these bands as the (0,0) and (1,0) bands of the E′3Σu−←X3Σg− system, where the E′ state is a mixed state resulting from the coupling of 4pπu and 5pπu 3Σu− Rydberg states with the lowest 3Σu− valence state. In contrast to the E3Σu−←X3Σg− bands seen at lower energies, these bands show resolved rotational structure. Spectroscopic parameters for the upper levels have been derived using an effective Hamiltonian and cross-section band models. The observed levels are perturbed by spin-orbit interactions with nearby levels (v=1 and 2) of the 4pπu D′3Σu+ Rydberg state. In addition, transitions to the v=2 level of the perturber state have been observed near 90920 cm−1.
Magnetic circular dichroism of Na3Nd(ODA)3⋅2NaClO4⋅6H2O
Authors: Fluyt L. , Couwenberg I. , Lambaerts H. , Binnemans K. , Görller-Walrand C. , Reid M. F.
Published: July 2002

This paper emphasizes the complementarity between optical and magneto-optical measurements on f elements. The magnetic circular dichroism, circular dichroism, and polarized absorption spectra of Na3Nd(ODA)3⋅2NaClO4⋅6H2O (NdODA), have been recorded at 4.2 K between 5000 and 30 000 cm−1. The electric dipole contributions to the f–f transitions are analyzed using a parametric model which contains 12 parameters which are determined by fitting the calculated to the empirical dipole strengths of 50 α and 16 π polarized absorption lines. The reliability of these intensity parameters is tested by simulating the more sensitive magneto-optical (magnetic circular dichroism) and chiroptical (circular dichroism) spectra.
Molecular orientational vibrations and thermodynamics in monolayers of chain molecules
Authors: Wang Zhenghao , Gong Changde
Published: July 2002

We study the molecular orientational vibrations (MOVs), using analytical method, in Langmuir (Langmuir–Blodgett) monolayers. At the present stage of our study, we fix the hydrophilic heads of the amphiphilic molecules and model the hydrophobic alkane chains as rigid rods. We have considered different tilt orders in our study and self-consistently determined the tilt angle at various surface pressures and temperatures. The study of these MOVs leads us to a breakthrough in the analytical theory for monolayers, with which we are enabled to analytically investigate a wide range of thermodynamical properties, such as angular correlation functions and specific heat, and to calculate structural functions in scattering experiments. As to MOV itself, a universal frequency gap at k=0 exists (except at the upright-tilted phase transition point). This frequency gap is closely related to the upright-tilted phase transition as we explain in the text. Comparison of our results with those from previous Monte Carlo simulations is made and shows good agreement.
Normal to tangential velocity conversion in cluster-surface collisions: ArN on graphite
Authors: De Martino A. , Benslimane M. , Châtelet M. , Pradère F. , Vach H.
Published: July 2002

Collisions between large neutral argon clusters and a pyrolytic graphite surface have been investigated at normal incidence by measuring angular distributions of density and angularly resolved time-of-flight distributions of scattered species (essentially monomers). These data have been taken for two surface temperatures (440 and 660 K) with cluster sizes N ranging from 400 to 8200 atoms per cluster, and at an incoming cluster energy of 72 meV/atom. A higher incident kinetic energy (115 meV/atom) has been investigated also for N=900 atoms per cluster. The experimental results have been analyzed by considering two components. First, an ‘‘ejection’’ contribution, dominant at large scattering angles, has been assigned to the evaporation of monomers from cluster material having acquired a flow velocity parallel to the surface. The average value of this flow velocity increases sharply with the incident cluster velocity in the investigated range. Second, a thermal contribution, dominant close to the surface normal, has been assigned to the trapping-desorption of monomers on the graphite surface for the smallest values of N, and to direct evaporation from the cluster when N increases.
Tunneling dynamics, symmetry, and far-infrared spectrum of the rotating water trimer. II. Calculations and experiments
Authors: Olthof E. H. T. , van der Avoird A. , Wormer P. E. S. , Liu Kun , Saykally R. J.
Published: July 2002

With the Hamiltonian derived in the preceding paper and the ab initio potentials of T. Bürgi, S. Graf, S. Leutwyler, and W. Klopper [J. Chem. Phys. 103, 1077 (1995)] and of J. G. C. M. van Duijneveldt-van de Rijdt and F. B. van Duijneveldt [Chem. Phys. Lett. 237, 560 (1995)], we calculate the pseudo-rotation tunneling levels in a rotating water trimer. The internal motions are treated by a three-dimensional discrete variable representation and the Coriolis coupling with the overall rotation is included. Also the effects of donor tunneling are included, by introducing semi-empirical coupling matrix elements. New experimental data are presented for the c-type band at 87.1 cm−1 in (H2O)3, which show that specific levels in the donor tunneling quartets of this band are further split into doublets. With the results of our quantitative calculations and the model of the preceding paper we can understand the mechanisms of all the splittings observed in the earlier high-resolution spectra of (H2O)3 and (D2O)3, as well as these new splittings, in terms of pseudo-rotation tunneling, donor tunneling and Coriolis coupling. An unambiguous assignment is given of all the bands observed and analyzed. The ab initio potential of the Van Duijneveldts yields accurate energies of the lower pseudo-rotation levels, the potential of Bürgi et al. performs better for the higher levels. With our analysis we can deduce from the spectra that donor tunneling involves inversion of the trimer.
The role of nonadditive effects in the first solvation shell of Na+ and Mg2+ in liquid ammonia: Monte Carlo studies including three-body corrections
Authors: Hannongbua Supot
Published: July 2002

The solvation structure of Na+ and Mg2+ in liquid ammonia has been investigated using the Metropolis Monte Carlo scheme. Simulations have been carried out for four systems namely, one Na+ in 201 NH3 and one Mg2+ in 201 NH3 with and without three-body corrections. With the experimental density of pure liquid ammonia at 277 K and 1 atm of 0.9893 g cm−3, a periodic cubic volume of side length 20.99 Å was yielded. The Na+–NH3 and Mg2+–NH3 pair potentials as well as the corresponding three-body correction functions have been newly developed based on ab initio calculations with a basis set of double zeta quality. The results indicate the failure of the pair approximation in predicting structural properties of both systems: the three-body corrections reduce the first shell coordination number of Na+ and Mg2+ in liquid ammonia from 9 to 8 and from 10 (two of them lying in small peak beyond the first maximum) to 6, respectively.