bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetic investigation of the quenching of Mg(3s3p 1P1) atoms in collisions with CH4 over the temperature range from 660 to 850 K
Authors: Nien Chia-Fu , Lin King-Chuen
Published: July 2002

Temperature dependence of the quenching efficiency of Mg(3s3p 1P1) by CH4 collision in the Ar bath gas has been studied over the temperature range of 660–850 K, using a pump–probe technique with time-resolved laser-induced fluorescence (LIF) as detection. The obtained thermal rate coefficients are attributed to physical and chemical quenching. The former contribution is evaluated to be less than 6.7×10−11 cm3 molecule−1 s−1, while the latter one is (0.59−1.12)×10−11 exp(5.75–6.54 kcal mol−1/RT) cm3 molecule−1 s−1 (R is gas constant; T is temperature). The chemical reaction dominates over the exit channels, especially in the low temperature. The negative temperature dependence indicates the existence of a depth-well intermediate. The observed depth-well energy below the reactants is about twice as small as that evaluated theoretically. This bound state is anticipated to locate in the region of surface crossing between the excited and the ground states. We adopted several models to account for the kinetic data as a function of temperature. The orbiting and absorbing-sphere models cannot be validly applied to the current system. The angle-dependent line of normals (ADLN) model takes into account the angular dependence of the threshold energy. The ADLN fit to the measured kinetic data is consistent with the insertion mechanism reported previously, which shows energetic preference to the end-on attack.
Laser ablation of graphite in an oxygen jet
Authors: Park Seung Min , Moon Ji Youn
Published: July 2002

Mass spectroscopic studies on a reactive laser ablation of graphite at 1064 nm in a pulsed oxygen jet have been performed. Significant changes in the relative enrichment of carbon ions including anomalous enrichment of C2+ ions as well as formation of CO+, C2O+, and C3O+ ions are observed. Collision-induced dissociation and reactive scattering of carbon ions prevail by injection of the oxygen jet to the carbon plume. Energetics of the ions is studied by using a time-of-flight quadrupole mass spectroscopy. Time-of-flight (TOF) spectra of each carbon ion consist of a fast and a slow component, which are considered to represent different formation mechanisms. By analyzing the changes of TOF spectra as a function of the delay time between the laser pulse and oxygen jet, we examine the effects of collisions on the formation of carbon ions.
Quasiclassical dynamics of benzene overtone relaxation on an ab initio force field. I. Energy flow and survival probabilities in planar benzene for CH(v=2,3)
Authors: Minehardt Todd J. , Wyatt Robert E.
Published: July 2002

Computational studies based upon ensembles of quasiclassical trajectories are presented for 21-mode (planar) benzene, with which we examined relaxation of the first and second (v=2,3) CH stretch overtones. The most complete ab initio force field available for this molecule was used for short time (t<240 fs) and long time (t=2.4 ps) dynamical calculations. Local mode analyses indicate that energy flow from the initially excited CH stretch is fast (t<240 fs) and irreversible. For both overtones examined, energy from the CH chromophore activates the following local modes in sequence: ipso in-plane CH wag, adjacent CC stretch, terminal CC stretch, ortho CH in-plane wag, meta CH in-plane wag, intermediate CC stretch, meta CH stretch, para CH in-plane wag, and para CH stretch. Long time calculations indicate that the in-plane CH wag and CH stretching local modes are at steady-state quantities by 2.4 ps, while the CC stretches and CCC bends continue to take on energy. The ortho and meta CH stretches were of minor importance in intramolecular vibrational energy redistribution in benzene, gaining little or no energy over the time scales we studied. Survival probabilities indicate that the half life of the excited CH stretch decreases by almost two thirds as the overtone energy is raised. The results of our study are compared to other classical and quantum dynamical investigations of benzene and are found to be in good qualitative agreement.
Early stages of dewetting of microscopically thin polymer films: A molecular dynamics study
Authors: Liu Hong , Bhattacharya Aniket , Chakrabarti Amitabha
Published: July 2002

We present results from a molecular dynamics simulation of the dewetting process of microscopically thin polymer films cast on a solid surface. The chain length and time scales accessed in the present simulation would correspond to early stages of the dewetting process before hydrodynamic interactions become important. Even though the surface in contact with the film is purely repulsive, we find that there is a “nucleation time” for dry patches to appear due to the competition between the interfacial energy and the surface energy contributions. Kinetics of growth of dry patches during the early stages of the dewetting process is controlled by surface diffusion of polymers.
Multidimensional wave packet dynamics within the fluid dynamical formulation of the Schrödinger equation
Authors: Dey Bijoy K. , Askar Attila , Rabitz H.
Published: July 2002

This paper explores the quantum fluid dynamical (QFD) representation of the time-dependent Schrödinger equation for the motion of a wave packet in a high dimensional space. A novel alternating direction technique is utilized to single out each of the many dimensions in the QFD equations. This technique is used to solve the continuity equation for the density and the equation for the convection of the flux for the quantum particle. The ability of the present scheme to efficiently and accurately describe the dynamics of a quantum particle is demonstrated in four dimensions where analytical results are known. We also apply the technique to the photodissociation of NOCl and NO2 where the systems are reduced to two coordinates by freezing the angular variable at its equilibrium value.
Binary nucleation of water–sulfuric acid system: A reexamination of the classical hydrates interaction model
Authors: Noppel Madis
Published: July 2002

It is shown that the hydration correction for the free energy of cluster formation in the classical binary nucleation theory is not applicable for the thermodynamic data of the water–surface acid system commonly used in nucleation calculations. A new form of the hydration correction is presented. For the commonly used thermodynamic data the nucleation rates of the new hydration correction are in the temperature range of 248–323 K, and at a relative humidity greater than 40%, 103–106 times lower, respectively, than the nucleation rates of the former theory. The predictions of acid–water nucleation rates of the thermodynamically consistent version of the classical binary homogeneous theory [G. J. Wilemski, Chem. Phys. 80, 1370 (1984)] with the new hydration correction are in accordance with experimental results [B. E. Wyslouzil, J. H. Seinfeld, R. C. Flagan, and K. J. Okuyama, Chem. Phys. 94, 6842 (1991)] obtained at 298 K and with experimental results [D. Boulaud, G. Madelaine, D. Vigla, and J. Bricard, J. Chem. Phys. 66, 4854 (1977)] obtained at 293 K, but at a given number concentration of acid and relative humidity, they predicted nucleation rates are rather more stronger functions of temperature than the experimental rates. The theoretical nucleation rate of water–sulfuric acid vapor is due to the hydration correction sensitive to the thermodynamic properties of dimers and timers.
S 2p excitation and fragmentation of sulfur aggregates
Authors: Teodorescu C. M. , Gravel D. , Rühl E.
Published: July 2002

Synchrotron radiation from the storage ring BESSY-I is used to excite the S 2p regime (150–190 eV) of sulfur aggregates Sn, n=2–8. Valence states are identified in photoion yield spectra of mass selected Sn+ cations in combination with extended Hückel calculations. Mass spectra, partial ion yields, and photoelectron–photoion–photoion (PEPIPICO) coincidence spectra are reported, yielding a consistent picture of massive decay processes occurring in the S 2p regime. PEPIPICO spectra especially give detailed information on the fragmentation mechanisms of doubly charged aggregates. It is shown that most cation pairs are formed via charge separation originating from S8++. We derive values for the kinetic energy releases in multistep fragmentations assuming that charge separation is driven by Coulomb explosion.
Characterization of the surface structure of CH3 and CF3 terminated n-alkanethiol monolayers self assembled on Au{111}
Authors: Houssiau L. , Graupe M. , Colorado R. , Kim H. I. , Lee T. R. , Perry S. S. , Rabalais J. W.
Published: July 2002

Surface elemental and structural characterization of hexadecanethiol and heptadecanethiol (C16 and C17 for short) and 16,16,16-trifluorohexadecanethiol (FC16) self-assembled monolayers (SAMs) on a Au{111} surface have been obtained from time-of-flight scattering and recoiling spectrometry. The clean Au surface was also characterized in order to identify the azimuthal orientation of the SAMs with respect to the substrate. Classical ion trajectory simulations were used to relate the experimental scattering and recoiling data to the surface structure. The scattered and recoiled atoms originate from the outermost five–six atomic layers, and azimuthal anisotropy was observed in the measurements. The results provide a model for the SAMs in which the alkyl chains chemisorb with the S atoms situated above the face-centered-cubic (fcc) threefold sites of the Au{111} surface to form a continuous film with a (√3×√3)R30° structure that fully covers the Au surface. The orientation of the molecular axis azimuth of the SAMs relative to the Au azimuthal directions was determined. The data indicate that the molecular chains have specific tilt and twist angles relative to the Au surface and six coexisting domains resulting from the six equivalent tilt directions of the molecular axis. Dramatic changes in the anisotropic patterns of the ion scattering azimuthal scans from the surfaces of the SAMs with different terminations were observed. These phenomena result from the different tilt angles of the CH3 and CF3 groups. The data are consistent with free rotation of both the CH3 and CF3 groups. The C16 SAM exhibited the best azimuthal features and was more resistant to radiation damage from the incident Ar+ scattering beam than the other films. Due to the tilt angle of the SAMs, an “ion’s eye view” of the structure, i.e., the positions of the atomic cores as experienced by the incoming keV ions, reveals a regular array of sloping cavities within each unit cell.
Near-infrared absorption spectrum of the Ar–HD complex: Confrontation of theory with experiment
Authors: Mrugała Felicja , Moszynski Robert
Published: July 2002

Converged close-coupling calculations of the bound rovibrational levels, positions and widths of metastable states that predissociate rotationally, vibrationally, and via tunneling, transition intensities, and spectrum shape have been performed starting from accurate ab initio and empirical potential energy surfaces for Ar–HD. The computed transitions frequencies agree very well with the observed positions of lines in the recorded near-infrared spectra [A.R.W. McKellar, Faraday Discuss. Chem. Soc. 73, 89 (1982); J. Chem. Phys. 105, 2628 (1996)]. The agreement of the measured and calculated linewidths in the S1(0) band is also good. Surprisingly, the ab initio potential reproduces the observed linewidths somewhat better than the empirical potential fitted to high-resolution infrared data of Ar–H2 and Ar–D2 [C. Bissonette et al., J. Chem. Phys. 105, 2639 (1996)]. As a result of the inclusion of some important transitions between the continuum states of the complex, a very good agreement with experiment is achieved for the shape of the S1(0) band of the spectrum. Reliable theoretical predictions of the transition energies in the Q1(1) band are also reported.
Thermally driven escape over a barrier of arbitrary shape
Authors: Drozdov A. N. , Brey J. J.
Published: July 2002

The Kramers theory for the thermally activated rate of escape of a Brownian particle from a potential well is extended to a barrier of arbitrary shape. The extension is based on an approximate solution of the underlying Fokker–Planck equation in the spatial diffusion regime. With the use of the Mel’nikov–Meshkov result for the underdamped Brownian motion an overall rate expression is constructed, which interpolates the correct limiting behavior for both weak and strong friction. It generalizes in a natural way various different rate expressions that are already available in the literature for parabolic, cusped, and quartic barriers. Applications to symmetric parabolic and cusped double-well potentials show good agreement between the theory and estimates of the rates from numerical calculations.
Pump-probe spectroscopy of H-bonds: Saturation kinetics and spectral transformation during and after pumping
Authors: Burshtein A. I. , Chernobrod B. M. , Sivachenko A. Yu.
Published: July 2002

The saturation kinetics of the H-bond absorption spectra in a strong laser field is described as a level-crossing problem with diffusional motion along the reaction coordinate. The full analogy with kinetics and rate of electron transfer reactions is emphasized when vibrational relaxation to the ground state is negligible. In this case, the transition rate saturates with a field strength reaching the diffusional regime limited by the friction in the ground or excited state. Taking vibrational relaxation into consideration changes qualitatively the kinetics of approaching the stationary level populations and absorption of pumping light. The fast relaxation results in significant desaturation of the light induced transitions and a shortening of the transient time. The absorption spectra of the weak probe light in the context of two-level and three-level models show either loss or gain in the red wing, as well as bleaching and superabsorption in the corresponding transmission spectra. The time evolution of these spectra, after switching the strong field on and off, was studied.
Kinetics of chain aggregation of poly(methyl methacrylate) in isoamyl acetate
Authors: Nakata Mitsuo , Nakagawa Tomohide , Nakamura Yoshiki , Wakatsuki Syogo
Published: July 2002

For dilute solutions of poly(methyl methacrylate) in isoamyl acetate with the molecular weight Mw=2.35×106, the phase separation process was observed as an aggregation process of polymer chains by light-scattering measurements. The aggregation process was measured for a period of hours at four polymer concentrations and at about 15 K below the phase separation temperature. The light-scattering data analysis by Guinier plot yielded the average molecular weight 〈M〉w and radius 〈R2〉z1/2 for polymer aggregates as a function of time t and revealed the exponential growth of 〈M〉w∼egt and 〈R2〉z∼eht. The coefficients g and h were proportional to the polymer concentration. A shape of the observed scattering function was independent of the concentration and time. The fractal dimension for 〈M〉w∼〈R2〉zD/2 was determined to be D=2.86±0.03. These characteristic features of the polymer aggregation were represented by the Smoluchowski equation for cluster–cluster aggregation with the collision kernel (i+j) for imer and jmer. The observed scattering function and fractal dimension were analyzed by the Smoluchowski equation with the assumed value D=3 for monodisperse clusters.
Spectator- and participant-like behavior of a Rydberg electron on predissociation of superexcited states of OCS
Authors: Hikosaka Yasumasa , Hattori Hideo , Mitsuke Koichiro
Published: July 2002

Predissociation of superexcited states of OCS is studied by two-dimensional photoelectron spectroscopy using synchrotron radiation in the photon energy range of 15–16.5 eV. A two-dimensional photoelectron spectrum exhibits two kinds of characteristic patterns both of which are ascribed to autoionization of sulfur atoms. This superexcited atom S* is produced by predissociation of a Rydberg state OCS*(RB) converging to OCS+(B̃ 2Σ+). The pattern of the first kind results from predissociation processes in which the effective principal quantum number n of the Rydberg electron is almost conserved. This suggests that the Rydberg electron behaves as a spectator because of its negligibly weak interaction with the ion core (spectator predissociation). On the contrary, n of S* does not accord with that of OCS*(RB) in the pattern of the second kind, indicating that the Rydberg electron participates directly in the electron exchange mechanism controlling conversion from OCS*(RB) to a predissociating state (participant predissociation). With increasing n, OCS*(RB) decays more preferentially by the spectator than by the participant predissociation. The spectator predissociation of OCS*(RB) proceeds through a two-step conversion which involves Rydberg states converging to OCS+(Ã 2Π and X̃ 2Π) and a dissociative multiple-electron-excited satellite state OCS* (SAT) asymptotically correlating with S*+CO(X̃ 1Σ+). In contrast, the participant predissociation may be accounted for by a direct conversion from OCS*(RB) to OCS*(SAT). The quantum yields are estimated to be 0.06 and 0.02 for the spectator and participant predissociation, respectively, at the incident photon energy of 15.95 eV where OCS*(RB) states with n∼12 lie. A simulation is performed to reproduce the partial cross section curve for the spectator predissociation by using a model in which the decay rates for the participant and spectator predissociation are assumed to be proportional to n−3 and n0, respectively. The simulated and experimental cross section curves are in good agreement with each other in the photon energy range of 15.8–16.04 eV.
Gradients for the partitioned equation-of-motion coupled-cluster method
Authors: Gwaltney Steven R. , Bartlett Rodney J.
Published: July 2002

Gradients for the partitioned equation-of-motion coupled-cluster method based on a second-order many-body perturbation theory ground state have been derived. The accuracy of this new method has been compared to the accuracy of other single reference methods for excited states, namely configuration interaction limited to single excitations (CIS), CIS with a perturbative doubles correction, and equation-of-motion coupled-cluster theory based on a coupled-cluster singles and doubles ground state. These have been compared based on calculations of low lying singlet excited states of H2, BH, BF, C2, CO, and N2, as well as the lowest singlet excited states of ammonia, formaldehyde, acetaldehyde, and acetone. Also, a question about the assignments of the vibrational spectrum for the first excited state of acetone is resolved.
Reactive dynamics for Zn(3P)+H2/D2/HD→ZnH/ZnD+H/D: Rotational populations in ZnH/ZnD products
Authors: Salazar Michael R. , Simons Jack
Published: July 2002

Using ab initio multiconfigurational potential-energy surfaces pertinent to the reaction Zn(3P)+H2→ZnH(2∑)+H and local surface tessellation and interpolation methods developed earlier, we carried out classical trajectory simulations of the title reactions, running large ensembles of trajectories with initial conditions representative of full-collision experiments. Then, using binning techniques, the ZnH/ZnD vibrational and rotational populations have been extracted. Our simulation results duplicate certain unexpected findings noted in experiments of Breckenridge and Wang [Chem. Phys. Lett. 123, 17 (1986); J. Chem. Phys. 87, 2630 (1987)]. Specifically, it is observed that the rotational profiles found for ZnH produced from H2 or from HD are nearly identical, as are the ZnD populations obtained in reactions with D2 or HD. By interrogating the progress of reactive trajectories, we have been able to identify the origin of (much of) the rotational angular momentum in the ZnH/ZnD products, which then allows us to put forth a physical model which, we believe, explains the unexpected ZnH/ZnD isotope effects in the product rotational profiles.
Computer simulation study of the subquadratic quantum number dependence of vibrational overtone dephasing: Comparison with the mode-coupling theory predictions
Authors: Gayathri N. , Bagchi Biman
Published: July 2002

Experimental studies have demonstrated that the vibrational dephasing of overtones do not always follow the quadratic quantum number (n) dependence predicted by the Kubo–Oxtoby theory of vibrational line shapes. While the reason for this failure of the theory is not quite clear yet, a recent theory suggested that the pronounced Gaussian time dependence of the frequency-modulation time-correlation function (tcf) could be a possible reason [Gayathri et al., J. Chem. Phys., 107, 10381 (1997)]. The theoretical study was based on a mode coupling theory calculation of the force–force time-correlation function that is required in the calculation of the frequency-modulation tcf. In order to test this and other predictions of the above study, detailed computer simulations of two neat liquids have been carried out. The systems studied are N–N stretch in liquid N2 and the C–I stretch in CH3I. It is found that although the frequency-modulation time-correlation function is largely Gaussian in both the cases, the overtone dephasing remains largely quadratic in n for N2. For methyl iodide, on the other hand, a pronounced sub-quadratic n dependence has been observed. Both the theory and the computer simulations suggest that this nonquadratic dependence can be expected when not only the decay of the frequency time-correlation function is Gaussian but the time scale of decay of the frequency-modulation tcf is comparable to that of the normal coordinate. The latter can happen when the following conditions are satisfied. First, the frequency of the normal mode should not be too large. Second, the mean-square fluctuation of the frequency-modulation and the anharmonicity coefficient of vibration should be large. It is found that both for N2 and CH3I, the resonant energy transfer between different molecules is significant. The effect of rotational–vibrational coupling, on the other hand, is found to be negligible for the systems studied.
Neighborship structure and dynamics in supercooled liquids
Authors: Keyes T.
Published: July 2002

The neighbors of a central atom in the supercooled, unit-density Lennard-Jones liquid are sorted by “neighborship” (first neighbor, second neighbor, etc.), and an analysis of static and dynamical properties is presented. A preliminary model is that neighbors n=1–12 fall in the first shell S1, that n=13,14 are transitional neighbors, and that S2 begins at n=15. S1 is identified as the cage of the central atom, and S1 plus the central atom is considered as a possible cluster; diffusion is proposed to occur via S1→S2 transitions. The radial probability distribution functions, P(n,r), for the nth neighbor are calculated. With decreasing T the shells pull away from each other and from the transitional neighbors, and a mean-field theory of P(n,r) breaks down. It is suggested that such behavior correlates with a dynamical slowing down. Similarly, a diffusive model for the number of original S1 neighbors still in S1 at time t fails for (reduced) T⩽0.80, indicating the onset of collective slow cluster dynamics. Static and dynamic evidence points to T∼0.8 as a temperature below which the liquid becomes more complex. The need to separate fast vibrational dynamics from measures of diffusion is discussed; one atom makes a first passage S1→S2 very quickly. The two-atoms first passage time τ2 is therefore proposed as an approximate single-atom diffusive time. The rate τ2−1 is in excellent agreement with the barrier hopping rate ωh calculated from instantaneous normal mode theory.
Selective excitation of high vibrational states using Raman chirped adiabatic passage
Authors: Davis J. C. , Warren W. S.
Published: July 2002

Calculations indicate that high vibrational states of oxygen and chlorine can be excited using a series of nonresonant Raman pulses, where both the pump and the Stokes pulses are chirped with linear frequency sweeps. Most of the previously reported coherent processes (such as simple adiabatic passage) are seriously degraded when rotational effects are included. However, we find that the laser pulse parameters (intensity and bandwidth) required to invert population into high vibrational states via Raman chirped adiabatic passage are achievable using technology that is currently available. Applications to homonuclear diatomic molecules are discussed in detail.
Monte Carlo simulation of F−(H2O)4 using an ab initio potential
Authors: Vaughn Simon J. , Akhmatskaya Elena V. , Vincent Mark A. , Masters Andrew J. , Hillier Ian H.
Published: July 2002

We present results concerning the structure and energetics of the cluster F−(H2O)4 at 0 K, using high quality ab initio methods, and at a temperature of 300 K, using an ab initio Monte Carlo simulation. At 0 K, we find the global energy minimum corresponds to three waters solvating the fluoride ion and with the fourth water in an outer hydration shell, hydrogen bonding to the other water molecules. Structures involving four waters hydrogen bonding to the fluoride are, however, of only slightly higher energy. At 300 K, the simulation results indicate that the fluoride is mostly to be found within a tetrahedron of solvating water molecules. The cluster is mobile, however, and a wide variety of structures are sampled.
Energy-resolved collision-induced dissociation of Aln+ clusters (n=2–11) in the center of mass energy range from few hundred meV to 10 eV
Authors: Ingólfsson Oddur , Takeo Harutoshi , Nonose Shinji
Published: July 2002

Energy-resolved collision-induced dissociation (CID) of Aln+ (n=2–11) in collision with argon is presented for the energy ranges from few hundred meV to 10 eV in the center of mass frame. The experiments were carried out with a recently constructed secondary ion tandem mass spectrometer, that is described in detail. The collision energy dependence is measured for the total and the partial dissociation cross sections, and the dissociation thresholds for the individual processes are estimated. The release of Al+ is found to be the dominating channel for n<8. For n>8, the cross section for the release of Al+ and Al are comparable. The release of more than one neutral atom from the larger clusters (n>6) is found to be in good agreement with sequential atom loss. In the case of the smaller clusters, on the other hand, fission is the energetically favorable process. The closed shell cluster, Al7+ (20 valence electrons), is found to be exceptionally stable and the adiabatic ionization potential of Al7 is found to be lower than that of the monomer. The stability of Al7+ is further reflected in the dissociation dynamics of the next neighbor, Al8+. The high stability of Al7+ as well as the dissociation dynamics of Al8+ are treated in the simple frame of the electronic shell model. Unlike Al7+, Al3+ (with 8 valence electrons) shows no sign of increased stability, and the dissociation dynamics seems to be controlled by the spin selection rules, rather than the energetics. In the present work, general trends and the dissociation dynamics of individual clusters are discussed. Qualitative information on the development of the geometric and electronic structure, with increasing cluster size, is deduced and discussed in terms of a transition from a covalent to a metallic character. Finally, this work is compared to earlier theoretical and experimental approaches to Aln+ clusters.
Coherence from fluorescence correlations: Oscillatory femtosecond fluorescence in pentacene/p-terphenyl
Authors: Tortschanoff A. , Brunner K. , Warmuth Ch. , Kauffmann H. F.
Published: July 2002

Spontaneous fluorescence has been used to measure the coherent femtosecond response of the organic, binary crystal pentacene/p-terphenyl. By using two-pulse excitation with phase-randomized pulses in an interferometric setup and analyzing the variance of the fluctuating intensity of correlated fluorescence photons, femtosecond beatings have been observed. The pattern of these terahertz oscillations is strongly dependent on the detuning frequency range of the exciting pulses, but is rather invariant with regard to the spectral position of the fluorescence probe window. In the interferometric regime of freely propagating pulses novel, ultrafast fluorescence carrier-wave oscillations superimposed to the beat structure have been obtained. The oscillatory signals evolve from a coherent superposition of optical free induction decays, caused by the different electronic transition energies of the pentacene absorber sites O1, O2, O3, and O4, respectively, are monitored as intrinsic, heterodyne beats by the fluorescence square detector. The major part of oscillations is thus considered to result from typical polarization interferences, but vibrational quantum beats are also extractable from the interferogram in the ultimate frequency regime of the pentacene S0→S1ν=1, S0→S1ν=2 resonances. The early picosecond-decay of the fluorescence beats reveals the mechanism of the loss of coherence to be mainly controlled by inhomogeneous dephasing at low phonon temperatures. A model of uncoupled two-level systems that includes Gaussian inhomogeneous broadening of the individual absorber sites and a Gaussian frequency distribution for the excitation pulses can account for the experimental data, quite adequately.
Effects of restricted rotational diffusion on 2H magic angle spinning nuclear magnetic resonance spectra
Authors: Kristensen J. H. , Hoatson G. L. , Vold R. L.
Published: July 2002

A general formalism is developed for describing the effects of restricted rotational diffusion on deuteron (2H) magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra. The approach is based on the Smoluchowski model that describes restricted rotational diffusion in an arbitrary ordering potential with an arbitrary diffusion tensor. It is shown that the Smoluchowski model gives a physically more reasonable description of molecular motion than the discrete Markov (jump) model. The models are shown to be mutually consistent for high ordering potentials and (or) low temperatures provided the diffusion coefficient is sufficiently high. However, for low ordering potentials and (or) high temperatures the discrete Markov model is not a useful approximation and the spectra can only be simulated with restricted rotational diffusion. This is also the case for small diffusion coefficients independent of the ordering potential and the temperature. The formalism is based on finite difference solutions to the stochastic Liouville–von Neumann equation. This defines a linear homogeneous system of coupled parabolic partial differential equations which includes both first- and second-order spatial derivatives. Numerical solutions are very difficult to obtain and some useful finite difference methods are presented. The results are elaborated for 2H MAS NMR spectroscopy. Solutions are obtained both in the presence and absence of radio frequency (rf) irradiation and effects of finite pulse width are included. The method is applied to the investigation of motional effects on 2H MAS NMR spectra of L-alanine-N,N,N-2H3. The orientational dependence of the ordering potential and the quadrupole parameters is determined from the Smoluchowski model. The activation energies are found to be temperature dependent. These effects have not previously been observed and give new information on molecular motion in this system. The rotational diffusion results are compared with the discrete Markov model and it is found that in this case the two models are consistent. The most important difference is that the Markov model does not map out the orientational dependence of the ordering potential and the quadrupole parameters. Another advantage of the rotational diffusion model is that it is physically more reasonable than the Markov model and that the parameters may be interpreted in terms of molecular properties.
Reaction dynamics of the As-rich GaAs(001)-2×4 surface with monoenergetic Br2 molecules: A scanning tunneling microscopy study
Authors: Liu Yong , Komrowski Andrew J. , Kummel Andrew C.
Published: July 2002

The adsorption of 0.15-eV and 0.89-eV Br2 onto the As-rich GaAs(001)-2×4 surface at 300 K has been investigated on the atomic scale over a wide range of total Br coverage using scanning tunneling microscopy (STM). It is found that the strained As–As dimer bonds and the As–Ga back bonds are significantly weakened and chemically activated by the Br atoms site-selectively adsorbed onto the second-layer Ga atoms. Consequently, at higher Br coverages, incident Br2 will preferentially react with these As–As and As–Ga bonds, removing (etching) As–As dimers and forming AsBr(s) or AsBr2(s) species, GaBr(s) chains, and etching pit islands. The incident energy of Br2 has a strong effect on the adsorption mechanisms.
Comparison of chemical selectivity and kinetic energy release in Si(s)+ICl(g) and H(g)+ICl(g)
Authors: Pettus Kharissia A. , Ahmadi Temer S. , Lanzendorf Eric J. , Kummel Andrew C.
Published: July 2002

ICl chemisorbs onto Si(111)–7×7 by two mechanisms: dissociative chemisorption and abstractive chemisorption. Abstractive chemisorption, in which one halogen atom of ICl bonds to the silicon surface while the other is ejected into the gas phase, is the dominant chemisorption mechanism for ICl/Si(111)–7×7. Multiphoton ionization (205 nm MPI) spectroscopy and time-of-flight (TOF) mass spectrometry were used to determine that the ratio of iodine-selective abstraction to chlorine-selective abstraction is at least 34±4: 1. The ICl and Si(111)–7×7 reaction can be compared to the ICl and atomic hydrogen (deuterium) reaction which has been studied extensively by others. The chemical selectivity of ICl+Si(111) is greater than the chemical selectivity of the gas phase reaction of H+ICl where the ratio of formation of HI to HCl is only 4:1. In both reactions, the iodine atom of ICl molecules is more reactive than the chlorine atom because the πx,y* antibonding orbital (the orbital that covalently reacts with other species) consists primarily of atomic iodine orbitals. The difference in the chemical selectivities of the silicon surface and gaseous hydrogen reactions with ICl is due to the ability of the silicon surface to rotationally steer ICl molecules, and the inability of silicon to migrate between the iodine and chlorine atoms. The median translational energies of ejected halogen atoms were determined to be 0.18±0.04 eV for chlorine atoms and 0.53±0.27 eV for iodine atoms which are a small fraction (14% for ejected iodine atoms and 9% for ejected chlorine atoms) of the total reaction exothermicities. The low translational energies of ejected atoms is due to the fact that the iodine–chlorine bond of ICl lengthens as the Si–I bond contracts; thus, there is little repulsion energy attributed to the Si–I–Cl transition state.
Molecular dynamics simulation of vibrational energy relaxation of highly excited molecules in fluids. I. General considerations
Authors: Vikhrenko V. S. , Heidelbach C. , Schwarzer D. , Nemtsov V. B. , Schroeder J.
Published: July 2002

Methods of implementation of classical molecular dynamics simulations of moderate size molecule vibrational energy relaxation and analysis of their results are proposed. Two different approaches are considered. The first is concerned with modeling a real nonequilibrium cooling process for the excited molecule in a solvent initially at equilibrium. In addition to the solute total, kinetic, and potential energy evolution, that define the character of the process and the rate constant or relaxation time, a great deal of important information is provided by a normal mode specific analysis of the process. Expressions for the decay of the normal mode energies, the work done by particular modes, and the vibration–rotation interaction are presented. The second approach is based on a simulation of a solute–solvent system under equilibrium conditions. In the framework of linear nonequilibrium statistical thermodynamics and normal mode representation of the solute several expressions for the rate constant are derived. In initial form, they are represented by integrals of the time correlation functions of the capacities of the solute–solvent interaction atomic or normal mode forces and include the solute heat capacity. After some approximations, which are adequate for specific cases, these expressions are transformed to combinations of those for individual oscillators with force–force time correlation functions. As an attempt to consider a strongly nonequilibrium situation we consider a two-temperature model and discuss the reason why the rate constant can be independent on the solute energy or temperature. Expressions for investigation of the energy redistribution in the solvent are derived in two forms. One of them is given in the usual form of a heat transfer equation with the source term describing the energy flux from the excited solute. The other form describes the energy redistribution in the solvent in terms of capacity time correlation functions and can be more convenient if memory effects and spatial dispersion play an important role in energy redistribution in the solvent.
Observation of an energy threshold for large ΔE collisional relaxation of highly vibrationally excited pyrazine (Evib=31 000–41 000 cm−1) by CO2
Authors: Elioff Michael S. , Wall Mark C. , Lemoff Andrew S. , Mullin Amy S.
Published: July 2002

Energy dependent studies of the collisional relaxation of highly vibrationally excited pyrazine through collisions with CO2 were performed for initial pyrazine energies Evib=31 000–35 000 cm−1. These studies are presented along with earlier results for pyrazine with Evib=36 000–41 000 cm−1. High-resolution transient IR laser absorption of individual CO2 (0000) rotational states (J=56–80) was used to investigate the magnitude and partitioning of energy gain into CO2 rotation and translation, which comprises the high energy tail of the energy transfer distribution function. Highly vibrationally excited pyrazine was prepared by absorption of pulsed UV light at seven wavelengths in the range λ=281–324 nm, followed by radiationless decay to pyrazine’s ground electronic state. Nascent CO2 (0000) rotational populations were measured for each UV excitation wavelength and distributions of nascent recoil velocities for individual rotational states of CO2 (0000) were obtained from Doppler-broadened transient linewidth measurements. Measurements of energy transfer rate constants at each UV wavelength yield energy-dependent probabilities for collisions involving large ΔE values. These results reveal that the magnitude of large ΔE collisional energy gain in CO2 (0000) is fairly insensitive to the amount of vibrational energy in pyrazine for Evib=31 000–35 000 cm−1. A comparison with earlier studies on pyrazine with Evib=36 000–41 000 cm−1 indicates that the V→RT energy transfer increases both in magnitude and probability for Evib>36 000 cm−1. Implications of incomplete intramolecular vibrational relaxation, electronic state coupling, and isomerization barriers are discussed in light of these results.
Infrared absorption line strengths of the Na⋯FH van der Waals molecule
Authors: Topaler Maria , Piecuch Piotr , Truhlar Donald G.
Published: July 2002

The dipole moment function of the Na⋯FH van der Waals molecule obtained in accurate multireference configuration interaction calculations and the recently published ab initio potential energy surface for NaFH [Topaler et al., J. Chem. Phys. 108, 5349 (1998)] have been used to predict the infrared absorption line strengths of Na⋯FH for several fundamental and overtone transitions. The fundamental excitation of the HF stretch mode in Na⋯FH is enhanced by a factor of 2.2 compared to the isolated HF molecule, and the first and second overtones are enhanced by factors of 21 and 53, respectively.
Excited states of van der Waals clusters by projector Monte Carlo, with application to excitations of molecules in small 4Hen
Authors: Blume D. , Mladenović M. , Lewerenz M. , Whaley K. B.
Published: July 2002

We demonstrate the extraction of excited rovibrational state energies for quantum systems with up to 15 dimensions, using the recently developed projection operator imaginary time spectral evolution (POITSE) method. The technique is applied to a series of model van der Waals complexes of the form AB-Xn, n=1−3, with heavy AB and a weakly bound, light atom X. The selectivity of several types of molecule-fixed and space-fixed projectors and their relationship to the nature of the accessible excited states are studied by making comparison with discrete variable-finite basis representation (DVR-FBR) calculations for n=1. These also allow identification of excitations dominant in ground state infrared absorptions. The results suggest a propensity for space-fixed projectors to access states where the angular momentum of the diatomic AB dominates the overall angular momentum, while molecule-fixed projectors access primarily bending and stretching modes. The relevance of this observation for the interpretation of the rovibrational spectra of molecules embedded in large liquid helium clusters is discussed.
Local vibrations in systems of interacting adsorbed molecules
Authors: Rozenbaum V. M. , Lin S. H.
Published: July 2002

A lattice system of adsorbed molecules is treated that is characterized by two bands of vibrational excitations. The first one originates from the collectivization of local high-frequency vibrations of individual molecules, which results from lateral intermolecular interactions. The second one arises due to the analogous collectivization of low-frequency resonance molecular modes, with their lifetimes governed by the coupling with substrate phonons. The temperature dependence of the spectral line shape for local vibrations is analyzed in the model that includes all kinds of cubic and quartic anharmonic coupling between high-frequency and low-frequency molecular modes in the fourth-order perturbation theory for the two-time retarded Green’s functions in the coordinate-momentum representation. As shown, various processes that involve four vibrational excitations and contribute to the line broadening for local vibrations are dominated by quartic anharmonic coefficients renormalized in terms of the cubic one; the renormalization is caused by the effective anharmonic force acting on each harmonic oscillator. Based on the translation symmetry of the system in surface-parallel directions, the quasimode approximation is substantiated, which enables the spectral line shift and width for local vibrations to be expressed in terms of dispersion laws and lifetimes for low-frequency molecular modes. The results obtained permit spectral line characteristics of local vibrations to be estimated for H/Si(111) and H(D)/C(111) in nice accordance with the experimentally measured values. Lateral interactions of low-frequency modes are shown to result in their shorter lifetimes and hence in the additionally narrowed spectral lines. The contribution from lateral interactions of local vibrations proves to be significant for systems with the sufficiently wide local vibration band and low frequencies of resonance modes, as, for instance, in OH/SiO2 and 2×1 phase of CO/NaCl(100).
Laser induced fluorescence Doppler profiles of photofragments in the presence of v–J correlation: A density matrix formalism
Authors: Chen Kuo-mei , Pei Cheng-chih
Published: July 2002

1+1 LIF Doppler profile functions of photofragments in the presence of v–J correlation are derived in a formalism of density matrix. Radon transforms which are imposed by the resonance condition of Doppler spectroscopy have been implemented exactly. Independent parameters which characterize the angular momentum polarizations of photofragments in a specific state on an expanding Newton sphere have been examined carefully to compare the present treatment with the bipolar moment formalism of Dixon [R. N. Dixon, J. Chem. Phys. 85, 1866 (1986)]. To extract information on speed distributions of photofragments from a combined, isotropic Doppler profile, the proper procedure to take the linear combination of experimental profiles from various detection geometries and rotational branches of transitions has been presented. For future analyses of Doppler spectroscopic measurements of photofragmentation processes, it is recommended that the Doppler profile function in the present framework should be utilized.
Ultrafast dissociation dynamics of acetone: A revisit to the S1 state and 3s Rydberg state
Authors: Zhong Q. , Poth L. , Castleman A. W.
Published: July 2002

Because of the dispute in the literature over the dissociation rate and energy partitioning of the acetone molecule upon photoexcitation to the S1 state (π*←n) and 3s Rydberg state (3s←n), we have remeasured the lifetime of acetone (also d6-acetone) on the S1 and 3s surfaces by a femtosecond time-resolved multiphoton ionization technique, coupled with a reflectron time-of-flight mass spectrometer. The measured dissociation rate of acetone on the S1 surface is prompt, and the acetyl radical is long lived. The lifetime of acetone on the 3s surface is measured to be 3.2±0.4 ps (6.0±0.5 ps for d6-acetone). The dissociation rate of acetyl is approximately 1.7 ps (2.5 ps for d3-acetyl) from the curve fitting. This agrees well with the Rice–Ramsperger–Kassel–Marcus theory predicted lifetime of 1.0 ps (1.9 ps for d3-acetyl) when the internal excitation energy of the acetyl radical is treated by a statistical-adiabatic-impulsive model.
Vibrational predissociation dynamics in the vibronic states of the aniline–neon van der Waals complex: New features revealed by complementary spectroscopic approaches
Authors: Becucci M. , Lakin N. M. , Pietraperzia G. , Castellucci E. , Bréchignac Ph. , Coutant B. , Hermine P.
Published: July 2002

We report two independent sets of experimental spectroscopic data which both contain information about the vibrational dynamics occurring in the aniline–neon van der Waals complex in its S1 electronically excited state. The high resolution excitation spectra of the three vibronic bands, 6a01¯, I02¯, and 101¯, of the S1←S0 transition, exhibit lifetime broadening with respect to transitions to the corresponding states in the aniline monomer. The dispersed emission spectra taken under excitation of the same three vibronic bands give access to both the distribution of aniline monomer states produced by vibrational predissociation of the complex and to the rates at which this dynamics proceeds. The overall results are discussed in a consistent way, with emphasis being given to the role of the coupling between the intramolecular and the intermolecular vibrational states. In the case of I02¯ excitation, it is shown that this coupling is reflected in the shape of the van der Waals wavefunction, as accessed through the analysis of the high resolution spectra [M. Becucci, G. Pietraperzia, N. M. Lakin, E. Castellucci, Ph. Bréchignac, Chem. Phys. Lett. 260, 87 (1996).].
Connectedness-in-probability and continuum percolation of adhesive hard spheres: Integral equation theory
Authors: Chiew Y. C.
Published: July 2002

Integral equation theory was employed to study continuum percolation and clustering of adhesive hard spheres based on a “connectedness-in-probability” criterion. This differs from earlier studies in that an “all-or-nothing” direct connectivity criterion was used. The connectivity probability may be regarded as a “hopping probability” that describes excitation that passes from one particle to another in complex fluids and dispersions. The connectivity Ornstein–Zernike integral equation was solved for analytically in the Percus–Yevick approximation. Percolation transitions and mean size of particle clusters were obtained as a function of connectivity probability, stickiness parameter, and particle density. It was shown that the pair-connectedness function follows a delay-differential equation which yields analytical expressions in the Percus–Yevick theory.