bluuebunny/crossref_metadata_2025 huggingface dataset:
A comparative study of the scattering of highly energetic atomic and molecular beams from metallic surfaces
Authors: Bertino Massimo F. , Manson J. R. , Silvestri W.
Published: July 2002

Time-of-flight spectra (TOF) of supersonic He and D2 beams in the energy range 100⩽Ei⩽250 meV have been measured after scattering from a clean Cu(001) surface at surface temperatures between 100 and 950 K. The TOF spectra of both He and D2 exhibit broad featureless distributions over the whole range of incident beam energies and surface temperatures. The intensities of the He TOF spectra are a factor of 5 to 7 higher than those of D2 when the incident beam energies are the same and below 200 meV. For the highest incident beam energies Ei⩾200 meV and surface temperatures Ts>700 K the difference between the He and D2 TOF spectra reduces to about a factor of 3. A theoretical model is employed which reproduces the TOF spectra to a very good approximation. The comparison of the best-fit parameters for He and D2 provides valuable information on the interaction parameters and their dependence on incident energy. The analysis of the energy and temperature dependence of the peak intensities of the D2 TOF spectra allows for the separation of the contribution of rotational excitations in the collision mechanism.
Fitting basis sets for the RI-MP2 approximate second-order many-body perturbation theory method
Authors: Bernholdt David E. , Harrison Robert J.
Published: July 2002

The “resolution of the identity” integral approximation applied to second-order many-body perturbation theory, or RI-MP2, method offers improved computational performance compared to traditional (exact) second-order perturbation theory calculations, but introduces a new auxiliary or “fitting basis set” into the method. We develop fitting basis sets for use with the correlation consistent cc-pVDZ and cc-pVTZ atomic orbital basis sets for the atoms H-Ne. These fitting sets are designed to reproduce exact second-order results for a set of 32 test cases, including a variety of reaction energies, weak interactions, and electrostatic properties, to better than 1% error averaged across all tests and less than 2% error in any individual case. Although the RI-MP2 method is primarily targeted to large-scale calculations, it offers substantial performance improvements even for the small molecules used in these test cases.
A theoretical study on laser control of a molecular nonadiabatic process by ultrashort chirped laser pulses
Authors: Mishima Kenji , Yamashita Koichi
Published: July 2002

We have theoretically studied the usefulness of ultrashort chirped laser pulses for laser control of photodissociation dynamics with a one-dimensional model system involving a potential well in the excited electronic state. The molecular system we treat in this paper entails three electronic states, a ground electronic state, and two excited electronic states. The ground electronic state is radiatively dipole-coupled with the lower of the two excited electronic states, while the two excited electronic states are themselves connected by diabatic coupling potential. Previously, we found a single linearly chirped ultrashort (in the order of fs) laser pulse can be employed to control photodissociation dynamics according to the sign of the chirp rate: A positively chirped pulse traps the molecule in the excited state potential well, while a negatively chirped pulse photodissociates the molecule rapidly. In the present paper we take into consideration the influence of the strength of the diabatic coupling and the reduced mass of the molecular system and show such a selectivity works best when the diabatic coupling is moderate and the molecular system is light. We explain these phenomena by a state selective excitation in vibrational states under the condition of adiabatic rapid passage (ARP) population transfer. We solve the integrodifferential equation of the molecular system involving the vibrational states explicitly, which is a quite different approach from that for the atomic case.
Stochastic resonance in surface catalytic oxidation of carbon monoxide
Authors: Yang Lingfa , Hou Zhonghuai , Xin Houwen
Published: July 2002

Stochastic resonance is a nonlinear cooperative effect between external signal and noise, in which the noise can play a constructive role to increase the signal-to-noise ratio in the detection of a weak signal. A surface catalytic reaction model, to describe oxidization of carbon monoxide carrying out far from equilibrium, was adopted to study the stochastic resonance. By computer simulation, we found noise can induce state-to-state transitions, and stochastic resonance behavior may appear at narrow bistable states or near discontinuous Hopf bifurcations, while a weak periodic signal riding on noise is input controlling.
Proton transfer in benzoic acid crystals: Another look using quantum operator theory
Authors: Antoniou Dimitri , Schwartz Steven D.
Published: July 2002

We present a calculation of the rate of synchronous double proton transfer in benzoic acid crystals. Experiments on these systems have been performed over a wide range of temperatures (roughly 10–400 °K). Even though the energetic barrier for proton transfer is rather high, the observed activation energy is low, while kinetic isotope experiments seem to indicate classical transfer. The system exhibits significant quantum character even at high temperatures and we show that the observed low activation energies can be reproduced assuming that the reaction is “assisted” by a low-frequency intramolecular mode, as has been suggested in different contexts by Benderskii [V. A. Benderskii, S. Yu. Grebenshchikov, and G. V. Mil’nikov, Chem. Phys. 194, 1 (1995)], Hynes [D. Borgis and J. Hynes, J. Chem. Phys. 94, 3619 (1991)] and Silbey [A. Suarez and R. Silbey, J. Chem. Phys. 94, 4809 (1991)]. We use our previous work on the quantum Kramers problem to perform a fully quantum calculation that incorporates symmetric coupling to the intramolecular mode and coupling to the condensed environment to all orders. We calculate the activation energies for hydrogen and deuterium transfer and we show that our results are in quantitative agreement with the experiment.
The 2Π ← X2Π electronic spectra of C8H and C10H in the gas phase
Authors: Linnartz Harold , Motylewski Tomasz , Maier John P.
Published: July 2002

The 2Π ← X 2Π electronic transition of linear C8H/C8D and C10H/C10D has been detected in the gas phase. The carbon radical chains were produced at low temperatures in a pulsed slit nozzle, incorporating a discharge in a high pressure expansion. Cavity ring down spectroscopy is used as a sensitive technique to observe the band systems in absorption. The 000 band of the 2Π3/2 ← X 2Π3/2 electronic transition of C8H in the gas phase has its origin near 15 973.5 cm−1, whereas that of C10H is around 14 000 cm−1. Some transitions involving vibrational excitation in the upper 2Π electronic state have been also detected. These measurements were undertaken because carbon chains are among the appealing candidates as carriers of diffuse interstellar bands; the observed origin bands do not show matches with the hitherto reported wavelengths. However, these gas phase data now provide a firm basis for a specific astronomical search.
Dynamic nuclear polarization of diamond. I. Solid state and thermal mixing effects
Authors: Reynhardt Eduard C. , High Grant L.
Published: July 2002

The dynamic nuclear polarization of 13C nuclei in a suite of seven natural type Ia and Ib diamonds, using continuous wave S- and X-band microwave radiation, is described. The 13C signal enhancement and polarization time have been measured for one of the type Ib diamonds as a function of magnetic field in the vicinity of the resonance field. The total paramagnetic impurity concentration (P1 and other centers) in this diamond is 2×1018 cm−3 (23 atomic parts per million), while the concentration of P1 centers is 9.3 ppm. Since the central electron spin resonance (ESR) linewidth HL is comparable with H0γC/γe, flip–flip and flip–flop forbidden transitions take place simultaneously. Consequently thermal mixing plays an important role in the 13C signal enhancement. However, the 13C spin-lattice relaxation rate is determined to a large extent by the solid state effect (forbidden transitions). The 13C polarization rates have been measured for the suite of diamonds by executing dynamic nuclear polarization (DNP) experiments on both hyperfine and central ESR lines. It is shown that the polarization rate is proportional to the paramagnetic impurity concentration of the sample, in agreement with the existing theory. It has been found that in type Ib diamonds with relatively low nitrogen impurity concentrations the dynamic nuclear polarization of a single hyperfine line yields an equilibrium 13C polarization that is one-quarter of that obtained in the case of dynamic nuclear polarization of the central line. In samples containing P1 and P2 centers (type Ia), or in type Ib samples with relatively high concentrations of P1 centers, the same equilibrium 13C polarization is obtained for the DNP of hyperfine and central transitions. This phenomenon is explained in terms of a model in which thermal contact is established between the electron Zeeman reservoir and the nuclear spin reservoirs via the spin–spin interaction reservoir if HL⩾H0γC/γe.
Nonadiabatic bending dissociation in 16 valence electron system OCS
Authors: Suzuki Toshinori , Katayanagi Hideki , Nanbu Shinkoh , Aoyagi Mutsumi
Published: July 2002

The speed, angular, and alignment distributions of S(1D2) atoms from the ultraviolet photodissociation of OCS have been measured by a photofragment imaging technique. From the excitation wavelength dependence of the scattering distribution of S(1D2), the excited states accessed by photoabsorption were assigned to the A′ Renner–Teller component of the  1Δ and the A″(1Σ−) states. It was found that the dissociation from the A′ state gives rise to high- and low-speed fragments, while the A″ state only provides the high-speed fragment. In order to elucidate the dissociation dynamics, in particular the bimodal speed distribution of S atoms, two-dimensional potential energy surfaces of OCS were calculated for the C–S stretch and bending coordinates by ab initio molecular orbital (MO) configuration interaction (CI) method. Conical intersections of 1Δ and 1Σ− with 1Π were found as adiabatic dissociation pathways. Wave packet calculations on these adiabatic surfaces, however, did not reproduce the low-speed component of S(1D2) fragments. The discrepancy regarding the slow S atoms was attributed to the dissociation induced by nonadiabatic transition from A′(1Δ) to A′(1Σ+) in the bending coordinate. This hypothesis was confirmed by wave packet calculations including nonadiabatic transitions. The slow recoil speed of S atoms in the nonadiabatic dissociation channel is due to more efficient conversion of bending energy into CO rotation than the adiabatic dissociation on the upper state surface. By analyzing the experimental data, taking into account the alignment of S(1D2) atoms, we determined the yield of the nonadiabatic transition from the A′(1Δ) to the ground states to be 0.31 in the dissociation at 223 nm. Our theoretical model has predicted a prominent structure in the absorption spectrum due to a Feshbach resonance in dissociation, while an action spectrum of jet-cooled OCS measured by monitoring S(1D2) exhibited only broad structure, indicating the limitation of our model calculations.
Electron spectroscopy of resonance-enhanced vibrational excitations of gaseous and solid tetrahydrofuran
Authors: Lepage M. , Letarte S. , Michaud M. , Motte-Tollet F. , Hubin-Franskin M.-J. , Roy D. , Sanche L.
Published: July 2002

We report intramolecular vibrational excitation of tetrahydrofuran (THF) in both the gas and solid phase induced by 1–30 eV electrons. High-resolution electron-energy-loss spectra reveal selective vibrational enhancement of THF for different impact energies, indicating the presence of at least three resonances (i.e., temporary anion states), located near 4, 7.5, and 10 eV, as well as many possibly overlapping higher-lying resonances. We also report electron-energy-loss spectra for different THF film thicknesses. Besides fundamental vibrational excitation, we observe several energy loss peaks due to overtones and combination vibrational modes. The present vibrational losses are assigned in compliance with published optical measurements. The similarities between the solid THF and gaseous cyclopentane spectra suggest that the resonances in both molecules could have a common origin and hence need not be attributed to the presence of the oxygen nonbonding electrons in THF.
Optical response functions for condensed systems with linear and quadratic electron–vibration coupling
Authors: Toutounji Mohamad , Small Gerald J. , Mukamel Shaul
Published: July 2002

Understanding the similarities and differences between optical coherence loss of electronic transitions of chromophores in glasses and in the glass forming solvent requires, in part, linear response (2-point correlation) functions, J(t;T). An approximate excited state vibrational Hamiltonian (He) which accounts for both linear and quadratic electron–phonon coupling is derived that is acceptable for mode frequency changes smaller than 30%. The associated linear response function for the case of no damping is obtained. A response function that includes damping is proposed for systems whose modes are either linearly or quadratically coupled. It is the product of three response functions, two of which are phononic and associated with linear and quadratic modes. The third response function is electronic with a dephasing frequency γel that is the width of the zero-phonon line. The total response function yields single-site absorption spectra in which folding of the widths of multi-phonon and sequence transitions occurs. Applications of the new response functions are made to the temperature dependence of single-site absorption and hole-burned spectra of the special pair band of the bacterial reaction center and the temperature dependence of the single site absorption spectrum of Al-phthalocyanine tetrasulphonate in glassy ethanol.
Density functionals from the extended G2 test set: Second-order gradient corrections
Authors: Schmider Hartmut L. , Becke Axel D.
Published: July 2002

In a recent paper [H. L. Schmider and A. D. Becke, J. Chem. Phys. 108, 9624 (1998)], we applied a systematic method for the determination of exchange-correlation functionals within the generalized gradient approximation (GGA) to the extended G2 test set of standard heats of formation of Curtiss et al. [J. Chem. Phys. 106, 1063 (1997)]. In the present work, we apply a similar methodology that goes beyond the GGA by taking second-order gradients and the (noninteracting) kinetic-energy density into account. The resulting improvement in the reproduction of thermochemical data brings us very close to the quality of G2 theory itself. Our lowest mean absolute error for standard heats of formation, 1.60 kcal/mol, is only marginally greater than the G2 value (1.58 kcal/mol). The corresponding largest deviation is 9.97 kcal/mol, as compared to 8.2 kcal/mol for G2 theory.
Density functional and ab initio study of the ketene and methylketene radical cations
Authors: Huang Ming-Bao , Wang Zhi-Xiang
Published: July 2002

The present study for the ketene and methylketene radical cations involves geometry, conformation, and hyperfine structure. The geometry and conformation (for the methylketene cation) investigations were based on the B3LYP/6-311G(d,p) and MP2/6-311G(d,p) calculations, and the isotropic proton hyperfine coupling constants (a(H)) were calculated using various density functional theory (DFT) methods and ab initio UHF, UMP2, and multireference single and double excitation configuration interaction (MRSDCI) methods with large basis sets. The ketene cation is predicted to have a C2v geometry. The barriers to methyl group rotation in the methylketene cation is predicted to be very low (<1 kcal/mol) and the cis (staggered) conformation is the most stable. On the rotation, the sum of the B3LYP/6-311G(d,p) a(H) values on the three β protons in the methylketene cation is almost a constant approximately equal to three times the experimental β coupling value. All the three ab initio methods predict too small a(H) values for the β protons. The B3LYP (also B3P86 and B3PW91) calculations predict a(H) values for the α and β protons in the two radicals in excellent agreement with experiment and the DFT methods do not underestimate the β couplings. The UMP2(FULL)/6-311G(d,p) hyperfine structure calculations predict a(H) values for the α protons in excellent agreement with experiment and the UMP2 hfcc results are overall better than the MRSDCI results. As an ab initio method for hyperfine structure calculations, the UMP2 method is suggested.
Coupled cluster response calculation of natural chiroptical spectra
Authors: Pedersen Thomas Bondo , Koch Henrik , Ruud Kenneth
Published: July 2002

We present the first coupled cluster singles and doubles calculations of the scalar rotatory strength and the rotatory strength tensor, which determine the intensity of isotropic and oriented electronic circular dichroism spectra, respectively, for the ethylene chromophore of (−)-trans-cyclo-octene. Results for the oscillator strength of the ordinary electronic absorption spectra are also reported. The results are presented in pictorial form for seven different basis sets and two formally equivalent formulations of the intensity quantities. The theoretical results are compared with the experimental absorption and isotropic circular dichroism spectra of (+)-trans-cyclo-octene.
Real-space polariton wave packet imaging
Authors: Koehl Richard M. , Adachi Satoru , Nelson Keith A.
Published: July 2002

The propagation of collective lattice vibrational wave packets that move at lightlike speeds through their crystalline hosts has been monitored through a real-space imaging method based on Fresnel diffraction and phase-contrast microscopy. Coherent optic phonon–polaritons generated through impulsive stimulated Raman scattering in a ferroelectric crystal are imaged with 5 μm spatial resolution and 35 fs time resolution as they propagate out of the excitation region and into other parts of the crystal. The “spatiotemporal” imaging method will permit direct examination of nonlinear lattice dynamics and will provide feedback in coherent control systems that use spatiotemporal femtosecond pulse shaping to manipulate propagating excitations with ultrashort light pulses directed to specified sample locations at specified times. The method will also permit the examination of defects and fabricated structures in bulk or thin film crystals using visible light.
Single and multiple collision effects observed in the femtosecond spectroscopy of I2-rare gas collision complexes: a statistical description
Authors: Dietz H. , Engel V.
Published: July 2002

We use a simple statistical model to simulate recent femtosecond experiments on the caging dynamics of I2 in high pressure rare gas environments [Wan et al., J. Chem. Phys. 106, 4353 (1997)]. The collision-induced bound state motion in I2 and its appearance in the pump/probe signal is perfectly reproduced in our calculation. In the case where He is used as the I2 collision partner we find multiple collisions to be important. The numerical results can be explained analytically under simplified assumptions.
A model of exit-channel corrected state distributions in the products of three-atom barrierless unimolecular reactions
Authors: Bonnet L. , Rayez J. C.
Published: July 2002

A simple model is derived which, by including exit-channel effects into transition state theory (TST), allows a description of state distributions in the products of three-atom barrierless unimolecular reactions. Exit-channel effects, which reduce mainly to a transfer of energy between rotational and translational motions on the way from the transition state (TS) to the products, are described within the recently derived free-path integration model (FPIM). A transfer coefficient χ relating the magnitude of the transfer to the mechanical parameters of the system is evidenced. The model is applied to the reaction O2H→O2+H in the case where it involves small transfers; semiquantitative agreement is found with quasiclassical trajectory (QCT) calculations. However, the model fails at describing product state distributions in the case of stronger transfers, demonstrating the necessity to go beyond FPIM.
Laser-induced hole filling and spectral diffusion of aluminum phthalocyanine tetrasulfonate in hyperquenched glassy films
Authors: Reinot T. , Hayes J. M. , Small G. J.
Published: July 2002

Laser-induced hole filling and spectral diffusion for the dye aluminum phthalocyanine tetrasulfonate in hyperquenched glassy films of water, ethanol, and methanol are investigated. Burning multiple holes into these films reveals a dependence on the burn direction, which is explained by the asymmetry of the antihole produced in the burning process. Spectral diffusion rates are shown to be dependent on sample annealing at temperatures well below the glass transition temperature, Tg. This is interpreted in terms of a β-relaxation process of the glass and is identified with transport of free volume.
Short-time Chebyshev propagator for the Liouville–von Neumann equation
Authors: Guo Hua , Chen Rongqing
Published: July 2002

A Chebyshev interpolation scheme is proposed for the short-time Liouville–von Neumann propagator. For each propagation step, a small number of Chebyshev polynomials is used to construct the propagator. The method involves only matrix-vector multiplication and is memory efficient since the three-term Chebyshev recursion needs only two vectors stored. It is also numerically stable since neither matrix diagonalization nor inversion is involved. The short Chebyshev recursion ensures that the divergence due to the complex eigenvalues of the Liouville superoperator is kept under control. Numerical tests carried out for the Redfield equation of a one-dimensional dissipative harmonic system demonstrate that the short-time Chebyshev propagator is accurate and significantly more efficient than the commonly used fourth-order Runge–Kutta scheme.
A theoretical study of five water/ammonia/formaldehyde cyclic trimers: Influence of cooperative effects
Authors: Masella Michel , Flament Jean-Pierre
Published: July 2002

Ab initio computations at the MP2 level on five dimers and five cyclic trimers, drawn from water, ammonia, and formaldehyde are presented. Trimers have been drawn to present cyclic X–H---Y patterns. Particular attentions have been devoted in analyzing the energetic contributions resulting from cooperative effects in the trimer binding energies (BEs) and in analyzing the trends of several parameters from monomers to dimers and from dimers to trimers [in particular, the trends of the R(X–H) bond lengths, of the R(X---Y) distances, of the δvXH shifts in the vXH stretch vibrational frequencies, and of the electronic density ρc value at the XH---Y axis critical point when it exists]. The results have exhibited that cooperative effects represent from 10% to 16% of the trimer BEs and that they reinforce, from dimers to trimers, the trends observed for the above parameters from monomers to dimers. In particular, for “typical” X–H---Y HB (i.e., where X and Y atoms correspond to oxygen or nitrogen atoms), R(X–H) bond lengths are increased within 0.01 Å from monomers to dimers and from dimers to trimers, R(X---Y) distances shortened within 0.18 Å, ρc values increased by about 17% and vXH red-shifted from 18 to 164 cm−1 from dimers to trimers. As contrasted to those HBs the R(X–H) and δvXH parameters corresponding to C–H---Y interaction (with Y=O or N) follows an opposite trend from monomers to dimers and from dimers to trimers (i.e., they are respectively smoothly shortened and blue-shifted). All of these results therefore exhibit the great incidence of cooperative effects on the properties of X–H---Y interactions (corresponding to typical HBs or not), which are of importance to understand the properties of biochemical systems.
Nascent state distributions of NO(X  2Π) generated from the reaction of S(1D) with N2O: Intramolecular vibrational-energy redistribution in the reaction intermediate
Authors: Akagi Hiroshi , Fujimura Yo , Kajimoto Okitsugu
Published: July 2002

The nascent internal state distribution of NO(X 2Π) generated from the reaction, S(1D)+N2O→NO+NS, has been determined by utilizing the laser-induced fluorescence (LIF) technique. The average vibrational energy of NO relative to the statistically expected value is found to be 37%. This amount is obviously smaller than that of the fragment N 16O of the isovalent reaction O18(1D)+N2 16O→N 18O+N 16O, though it is still larger than that of OH18 produced from the O16(1D)+H2 18O reaction. To interpret the observed difference in the product energy partitioning, we have applied the quantal intramolecular vibrational-energy redistribution (IVR) representation to the energy mixing in the collision complex. Using a local-mode vibration model with momentum couplings, we have extracted the crucial factors determining the energy partitioning in these reactions. The reaction system consisting of only heavy mass atoms generally has a large vibrational coupling and a large density of states, both of which favor the rapid energy mixing during the short-lifetime of the intermediate complex.
Electronic-structure calculations by first-principles density-based embedding of explicitly correlated systems
Authors: Govind Niranjan , Wang Yan Alexander , Carter Emily A.
Published: July 2002

A first-principles embedding theory that combines the salient features of density functional theory (DFT) and traditional quantum chemical methods is presented. The method involves constructing a DFT-based embedding potential and then using it as a one-electron operator within a very accurate ab initio calculation. We demonstrate how DFT calculations can be systematically improved via this procedure. The scheme is tested using two closed shell systems, a toy model Li2Mg2, and the experimentally well characterized CO/Cu(111) system. Our results are in good agreement with near full configuration interaction calculations in the former case and experimental adsorbate binding energies in the latter. This method provides the means to systematically include electron correlation in a local region of a condensed phase.
Nonadiabatic molecular dynamics simulation of photoexcitation experiments for the solvated electron in methanol
Authors: Mináry Péter , Turi László , Rossky Peter J.
Published: July 2002

Nonadiabatic quantum molecular dynamics simulations have been performed to simulate the pump-and-probe photoexcitation experiments of the ground state equilibrium solvated electron in methanol carried out by Barbara et al. [Chem. Phys. Lett. 232, 135 (1995)]. We have characterized both the time evolution of the quantum solute, the solvated electron, and the solvation response of the classical methanol bath. The quantum energy gap provides an excellent tool to gain insight into the underlying microscopic details of the solvation process. The solvent response is characterized for both processes by a fast Gaussian component and a biexponential decay. The present results suggest that the residence time of the solvated electron in the first excited state is substantially longer than inferred from the cited experiments. The experimentally observed fast exponential portion of the relaxation more likely corresponds to the adiabatic solvent response than to the lifetime of the excited state electron. By comparing to photoexcitation simulations in water, it is shown that the simulated excited state lifetime is about three times longer in methanol than in water, predicting a less substantial increase than a recent calculation based on nonadiabatic coupling elements alone. Hydrogen-bonding statistical analysis provides interesting additional details about the dynamics. We find that the hydrogen-bonding network is significantly different in the first solvent shell around the electron in ground and first excited states, the distribution around the latter, larger and more diffuse, ion resembling more that of the pure liquid. Transformation of the corresponding hydrogen bonding structures takes place on a 1 ps time scale.
Differential cross sections for H+D2→HD(v′=1, J′=1,5,8)+D at 1.7 eV
Authors: Fernández-Alonso Félix , Bean Brian D. , Zare Richard N.
Published: July 2002

A 1:4 mixture of HBr and D2 is expanded into a vacuum chamber, fast H atoms are generated by photolysis of HBr ca. 210 nm, and the resulting HD (v′, J′) products are detected by (2+1) resonance-enhanced multiphoton ionization (REMPI) in a Wiley–McLaren time-of-flight spectrometer. The photoloc technique allows a direct inversion of HD (v′, J′) core-extracted time-of-flight profiles into differential cross sections for the H+D2→HD(v′=1, J′=1,5,8)+D reactions at collision energies ca. 1.7 eV. The data reveal a systematic trend from narrow, completely backward scattering for HD (v′=1, J′=1) toward broader, side scattering for HD (v′=1, J′=8). A calculation based on the line of centers model with nearly elastic specular scattering accounts qualitatively for the observations.
Fifth-order nonlinear Raman processes in molecular liquids using quasi-cw noisy light. II. Experiment
Authors: Kirkwood Jason C. , Albrecht A. C. , Ulness Darin J. , Stimson Michael J.
Published: July 2002

Fifth-order analogs of coherent Raman scattering generated in a number of molecular liquids using broadband quasi-cw noisy light are presented. It is seen how the signal for the direct fifth-order process, which probes the dynamics of both a fundamental vibration and its overtone, is often contaminated by a sequential process, that is only capable of probing the vibrational dynamics of the fundamental. Although these two processes are virtually indistinguishable when a single Raman resonance is excited, we find that when a second Raman resonance is available within the experimental window governed by the bandwidth of the noisy light, new frequency components in the signal arise and the two competing fifth-order processes become distinguishable. These new frequency components, as well as their decay, are explained in terms of spectral filtering of the noisy light by the Raman resonances. This spectral filter analogy predicts which of the two competing processes dominates in an equimolar mixture of benzene-h6 and benzene-d6, and also in neat pyridine (each providing two available fundamentals). It is found by this new method that the sequential event is present and apparently fully dominates the direct process for all molecular liquids studied here.
A high resolution electron energy loss spectroscopy study of the Fermi resonance of CO2 adsorbed on a Ag(110)/CO3 layer
Authors: Krenzer B. , Constant L. , Conrad H.
Published: July 2002

The adsorption of CO2 on a Ag(110) surface precovered with a saturated CO3 layer has been studied by high resolution electron energy loss spectroscopy (HREELS). A weakly bound state of CO2 is stabilized by the presence of carbonate. In addition to the losses of the bend (658 cm−1) and the antisymmetric stretch (2343 cm−1) fundamental modes, the presence of two peaks in the frequency region of the symmetric stretch mode have been observed. Such a loss pair is characteristic for the existence of a Fermi resonance, between the overtone of the bend and the symmetric stretch vibration, which had been studied in detail for the gas phase CO2 molecule. By use of three CO2 isotopomers the existence of the Fermi resonance in the adsorbed state has been verified and the Fermi constant and the energy of the unperturbed vibrations have been determined from the isotopic shifts. The character of the CO2 molecule is almost unaffected by the bonding and it adsorbs in a tilted geometry.
Construction of interpolated potential energy surfaces using constrained dynamics: Application to rotational inelastic scattering
Authors: Duncan Alexander H. , Collins Michael A.
Published: July 2002

An established method for constructing molecular potential energy surfaces by interpolation of ab initio data has been coupled with a new approach to sampling molecular configurations which uses constrained classical dynamics. To illustrate this approach, model surfaces for the scattering of two rigid diatomic molecules are derived and shown to accurately predict rotational inelastic scattering cross sections.
Differential cross sections for H+D2→HD (v′=2, J′=0,3,5)+D at 1.55 eV
Authors: Fernández-Alonso Félix , Bean Brian D. , Zare Richard N.
Published: July 2002

The photoloc technique with core extraction of the nascent product laboratory speed distribution in a Wiley–McLaren time-of-flight spectrometer has been used to measure differential cross sections for the reaction H+D2→HD (v′=2, J′=0,3,5)+D at collision energies ∼1.55 eV. We find that the peak of each angular distribution shifts from complete backward scattering toward side scattering as the rotational excitation of the product increases. We found the same trend in our previous study of H+D2→HD (v′=1, J′=1,5,8)+D at ∼1.70 eV. We conclude that the same type of correlation exists between impact parameter and rotational quantum number in both product vibrational manifolds. Further analysis of the HD (v′=2, J′) differential cross section data reveals, however, a clear tendency of this vibrational manifold to scatter sideways at lower J′ than HD(v′=1, J′). Within the framework of a line-of-centers model with nearly elastic specular scattering, this result implies that smaller impact parameters lead to more vibrationally excited products.
Polyethylene under tensil load: Strain energy storage and breaking of linear and knotted alkanes probed by first-principles molecular dynamics calculations
Authors: Saitta A. Marco , Klein Michael L.
Published: July 2002

The mechanical resistance of a polyethylene strand subject to tension and the way its properties are affected by the presence of a knot is studied using first-principles molecular dynamics calculations. The distribution of strain energy for the knotted chains has a well-defined shape that is very different from the one found in the linear case. The presence of a knot significantly weakens the chain in which it is tied. Chain rupture invariably occurs just outside the entrance to the knot, as is the case for a macroscopic rope.
High symmetry effects on hydrogen bond rearrangement: The 4.1 THz vibrational band of (D2O)4
Authors: Brown Mac G. , Keutsch Frank N. , Braly Linda B. , Saykally Richard J.
Published: July 2002

Vibration–rotation–tunneling (VRT) spectroscopy has been extended to the 4 THz spectral region through the observation of a second intermolecular vibration of (D2O)4. Analysis of the precisely measured perpendicular transition confirms the previously reported cyclic homodromic structure and reveals a dramatically increased (30×) hydrogen bond rearrangement rate in the excited state.
The microwave spectrum of a new phosphorus-bearing radical CH2P(2B2)
Authors: Saito Shuji , Yamamoto Satoshi
Published: July 2002

The microwave spectrum of the CH2P radical in its B22 ground electronic state was detected for the first time using a source-modulated spectrometer equipped with a free-space cell. The CH2P radical was generated in the cell by a dc-glow discharge in a mixture of PH3, CH4, and He, and its isotopic species, CD2P, in a mixture containing CD4 instead of CH4. Forty four a-type R-branch rotational transitions of CH2P were measured for the transitions N=8–7 to N=12–11 with Ka=0 to 5 in the frequency region of 260–400 GHz, for which fine and hyperfine structures due to the phosphorus and hydrogen nuclei were resolved. For CD2P, 39 rotational transitions from N=9–8 to N=13–12 with Ka=0 to 4 were measured in the 240–355 GHz with fine structure and phosphorus hyperfine structure. Molecular constants of CH2P, including the fine structure constants and hyperfine coupling constants due to both the nuclei of P and H, were precisely determined by least-squares methods from 158 observed spectral lines. Those of CD2P were similarly obtained from 70 observed spectral lines. The rotational constants of both species were used to derive the molecular structural parameters for CH2P: r0(CP)=1.6576(28) Å, r0(CH)=1.0912(61) Å, and ∠HCH=115.96(96)°. The spin densities of unpaired electrons on the P and H nuclei were estimated from the hyperfine coupling constants, and found to be consistent with the molecular structure determined.
Ab initio calculations of structural and electronic properties of small silver bromide clusters
Authors: Rabilloud F. , Spiegelmann F. , Heully J. L.
Published: July 2002

Ab initio configuration interaction (CI) calculations are performed to study the ground state of small neutral and singly charged silver bromide clusters AgnBrp(±) (n,p⩽2). The results are obtained at complete active space self-consistent field and also at variational plus second order perturbational multireference CI (MRPT2) levels of approximation. We discuss more particulary the structural properties and the stability of the lowest isomers. Adiabatic and vertical ionization potentials and electron affinities have also been determined.
Intramolecular vibrational energy redistribution and intermolecular energy transfer in the (d, d) excited state of nickel octaethylporphyrin
Authors: Mizutani Yasuhisa , Uesugi Yuki , Kitagawa Teizo
Published: July 2002

The formation of a vibrationally excited photoproduct of nickel octaethylporphyrin (NiOEP) upon (π, π*) excitation and its subsequent vibrational energy relaxation were monitored by picosecond time-resolved resonance Raman spectroscopy. Stokes Raman bands due to the photoproduct instantaneously appeared upon the photoexcitation. Their intensities decayed with a time constant of ∼300 ps, which indicates electronic relaxation from the (d, d) excited state (B1g) to the ground state (A1g), being consistent with the results of transient absorption measurements by Holten and co-workers [D. Kim, C. Kirmaier, and D. Holten, Chem. Phys. 75, 305 (1983); J. Rodriguez and D. Holten, J. Chem. Phys. 91, 3525 (1989)]. The Raman frequencies of NiOEP in the (d, d) excited state are shifted to lower frequencies compared to those of the ground state species, and it is reasonably interpreted by the core size expansion of the macrocycle by 0.05 Å upon the electron promotion from the dz2 to the dx2−y2 orbital. Anti-Stokes ν4 intensity in the vibrationally excited (d, d) state of NiOEP appeared promptly and decayed with time constants of 11±2 and 330±40 ps. The former is ascribed to vibrational relaxation, while the latter corresponds to the electronic relaxation from the (d, d) excited state to the electronic ground state. In contrast, the rise of anti-Stokes ν7 intensity was not instantaneous, but delayed by 2.6±0.5 ps, which indicates that intramolecular vibrational energy redistribution has not been completed in subpicosecond time regime. The peak position of the ν4 band shifted by nearly 5 cm−1 between 0 and 50 ps. The time constant for the shift of the ν4 band was 9.2±1.3 ps, which was close to that for the fast component of intensity decay of anti-Stokes bands. The ν4 band became narrower and symmetric as the delay time increases. These can be ascribed to intramolecular anharmonic coupling of the ν4 mode with the low frequency modes. The intra- and intermolecular vibrational energy relaxation in the metal excited state will be discussed.
The infrared spectrum of the benzene–Ar cation
Authors: Satink Rob G. , Piest Hans , von Helden Gert , Meijer Gerard
Published: July 2002

The infrared (IR) absorption spectra of the jet-cooled C6H6 and C6D6 cations, complexed with Ar, are measured throughout the 450–1500 cm−1 region via IR-laser-induced vibrational dissociation spectroscopy. The IR spectrum of the C6H6–Ar cation is dominated by a Fermi resonance between the IR active ν11 mode and two components of the combination mode of the lowest frequency modes ν6 and ν16. A stringent upper limit of 316 cm−1 is found for the value of the dissociation limit D0 of the neutral C6D6–Ar complex.
Modeling of dynamic molecular solvent properties using local and cavity field approaches
Authors: Macak Peter , Norman Patrick , Luo Yi , Ågren Hans
Published: July 2002

Current models of solvent properties using cavity reaction field methods are analyzed and categorized in two groups according to the use of local fields or cavity fields as the perturbing fields. Analytical connections between the two approaches are derived.
Quantum control of I2 wave packet localization in solid argon matrix
Authors: Guiang Chona S. , Wyatt Robert E.
Published: July 2002

A hybrid quantum/classical approach for treating the vibrational and translational motion of the I2 molecule inside a cold Ar matrix is implemented in the control of vibrational wave packet localization on the excited (A) electronic surface of I2. Quantum control was performed in the weak-field regime at six different temperatures to examine thermal effects on the dynamics of I2 inside the lattice and on the degree of control that can be achieved for this system. It was found in this study that an increase in temperature from 0 to 75 K leads to a moderate decrease in the degree of control achieved. The role played by I2 rotation on control was also shown to be minimal under the conditions examined in this work.
Intermolecular vibrations and asymmetric-top pendular states in 1-naphthol–H2O and –NH3
Authors: Kim Wousik , Lee Soohyun , Felker Peter M.
Published: July 2002

We report the results of nonlinear Raman spectroscopy measurements of intermolecular transitions in the hydrogen-bonded complexes 1-naphthol–H2O and 1-naphthol–NH3. Bands are assigned, primarily by pendular band-contour analysis and by reference to calculated results from Knochenmuss et al. [J. Phys. Chem. 102, 1935 (1998)], to transitions involving the three hydrogen-bond vibrations in which there is relative translational motion between the two molecules in the complex. Splittings due to tunneling between hydrogen-bond torsional states are also identified.
Bond-forming reactions of molecular dications with rare gas atoms: Production of ArC2+ in the reaction CO2++Ar
Authors: Lu Wenyun , Tosi Paolo , Bassi Davide
Published: July 2002

Integral cross sections for the bond-forming reaction CO2++Ar→ArC2++O have been measured as a function of collision energy in a guided-ion beam mass spectrometer. The energy dependence is consistent with an endoergic reaction. Since the title reaction is in competition with several charge-transfer processes, the cross section at the maximum is only 0.023 Å2 at a collision energy of about 3 eV. Simple kinematics considerations suggest that the falloff of the cross section at higher energies might be due to the vibrational predissociation of ArC2+. State correlation diagrams are used for discussing the reaction mechanism.
Phase behavior of comblike copolymers: The integral equation theory
Authors: Khalatur Pavel G. , Khokhlov Alexei R.
Published: July 2002

We study the phase behavior of self-assembling grafted comblike copolymers with strongly attractive side chains. An off-lattice, microscopic integral equation theory is applied to investigate intermolecular correlations, collective scattering intensities, and thermally induced macro- and microphase separation transitions in the systems of different comb models having various architectures and chain stiffness. The properties of these models are analyzed as a function of molecular density, the number of side chains, the length of the side chains and backbone. Detailed calculations reveal the main factors that control micro- versus macrophase separation and thereby yield guideline for controlling the phase behavior of comblike copolymer systems in solutions and in melts.
Nonresonant photofragmentation/ionization dynamics of O2 using picosecond and femtosecond laser pulses at 248 nm
Authors: Bakker Bernard L. G. , Parker David H. , Samartzis Peter C. , Kitsopoulos Theofanis N.
Published: July 2002

Photodissociation/ionization of molecular oxygen in a cold molecular beam is studied using a short-pulse laser beam at 248 nm and velocity map imaging. Both photoelectron and O+ images are recorded for laser pulsewidths of 5 and 0.5 ps. Most of the observed ionization dynamics takes place after absorption of four laser photons, equivalent to 20 eV excitation energy, in an above threshold ionization process. Two main channels are identified: postdissociative ionization creating O(3P)+O**3s″(1P, 3P) atoms where O** is an electronically excited autoionizing atom, and molecular (auto)ionization to create a range of highly vibrationally excited ground electronic state O2+ ions. The observed O+ signals then arise from resonance-enhanced two-photon dissociation of O2+ or autoionization of O** atoms, while the electron signals arise from ionization of O2 or autoionization of O**. The latter channel can be used to directly scale the photoelectron and O+ signal strengths. The O+ images show strong differences for 0.5 ps and 5 ps pulse which could arise from ac Stark shifting of levels involved in the A 2Πu←X 2Πg transition of O2+. With resonance enhancement, two-photon dissociation of O2+ is the dominant process producing O+ ions. The O+ angular distributions show an anisotropy that is more extreme than a simple two-step dissociation, which is attributed to alignment effects.
Quantum scattering study of collisional energy transfer in He+NO2: The importance of the vibronic mixing
Authors: Petrongolo Carlo , Schatz George C.
Published: July 2002

We present the results of a quantum scattering study of collisional energy transfer in the title reaction, considering energies up to 14 000 cm−1 above the NO2 zero point energy. The collisions are described using the VCC–IOS (vibrational-coupled-channel infinite-order-sudden) quantum scattering method, with two coupled potential surfaces and as many as 329 total states in the basis expansion. The intramolecular potentials describe the X 2A′(2A1) and A 2A′(2B2) states of NO2, and their coupling through a conical intersection. The intermolecular potential is based on empirical sums of atom–atom potentials. We find that vibronic mixing between the X and A states of NO2 strongly enhances collisional energy transfer and, as a result, there is a noticeable change in the energy transfer efficiency as vibronic energy in NO2 is increased above the threshold for A state excitation. This change in efficiency occurs even if the same intermolecular potential is used for both NO2 electronic states. Many transitions are enhanced because the energy gaps between vibronically mixed and unmixed states are smaller. Energy transfer is also enhanced between states which are both vibronically mixed, whenever a significant component of each state involves the same zeroth order level of the excited electronic state. The calculated results are in good agreement with recent measurements.
Ab initio determination of the far infrared spectra of some isotopic varieties of ethanol
Authors: Senent M. L. , Smeyers Y. G. , Domı́nguez-Gómez R. , Villa M.
Published: July 2002

The far infrared (FIR) spectra of various isotopic species of ethanol (-h6, -d1, and -d3) are analyzed from MP4((SDQT) ab initio calculations using models in one and two dimensions. From the calculated frequencies and intensities, previous assignments of ethanol-h6 and -d1 bands are reviewed. The position of several combination bands are predicted. Ethanol shows two conformers, trans and gauche, and two interacting torsional modes. The torsional barriers have been calculated to be V3(trans)=1226.7 cm−1, V3(gauche)=1296.3 cm−1, VOH(α=62°)=404.1 cm−1, and VOH(α=180°)=423.3 cm−1. The flexible models in one and two dimensions yields the same OH torsional frequencies, whereas they differ in the methyl group state calculations. The fundamental bands of the ethanol-h6 have been evaluated at 205.5 cm−1 (OH torsion) and 257.0 cm−1 (CH3 torsion) and their corresponding intensities to be 18.650×10−4 and 0.662×10−4.