bluuebunny/crossref_metadata_2025 huggingface dataset:
Trajectory studies of unimolecular reactions of Si2H4 and SiH2 on a global potential surface fitted to a
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o and experimental data
Authors: Agrawal Paras M. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

The unimolecular decomposition dynamics of Si2H4 have been investigated using classical trajectory methods on a global potential-energy surface fitted to the results of ab initio calculations and the available experimental data. The required phase-space averages are computed using Metropolis sampling techniques. It is found that unless the parameters of the Markov walk are adjusted for each different type of atom present, extremely long Markov walks are required to adequately cover the phase space of the system. Microcanonical rate coefficients for the decomposition of Si2H4 into all open channels are reported at energies in the range 5.0<E<9.0 eV. The most important dissociation channel over this energy range is three-center elimination of molecular hydrogen leading to H2 Si=Si. At energies below 7.0 eV, the other channels are, in order of importance, Si–Si bond rupture, four-center H2 elimination, and simple Si–H bond rupture. At or above 8.0 eV, four-center H2 elimination replaces Si–Si bond rupture as the second most important decomposition channel. The energy dependence of the rate coefficients is well described by an RRK expression. Three-center H2 elimination involves a simultaneous rupture of both Si–H bonds whereas the four-center elimination is found to proceed by a hydrogen atom transfer process followed by H2 elimination. Except for a small propensity to form H2 with excess rotational energy, the energy partitioning among the products is nearly statistical. A comparison study of the decomposition of Si2H4 complexes formed by the recombination of two SiH2 molecules shows that the rates for both three- and four-center H2 elimination are in agreement with those computed using a statistical distribution of the same internal energy. The rate for Si–Si bond rupture, however, is significantly larger for Si2H4 complexes formed by SiH2 recombination than for Si2H4 molecules with the same internal energy randomly distributed. The decomposition dynamics of SiH2 on the global surface are also reported.
Methyl quantum rotation in solid CH3F: An inelastic neutron scattering study
Authors: Prager M.
Published: July 2002

Inelastic incoherent neutron spectra of solid CH3F were measured in the μeV and in the meV range. The methyl groups display the effect of rotational tunneling with a splitting of the librational ground state ℏωt =23.1 μeV. All CH3 groups are equivalent. The tunnel splitting and the transition energy to the first excited librational state E01=10.1 meV are reproduced by a rotational potential V(φ)=11.8 meV⋅cos 3φ. A crystal structure different to those of the other halides has to be postulated to understand this potential.
A classical trajectory study of inelastic scattering of NO from graphite surfaces: Rotational energy distributions
Authors: Pettersson Jan B. C. , Nyman Gunnar , Holmlid Leif
Published: July 2002

The inelastic scattering of NO molecules from graphite surfaces is studied by classical trajectory methods. The experimental results from Frenkel et al. (1982), Segner et al. (1983), and Häger and Walther (1984) are analyzed. A model using a small isolated part of the graphite surface in interaction with the NO molecule gives results in good agreement with experiment. The parameter values in the model are fixed at the values previously found to reproduce the angular distributions well [Nyman and Pettersson (1987)]. For this system, the experimental results give a ‘‘rotational cooling’’ such that the rotational temperature of the inelastically scattered molecules becomes smaller than the surface temperature. This effect is reproduced accurately by the calculations, giving a rotational temperature of 250 K, independent of the surface temperature above 300 K. The main factor controlling this inelastic rotational cooling is the low initial value of the normal component of the total angular momentum. A ‘‘rotational rainbow’’ structure is found in the calculations in many cases, primarily at high surface temperatures. The final energy distributions are shown to be mainly statistical by application of a unimolecular decomposition picture, similar to the common RRK type model used for gas phase reactions.
Electron binding energies of anionic alkali metal triatomics from partial fourth order electron propagator theory calculations
Authors: Ortiz J. V.
Published: July 2002

Vertical ionization energies of Li−3, Na−3, LiNa−2, and Li2Na− are calculated with ab initio electron propagator theory. D∞h and C∞v isomers for the heteronuclear triatomics are considered. Two doublet final states with Σ symmetry are considered for each case. Koopmans’s theorem, second order, third order, and partial fourth order results form a steadily converging series. Outer valence approximation results are not similar and are probably inferior to the partial fourth order results. Convergence of results with respect to the order of electron interaction in the propagator self-energy and with respect to basis set saturation is achieved to within 0.1 eV.
Quasiclassical trajectory study of rotational energy transfer in the scattering of NH3 from a flat, rigid gold surface
Authors: Coltrin Michael E. , Kay Bruce D.
Published: July 2002

Quasiclassical trajectories are used to study rotational energy transfer in the collision of a rigid-rotor NH3 molecule with a flat, rigid gold surface. The anisotropic term in the long-range attractive potential causes the NH3 to preferentially reorient with its dipole moment normal to the surface plane as it approaches the surface. This reorientation decreases the rotational energy transfer and gives rise to a sharp rotational rainbow at zero rotational energy. Trajectory results predict that the molecule preferentially scatters into low K states (tumbling) rather than the J=K states (spinning). This prediction is in qualitative agreement with recent molecular beam/surface scattering experiments.
Spectra in the chaotic region: A quantum analysis of the photodissociation of H+3
Authors: Gomez Llorente Jose M. , Zakrzewski Jakub , Taylor Howard S. , Kulander Kenneth C.
Published: July 2002

A quantum theory of periodic orbit based resonances is presented and applied to the photodissociation of highly excited H+3. Ab initio stabilization computations are performed to show that periodic orbits are the origin of stable roots producing scars along the orbits in the system’s wave functions. Spacings and widths of the resonances are in satisfactory agreement with the experiment and verify the mechanism proposed by Gomez and Pollak. The validity and utility of the PO based resonance theory to study the dynamics of highly excited systems is demonstrated.
High temperature photoelectron spectroscopy: A study of U, UO, and UO2
Authors: Allen G. C. , Baerends E. J. , Vernooijs P. , Dyke J. M. , Ellis A. M. , Fehér M. , Morris A.
Published: July 2002

The He i photoelectron spectrum of atomic uranium has been recorded in the vapor phase. Bands associated with metal (6d)−1, (7s)−1, and (5f)−1 ionizations were observed. The relative intensities of the (6d)−1 and (5f)−1 bands are consistent with computed relative cross sections at the He i energy, whereas the intensity of the (7s)−1 band is larger than expected from computed cross sections, implying enhancement of this feature by autoionization effects. HeI photoelectron spectra have also been recorded for the vapor above a heated equimolar UO2(s):U(s) mixture. By studying the spectra obtained as a function of temperature, bands associated with UO and UO2 were identified and these were assigned using relativistic Hartree–Fock–Slater calculations on these molecules.
The effect of electron correlation on the electron density distributions of molecules: Comparison of perturbation and configuration interaction methods
Authors: Wang Liang-Chen , Boyd Russell J.
Published: July 2002

Two methods for including the effects of electron correlation on the electron density distributions of molecules are compared. Second-order Mo/ller–Plesset perturbation theory (MP2) is shown to give electron density distributions in good qualitative agreement with much more time-consuming configuration interaction calculations. The latter include all single and double substitutions from the Hartree–Fock reference configuration. The two methods are compared by means of difference density maps calculated with large basis sets for a few representative molecules. From an analysis of the relative importance of single and double substitutions, it is shown that double substitutions are dominant near nuclei, but in other regions of space the single substitutions are more important. The MP2 method is shown to provide a practical method for calculating the correlation corrections to the electron density distributions of molecules of intermediate size.
Polar molecule in a nonpolar liquid. A molecular dynamics study
Authors: Zhu S.-B. , Robinson G. W.
Published: July 2002

Molecular dynamics techniques are used to study the structural changes in liquid carbon disulphide resulting from the introduction of a polar solute. It is found that both translational relaxation and rotational relaxation of the solvent in the shell surrounding the solute are faster than in the bulk liquid. Moreover, shifts in the intramolecular vibrational spectra and variations of the dielectric properties of the cage solvent are in evidence. The dynamical and spectroscopic behavior of the solute is also discussed. Microscopic friction experienced by the solute molecule for both translational and rotational motions is computed using the generalized stochastic theory. Information of this type is needed for constructing valid theories of ultrafast chemical reaction dynamics in condensed matter.
The radiative lifetime of N2(a 1Πg, v=0–2)
Authors: Marinelli William J. , Kessler William J. , Green Byron David , Blumberg William A. M.
Published: July 2002

We have employed direct two-photon laser excitation of specific vibrational levels of N2(a 1 Πg) to measure the lifetime of this state. Direct observation of emission from the a 1Πg –X 1∑+g transition in a large cell was employed to follow fluorescence decays. Experiments were conducted to verify that the effects of collisional transfer and diffusion were not contributing to the observed lifetime. Our experiments showed that the radiative lifetime of vibrational levels 0–2 is 56±4 μs and is independent of vibrational level, within experimental error. The observed lifetimes are in good agreement with recently reported theoretical calculations.
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o self-consistent field and configuration interaction study of Cu5O and Ag5O as models for oxygen chemisorption on Cu(100) and Ag(100)
Authors: Illas F. , Bachs M. , Rubio J. , Ricart J. M.
Published: July 2002

The lowest electronic states of Cu5O and Ag5O arising from interaction of atomic oxygen (3P) and the isolated cluster model on the 2E or 4A2 electronic states have been studied at the SCF, CASCI, MP2, and CIPSI levels using nonempirical pseudopotentials for the Ar (Kr) cores of Cu (Ag). The ground state of Cu5O and Ag5O is found to be 2E but only after inclusion of correlation effects. However, the energy difference between 2E and the 4A2 or 2A1 electronic states is rather small. The effects of electronic correlation have been analyzed in terms of nondynamical and dynamical contributions. It is shown that SCF values account for only about 25%–30% of the total binding energy, whereas inclusion of nondynamical correlation contributions allow to recover 50% of that value. Thus, dynamical correlation effects must be included in order to describe properly oxygen-cluster interaction occurring in these systems. Results are in good agreement with recent all electron CI studies carried out for Cu5O (2E).
Radiative lifetimes of metastable O+2(a 4Πu) and NO+(a 3Σ+)
Authors: Kuo Chau-Hong , Wyttenbach Thomas , Beggs Cindy G. , Kemper Paul R. , Bowers Michael T.
Published: July 2002

The spin-forbidden radiative decays of the lowest energy metastable electronic states of O+2 and NO+ have been studied using a Fourier transform ion cyclotron resonance mass spectrometer. The metastable species are stored in the mass spectrometer for several hundreds of milliseconds and then selectively detected by pulsing CO2 into the ion trap and monitoring the formation of CO+2. By varying the delay time between formation of the metastable ions and injection of CO2, apparent radiative lifetimes of 101 ms for O+2(a 4Πu) and 530 ms for NO+(a 3Σ+) are measured. Our results are compared with literature values and with a theoretical calculation. The theoretical model, demonstrated for O+2, is based on spin–orbit coupling between the a 4Πu and A2Πu states and upon spin–rotation mixing of the different spin components within the a 4Πu state. The results allow quantitative interpretation of our results and those available in the literature.
Quantum reactive scattering in three dimensions using hyperspherical (APH) coordinates. IV. Discrete variable representation (DVR) basis functions and the analysis of accurate results for F+H2
Authors: Bačić Z. , Kress J. D. , Parker G. A. , Pack R. T
Published: July 2002

Accurate 3D coupled channel calculations for total angular momentum J=0 for the reaction F+H2→HF+H using a realistic potential energy surface are analyzed. The reactive scattering is formulated using the hyperspherical (APH) coordinates of Pack and Parker. The adiabatic basis functions are generated quite efficiently using the discrete variable representation method. Reaction probabilities for relative collision energies of up to 17.4 kcal/mol are presented. To aid in the interpretation of the resonances and quantum structure observed in the calculated reaction probabilities, we analyze the phases of the S matrix transition elements, Argand diagrams, time delays and eigenlifetimes of the collision lifetime matrix. Collinear (1D) and reduced dimensional 3D bending corrected rotating linear model (BCRLM) calculations are presented and compared with the accurate 3D calculations.
Coherent control of bimolecular chemical reactions
Authors: Krause Jeffrey L. , Shapiro Moshe , Brumer Paul
Published: July 2002

We show how the method of coherent control can be applied to control product yield in bimolecular chemical reactions. First, a laser pulse prepares a coherent superposition of two bound levels of an electronically excited state of the reactants. Then, after a variable delay, a second laser pulse photodissociates the system to the ground state continuum. By varying the detuning of the excitation pulse and the delay between the two pulses, we show that significant control of the product ratio can be achieved. As a first application of the method we demonstrate, with exact quantum-mechanical calculations, the control of the collinear H+H2 and D+H2 reactions. The pulses and delays used in these calculations are on the order of picoseconds, a range of time scales well within that achievable with conventional laser systems.
Approximate calculations of correlation energy using one-electron density-functional procedures: Application to Lin and Nan clusters
Authors: Fantucci P. , Polezzo S. , Bonačić-Koutecký V. , Koutecký J.
Published: July 2002

Nonlocal-density-functional (NLDF) procedures using restricted Hartree–Fock (HF) electron density have been adopted to determine the geometries and stabilities of neutral and cationic Lin and Nan (n=1–9) clusters and their ionization potentials. The NLDF yields parallel results to those previously obtained from all-electron configuration-interaction (CI) procedures, and as expected it accounts for the contributions of the correlation energy which are not present in truncated CI. In addition, a comparison between the results obtained from the local-density functional (LDF) without and with self-interaction correction has been carried out. The LDF procedure yields results for cluster stabilities which differ up to 10%–15% from those obtained from the NLDF schemes. This shows clearly that the results obtained from LDF and NLDF schemes are substantially different for calculated properties involving mainly correlation energy differences, provided that an exact HF exchange potential is used.
Accurate configuration interaction calculations of the hyperfine interactions in the benzene cation
Authors: Huang Ming-Bao , Lunell Sten
Published: July 2002

The Jahn–Teller distorted states 2B2g and 2B1g of the benzene cation have been studied at the ab initio CI level. Relative energies indicate that the 2B2g state is more stable (the ground state), and the calculated isotropic and anisotropic hyperfine coupling constants for the 2B2g state are in good agreement with the values observed at 4.2K.
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o approach to molecular crystals: A periodic Hartree–Fock study of crystalline urea
Authors: Dovesi R. , Causa’ M. , Orlando R. , Roetti C. , Saunders V. R.
Published: July 2002

The electronic structure of crystalline urea (two molecules, 16 atoms per unit cell) is investigated at an ab initio level with CRYSTAL, a Hartree–Fock linear combination of atomic orbitals (LCAO) program for periodic systems. The influence of the basis set and of the computational parameters which control the treatment of the Coulomb and exchange series and the reciprocal space integration is documented; results include total and interaction energy, Mulliken analysis data and interaction (solid minus molecules) density maps, band structure, and density of states. The crystal field modifies the electronic structure of the isolated molecule, the main effect being an increase in the ionicity of bonds. The interaction energy obtained with a 6-21** basis set is 28 kcal/mol, (16 kcal/mol after a correction of the basis set superposition error by using the counterpoise method) to be compared with 21±0.5 kcal/mol from experiment. This preliminary application shows that accurate ab initio calculations of hydrogen bonded molecular crystals with a relatively small unit cell (less than 30 atoms/cell) are now within reach.
Dissociative attachment in silane
Authors: Haaland Peter
Published: July 2002

Cross sections for production of SiH−n from silane by electron impact from 6–12 eV have been measured with Fourier transform mass spectrometry techniques. The peak cross sections decrease and thresholds increase for the more extensively dissociated silicon hydride anions: σa(SiH−3)=1.9×10−18 cm2; σa(SiH−2)=0.8×10−18 cm2; σa (SiH−)=0.2×10−18 cm2. The cross sections for production of SiH−4 and H− are less than 8×10−21 cm2 over the 6–12 eV energy range. Ab initio calculations of the anion thermochemistry show that if the neutral products include molecular hydrogen substantial excitation of the anion or neutral fragments is required. The rates of anion production calculated from dissociative attachment cross sections show that attachment of electrons to radicals or excited species are the most important source of negative ions in silane plasmas.
Picosecond energy transfer of vibrationally hot molecules in solution: Experimental studies and theoretical analysis
Authors: Sukowski U. , Seilmeier A. , Elsaesser T. , Fischer S. F.
Published: July 2002

The cooling of vibrationally hot azulene is studied in different solvents by picosecond spectroscopy. Excitation to the electronic S1 state generates molecules with a vibrationally hot ground state by rapid internal conversion. The subsequent cooling is monitored by the temperature-dependent change of the S0–S1 absorption edge and occurs via interaction with the solvent on a time scale of several tens of picoseconds. A theoretical model of intermolecular energy transfer in the liquid phase is developed. The vibrational excess energy of azulene is transferred to the solvent molecules by isolated binary collisions, where the multimode vibrational system of the molecules is considered explicitly. The dissipation of energy within the solvent is simulated by the macroscopic conduction of heat. The temporal development of the vibrational temperature of the azulene molecules and the concommitant changes of absorption are calculated taking into account the properties of the specific solvent. The results of the theory show quantitative agreement with our data.
Rotational partition functions for symmetric-top molecules
Authors: McDowell Robin S.
Published: July 2002

A simple, accurate closed-form expression for the rotational partition functions of symmetric-top molecules is derived, which includes the effects of nuclear-spin statistics (significant at very low temperatures), quartic and sextic centrifugal distortion terms (moderate and high temperatures), and inversion (all temperatures): Qr≂σ*(πm)1/2  exp[β(4−m)/ 12]β−3/21+β2(1−m)2/90+⋅⋅⋅](1+δ) (1+ρ1β−1+ρ2β−2+ρ3β−3), where β≡hcB/kT, m≡B/A for prolate tops or B/C for oblate tops (A>B>C are the rotational constants), and σ*≡Πi(2Ii+1)/σ, where Ii is the spin of nucleus i and σ is the classical symmetry number. For C3vXY3 and WXY3 molecules, the low-temperature nuclear-spin correction is δ≊2(2IY+1)−2 exp[π2m(βm−6)/54β]; similar expressions are given for other molecular models. The ρ’s are functions of the centrifugal distortion constants DJ, DJK, DK, HJ, etc. If inversion doubling occurs, with a splitting ΔG in the ground vibrational state, this expression for Qr is to be multiplied by (1−βΔG/2B+⋅⋅⋅). The molecules NH3, CH3D, CHD3, allene (C3H4), and ethane (C2H6), including isotopic variants, are discussed and illustrative calculations presented for T=10 to 2000 K.
High precision calculations on the 2S ground state of the lithium atom
Authors: King Frederick W. , Bergsbaken Michael P.
Published: July 2002

The results of a detailed variational calculation on the 2S ground state of the lithium atom are reported. The wave function was constructed using Hylleraas-type functions with extensive exponent optimization being employed. The calculated nonrelativistic ground state energy obtained was −7.478 059 53 a.u., which is the lowest upper bound estimate for this quantity obtained to date. The discrepancy with semi-empirical estimates of this quality is discussed. The hyperfine coupling constant for 7Li is calculated to be 401.795 MHz, which compares favorably with the experimental result of 401.752 043 3 MHz. Expectation values are reported for the individual energy components, as well as the electron density at the nucleus, and the specific mass shift operator. The transition isotope shift is also evaluated. The convergence characteristics of the calculations are discussed, along with the effect of retaining only one of the two possible spin eigenfunctions in the basis set.
Collision dynamics of OH(X 2Πi,v=12)
Authors: Sappey Andrew D. , Copeland Richard A.
Published: July 2002

Collisional removal rate constants for the OH radical in v=12 of the ground electronic state are measured for the colliders CO2, O2, N2, H2, He, and Ar. OH molecules, generated in v=8 by the reaction of hydrogen atoms with ozone, are excited to v=12 by direct overtone excitation with pulsed infrared laser light. The temporal evolution of the v=12 radicals is probed as a function of collider gas pressure by a time-delayed pulsed ultraviolet probe laser. The probe laser is used to excite the molecules via the B 2Σ+–X 2Πi(0,12) electronic transition, and the resulting B 2Σ+–A 2Σ+ fluorescence is detected. We measure rate constants for CO2:(5.6±1.5)×10−11; O2:(1.6±0.2)×10−11; He:(3.6±0.6)×10−12; H2:(3.0±0.8)×10−12; Ar:(2.6±0.5)×10−12; N2:(2.5±0.7)×10−12 (all in units of cm3 s−1). These rate constants are over fifty times faster in all cases than the vibrational relaxation rate constants for the lower levels (v=1 and v=2) of the ground state.
The van der Waals rovibrational states of the Ar–NH3 dimer
Authors: van Bladel J. W. I. , van der Avoird A. , Wormer P. E. S.
Published: July 2002

Starting from an ab initio anisotropic potential surface, we have calculated the bound rovibrational states of the Ar–NH3 dimer for J=0, 1, 2, and 3. There is good agreement with data available from molecular beam spectroscopy in the microwave and far-infrared regions. The nature of the states is discussed and they are correlated with free internal rotor states. In spite of the substantial barriers to internal rotation, Ar–NH3 resembles much more a free internal rotor complex than a nearly rigid dimer. Still, the van der Waals vibrations show characteristic differences from free internal rotations. In particular, there is strong mixing between the fundamental stretch vibration of the dimer and the lowest bending mode. We assign the band observed in the far-infrared region to the lowest, primarily bending, of these modes. Further transitions in the yet unexplored far-infrared regions are predicted.
Far-infrared spectra and two-dimensional potential energy surface for the out-of-plane vibrations of 2-cyclopenten-1-one and its deuterated isotopomers
Authors: Cheatham C. M. , Laane J.
Published: July 2002

The far-infrared spectra of 2-cyclopenten-1-one and its 5-d1 and 5,5-d2 derivatives have been recorded and analyzed. Each molecule has a ring-puckering series of bands characteristic of mixed quartic–quadratic potential energy functions. The fundamental puckering frequencies were observed at 94.4, 89.0, and 84.9 cm−1, respectively, for the d0, d1, and d2 species. In addition to the main puckering series for each molecule, side bands arising from the ring-twisting excited states were also observed. Bands resulting from ring-twisting transitions from various puckering states were observed for all three species in the 280–290 cm−1 region. The data demonstrate that the molecular skeleton is planar. A two-dimensional potential energy function of the form V=a1x41 +b1x21+a2x42+b2x22 +cx21x22, where x1 and x2 are the ring-puckering and ring-twisting coordinates, was determined. For all three isotopomers, this gives rise to calculated frequencies which are in excellent agreement with the observed values. The interaction term c is larger than in related molecules, reflecting the effects of asymmetry and conjugation.
Electronic and structural properties of polyparaphenylenevinylene from the vibrational spectra
Authors: Tian B. , Zerbi G. , Müllen K.
Published: July 2002

The infrared and Raman spectra of oligomeric model molecules of polyparaphenylenevinylene (PPV) and of the polymer in the pristine state, and the infrared spectra of doped or photoexcited PPV are analyzed in terms of the theory of the effective conjugation coordinate (ECC). It is shown that all the spectroscopic observations (frequencies and intensities) are accounted for by ECC theory which shows that most of the observed features are due to the delocalization of π electrons. Measurements of the lower limit of the effective conjugation length and of the molecular chain length are made and the structural relevance of such concepts are discussed in terms of the conformation of the oligomeric or polymeric chain. Evidence is found that the charge carrier produces ‘‘quinoid’’ type defects, hence fully coplanar.
Optical-heterodyne-detected induced phase modulation for the study of femtosecond molecular dynamics
Authors: Hattori Toshiaki , Terasaki Akira , Kobayashi Takayoshi , Wada Tatsuo , Yamada Akira , Sasabe Hiroyuki
Published: July 2002

A new experimental technique for the study of ultrafast optical response of nonlinear materials, which detects nonrelaxational response in the media with high sensitivity, is introduced. Polarization-selective optical-heterodyne-detection scheme is applied to the measurement of the spectral shift of a probe pulse which is caused by induced phase modulation (IPM) brought about by a pump pulse. This technique, being sensitive to the high-frequency component of the nonlinear response, is especially applicable to the study of intermolecular dynamics in disordered media. The temporal responses of the optical Kerr effect in liquid carbon tetrachloride, benzene, and carbon disulfide are studied using this technique with femtosecond optical pulses. In the response of benzene, damped oscillations are clearly observed which are due to intermolecular vibrational motions of the molecules. The data obtained from benzene and carbon disulfide are analyzed using a Fourier-transform method. The response functions for the IPM measurements are reconstructed, and the Fourier spectra are also obtained. They are discussed in terms of inertial molecular motions and the local structures in these liquids.
Monolayer structures of carbon monoxide adsorbed on sodium chloride: A helium atom diffraction study
Authors: Schmicker D. , Toennies J. P. , Vollmer R. , Weiss H.
Published: July 2002

The monolayer CO adsorbed on NaCl(001) was studied with helium atom diffraction. At Tsample = 45 K the adlayer is commensurate to the substrate and of (1×1) symmetry. At Tsample ≤ 30 K a commensurate (2×1) superstructure is formed. The unit cell of this low-temperature phase contains two molecules and a glide plane.
Torsion–rotation interactions in a two-top molecule: High resolution S1←S electronic spectrum of 2,3-dimethylnaphthalene
Authors: Tan X.-Q. , Clouthier D. J. , Judge R. H. , Plusquellic D. F. , Tomer J. L. , Pratt D. W.
Published: July 2002

A rotationally resolved fluorescence excitation spectrum of the O00 band in the S1←S0 transition of 2,3-dimethylnaphthalene has been obtained using a cw laser/molecular beam spectrometer. More than 3000 lines were observed, each exhibiting a width of about 3 MHz. Despite the proximity of the two methyl groups, the observed rotational structure can be interpreted satisfactorily using an uncoupled rotor model. However, extensive torsion–rotation interactions are observed. Accurate measurements of these perturbations are used to determine the effective threefold hindering potentials in both electronic states, V3(S0)=652 cm−1 and V3(S1)=391 cm−1, respectively. Rotor–rotor couplings do influence these barriers as well as the patterns of lines observed in the rotationally resolved spectra of higher vibronic bands.
Fine structure branching ratios and Doppler spectra of O(3P
j) produced by the reaction of H+O2→OH+O
Authors: Matsumi Yutaka , Shafer Neil , Tonokura Kenichi , Kawasaki Masahiro , Kim Hong Lae
Published: July 2002

O(2p3Pj) atoms produced from the reaction between H(1s2S) and O2(X3Σ−g) at collision energies (Ecoll) of 2.5 and 2.1 eV are detected by a resonance enhanced multiphoton ionization technique. From the Doppler profile measurements of O(3Pj), the average kinetic energy released to the products is estimated. The results suggest that the average fraction of the kinetic energy disposal at Ecoll =2.1 eV is larger than that at Ecoll =2.5 eV. The obtained branching ratios of O(3Pj) j=2, 1, and 0 at Ecoll =2.5 eV are (0.70±0.02), (0.21±0.02), and (0.09±0.02), and those at Ecoll =2.1 eV are (0.63±0.02), (0.25±0.04), and (0.12±0.02), respectively, with error bars of ±σ. The results suggest that the nonadiabatic couplings at the last stage of the reaction are important for the determination of the ratios.
Bending fluctuations of an elastic line on a curved surface
Authors: Manning Gerald S.
Published: July 2002

In an ongoing effort to model the mechanical behavior of the nucleosome core particle in terms of elasticity theory, we study the bending fluctuations of an elastic line (nucleosomal DNA) confined to a curved surface (outside surface of the histone octamer). We define what is meant by a bending fluctuation by generalizing from the plane, where a relaxed elastic line is a segment of a straight line, a fluctuation carries the straight segment to an arc of a circle, and the measure of the extent of bending is the total curvature of the arc. It is then natural to measure the extent of a bending fluctuation on a general surface by the total geodesic curvature of the arc into which the elastic line is carried by the fluctuation. The path of the bent elastic line must satisfy a differential equation on the surface, which is derived by variational methods. The bending energy per unit length scales with the square of the extent of bending (Hooke’s law) if the surface is a plane or a sphere, but not generally. For application to the nucleosome core particle, bending fluctuations on a cylinder are analyzed. A relaxed elastic line on a cylinder can follow a circumferential trajectory only if its length l does not exceed a critical value lcrit. For subcritical lengths, a fluctuational bend away from the circumferential path requires an amount of energy that does not conform to Hooke’s law. The energy vanishes at the critical length (to lowest order in the square of the bending angle) and the mean-square thermal bending amplitude diverges, an event that could be related to conformational changes of the nucleosome core particle. A formal analogy between bending of an elastic line on a cylinder and the Landau theory of second-order phase transitions is explored in detail.
Photofragmentation dynamics of submonolayers of CH3Br adsorbed on Pt(111)
Authors: Ukraintsev V. A. , Long T. J. , Harrison I.
Published: July 2002

Angle-resolved, photofragment translational energy distributions, Pϑ(ET)s, arising from 308 nm laser irradiation of CH3Br adsorbed on Pt(111) are presented. Methyl photofragments were observed with translational energies extending to 2 eV and with Pϑ(ET)s which varied sharply with angle of exit from the surface. The fragmentation dynamics were consistent with a mechanism of dissociative electron attachment of subvacuum level, photoexcited substrate electrons to adsorbed CH3Br. The CH3 Pϑ(ET)s and the angular variation of the CH3 yield gave evidence that submonolayers of CH3Br form islands on Pt(111).
Comparison of variational Rice–Ramsperger–Kassel–Marcus theory with quantum scattering theory for the He+H+2 →HeH++H reaction
Authors: Klippenstein Stephen J. , Kress Joel D.
Published: July 2002

Three-dimensional quantum-scattering-theory calculations of the cumulative reaction probability (CRP) for the He+H+2→HeH++H reaction are reported for total angular momentum J=0. The results of these calculations are compared with variational calculations of the number of available states at the transition state for the dissociation of HeH+2 into HeH+ and H. For this reaction at zero total angular momentum two separate transition-state regions are found to exist throughout a broad energy range. The long-range transition state is found to be dominant for total energies up to at least 1.5 eV. As a result the energy spacings of the steps observed in the averaged CRP roughly correlate with the product rotational energy spacings.
Nonperturbative susceptibility of a three-level system interacting with a monochromatic field
Authors: Kavanaugh Theresa C. , Silbey Robert J.
Published: July 2002

We derive an exact, intensity-dependent expression for the susceptibility of a three-level system interacting with one monochromatic electric field. This expression, once expanded, is equivalent to the usual perturbative series χ=χ(1)+χ(3)E2+⋅⋅⋅ within the rotating wave approximation. We consider two types of resonant processes: one- and two-photon resonances corresponding to the intensity-dependent susceptibility χ(3)(−ω;ω,−ω,ω), and one-, two-, and three-photon resonances corresponding to χ(3)(−3ω;ω,ω,ω). As an example of current interest, we use a model three-level system that mimics the excited electronic states of typical nonlinear optical polymers and show that near resonance, successive terms in the perturbative series approach the same order of magnitude for experimentally realizable fields.
An accurate ab initio quartic force field for ammonia
Authors: Martin J. M. L. , Lee Timothy J. , Taylor Peter R.
Published: July 2002

The quartic force field of ammonia is computed using basis sets of spdf/spd and spdfg/spdf quality and an augmented coupled cluster method. After correcting for Fermi resonance, the computed fundamentals and ν4 overtones agree on average to better than 3 cm−1 with the experimental ones except for ν2. The discrepancy for ν2 is principally due to higher-order anharmonicity effects. The computed ω1, ω3, and ω4 confirm the recent experimental determination by Lehmann and Coy but are associated with smaller error bars. The discrepancy between the computed and experimental ω2 is far outside the expected error range, which is also attributed to higher-order anharmonicity effects not accounted for in the experimental determination. Spectroscopic constants are predicted for a number of symmetric and asymmetric top isotopomers of NH3.
Correlated sampling of Monte Carlo derivatives with iterative-fixed sampling
Authors: Sun Zhiwei , Lester William A. , Hammond Brian L.
Published: July 2002

A correlated sampling method for determining the energy and other property derivatives by finite difference is implemented within variational Monte Carlo. Determination of derivatives takes place over a fixed sample of electronic coordinates, so it is possible to distinguish small energy or other property differences accurately. Using finite differences avoids the evaluation of complicated derivative expressions and can be applied directly to Green’s function Monte Carlo methods without the need for derivatives of the Green’s function. The algorithm can be used to evaluate derivatives with respect to any parameters in the Hamiltonian or in the trial function. In this paper, it is applied to H2 and Li2 for their energy derivatives with respect to nuclear coordinates. Results are in agreement with experimental data.
Translation-to-vibrational excitation in the dissociative adsorption of D2
Authors: Darling G. R. , Holloway S.
Published: July 2002

In this work we have studied the dissociation dynamics of deuterium on two potential energy surfaces. In each case, there is significant translational-to-vibrational coupling that results in molecules that emerge vibrationally excited. In each case, the translational energy dependence of the onset of inelastic scattering is similar. The surfaces also result in dissociative adsorption but the thresholds in this case are well separated. The particular topological differences between the two surfaces that result in these findings are discussed.
Electrolyte effects on molecular radiationless decay: The photophysics of 3-aminofluorenone in acetonitrile-salt solutions
Authors: Thompson Peggy A. , Simon John D.
Published: July 2002

Salt effects on the photophysics of 3-aminofluoren-9-one (3-AF) are examined in acetonitrile solution. Infrared spectroscopy reveals no specific interactions between 3-AF and the dissolved ions. Combined steady state and time-resolved studies are used to determine the S0←S1 nonradiative and radiative rate constants. Added salt has a small effect on the radiative rate, which can be quantitatively accounted for by the Strickler–Berg equation. In contrast, electrolytes strongly influence the nonradiative decay rate which are quantitatively accounted for in terms of the energy gap law. No correlation is observed between the energy gap and the ionic strength of the solution. The results are compared to theoretical models for molecular solvation in ionic solutions.
Isolation of stable intermediates in reactive gas mixtures: Rotational spectrum of H3P...Cl2 in a pulsed jet
Authors: Legon A. C. , Warner H. E.
Published: July 2002

The weakly bound dimer H3P...Cl2 has been isolated in a pulsed jet by keeping the highly reactive components H3P and Cl2 separate until the point at which they expand into the evacuated Fabry–Pérot cavity of a pulsed-nozzle Fourier-transform microwave spectrometer. The rotational spectra of the three symmetric-top isotopomers H3P...35Cl 35Cl, H3P...35Cl 37Cl, and H3P...37Cl 35Cl detected in this way have been analyzed to give the ground-state spectroscopic constants B0, DJ, DJK, χ1(Cl), and χ2(Cl). Comparison of the dimer properties determined from the spectroscopic constants of H3P...Cl2 with those of the hydrogen-bonded dimer H3P...HCl indicates that charge transfer is not predominant in the interaction of PH3 and Cl2. Some parallels in the properties of B...Cl2 and B...HCl are discussed for B=PH3 and HF.
Dissociative electron attachment of CH3Br on GaAs(110) by thermalized photoexcited substrate electrons
Authors: Yang Q. Y. , Schwarz W. N. , Osgood R. M.
Published: July 2002

By using kinetic-energy-resolved measurement of adsorbate fragmentation, we observe an electron-transfer reaction from the conduction band minimum of a semiconductor surface to a molecular adsorbate. Bond cleavage in the molecular system, CH3Br on GaAs(110), occurs through a mechanism analogous to gas-phase dissociative electron attachment.
Vibrational and K′a dependencies of the multidimensional tunneling dynamics in the 82.6 cm−1 intermolecular vibration of the water dimer-d4
Authors: Pugliano N. , Cruzan J. D. , Loeser J. G. , Saykally R. J.
Published: July 2002

Using tunable far infrared laser absorption spectroscopy, 12 vibration–rotation-tunneling (VRT) subbands, consisting of approximately 230 transitions have been measured and analyzed for an 82.6 cm−1 intermolecular vibration of the water dimer-d4. Each of the VRT subbands originate from K″a = 0 and terminate in either K′a = 0 or 1. These data provide a complete characterization of the tunneling dynamics in the vibrationally excited state as well as definitive symmetry labels for all VRT energy levels. Furthermore, an accurate value for the A′ rotational constant of 122.9 GHz is found to agree well with its corresponding ground state value. All other excited state rotational constants are fitted, and discussed in terms of the corresponding ground state constants. In this vibration, the quantum tunneling motions are determined to exhibit large dependencies with both the K′a quantum number and the vibrational coordinate, as is evidenced by the measured tunneling splittings. The generalized internal-axis-method (IAM) treatment, which has been developed to model the ground state tunneling dynamics, is considered for the qualitative description of each tunneling pathway, however, the variation of tunneling splittings with vibrational excitation indicate that the high barrier approximation does not appear to be applicable in the excited state. The data are consistent with a motion possessing a′ symmetry, and the vibration is assigned as the ν8 acceptor bending coordinate. This assignment is in agreement with the vibrational symmetry determined from the data, the results of high level ab initio calculations, and preliminary data assigned to the analogous vibration in D2O–DOH.
Simple model of dehydration transitions in lamellar systems, such as lipid membranes
Authors: Cevc G. , Kornyshev A. A.
Published: July 2002

A simple, exactly solvable ‘‘Gaussian’’ model for the hydrophilic surfaces with an intrinsic lateral periodicity is introduced. This model describes the crossover from a repulsive mode of the hydrational interaction between such surfaces to an attractive one. The resulting transition takes place gradually upon decreasing temperature. Even more importantly, this transition is promoted by the frustration of the water structure near the surfaces and by the increased water binding to the hydrophilic surface residues. Model conclusions and experimental conditions under which the dehydration transitions occur, or might occur in the lipid lamellar systems are discussed.
Exciton transport and degenerate four wave mixing in topologically disordered systems
Authors: Wang N. , Leegwater J. A. , Mukamel S.
Published: July 2002

The static and dynamical properties of excitons in a lattice with a random distribution of polarizable atoms are studied using Green’s function techniques. Exciton transport is related to the configurationally averaged particle–hole Green’s function which is calculated using the ladder diagram approximation. Degenerate four wave mixing (D4WM) using resonant pump beams and an off resonant probe is shown to provide a direct probe for exciton transport. A disorder induced extra resonance is predicted whose width is proportional to the exciton diffusion coefficient. Numerical calculations are presented for the diffusion coefficient and the D4WM signal as the excitation energy is tuned across the exciton band.
Surface plasmon enhanced Raman spectroscopy with HS(CH2)21OH on different metals
Authors: Nemetz Axel , Fischer Thomas , Ulman Abraham , Knoll Wolfgang
Published: July 2002

A method is presented that expands plasmon surface polariton field enhanced Raman spectroscopy (PSPR) to arbitrary metal substrates. Using this technique the ratio of the Raman tensor elements αzz/αxx for the alkane chain in HS(CH2)21OH on silver is determined. Based on this knowledge we also investigate PSPR spectra of HS(CH2)21OH adsorbed on copper and gold in order to determine its tilt angle with respect to the metal surface.
Photoacoustic Fourier transform infrared spectroscopy of gases: A nonequilibrium approach
Authors: Huang Jingbing , Urban Marek W.
Published: July 2002

The first order approximations of the nonequilibrium mass balance and momentum balance equations allow predictions of the nonequilibrium temperature–pressure relationships in a photoacoustic effect. This is achieved by setting appropriate boundary conditions for the case when frequency of the normal vibrational modes in infrared is significantly greater than the photoacoustic modulation frequency. Based on the internal energy balance equations, the temperature distributions in the gas phase are derived and tested experimentally, allowing quantitative analysis of the photoacoustic FT-IR spectra.
Nonlocal thermodynamic equilibrium effects of vibrationally excited NO
Authors: Moralejo C. , Cody R. J. , Allen J. E.
Published: July 2002

The vibrational distribution of NO(X 2π) from the reaction O(3P)+NO2(X 2A1)→NO(X 2π,v″)+O2 was studied at room temperature in a fast-flow system. Oxygen atoms were produced via the microwave-discharge dissociation of molecular oxygen with argon as the carrier gas. The reaction time between the NO2 molecules and the oxygen atoms was kept at ∼3 ms to minimize vibrational energy quenching. NO(X) radicals were detected via laser-induced fluorescence of the A 2Σ+←X 2π electronic transition. The relative vibrational population ratio of NO in the ground electronic state was determined to be 87.4±2.6% and 12.6±2.7% in the v″=0 and v″=1 levels, respectively. No higher vibrational states were observed. The NO vibrational population ratio measured in this study correlates well with the earlier observation of high vibrational excitation in O2.
Electronic-to-vibrational, -rotational, and -translational energy transfer: S(1D)+CO, N2, O2, and CO2 measured by Doppler spectroscopy
Authors: Nan G. , Neyer D. W. , Houston P. L. , Burak I.
Published: July 2002

Collisions of S(1D) with CO, N2, O2, and CO2 have been investigated to estimate the fraction of the sulfur electronic energy that is deposited in the internal degrees of freedom in the collision partner during the quenching of S(1D) to S(3P). The experiment measures the Doppler profile of the S(3P) product, a profile that depends both on the amount of energy disposed into the internal degrees of freedom and on the differential scattering cross section for the inelastic collision. For CO and N2 the results are consistent with a collision complex model for which the scattering is assumed to be isotropic in the collision plane and for which the energy is partitioned statistically into the degrees of freedom. Under the assumption of isotropic scattering, the results suggest that less energy than the statistical prediction is partitioned into translation for collisions with O2, whereas more energy is partitioned into translation for CO2.
Reactive collisions of atoms with diatomic molecules
Authors: Wolniewicz L. , Hinze Juergen , Alijah Alexander
Published: July 2002

The theory of the reactive collision of an atom with a diatomic molecule is formulated in ‘‘democratic’’ hyperspherical coordinates. An adiabatic ansatz is used to separate the distance coordinate from the angular coordinates. The angular eigenvalue problem is solved, using the hyperspherical harmonics as basis functions, while the R-matrix propagation method is used to integrate the resulting coupled equations along the distance coordinate. As an example, reactive collision probabilities for H+H2 are computed, using the Porter–Karplus surface. The symmetry requirements, when dealing with three identical Fermions in the collision, are considered explicitly.