bluuebunny/crossref_metadata_2025 huggingface dataset:
A Fourier transform infrared study of the D and 13C substituted C2H3 vinyl radical in solid argon
Authors: Shepherd Richard A. , Doyle T. J. , Graham W. R. M.
Published: July 2002

A Fourier transform infrared (FTIR) isotopic matrix study of the C2H3 vinyl radical produced by the vacuum UV photolysis of ethylene has been carried out in the midinfrared from 250 to 4800 cm−1. A variety of carbon-13 and deuterium substituted ethylene parent molecules were used to form various isotopomers of C2H3. On the basis of its isotopic behavior and by comparison with Dupuis and Wendoloski’s GVB ab initio calculations, an absorption at 900 cm−1 has been assigned to the ν7, out-of-plane bending mode of C2H3. The following values for ν7 have been obtained for vinyl isotopomers: 887 cm−1 for 12CH2=12CD; 797 cm−1 for 12CD2=12CH; 704 cm−1 for 12CD2=12CD; 889 cm−1 for 13CH2=12CH; 895 cm−1 for 12CH2=13CH; and 894 cm−1 for 13CH2=13CH.
Electron spin echo envelope modulation amplitudes: A perturbation treatment of I=1 nuclei in extreme quadrupole coupling limits
Authors: Flanagan Heather L. , Singel David J.
Published: July 2002

ESEEM (electron spin echo envelope modulation) amplitudes for an S=1/2, I=1 system with isotropic hyperfine interaction in an orientationally disordered solid are derived by perturbation treatment in the limits of extreme quadrupole coupling. The derived expressions are shown to account for peak absences and other features noted in simulated spectra [H. L. Flanagan and D. J. Singel, J. Chem. Phys. 87, 5606 (1987)].
Very low temperature spectroscopy: The pressure broadening coefficients for CO–He between 4.3 and 1.7 K
Authors: Willey Daniel R. , Crownover Richard L. , Bittner D. N. , De Lucia Frank C.
Published: July 2002

The attributes of very low temperature spectroscopy in the context of a simple collisionally cooled cell are described. Among them are narrow linewidths, large absorption coefficients, and the simplification of complex spectra. Most importantly, the technique makes possible the study of gas phase collisional processes under equilibrium conditions in a variable temperature environment below 4 K. In this regime the rotational energy level spacing is typically larger than kT, thereby both significantly simplifying the most exact theoretical calculations and giving a more direct link between experimental observables and fundamental molecular parameters. The results of a pressure broadening experiment in the CO–He system in the region between 4.24–1.78 K are described. It was found that the pressure broadening parameters varied from 21.3 to 40.0 MHz/Torr over this temperature region. Comparison with earlier theoretical calculations showed excellent agreement at the higher temperatures; however, at the lowest observed temperature the theoretical value was about 20% greater than the measurement.
Collision energy dependence of the chemiluminescent reaction: Ba+N2O→BaO+N2
Authors: Alcaraz C. , de Pujo P. , Cuvellier J. , Mestdagh J. M.
Published: July 2002

The chemiluminescence spectrum of the reaction Ba+N2O→BaO+N2 has been studied using a crossed beam apparatus as a function of the collision energy over the range 0.1–0.6 eV. The relative importance of its red wing increases as the collision energy is raised. Moreover, the cross section associated to chemiluminescence at a wavelength λ of the range 450–700 nm goes through a maximum as the collision energy is varied, the position of which depends significantly on the value of λ. The importance of this effect cast some doubt about works, where the energy dependence of the ‘‘total’’ chemiluminescence is measured using a detector which does not have a constant response over the wide range of the chemiluminescence. The analysis of the present results reveals that increasing the collision energy results in lowering the average vibrational excitation present in the emitting states A 1Σ+ and A′ 1Π of the product BaO.
Lanczos recursion, continued fractions, Padé approximants, and variational principles in quantum scattering theory
Authors: Duneczky Csilla , Wyatt Robert E.
Published: July 2002

Connections are established between the Schwinger and Newton variational principles and recursive generation of the remainder in Born series expansions of individual K-matrix elements. It is shown that Lanczos development of the remainder yields results identical to either of these variational principles, depending upon the starting vectors that are used to initiate the recursion sequence. In all cases, the correction to the Born series is computed from the 1,1 element of the inversion of a small tridiagonal matrix. The relationship to Padé approximants and continued fractions is also noted. Numerical results on the convergence of elastic and inelastic K-matrix elements are presented for a model system.
Thermodynamic properties and equation of state of dense fluid nitrogen
Authors: Belak J. , Etters R. D. , LeSar R.
Published: July 2002

Results of constant-pressure Monte Carlo calculations on dense fluid nitrogen over a pressure range of 2 to 300 kbar and a temperature range of 300–3000 K are presented. From analytic fits to the calculated volumes, enthalpies and vibrational frequency shifts, a comprehensive set of thermodynamic quantities is derived, including: thermal expansivity, compressibility, specific heat, Grüneisen parameter, and speed of sound. Comparison of the theoretical results to experiment at room temperature shows very good agreement (within 0.3% in volume and 1% in speed of sound, for instance). Good agreement is also obtained with earlier simulation data. In agreement with experimental studies of fluid metals, we find that the speed of sound varies linearly with density; along isotherms as well as along the Hugoniot. We find that ργG, the density times the Grüneisen parameter, which is assumed to be a constant in an often-used phenomenological equation of state, varies considerably with density and temperature. Comparison is made with results from a ‘‘soft sphere’’ model presented in an accompanying paper.
Reinterpretation of the main absorption band of 1,3-butadiene
Authors: McDiarmid Ruth , Sheybani Abdol-Hakim
Published: July 2002

We have measured the near ultraviolet absorption spectrum of 2,3-dideuterobutadiene to provide a complete set of experimental Bu←X vibrational intervals and bandwidths for all symmetrically deuterated butadienes. These vibrational intervals and bandwidth ratios are compared with the ground state vibrational frequencies and frequency ratios of the molecules. The prominent vibrational frequency interval observed in transitions to the Bu state of butadiene is demonstrated to arise predominantly from a kinetic coupling of the C=C stretching and CH wagging vibrations. The experimental bandwidth ratios are shown to correlate with single quanta of the ground state au CH2 twist frequency interval ratios. From the latter, a plausable decay path for the Bu excited state of butadiene is deduced. The implications of these conclusions on prior and present attempts to determine the butadiene Bu equilibrium geometry and to understand polyene spectroscopy, photochemistry, and photophysics are discussed.
A molecular beam scattering investigation of the oxidation of CO on Rh(111). I. Kinetics and mechanism
Authors: Brown L. S. , Sibener S. J.
Published: July 2002

The catalytic oxidation of carbon monoxide on a Rh(111) surface has been investigated using modulated molecular beam techniques. Reaction proceeds via a Langmuir–Hinshelwood mechanism. Under experimental conditions which provide a high coverage of oxygen adatoms and near zero coverage of adsorbed CO, an activation energy of 24.5±0.4 kcal/mol and a preexponential factor of 2±1×10−3 cm2 s−1 were obtained. The angular distribution of the product CO2 is sharply peaked toward the surface normal, and cannot be described by a simple cosnθ expression. Present results are discussed in relation to previous work on platinum and palladium surfaces.
The fractal nature of the surfaces of porous silicas as revealed in electronic energy transfer between adsorbates: Comparison of three donor/acceptor pairs
Authors: Pines Dina , Huppert Dan , Avnir David
Published: July 2002

Electronic energy transfer between adsorbed rhodamine 6G (R6G, donor) and malachite green (MG, acceptor) was studied on nonporous silica, on silica with average pore diameter of 60 Å (Si–60), on Si-100 and on Si-200. The decay profiles obey the Klafter–Blumen equation for the survival probability in fractal environments, with fractal dimensions D of 2.1, 2.3, 2.5, and 2.7, respectively, in good agreement with the results obtained with two other pairs, rhodamine-B/MG and R6G/R6G (in a depolarization experiment). The invariance of D to changes in the critical Förster radius, suggests that the derived D values reflect structural scale invariance properties of the supports and in particular the surface fractal dimension as ‘‘seen’’ by surface/molecule interaction. This interpretation compares favorably with other surface studies of SiO2.
Collisionally induced population transfer effect in infrared absorption spectra. I. A line-by-line coupling theory from resonances to the far wings
Authors: Boulet C. , Boissoles J. , Robert D.
Published: July 2002

The line-by-line coupling for a pressure broadened rovibrational band is formulated in the far-wing limit. The present quasistatic theory assumes that the wave frequency is displaced from the line centers by an amount that is large compared with both the reciprocal duration of a typcial binary collision and frequency separation between strongly coupled lines. This theory generalizes that of Rosenkranz [J. Chem. Phys. 83, 6139 (1985)] where the Fano’s relaxation operator was reduced to a scalar parameter through a band average. The present approach permits computation of far-wing absorption more specifically tailored to individual lines. Such a line-by-line approach is needed for rovibrational bands where some far lines contribute significantly to the total absorption. In order to obtain a qualitative picture of the line coupling as a function of the frequency displacement, calculations for collisions of CO2 with Ar have been performed for some lines. The results are compared with the corresponding variation of the diagonal relaxation matrix element.
Unimolecular reactions in isolated and collisional systems: Deuterium isotope effect in the photoisomerization of stilbene
Authors: Courtney Scott H. , Balk Michael W. , Philips Laura A. , Webb Steven P. , Yang Ding , Levy Donald H. , Fleming Graham R.
Published: July 2002

The isomerization of t-stilbene (stilbene h12 ) and three deuterated derivatives has been studied in a supersonic expansion, the thermal gas phase, and solution. In the jet we find that almost all effect of full deuteration (stilbene d12 ) is produced by deuteration of the two ethylinic hydrogens only (stilbene d2 ). Complete deuteration of the phenyl rings (stilbene d10 ) has rather little influence on the decay of the jet-cooled molecule. Nonexponential decays are found at intermediate excess energies in the jet-cooled system, with the degree of nonexponentiality decreasing with increasing excess energy. The ordering of the decay rates observed in the jet is not consistent with previous RRKM calculations of the isomerization rates of stilbene h12 and d2. Using similar parameters the calculations consistently place the stilbene d2 and stilbene d10 curves in the wrong order. Our results suggest extensive but not complete vibrational relaxation in the isolated molecule. Vibrational redistribution rapidly becomes complete in the presence of buffer gas. In thermal samples the isomerization rates of stilbene h12 and stilbene d10 are identical over a wide range of solvents and temperatures. By contrast the isomerization rates in stilbene d2 and stilbene d12 are 1.4 and 1.5 times slower than in stilbene h12. Again, these ratios appear constant over a wide range of experimental conditions.
Predissociation of the d 1Πg Rydberg state in O2: Nature of the Rydberg–valence interactions
Authors: van der Zande Wim J. , Koot Wouter , Los J. , Peterson James R.
Published: July 2002

Predissociation of the higher vibrational states (v=4–8) of the d 1Πg Rydberg state is investigated experimentally using translational spectroscopy. These states predissociate to the O(1D)+O(1D) dissociation limit. Individual rotational levels of the v=4 state are observed for the first time, and the energies and rotational constants for both 16O2 and 18O2 isotopes are established. The observed predissociation behavior and natural linewidths are explained by invoking both Rydberg–valence and valence–valence interactions. The nearly diabatic nature of the 1 1Πg and 2 1Πg valence states is deduced from these observations. One of the‘‘diabatic’’ valence states is ‘‘bound,’’ and its perturbations on the v=1 to 3 of the Rydberg state explain observations reported from resonance enhanced multiphoton ionization experiments.
Resonance Raman depolarization in the S3 state of CS2 vapor
Authors: Li Bulang , Myers Anne B.
Published: July 2002

An approach is presented and demonstrated for determining predissociation lifetimes of small molecules in the vapor phase at room temperature through measurement of resonance Raman depolarization ratios. Depolarization ratios for low-pressure CS2 vapor have been measured using ten excitation wavelengths between 1995 and 2001 Å, spanning one of the strongest vibrational features of the S3←S0 electronic transition. The excitation wavelengths are obtained by anti-Stokes Raman shifting in H2 the fourth harmonic of a Nd:YAG laser and the second harmonic of a YAG-pumped dye laser. The Raman depolarization ratio ρ(I⊥/I∥) varies from less than 0.4 with excitation on the high-frequency side of the vibronic band to nearly 0.6 on the low-frequency side. Numerical simulation of the depolarization dispersion curve using literature values for the rotational constants shows that this behavior can be attributed straightforwardly to partially selective excitation of initial J states for which the S3 predissociation lifetime is either long or short relative to rotational periods. The best fit between the experimental and calculated depolarization dispersion curves is obtained with a homogeneous linewidth of 6 cm−1 (excited state lifetime 0.9 ps). The relevance of this result to measurements of dissociation times through photofragment anisotropy analysis is discussed.
Random tempering of Gaussian-type geminals. III. Coupled pair calculations on lithium hydride and beryllium
Authors: Alexander S. A. , Monkhorst H. J. , Szalewicz K.
Published: July 2002

We use random tempering formulas and a sorting procedure to produce basis sets of explicitly correlated Gaussian-type geminals for the calculation of the coupled pair energies of the beryllium atom and lithium hydride. These energies show rapid convergence and are comparable to calculations which use a basis set where all nonlinear parameters have been optimized. Our complete coupled pair energies are 92.95 and 82.65 mhartree for Be and LiH, respectively. When combined with literature values for single and triple excitations, our results provide the most accurate ab initio calculated energies to date for Be and LiH.
Solvent influenced nonadiabatic transitions: A classical kinetic theory
Authors: Dardi Peter S. , Cukier R. I.
Published: July 2002

We derive an Enskog level kinetic equation for a time correlation function appropriate to the description of weak quantum transitions induced by collisions in a classical equilibrium fluid. This time correlation function involves propagation on two potential surfaces, reflecting the dependence of the elastic potentials on the quantum state. The dynamical variable of the correlation function is the phase space density of a tagged pair of molecules; one member of the pair undergoes the transition and the other induces it. The original motivation for this work was the calculation of vibrational relaxation rates in fluids [P. S. Dardi and R. I. Cukier, J. Chem. Phys. 86, 2264, 6893 (1987)], but the results derived here should also be applicable to other weak nonadiabatic processes, e.g., electronic transitions. The kinetic equation includes the interactions between the tagged pair exactly. In its low density limit, the rate constant based on the kinetic equation corresponds to the thermally averaged, semiclassical distorted wave Born approximation result. The solvent-tagged particle interactions are described at the Enskog level, which includes the effects of all uncorrelated collisions. The tagged particle–solvent collision operators depend on the quantum state of the molecule undergoing the transition. This new effect is not incorporated in more phenomenological descriptions of solvent influenced nonadiabatic transitions. Our derivation is based on a binary collision expansion. We also derive Enskog level kinetic equations, using the binary collision expansion, for standard one potential surface, tagged single particle and tagged pair time correlation functions. We compare our results for the standard time correlation functions with results from renormalized kinetic theory to show that the correct Enskog kinetic theory for both hard and soft forces is obtained.
Dynamics of molecular reactions in solids: Photodissociation of HI in crystalline Xe
Authors: Alimi R. , Gerber R. B. , Apkarian V. A.
Published: July 2002

The photodissociation of HI impurities in a crystalline Xe host is studied by molecular dynamics simulations. From the calculated trajectories, analyses are given for: The behavior of the HI molecule before dissociation, the motions of the fragments following photon absorption, and the sites and vibration dynamics of the H fragments long after dissociation. The main findings include: (i) The photodissociation yield as a function of temperature is not monotonic (0 at 0 K, 0.2 at 17 K, 0.1 at 35 K). (ii) The nascent H atoms, at early time (∼0.5 ps), exhibit well defined, high frequency (∼900 cm−1) vibrations in the cage. The H trajectories acquire increasingly a random walk character with the progress of time. (iii) H exit from the cage is virtually never direct. (iv) After relaxation to equilibrium, the product H atoms occupy dilated interstitial sites (nearest-neighbor xenons displace by 0.3 and 0.6 Å at the octahedral and tetrahedral interstitial sites, respectively). (v) The H atom dynamically distorts the octahedral site and exhibits three well-separated local vibrational frequencies, corresponding to motions along well-defined axes of the site. (vi) The reagent HI molecular rotations are strongly hindered at low temperatures, and are more aptly described as large amplitude bendings associated with the complex Xe⋅⋅⋅HI. The experimental implications of the above findings and the possible consequences of quantum effects are discussed.
The structure of the nitrous oxide dimer from sub-Doppler resolution infrared spectroscopy
Authors: Huang Z. S. , Miller R. E.
Published: July 2002

The opto-thermal detection method has been used to obtain rotationally resolved infrared spectra for both (14N14N16O)2 and (15N14N16O)2. The vibrational band observed in each case correlates with the ν1+ν3 band of the N2O monomer. A large number of A- and B-type transitions have been assigned and fit for both species yielding accurate molecular constants for both the ground and excited vibrational states. From intensity alternations associated with nuclear spin statistics and the small inertia defect [Δ″=0.611 amu Å2 for (14N14N16O)2], it is clear that the nitrous oxide dimer has a slipped parallel geometry which possesses a center of symmetry. Isotopic substitution shows that of the two such structures, only the one having the oxygen atoms close to one another is experimentally observed. The structural constants are determined to be RNN=3.493(4) Å and θNNO=59.2(5)°.
Competitive charge–transfer and vibrational quenching of N+2 (X,v=1) in collisions with O2 and NO
Authors: Ferguson E. E. , Richter R. , Lindinger W.
Published: July 2002

The competitive charge–transfer and vibrational quenching of N+2 (X,v=1) ions in collision with O2 and NO molecules have been found to have comparable rate constants. The vibrational relaxation rate constants are rationalized in terms of a vibrational predissociation mechanism occurring in relatively long-lived complexes (τ∼10−10−10−11 s). The large magnitudes are attributed to unusually large attractive ion–neutral interaction potentials resulting from a charge–transfer stabilization interaction resulting from the near resonance of the N+2 +NO→NO+(a 3Σ+)+N2 and N+2 +O2→O+2 (a 4Πu) +N2 interactions. The more favorable orbital overlap of the O+2(a 4πu) with N2 relative to that for O+2(X 2Πg) with N2 may explain the slight preference for vibrational quenching over charge transfer and also the order of magnitude faster O+2(a 4πu) charge–transfer with N2 compared to N+2 charge–transfer with O2.
Effects of solute–solvent coupling and solvent saturation on solvation dynamics of charge transfer reactions
Authors: Hwang J.-K. , Creighton S. , King G. , Whitney D. , Warshel A.
Published: July 2002

The effect of the solute–solvent coupling on the dynamics of adiabatic charge transfer reactions is examined. A two-state description of the solute electronic configuration is implemented to adequately treat the gradual transfer of charge from the donor to acceptor as the reaction progresses. A formalism is developed for separating the solute and solvent contributions to the reaction coordinate (defined here as the difference in energy between the reactant and product states) and equations of motion are derived for these coordinates. It is found that these equations of motion are far more elaborate than the ones considered in the diabatic case (weak coupling between solute electronic states). In particular, they must reflect the effect of the solvent polarization on the solute polarization. The corresponding coupling term is missing in most treatments of charge transfer processes. To explore the coupled dynamics of the reactive trajectories we use both direct downhill simulations and a new approach which evaluates the average solvent coordinate using the linear response approximation. Our study indicates that the reactive fluctuations are characterized by both the polarization time of the solute dipole and the relaxation time (or times) of the solvent. The nature of the relevant solvent relaxation time is explored on a microscopic level for different charge configurations and different solute radii. The results demonstrate that the autocorrelation function of the solvent coordinate (which is proportional to the solvent reaction field) reflects saturation effects and depends on the solute size and charge distribution. Thus we conclude that the microscopic dynamics of the solvent can deviate from the corresponding macroscopic estimate.
Order–disorder transitions in Langmuir–Blodgett films. IV. Structure of [F(CF2)8(CH2)1COO−]2 Cd2+ multilayers at ambient and elevated temperatures
Authors: Naselli C. , Swalen J. D. , Rabolt J. F.
Published: July 2002

Langmuir–Blodgett multilayered (6–12) films of a semifluorinated fatty acid, F(CF2)8(CH2)10COOH, have been deposited on transmission and reflection substrates and investigated using polarized infrared (IR) spectroscopy. The spectra revealed that the cadmium salt of this fatty acid was transferred and not the acid itself. Unlike its hydrocarbon analog, [F(CF2)8(CH2)10COO−]2 Cd2+ is not oriented normal to the surface but is inclined due to the packing constraints dictated by the considerably larger cross section of the fluorocarbon part of the molecule. At elevated temperatures, the orientation of the fluorocarbon chain is observed to change and this has been attributed to disordered conformations preferentially introduced in the hydrocarbon portion of the molecule due to its inherent flexibility relative to the rigid fluorocarbon helix.
Time-dependent Hartree wave packet dynamical techniques for computation of electronically excited state optical spectra of many-body quantum systems
Authors: Messina Michael , Coalson Rob D.
Published: July 2002

An approximate solution technique for computing spectra of many-body molecular systems is proposed. We focus for concreteness on 0 K electronic absorption and emission spectra. From a time-domain perspective, it is necessary to propagate a well-defined initial Schrödinger wave packet on a specified potential energy surface in order to extract such spectra. In order to perform this task for systems with many degrees of freedom, we investigate the utility of a time-dependent Hartree factorization, in which the wave packet for the complete system is variationally factorized into a product of wave packets of smaller dimensionality. This method is shown to be both flexible and reliable for prototypical model systems associated with the physical problem of impurity spectra in host crystals. Successful application is made to a recently measured emission spectrum of I2 embedded in an argon matrix.
Laser induced photodissociation and desorption. I. CH2I2 adsorbed on Al2O3
Authors: Domen K. , Chuang T. J.
Published: July 2002

CH2I2 molecules adsorbed on Al2O3 and Ag surfaces are used as model systems for studying the photofragmentation and desorption processes on solid surfaces involving electronic excitation of the adsorbate. The molecules are activated into an antibonding state by a ultraviolet (UV) light pulse and the desorption behavior is monitored by time-of-flight mass spectrometry. The surface coverage and chemical changes due to laser irradiation are determined by x-ray photoemission and thermal desorption spectroscopies. The desorption yields as well as the mass and the translational energy distributions are measured as a function of the surface coverage, the laser fluence, and the number of laser pulses. The UV radiation effect on a mixture of CH2I2 and NH3 as coadsorbates on Al2O3 is also examined. The results reveal the characteristic electronic, thermal, and ‘‘explosive’’ desorption effects depending on both the gaseous exposure and the laser irradiation conditions. The quantum yields, the molecular selectivities, the energetics of dissociation and the desorption mechanisms are discussed based on the experimental observations. For clarity, the results and analyses are presented in two parts: CH2I2/Al2O3 in paper I and CH2I2/Ag in paper II of this series.
Investigations of transformed mass-scaled Jacobi coordinates for vibrations of polyatomic molecules with application to H2O
Authors: Bowman Joel M. , Zûñiga Josê , Wierzbicki Andrzej
Published: July 2002

We present an investigation of orthogonal transformations of Delves coordinates, i.e., mass-scaled Jacobi coordinates, in applications to polyatomic molecule vibrations. First, we show for triatomic molecules with a stationary point of C2v symmetry that these coordinates can be transformed exactly to yield normal coordinates in the small displacement limit. A partial transformation to normal coordinates is also given for tetraatomic molecules with C2v symmetry. Second, we show that this transformation can be used to optimize a set of coordinates for use in variational calculations of vibrations of polyatomic molecules, and demonstrate its utility in variational calculations of vibrational energies for H2O.
State resolved cross sections for rotationally inelastic collisions of NH2(X̃ 2B1) with helium
Authors: Dagdigian Paul J.
Published: July 2002

Integral cross sections for rotational transitions from the lowest orthorotational level (00,0) of NH2 induced by collisions with helium have been measured in a crossed beam experiment. A pulsed supersonic beam of rotationally cold NH2 was produced by 193 nm photolysis of a dilute mixture of ammonia in argon seed gas at the tip of a nozzle; the helium target was also prepared as a pulsed beam. The final rotational state populations (without spin-state resolution) in the Ka=0 and 1 manifolds of NH2 were interrogated in the collision zone by dye laser fluorescence excitation in the Ã 2A1←X̃ 2B1 band system. Because of the rotational selection rules in this band system, it was not possible to put the cross sections for the two Ka manifolds on the same scale. Within a given Ka manifold, there is a marked preference for collisional transitions with the smallest ΔN change. These experimental results are compared to calculated close-coupling rotationally inelastic cross sections for H2O(00,0)–He collisions.
Dissociative photoionization of N2O: Analytical photoion spectroscopy
Authors: Masuoka Toshio , Mitani Shichiro
Published: July 2002

Branching ratios of ions produced from the dissociative photoionization of N2O have been measured by using a time-of-flight mass spectrometer and synchrotron radiation in the 15–36 eV region. The branching ratios in the Franck–Condon gap (15.0–16.4 eV) indicate that the autoionizing Rydberg states in this region predissociate to NO+ and O+ through various dissociative states. The results obtained by the analytical photoion spectroscopy above the gap show dissociation pathways of the N2O+ ions. These pathways are discussed by comparing with the reported electronic states of ions and super excited neutrals.
Theoretical studies of DNA during gel electrophoresis
Authors: Deutsch J. M. , Madden T. L.
Published: July 2002

The dynamics of a long chain macromolecule in a gel is examined using numerical and analytical approaches. In contrast with previous conjectures, it is found that the chain does not move entirely in a ‘‘tube.’’ For strong fields, an instability produced by the applied field causes the chain to frequently contract on itself. This is followed by it unwinding into an extended configuration. For lower fields, portions of the chain frequently double up on themselves and move through the gel thereby violating the tube constraint. The results can be explained by breaking down the motion into various stages, each of which can be further analyzed. The results of the simulation appear in accord with experiment and provide a more realistic description of DNA motion then tube models.
Molecular dynamics simulation of the optical absorption spectrum of the hydrated electron
Authors: Romero C. , Jonah C. D.
Published: July 2002

The optical absorption spectrum of the hydrated electron was computed at 300 K using the Feynman path integral formulation of quantum statistical mechanics in conjunction with molecular dynamics simulations. In addition, the potential energy of the hydrated electron was studied at 700 K as a function of the liquid density between 0.02 and 1.0 g/cm3 and correlated with the maximum of the absorption spectrum. The procedure to calculate the optical spectrum makes use of the solution of the Schrödinger equation for an ensemble of model potentials which span the region allowed by the fluctuations of the potential well which traps the electron in thermodynamic equilibrium. The results indicate that the absorption band is due to a strongly allowed 1s→2p transition and the breadth of the spectrum is a consequence of the fluctuations in the trap dimensions. Moreover, these calculations imply that the long tail of the absorption spectrum in the UV arises from electrons which are in deep traps. Such traps occur with low but finite probability in the ensemble of solvent configurations which are accessible to the system in thermal equilibrium. Calculations of the average potential energy of the electron as a function of the density reproduced well the measured shift in the absorption spectrum of the excess electron in water.
Brownian dynamics of a flexible polymer. Internal modes and quaiselastic scattering function
Authors: Rey Antonio , Freire Juan J. , Garcia de la Torre José
Published: July 2002

Brownian Dynamics trajectories have been simulated by means of the Ermak and McCammon algorithm for Gaussian polymer chains of different lengths with hydrodynamic interactions. The results for relaxation times and dynamic scattering functions of chains with 2–20 statistical units are close to those calculated from the preaveraged Rouse–Zimm theory. The differences between both sets of results are slightly higher than the range of uncertainties of the simulation values (about 5% in most typical cases). Moreover, the simulated scattering functions have been fitted to sums of exponentials in order to extract translational diffusion coefficients and first relaxation times, following the Pecora procedure. The values obtained this way are in satisfactory agreement with the results directly calculated from the trajectories.
Energy partitioning in Ar++O2 collisions at low energies: Analysis of product states by laser-induced predissociation
Authors: Scherbarth S. , Gerlich D.
Published: July 2002

Combining the versatility of a guided ion beam (GIB) apparatus with a tunable dye laser system, we have studied in detail the ion–molecule reaction Ar++O2→O+2+Ar at collision energies ranging from 0.04 to 3 eV center of mass (c.m.). The results include integral cross sections and product angular distributions. The extracted kinetic energy distributions provide medium resolution information about the energy partitioning, and are indicative of a significant change of the reaction mechanism between 0.05 and 0.5 eV collision energy. Nascent rovibrational state distributions of metastable O+2(a 4Π) products are obtained from 0.5 to 1.4 eV by photofragmentation. In contrast to what is generally expected from charge transfer processes, a preference for excitation of high rotational states at the expense of vibrational states has been observed. The results are discussed qualitatively on the basis of a diatomics-in-molecules (DIM) surface.
Spectroscopy and photodissociation dynamics of H2O: Time-dependent view
Authors: Zhang Jinzhong , Imre Dan G.
Published: July 2002

We present a study of H2O photodissociation dynamics in the first excited state. The absorption and emission spectra are calculated quantum mechanically using a time-dependent wave packet propagation technique. The excited state potential energy surface is obtained from ab initio data generated by Palma et al., while the ground state is a model by Reimers and Watts cast in a simple analytical form and constructed by fitting to infrared data. The variation of the transition dipole moment is taken into account by using an analytical functional form which is adjusted to fit the low resolution experimental emission spectrum. The calculated absorption spectrum is very similar to both the experimental one and results obtained with other methods. The high resolution emission spectrum is calculated for the first time and shows excellent agreement with the recently measured experimental spectrum by Hudson and co-workers. Dynamics on both the ground and excited states are examined.
Quantum analysis of high vibrational overtones of hydrogen cyanide using optimal modes
Authors: Fleming Patrick R. , Hutchinson John S.
Published: July 2002

An accurate two mode model of HCN is used to demonstrate a technique for and the utility of using optimized linear combinations of bond modes to describe molecular vibrational states. While normal modes are exact at zero energy and bond modes describe dissociations conveniently, neither set of coordinates describe accurately the motions of most of the spectroscopically accessible molecular vibrational states of HCN. We present a newly developed method of describing molecular vibrations with an optimized set of linear combinations of bond modes. The Hamiltonian is canonically transformed to these coordinates using the transformation method. Subsequent diagonalization of the rotated Hamiltonian matrix gives the unaltered energies and a much improved description of the molecular vibrations as measured by the projection of the eigenstates onto the zero order states. This technique improves spectroscopic assignments and simplifies visualization of the quantum vibrational motions of molecules. This is illustrated by resolving an apparent intensity anomaly in the overtone spectrum of HCN.
Mode specific vibrational relaxation in the acetylene–hydrogen fluoride binary complex
Authors: Huang Z. S. , Miller R. E.
Published: July 2002

The optothermal molecular beam technique has been used to record the near infrared spectrum of the acetylene–HF binary complex associated with the ν7 vibrational mode (asymmetric C–H stretch). A fit to this perpendicular band spectrum gives accurate rotational constants for both the ground and excited vibrational states, as well as the vibrational origin, namely A″=1.192 60(5) cm−1 , B″=0.157 34(1) cm−1 , C″=0.138 10(1) cm−1 , A′=1.186 90(5) cm−1 , B′=0.157 34(1) cm−1 , C′=0.138 08(1) cm−1, and ν0 =3276.2889(30) cm−1 . From the homogeneous linewidths of the observed transitions the lifetime of the excited vibrational state has been determined to be 3.6 ns. When compared with the lifetime determined previously for the ν1 mode (0.8 ns) and the data obtained previously for several other systems, it is clear that vibrational relaxation in this molecule is mode specific. A discussion is given concerning the nature of this relaxation process.
Structure and tension of the interface between dilute polymer solutions
Authors: Szleifer I. , Widom B.
Published: July 2002

We treat a lattice model of phase separation in polymer solutions in mean-field (Flory) approximation, taking account of anisotropic biases in the chain conformations in the interface, as in an earlier theory of Helfand. Near the critical point of the phase separation those biases lead to a square-gradient contribution to the free-energy density that is of de Gennes’ form, and thus to an interfacial tension that varies with polymerization index N and temperature distance below the critical point Tc−T as in the theories of Nose and of Vrij and Roebersen. In the scaling regime N→∞ and Tc−T→0 at fixed x=const N1/2(Tc−T) the surface tension σ is of the form N−1 Σ(x), with Σ(x) a scaling function that we display and that has the asymptotic behavior Σ(x)∼const x3/2 for x→0 and Σ(x)∼const x2 for x→∞. The latter contrasts with the x5/2 found earlier when no account was taken of the chain-conformation biases. These biases are displayed as functions of location in the interface. On the concentrated-phase side of the interface the concentration of horizontal links is greater, and on the dilute-phase side it is less, than in a random chain.
Comment on: ‘‘Predissociation of the d 1Πg Rydberg state in O2: Nature of the Rydberg–valence interactions’’
Authors: Friedman R. S. , Dalgarno A.
Published: July 2002

The rotational constants of the lowest vibrational levels of the d 1Πg state of O2 display an anomalous behavior which has been attributed to interactions between the Rydberg and valence 1Πg states. We confirm the suggestion by carrying out a theoretical analysis in which we solved numerically the pair of coupled equations that describe the interacting states.
Reply to Comment on: Accurate three-dimensional quantum scattering calculations for F+H2→HF+H
Authors: Zhang John Z. H. , Miller William H.
Published: July 2002

The fluorine + hydrogen reaction is studied quantum mechanically. The total reaction probability seems to be in good agreement with Yu et al. (ref. 1). Some of the calculations are repeated and found to agree well with other groups.(AIP)
A molecular dynamics study of the crystalline phases of potassium nitrate
Authors: Smith W. , Clarke J. H. R.
Published: July 2002

Constant pressure molecular dynamics simulations have been made for a simple ionic model for crystalline potassium nitrate at several pressures and temperatures which span the known phase diagram. The nitrate ion was modeled as a rigid four-site entity with distributed charges and a quaternion algorithm was used to solve the rotational equations of motion. Several structural features of KNO3 are reproduced in the simulations. The 300 K, 0.1 GPa aragonite-like phase II of KNO3 has been reproduced although in the model it appears at a pressure between 1 and 2 GPa. The disordering of the nitrate orientations which occurs at high temperatures has also been observed. At 3 GPa the stable phase of the model is very similar to the recently characterized phase IV of KNO3. Spontaneous phase transitions in the simulations did not always produce well ordered structures and this appears to be a problem inherent in the method.
Survival, relaxation, and excitation of vibrational energy during the scattering of NO molecules from a graphite surface
Authors: Vach H. , Häger J. , Walther H.
Published: July 2002

The rotational, angular, and translational scattering behavior of NO molecules prepared in a single ro-vibrational state before their interaction with a cleaved graphite surface was determined (v=1→v=1 process). Moreover, the deactivation probability of the vibrational energy into the rotational and translational motion of the molecule could be evaluated (v=1→v=0 process). Finally, it was possible to investigate the scattering behavior of NO molecules that experienced vibrational excitation during their surface encounter (v=0→v=1 process).
Two-photon spectroscopy of Rydberg states of NO
Authors: Pratt S. T. , Jungen Ch. , Miescher E.
Published: July 2002

Two-photon spectroscopy of supersonically cooled NO in the region between the NO+ X 1Σ+, v+=0 and 1 ionization thresholds reveals the n=7–12 members of the ndπ−, v=1 Rydberg series. The n=5 member of the same series is observed below the two-photon ionization threshold by using two-photon resonant, three-photon ionization. The data are analyzed by using multichannel quantum defect theory to extract rotationless quantum defects and to reveal perturbation by the nf, v=1 series. Theoretical two-photon rotational line strengths are in satisfactory agreement with the observed data.
Vibration–rotation spectrum of the acetylene–carbon monoxide van der Waals molecule in the 3 μ region
Authors: Marshall Mark D. , Prichard Diana G. , Muenter J. S.
Published: July 2002

The infrared absorption spectrum of the acetylene–carbon monoxide van der Waals molecule has been obtained in the 3 μ region. A color center laser was used to excite the vibration of the complex which corresponds to the antisymmetric hydrogen stretching mode in the monomer in a pulsed molecular jet. The form of the spectrum indicates that the molecule is a linear, hydrogen bonded complex, although excited state perturbations are present. The vibrational origin is at 3279.898(2) cm−1 and the ground state B value is 1397.4(6) MHz, corresponding to a separation of 5.011 Å for the monomer centers of mass. The spectrum cannot directly distinguish between the HCCH–CO and the HCCH–OC configurations, but the combined evidence of other hydrogen bonded complexes of carbon monoxide, the expectation that the observed hydrogen bond should be slightly shorter than the sum of van der Waals radii, and a distributed multipole analysis strongly favors the HCCH–CO configuration.
One- and two-photon photodissociation of Fe(CO)5 at 248 nm. Application of an accurate method for calculating angle resolved velocity distributions for multiple sequential bond rupture processes
Authors: Venkataraman B. K. , Bandukwalla G. , Zhang Zhuangjian , Vernon Matt
Published: July 2002

A method is presented for computing the effective center-of-mass velocity distribution for photofragments produced by sequential bond ruptures using Fourier transforms. The method has the advantage that energy and linear momentum conservation are correctly accounted for while remaining computationally feasible. To illustrate the method, the one- and two-photon photodissociation of Fe(CO)5 at 248 nm has been experimentally investigated using the crossed laser-molecular beam method and the measured velocity distributions compared to the prediction of various statistical models for the photodissociation process calculated by the Fourier transform method. The strength of the Fourier transform method is illustrated by the two-photon channel which involves five sequential bond ruptures. The main conclusion regarding the photodissociation mechanism is that a modified form of the separate statistical ensemble theory developed by Wittig and co-workers can quantitatively explain the observed velocity distributions for the one-photon process. Fluxional interchange of the CO ligands is shown not to be important on the CO elimination time scale. The two-photon products have a faster than statistical translational energy distribution which is rationalized by both a dynamical constraint on CO rotational excitation and on the change in orbital– and spin–coupling configuration of the Fe atom as the last two CO ligands are removed.
The laser photoelectron spectrum of gas phase p-difluorobenzene
Authors: Sekreta Ellen , Viswanathan K. S. , Reilly James P.
Published: July 2002

Gas phase p-difluorobenzene (PDFB) has been laser ionized from several vibronic levels of its 1B2u first excited electronic state. The vibrational state distribution of the ions generated depends on the vibronic level from which ionization occurs. The fundamental frequencies of five symmetric vibrations; ν2, ν3, ν4, ν5, and ν6, and three asymmetric vibrations; ν8, ν29, and ν30, of the lowest electronic state of PDFB+ have been assigned. Effects due to electronic configuration interaction are observed in the photoelectron spectra. Small peaks are also observed in the spectra which may reflect a change in geometry of the ground state cation.
The dipole moment function and vibrational transition intensities of OH
Authors: Nelson David D. , Schiffman Aram , Nesbitt David J.
Published: July 2002

The relative intensities of nine pairs of rovibrational transitions of OH in the v=1←0 fundamental have been measured by flash kinetic infrared absorption spectroscopy. Each pair of transitions originates from a common rotational and spin–orbit state, so that relative intensities are independent of the OH number density and quantum state distribution. The relative intensities are strongly J dependent and this dependence provides detailed information about the shape of the OH dipole moment function, μ(r), and hence the absolute infrared transition strengths. In an accompanying paper we present the theoretical basis for extracting μ(r), for an open shell diatomic like OH, from relative infrared intensities and permanent dipole moment measurements (Peterson et al.). In this work we implement those ideas and determine the OH dipole moment function to be: μ(r)=1.6498(6) D+0.561(32) D/Å (r−re )−0.75(17) D/Å2 (r−re )−1.5(11) D/Å3(r−re )3. The accuracy of μ(r) is excellent near re (re =0.970 Å), since the data used to derive it are from low vibrational states. The useful range of this function extends from approximately 0.75 to 1.35 Å. The rotationless Einstein A coefficient for the OH fundamental is determined from μ(r) to be 16.7(19) Hz. This is in considerable disagreement with most other experimental and theoretical results, but is in good agreement with theoretical calculations by Mies (18.3 Hz) and by Langhoff et al. (13.8 Hz).
Dissociative desorption from CH3I by ultraviolet-laser radiation
Authors: Kutzner J. , Lindeke G. , Welge K. H. , Feldmann D.
Published: July 2002

CH3 I molecules condensed in thick layers are exposed to pulsed UV-laser radiation at a wavelength of 266 nm. Atoms and molecules emitted from the surface are detected after a flight path by species and state selective resonant multiphoton ionization combined with a time-of-light mass spectrometer. This detection method is tested by photodissociation experiments of CH3 I in the gas phase. The two dissociation channels known from gas phase dissociation, yielding I(2 P3/2 ) and I*(2 P1/2 ) are identified in the velocity distribution of CH3 fragments desorbed from the surface. At low UV fluences this ‘‘direct’’ dissociative photodesorption dominates. Its similarity with gas phase photodissociation is studied in some detail. At UV fluences above 1.5 mJ/cm2 a strong increase of the overall desorption yield is observed and the composition of desorbed species changes. The process becomes similar to UV-laser photoablation of polymers.