bluuebunny/crossref_metadata_2025 huggingface dataset:
Rovibrational state mixing in the C–H stretch fundamental region of norbornadiene
Authors: Minton T. K. , Kim H. L. , McDonald J. D.
Published: July 2002

Infrared laser induced fluorescence has been used to probe the extent of state mixing, as well as its rotational dependence, in the C–H stretch fundamental region of norbornadiene in the electronic ground state. Both total and dispersed IR fluorescence were collected from molecules, cooled, and isolated in a molecular beam, after excitation with an infrared optical parametric oscillator. The dilution factor, which is inversely related to the number of coupled states, has been measured for four C–H stretch bands, and in addition, the rotational dependence of the dilution factor has been investigated by exciting different positions, or average J values, within a single vibrational band. The relative energy contents of various vibrational modes, whose fluorescence was observed, have also been determined. An approximate average coupling width of 0.05 cm−1 can be derived from the dilution factors and the calculated state density of the rovibrational bath states {‖li〉} that are permitted by symmetry and angular momentum to couple to the zeroth order optically active rovibrational state ‖s〉. The rotational dependence of the dilution factor shows that the number of mixed states scales linearly with J, where J is the total angular momentum quantum number for the ‖s〉 state. Allowing for both Coriolis and Fermi couplings within a width of 0.05 cm−1, we can adequately predict the extent of state mixing and the distribution of the excitation energy among the modes of the molecule by using a straightforward statistical model, which assumes equal coupling between all the zeroth order rovibrational states that have the correct energy, symmetry, and J to couple to a given optically active state.
Rotationally resolved hot atom collisional excitation of CO2 001 and 002 stretching vibrations by time-resolved diode laser spectroscopy
Authors: O’Neill J. A. , Wang C. X. , Cai J. Y. , Flynn G. W. , Weston Ralph E.
Published: July 2002

A tunable infrared diode laser has been used to monitor the time-resolved absorption of rovibrationally excited CO2 molecules produced in collisions with fast hydrogen atoms from the 193 nm excimer laser photolysis of H2S. Nascent relative rotational population distributions were measured for the 0001 and 0002 vibrational states in CO2 following collisional excitation. The nascent distributions for both of these states were found to be significantly displaced from the 300 K Boltzmann distribution with peaks at J=31 and J=34 for 0001 and 0002, respectively. An approximate diffusion treatment for the mass transport of fast moving hydrogen atoms out of the region of the diode laser probe beam was developed and used to obtain approximate absolute scattering cross sections. Sufficient energy is available in the H/CO2 collision process to excite 0001 rotational states with J greater than 140. The fall-off in excitation probability for J>31 is ascribed to the small impact parameter necessary to produce vibrationally excited CO2 (0001). This in turn limits the total angular momentum available in those collisions which actually produce 0001. These results are compared with previous experiments measuring the total probability for collisional excitation of all ν3 quanta as well as with various models for inelastic collisional scattering.
Infrared spectroscopy of the hydrogen cyanide dimer
Authors: Jucks K. W. , Miller R. E.
Published: July 2002

Sub-Doppler resolution infrared spectra have been obtained and assigned for four vibrational bands of the hydrogen cyanide dimer, namely the ν1 and ν2 C–H stretching fundamentals and two hot bands. Accurate molecular constants are reported for all of these. In the case of the free C–H stretch (ν1 ), Stark spectra have also been recorded in order to determine the dipole moments of this complex in both the ground and vibrationally excited states. The linewidths obtained for the individual rovibrational transitions indicate that the vibrational predissociation lifetime of this complex is highly mode specific.
Interactions of low energy reactive ions with surfaces. I. Dose and energy dependence of 3–300 eV C+, O+, and CO+ reactions with a Ni(111) surface
Authors: Kang Heon , Kasi Srinandan R. , Rabalais J. Wayne
Published: July 2002

Reactions of C+, O+, and CO+ ions with a clean Ni(111) surface are studied as a function of both kinetic energy (Ek) and ion dose for a range of chemically important energies (3–300 eV). An energy- and mass-selected, differentially pumped ion beam and ultrahigh vacuum spectrometer system which is designed to perform Ek dependence measurements of low-Ek ion–surface interactions is described. The Ni surface is exposed to low-Ek beams of these ions and concentrations of the resulting adspecies are monitored by Auger electron spectroscopy (AES) and x-ray and UV photoelectron spectroscopy (XPS and UPS). Reactions of C+ and O+ with Ni(111) yield nickel carbide (for monolayer doses) and oxide, respectively. These reactions are efficient at low Ek; reaction probabilities Pr =0.6–0.8 are observed for Ek <20 eV. Pr decreases slowly with increasing Ek until implantation of ions into the solid becomes significant. In the reaction of CO+ with Ni(111), the yield of molecular CO on the surface drops rapidly from 0.5 to below 0.1 in the range 3–10 eV. Collisional dissociation of CO+, which results in formation of surface NiC and NiO adspecies, increases with Ek and becomes the dominant process above 9 eV. Depth profile studies of the reacted surfaces indicate that implantation of C+ into subsurface Ni layers is more facile than implantation of O+ in the high energy portion of the range investigated. A Monte Carlo simulation of the atomic ion penetration into Ni has been carried out and the results are compared with experimental measurements. Reaction, desorption, and collision induced dissociation of the reactive ions and their reaction energetics are discussed in terms of qualitative potential energy diagrams in order to interpret the observed Ek dependence.
Comment on: ‘‘Quantum scattering via the S-matrix version of the Kohn variational principle’’
Authors: Zhang J. Z. H. , Miller William H.
Published: July 2002

Rescigno and Schneider(ref.2), have suggested a simplification in the S−matrix version of Kohn variational method. It is pointed out that their idea works not only for electron−atom/molecule scattering, but also for heavy particle reactive scattering. Applications to atomic hydrogen and fluorine reactions with molecular hydrogen are considered.(AIP)
Configuration entropy dependence of cooperative relaxation properties in glass-forming liquids
Authors: Mohanty Udayan
Published: July 2002

We propose a model based on separating the anharmonic vibrations from the stable packing configurations that elucidates how the loss of configurational entropy Sc gives rise to kinetic sluggishness as the temperature of a glass forming liquid is lowered towards T2, where Sc vanishes. The shear viscosity, the self-diffusion coefficient, and the structural relaxation time of a cooperative rearranging region are evaluated. We calculate the time necessary to establish an equilibrium distribution of subcritical clusters in the supercooled liquid. A comparison of the induction time with the structural relaxation time sheds light into the nature of the transition at T2.
An analytical six-dimensional potential energy surface for (HF)2 from a
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o calculations
Authors: Bunker P. R. , Kofranek Manfred , Lischka Hans , Karpfen Alfred
Published: July 2002

We have developed an analytical expression for the 6D potential energy surface of (HF)2 and fitted it to 1061 ab initio points covering an energy range of up to 25 000 cm−1 above equilibrium. The ab initio calculation used the coupled pair functional approach with an extended polarized basis set. We have adjusted 42 parameters (and constrained 7 others) in the fitting of the analytical function to the points; the standard deviation of the weighted fitting is 26.8 cm−1 . The minimum energy path from the linear saddle point (345 cm−1 above equilibrium), through the equilibrium point, to the C2h saddle point (332 cm−1 above equilibrium) has been determined, and several cuts through the potential are presented. We plan to use this potential surface in reaction path and close coupling calculations of rotation–vibration term values.
Slow manifolds and mixed-mode oscillations in the Belousov–Zhabotinskii reaction
Authors: Barkley Dwight
Published: July 2002

The mixed-mode oscillations observed at high flow rates in the Belousov–Zhabotinskii (BZ) reaction are considered and comparison is made between these oscillations and the dynamics of three different mathematical models based on slow manifolds. It is shown that the model proposed by Rössler for the generation of complex behavior in nonequilibrium chemical reactions is in conflict with the behavior of the BZ reaction. It is also shown that a slow-manifold model based on the hysteresis-Hopf normal form fails to accurately reproduce the oscillations found at high flow rates in the BZ system. A model of the type first proposed by Boissonade is presented; the model consists of the coupling of two simple systems. It is shown that this model naturally generates mixed-mode oscillations like those observed in the BZ reaction.
Picosecond study of the population lifetime of CO(v=1) chemisorbed on SiO2-supported rhodium particles
Authors: Heilweil E. J. , Cavanagh R. R. , Stephenson J. C.
Published: July 2002

Infrared pump–probe characterization of the excited state lifetimes reveals that CO bound to isolated metal sites (T1=140±20 ps) persists longer than the signal observed for CO bound to ≈35 Å diameter metal particles (≤18 ps), suggesting paticipation of electron–hole excitations in the larger metal particles.
Depolarization ratios of resonance Raman scattering in the gas phase
Authors: Ziegler L. D. , Chung Y. C. , Wang P. , Zhang Y. P.
Published: July 2002

All three scattering invariants, isotropic, antisymmetric, and anisotropic, contribute to the resonance Raman (RR) activity of randomly oriented gas phase scatterers as shown in a sum-over-all-rovibronic-states approach regardless of the symmetry of the vibrational transition. Thus, in the gas phase, totally symmetric modes may have Raman depolarization ratios (ρ)>3/4 and nontotally symmetric modes may have ρ≠3/4 for resonance excitation frequencies. Large ρ dispersion effects are predicted when the RR scattering cross section is dominated by the contribution of a single vibronic band. The extent of this purely rotational effect sensitively depends on the excitation frequency and the resonant dephasing rate. These depolarization ratio effects are demonstrated by the resonance Raman scattering of CH3I (B state) and NH3, ND3 (A state) for both a and e vibrational bands as well as for J (NH3) and K (CH3I) resolved rovibrational features.
Quantum reactive scattering in three dimensions using hyperspherical (APH) coordinates. III. Small θ behavior and corrigenda
Authors: Pack Russell T , Parker Gregory A.
Published: July 2002

We continue development of the theory of reactive (rearrangement) scattering using adiabatically adjusting, principal axis, hyperspherical (APH) coordinates. The behavior of the solutions for small θ (oblate symmetric top) configurations is examined, and it is shown that the exact surface functions need not be continuous at θ=0. A procedure for testing the importance of the small θ region and the adequacy of the usual APH centrifugal sudden basis of surface functions is presented and illustrated by example calculations on the even parity, J=1, H3 system. A list of corrections to earlier papers in the series is also given.
A theoretical study of the Ar2HCl van der Waals cluster
Authors: Hutson Jeremy M. , Beswick J. Alberto , Halberstadt Nadine
Published: July 2002

A theoretical method for treating the dynamics of polymeric van der Waals clusters is developed, based on an adiabatic separation of the heavy atom motions. The method is applied to the calculation of spectroscopic parameters for the complex Ar2HCl, and the results are compared with experimental data from high-resolution microwave studies. Potentials based on pairwise additivity and the known Ar–HCl pair potential are used. Small discrepancies between experiment and theory are observed, and it seems likely that these are attributable to the effects of three-body forces rather than to deficiencies of the pair potentials used. High-resolution spectroscopy of van der Waals clusters shows great promise as a tool for investigating nonadditive intermolecular forces.
Anisotropic displacive to order–disorder crossover of the orientational phase transition in anthracene-tetracyanobenzene crystal
Authors: Luty T. , Lefebvre J.
Published: July 2002

A microscopic model for the orientational instability in charge–transfer anthracene-tetracyanobenzene crystal has been proposed. The model is essentially for one degree of freedom but its properties are the same as that called the pseudospin-phonon model. The essential idea presented here is that a character of a degree of freedom is modulated by intermolecular coupling constants. For crystals, where the couplings are anisotropic, a given degree of freedom can change its character from small (angular) displacements for strong coupling directions (planes) to a discrete, pseudospin variable proper for weak coupling directions. The displacive to order–disorder crossover is seen as a change of character of a degree of freedom due to the strength (anisotropy) of intermolecular couplings. The concept has been elaborated for the anthracene-tetracyanobenzene crystal in order to solve the longstanding controversy concerning the mechanism of the orientational phase transition. The conclusion is that the instability is of a displacive type within (ab) planes of anthracene molecules, where an existing soft librational phonon acts as a modulator for interactions between the planes and triggers an order–disorder instability in the direction perpendicular to the planes. This microscopic picture can explain qualitatively experimental results for the anthracene-tetracyanobenzene crystal and is believed to be applicable to other anisotropic molecular crystals where the crossover can be regulated by pressure or impurities.
Rotational structure in an excited vibronic band of the dipole-supported state of cyanomethyl anion, CH2CN−
Authors: Wetzel Donna M. , Brauman John I.
Published: July 2002

Previous reports of threshold resonances occurring in the photodetachment spectra of molecular anions have provided detailed information about the nature of dipole-supported states and the dynamics of autodetachment from the vibrationless level. In this paper we report the first observation and analysis of rotational band structure in an excited vibrational level of a dipole-supported state. The 1 cm−1 resolution laser photodetachment spectrum of cyanomethyl anion (CH2CN−), the conjugate base of acetonitrile, was recorded in the 12 500–13 700 cm−1 region using ion cyclotron resonance spectrometry. Rotational assignment of the resonances occurring in this region provides evidence for vibrational-to-electronic coupling in the autodetachment process.
Hydrogen motion on a Cu surface: A model study of the rate of single and double site-to-site jumps and the role of the motion perpendicular to the surface
Authors: Haug Kenneth , Wahnström Göran , Metiu Horia
Published: July 2002

We use the Miller, Schwartz, and Tromp flux–flux correlation function formula to calculate the rate coefficient for site-to-site hopping by an adsorbed hydrogen atom on Cu(100). We examine several one- and two-dimensional models with a rigid surface. We reach several qualitative conclusions which are relevant to real systems: the motion perpendicular to the surface plays a very important role in determining the site-to-site hopping rate; there is substantial barrier recrossing which makes transition state theory inaccurate; at moderate temperatures multiple jumps become important.
Spectroscopy of phenylacetylene bound to clusters of ammonia and the surface cluster analogy
Authors: Breen J. J. , Kilgore K. , Tzeng W.-B. , Wei S. , Keesee R. G. , Castleman A. W.
Published: July 2002

The results of a systematic study of the spectra shifts of the S1–S0 transition in phenylacetylene (PA) [1B2(1Lb)←1A1(1A)] due to solvation by NH3 are reported based on two-photon REMPI. A unique aspect of the present work is comparison of the red shifts for clusters produced by coexpansion techiques with those where PA is attached to performed ammonia clusters. In both experiments the 1–1 complex is red shifted by 80 cm−1. In the case of the coexpansion experiments the red shift is about 50 cm−1 for PA(NH3)n, n=2–7. By contrast, the clusters produced by attachment display a much different and gradually increasing red shift which saturates at 150 cm−1 for n=13 and beyond. Structural differences for clusters produced by the two different techniques are believed to be responsible.
Theory of rotational transition in atom–diatom chemical reaction
Authors: Nakamura Masato , Nakamura Hiroki
Published: July 2002

Rotational transition in atom–diatom chemical reaction is theoretically studied. A new approximate theory (which we call IOS-DW approximation) is proposed on the basis of the physical idea that rotational transition in reaction is induced by the following two different mechanisms: rotationally inelastic half collision in both initial and final arrangement channels, and coordinate transformation in the reaction zone. This theory gives a fairy compact expression for the state-to-state transition probability. Introducing the additional physically reasonable assumption that reaction (particle rearrangement) takes place in a spatially localized region, we have reduced this expression into a simpler analytical form which can explicitly give overall rotational state distribution in reaction. Numerical application was made to the H+H2 reaction and demonstrated its effectiveness for the simplicity. A further simplified most naive approximation, i.e., independent events approximation was also proposed and demonstrated to work well in the test calculation of H+H2. The overall rotational state distribution is expressed simply by a product sum of the transition probabilities for the three consecutive processes in reaction: inelastic transition in the initial half collision, transition due to particle rearrangement, and inelastic transition in the final half collision.
Fermi resonances between CH stretching and bending vibrations in CHF3, CHCl3, and (CF3)3CH
Authors: Kauppi Esa , Halonen Lauri
Published: July 2002

A simple vibrational curvilinear internal coordinate Hamiltonian is used to account for Fermi resonance interactions between CH stretching and bending vibrations in CHF3, CHCl3, and (CF3)3CH. The eigenvalues of the Hamiltonian are obtained variationally with a Morse oscillator basis set for the stretch and two-dimensional harmonic oscillator basis set for the bend. Five or six potential energy parameters are found to describe well the experimental CH stretching and bending overtone levels of the studied molecules. In the case of CHF3 the potential energy surface obtained is in good agreement with ab initio surfaces. A bond dipole model is used to calculate infrared absorption intensities for the transitions from the ground state to the totally symmetric excited states.
Monte Carlo studies of hydrogen fluoride clusters: Cluster size distributions in hydrogen fluoride vapor
Authors: Zhang Changyin , Freeman David L. , Doll J. D.
Published: July 2002

An investigation into the structure and composition of hydrogen fluoride vapor is reported. Calculations are performed using a modified central force model potential developed by Klein and McDonald. Using a simulated annealing procedure, minimum energy structures for HF clusters are investigated ranging in size from n=2 to 7. Good agreement is found for the structural parameters obtained from the model potential and other theoretical and experimental information. The Monte Carlo method is used to determine the thermodynamic energy, entropy, and Gibbs free energy of the hydrogen fluoride clusters at 1 atm pressure and 100 and 273 K. A minimum in the Gibbs free energy change is found at n=4 implying that tetramers are very important in the vapor.
Theoretical characterization of the potential energy surface for H+O2→HO*2→HO+O. II. The potential for H atom exchange in HO2
Authors: Walch Stephen P. , Rohlfing Celeste McMichael
Published: July 2002

The results of CASSCF/multireference contracted CI calculations with large ANO basis sets are presented for the exchange region of the HO2 potential energy surface. The saddle point for H atom exchange is ≊13 kcal/mol below the energy of H+O2; therefore, this region of the surface will be accessible during H+O2 recombination and metathesis reactions.
An investigation of the annihilated unrestricted Hartree–Fock wave function and its use in second-order Mo/ller–Plesset perturbation theory
Authors: Baker J.
Published: July 2002

The recently introduced annihilated unrestricted Hartree–Fock (AUHF) wave function—in which the first spin contaminant in an unrestricted Hartree–Fock (UHF) wave function has been annihilated self-consistently—is discussed in some detail with particular attention to its use as a basis for a perturbation expansion. A series of calculations are presented highlighting the advantages and disadvantages of a second-order Mo/ller–Plesset (AUMP2) perturbation treatment.
Vibrational states of very floppy clusters: Approximate separability and the choice of good curvilinear coordinates for XeHe2, I2He
Authors: Horn T. R. , Gerber R. B. , Ratner Mark A.
Published: July 2002

A study is made of the vibrational energy levels and the corresponding oscillation dynamics of the clusters Xe4He2, Xe3He2, and I24He. XeHe2 is a representative of the ‘‘three ball’’ clusters, while I2He is a prototype of the ‘‘stick and ball’’ systems. The treatment is based on the vibrational self-consistent field (SCF) method, which introduces an approximate separation of the modes involved. Success of the method depends on an adequate choice of the coordinates that are being mutually separated. We use physical arguments, based on mass ratios and potential function considerations, as well as comparative SCF calculations in different coordinate systems, to determine the appropriate modes for each system. Numerically exact results are also obtained by configuration interaction (CI) calculations using a basis of SCF states. The SCF and CI calculations include all modes and employ realistic potentials. Several states that are both rotationally and vibrationally excited are also calculated. The main conclusions are: (1) Hyperspherical coordinates are the best modes for XeHe2; ellipsoidal coordinates are best for I2He. In each case, the ‘‘good modes’’ SCF gives energies in remarkable agreement with the exact (CI) ones. (2) XeHe2 resembles a quantum liquid drop: Even in the ground state, it is delocalized over and between the (two) classical equilibrium structures. (3) Structural distributions, rather than rigid geometry, are essential for the description of such floppy clusters. The single-mode SCF wave functions offer a highly accurate description of the structural distributions. (4) There is a sequence of bound, excited rotational states of I2He in which the He precesses around the I2 axis. The amplitude of the I2He bending vibrations are very large (θA<20°), but none of the bound states involves a full rotational motion around the I2 stick (with angular momentum normal to the axis). The SCF method with the ‘‘good coordinates’’ proposed here is expected to yield results of similar high accuracy for any cluster of the ‘‘three balls’’ or ‘‘stick and ball’’ types.
A coupled channel quantum scattering study of alignment effects in Na(2P3/2)+He→Na(2P1/2)+He collisions
Authors: Schatz George C. , Kovalenko Laurie J. , Leone Stephen R.
Published: July 2002

In this paper we present results of coupled channel quantum scattering calculations of the alignment selected  j=3/2→ j=1/2 fine structure changing integral cross section for Na(2P)+He. This cross section has in the past been written in terms of a coherent sum of partial wave amplitudes, but we have found that it can be expressed in terms of an incoherent sum of partial cross sections, each labeled by the total angular momentum J and by parity. It is also possible to define an alignment selected wave function for each J such that the azimuthal average of the square of this wave function projected onto each final state is proportional to the magnitude of the partial cross section into that state. This J labeled wave function is thus clearly related to the physical measurables, and we have used it to determine propensities for preservation of asymptotically prepared alignment during collisions. Using a potential surface based on Pascale’s ab initio calculations, we find that the alignment ratio σ⊥/σ∥ is an increasing function of energy, with a value less than unity at low energy (<0.01 eV), but increasing quickly to a value of about 2.0 at 0.04 eV and then more slowly at higher energy, up to a value of 2.7 at 0.2 eV (the highest energy considered). Above 0.02 eV, both the alignment ratio and the alignment selected integral cross sections are in good agreement with values calculated in an accompanying semiclassical study (Kovalenko, Leone, and Delos).

An examination of the J labeled alignment selected scattering wave functions and of the expectation values of 〈Ω〉, 〈Λ〉, and 〈Σ〉 indicates that at low J when the initial state is prepared with ∥ polarization, the dominant state at short range is Σ while with ⊥ polarization the dominant state is Π (i.e., asymptotic alignment is preserved). By way of contrast, this propensity for alignment preservation is not seen if fluxes or probability densities associated with alignment selected wave functions labeled by the initial orbital quantum number l (rather than J) are considered. This l labeled result is in accord with recent work by Pouilly and Alexander, but the lack of alignment preservation in this case has no relationship with the alignment cross sections, or with the alignment selected plane wave scattering wave function, since the l labeled wave functions must be coherently combined to generate this information. The orbital scrambling found for the l labeled solutions thus is not related to measurable properties, and instead the correct picture is provided by the J labeled solutions, which do show preservation of alignment. We find that even in the J labeled picture, alignment preservation does not by itself guarantee any specific trend in the alignment ratio for the fine structure transition.
Molecular dynamics of chain molecules in solution. Static and dynamic properties
Authors: Luque Jorge , Santamaría Jesús , Freire Juan J.
Published: July 2002

Molecular dynamics simulations have been performed for systems of chains composed of Nc =8–20 units immersed in 343-Nc solvent molecules. Two different types of forces (rigid and elastic) between neighboring chain units are considered. Also, different values of the interaction between chain units and solvent molecules are introduced. Distribution functions of distances, dimensions, time-correlation functions and translational diffusion coefficients are calculated from the simulation results. The values of these properties show that the elastic model corresponds to a more expanded chain. More expanded chains are also simulated with more favorable polymer–solvent interactions. Slower relaxation times are obtained for the expanded chains. Scaling laws for dimensions and diffusion coefficients in terms of the chain lengths are investigated.
From short-time molecular dynamics to long-time stochastic dynamics of proteins
Authors: Cartling Bo
Published: July 2002

To extend the time scales of descriptions of protein dynamics, beyond those accessible by the molecular dynamics method, theories of stochastic processes are utilized for both short- and long-time dynamics. A first step is the bridging from short-time fluctuations in conformational states to transitions between conformational states. Stochastic short-time dynamics of a reaction coordinate of a conformational transition is deduced starting from the classical equations of motion of a molecular system. The coupling strength between the reaction coordinate and the bath, that remaining degrees of freedom constitute, is determined by an analysis of the short-time fluctuations in molecular dynamics trajectories. An effective potential energy function of the reaction coordinate is obtained by an energy minimization method. The required transition rates are determined from the nonstationary solutions of the Fokker–Planck equation for Brownian motion. As a first application of this approach, dihedral transitions in the sidechain of an aromatic amino acid residue in an α-helix are studied. The rate constants of elementary conformational transitions constitute the basic parameters of a stochastic model of protein conformational relaxation dynamics. This model is useful for descriptions of the coupling between protein conformational dynamics and reactions involved in the functions of proteins.
Solvent influence on the magnetic field effect of polymethylene-linked photogenerated radical ion pairs
Authors: Busmann Hans-Gerd , Staerk Hubert , Weller Albert
Published: July 2002

The solvent viscosity and polarity dependence of the magnetic field effect in polymethylene-linked radical ion pairs, which were generated by photoinduced intramolecular electron transfer in compounds of the type pyrene–(CH2)n–N,N-dimethylaniline, has been studied. A stochastic Liouville equation is used, in which the dynamics of the polymethylene chain, the spin Hamiltonian as a function of the varying radical distance (exchange interaction), and a distance-dependent back electron transfer rate are incorporated. The results are compared with predictions made on the basis of the (static) subensemble approximation.
Frequency-domain observation of the ultrafast inertial response of the optical Kerr effect in CS2
Authors: Barker Charles E. , Trebino Rick , Kostenbauder A. G. , Siegman A. E.
Published: July 2002

Time-domain experiments using femtosecond pulses have recently revealed the inertial aspect of the nuclear-orientation component of the optical Kerr effect in carbon disulfide. We present frequency-domain measurements, performed with the tunable-laser-induced grating technique, which also demonstrate the need to incorporate inertial behavior in models of the nuclear-orientation contribution to this ultrafast response. No previously suggested model, however, yields a fit to our data that passes standard goodness-of-fit tests at an acceptable level.
Evidence for three interacting potential energy surfaces in the photodissociation of ICN at 249 nm
Authors: Black J. F. , Waldeck J. R. , Zare R. N.
Published: July 2002

Cyanogen iodide (ICN) is photodissociated at 249 nm. The CN X 2Σ+ photofragment is probed by sub-Doppler laser-induced fluorescence (LIF), allowing the extraction of recoil velocity anisotropies and branching ratios to the two iodine atom spin–orbit states I(2P1/2) and I(2P3/2) as a function of the CN (v=0) rotational state. The quantum yield for I(2P1/2) production ΦI* is found to be 43%±3%, in excellent agreement with the recent diode laser spectroscopic measurement of Hess and Leone. The population of the F1 and F2 spin–rotation doublet components shows nonstatistical behavior over a wide range of N for both I atom spin–orbit state exit channels. The results suggest that trajectories leading to I(2P1/2) evolve on an essentially collinear surface; the CN fragments being found in low rotational levels with almost limiting values of the system anisotropy parameter (β=1.85 to 1.9). This value of β yields an estimate for the dissociative lifetime [CN X 2Σ+v=0, N=0; I(2P1/2)] of 90±15 fs at this photolysis wavelength. There is evidence that trajectories leading to I(2P1/2)+CN(v=0) in intermediate N levels have sampled a bent surface, indicating that multiple curve crossings occur in this channel. The nature of trajectories correlated to I(2P3/2) is very complicated, with clear evidence for a mixed parallel and perpendicular initial transition and subsequent curve crossings. The CN (v=0) fragments formed in conjunction with this channel are found predominantly in medium to high rotational quantum states. The system anisotropy parameters vary as a function of N, being negative at low N and becoming positive at high N. An analysis of the correlation between fragment velocity and rotation yields results inconsistent with a pure parallel or perpendicular excitation. We present a model involving three interacting surfaces, by which we can reconcile all major experimental observations at this photolysis wavelength.
Liquid theory for band structure in a liquid. II. p orbitals and phonons
Authors: Xu Bing-Chang , Stratt Richard M.
Published: July 2002

Surprisingly, the ground-state quantum mechanical problem of calculating the set of single-electron states available to a liquid (its electronic band structure) can be turned into an exercise in ordinary classical liquid theory. We generalize our previous findings by showing that this statement continues to hold for bands constructed from a basis of atomic p orbitals and we use this idea to provide a simple mean field theory useful for p bands in liquids. In addition, there is a natural way of thinking about the normal modes of vibration of a liquid (its phonons) that is accessible through virtually the same formalism. We discuss the significance of these ‘‘instantaneous normal modes’’ and show that the same kind of mean field theory is helpful in understanding both this phonon spectrum and its implications for liquid-state dynamics.
The effect of multiple nucleation events on heterogeneous nucleation rates
Authors: Weinberg Michael C.
Published: July 2002

The influence on the overall heterogeneous nucleation rate of multiple nucleation events on impurity particles is investigated. A simple model based upon coupled consecutive chemical reactions is used to describe the time dependence of the heterogeneous nucleation rate. Also, the kinetics are illustrated for a particular example in which the intrinsic nucleation probability is assumed to vary inversely with the impurity particle occupancy.
Hydrogen motion on a rigid Cu surface: The calculation of the site to site hopping rate by using flux–flux correlation functions
Authors: Haug Kenneth , Wahnström Göran , Metiu Horia
Published: July 2002

We use the quantum flux–flux correlation function theory to calculate the rate coefficient for site-to-site hopping by a single hydrogen atom absorbed on a rigid Cu(100) surface. We investigate hydrogen dynamics during barrier crossing and determine the time scales on which the hydrogen atom crosses or recrosses the barrier, as well as the time scale on which double jumps occur. We define two kinds of transition state theory rate coefficients: one (Miller and Tromp) which assumes that only the short time dynamics contributes to the rate coefficient and another which includes the effect of the earliest recrossing. We examine numerically the accuracy of these approximations and compare them to other transition state theory calculations and to our ‘‘exact’’ calculations. The simulations are also used to study the contribution of multiple jumps to the diffusion coefficient, to calculate the isotope effect on the rate coefficient and to determine the role of dimensionality in modeling surface diffusion. We find that the motion of the adsorbed atom perpendicular to the surface influences strongly the migration dynamics because the energy is very rapidly transferred back and forth between motion parallel and perpendicular to the surface. In particular this energy exchange process enhances the frequency of recrossing events and diminishes the frequency of the multiple jumps. We also make an extensive comparison between classical and quantum simulations.
Photodissociation dynamics of formaldehyde: H2 (v,J) vector correlations
Authors: Carleton Karen L. , Butenhoff Thomas J. , Moore C. Bradley
Published: July 2002

Several bipolar moments of the H2 (v,J) correlated angular momentum and velocity distribution produced from the photodissociation of formaldehyde near the threshold for dissociation have been measured by analysis of Doppler-resolved LIF line shapes. It is determined that the fragment H2 〈v⋅J〉 correlation is not at the limit of v⊥J, but is closer to the limit of v⊥J than to v∥J. The rotation of the excited H2CO during the 10−7–10−8 s before dissociation does not completely wash out the lab-frame vector correlations. Anisotropy parameters as large as 0.85 and as small as −0.41 have been measured; these are outside the limits imposed by classical models of parent rotation. A quantum mechanical model for parent rotation is introduced that accounts for the large magnitude of the measured anisotropy parameters. Photolysis on the rR0(0) line of the 43 band produces fragments with β<0 while photolysis on the same rotational transition of the 2141 band produces fragments with β>0. It is not known if the different anisotropies are caused by differences in the parent transition dipole moment or by differences in the dissociation dynamics. The simple impulsive model that reproduces the fragment rotational distributions and product quantum-state correlations does not adequately describe the measured H2 (v,J) vector correlations.
Rotationally inelastic scattering of NH3 with H2: Molecular-beam experiments and quantum calculations
Authors: Ebel G. , Krohne R. , Meyer H. , Buck U. , Schinke R. , Seelemann T. , Andresen P. , Schleipen J. , ter Meulen J. J. , Diercksen G. H. F.
Published: July 2002

In crossed molecular-beam experiments, three type of cross sections have been measured: Total differential cross sections with well-resolved diffraction oscillations for oD2–NH3 at E=95.9 and 111.3 meV, differential energy-loss spectra for ND3–oD2 at E=118.3 meV which cover the center-of-mass (c.m.) angular range from 85° to 170° and are obtained by time-of-flight (TOF) analysis, and state-to-state integral cross sections for oNH3–H2 and pNH3–H2 at E=75 meV for many final rotational states which are detected by resonance enhanced multiphoton ionization. These data which are mainly sensitive to the anisotropy of the potential energy surface are well reproduced by quantum calculations in the coupled-states approximation. The potential is constructed by combining large basis-set self-consistent-field (SCF) calculations with damped long-range dispersion coefficients. The two free parameters of the damping function are fitted to a restricted set of configuration interaction (CI) calculations.
A general method for Doppler determination of cylindrically symmetric velocity distributions: An application of Fourier transform Doppler spectroscopy
Authors: Taatjes Craig A. , Cline Joseph I. , Leone Stephen R.
Published: July 2002

A general method is developed for determination of cylindrically symmetric velocity distributions from Doppler profile measurements. This method applies Kinsey’s Fourier transform Doppler spectroscopy [J. L. Kinsey, J. Chem. Phys. 66, 2560 (1977)] to distributions arising from photodissociation and uses an orthogonal polynomial expansion to perform the integral transforms analytically. This method is shown to offer an improvement in stability over direct numerical solution of the integral equation and to have applicability to distributions which are not ‘‘separable,’’ that is, which cannot be separated into a product of speed- and angle-dependent factors. The method is applied to experimental measurements of the collisional relaxation of a fast anisotropic distribution of I[2P1/2] atoms in a thermal bath (preceding paper). It is shown that the nascent distribution is separable, but the distribution does not remain separable throughout the relaxation process.
A crossed-beam study of the state-resolved dynamics of CH(X 2Π) + D2. II. The isotopic exchange channel
Authors: Macdonald R. Glen , Liu Kopin
Published: July 2002

The state-to-state integral cross sections for the isotopic exchange reaction of CH(X 2Π) with D2 to produce CD(X 2Π) have been measured in a crossed-beam apparatus by the laser-induced fluorescence method. Two types of measurements were performed: (1) the translational energy dependence of an individual quantum state of the product and (2) the state distribution of the products at fixed and well-defined translational energy. To understand some of the finer details of the reaction dynamics similar experiments were also carried out on the reaction CD(X 2Π) with H2 to give CH(X 2Π). For the isotopic exchange channel, the cross section decreased rapidly with increasing translational energy, signifying a complex formation reaction mechanism. The CD(CH) product rotational level distributions are substantially colder than a statistical expectation and are interpreted as the result of a multiple-impact collision between the receding products in the exit channel. A novel frequency-locking mechanism, similar to that proposed to understand the dynamics of inelastic collisions between CH and D2 (the preceding paper), is suggested to explain anomalous peaks in the product rotational level distribution for the isotopic exchange channel. However, the fine-structure state distributions for the reactive process, which are very different from that for the inelastic channel, cannot be rationalized by conventional interpretation. Some basic concepts about fine-structure selectivity in chemical reactions have been developed. With these concepts, it is conjectured that the fine-structure state distributions of open-shell molecules arising from reactive encounters could provide a ‘‘fingerprint’’ of the electronic wave function at the transition state.
The infinite-order-sudden-approximation calculations of reactive cross sections and product angular distributions for the F+H2 reaction and its isotopic variants on a modified London–Eyring–Polanyi–Sato potential energy surface
Authors: Takayanagi Toshiyuki , Tsunashima Shigeru , Sato Shin
Published: July 2002

The reactive cross sections and product angular distributions for the F+H2,F+D2 and F+HD reactions have been calculated using the infinite-order-sudden approximation on a modified London–Eyring–Polanyi–Sato potential energy surface which has a nonlinear saddle point. This surface was constructed previously so as to reproduce the experimentally obtained product angular distributions by the quasiclassical trajectory calculations. The calculated branching ratios of different vibrational states of products, HF(v′) and DF(v′) from above three reactions, were all in qualitative agreement with those experimentally obtained; however, the product angular distributions calculated were not better than those calculated by the quasiclassical trajectory method. These results are compared with those calculated on different potential surfaces which predict collinear transition states.
The vibrational spectra of molecular ions isolated in solid neon. V. N2O+ and NNO−2
Authors: Jacox Marilyn E.
Published: July 2002

When a Ne:N2O=200 or 800 sample is codeposited at approximately 5 K with a beam of neon atoms that has been excited in a microwave discharge, the infrared spectrum of the products includes absorptions contributed by the two stretching fundamentals of ground-state N2O+. An absorption near 1200 cm−1 also results from the stabilization of NNO−2 on reaction of O− with N2O. The two other stretching fundamentals of NNO−2 have also been identified. Detailed isotopic substitution studies support these identifications and permit a normal coordinate analysis for both N2O+ and NNO−2. The nitrogen–nitrogen bond of NNO−2 has approximately single bond character, and the nitrogen–oxygen bonds are relatively weak. The photodestruction threshold for NNO−2 in this experimental system is near 420 nm. Photolysis of NNO−2 is accompanied by growth in the absorptions of cis ONNO. Several other infrared absorptions in the initial sample are tentatively assigned to vibrations of N2O complexed to various ions.
The extraction of dynamics and the classical mechanical simulation of low resolution regular and chaotic spectra: HCN/HNC
Authors: Farantos S. C. , Gomez Llorente J. M. , Hahn O. , Taylor H. S.
Published: July 2002

The classical analog of the quantum survival probability function as well as low resolution spectra are calculated for HCN and its isomer HNC, and compared with available experimental ones and with the recent quantum mechanical SEP spectra obtained by Wyatt and co-workers for energies up to about 30 000 cm−1. The HCN(0,v,0)A 1A″→X 1Σ+ with the bending quantum number v=0–3, and the corresponding HNC (0,0,0) transitions were studied. It is shown that classical mechanics satisfactorily reproduces the main features of the survival probability functions and illustrates the dynamics, the results of which are the spectra. Specifically the observed spectral features are associated with the regular or chaotic motion localized in particular resonance zones. The spectroscopic agreement between classical and quantum mechanics reveals that there should be a correspondence between the regions of phase space where the classical motion is localized and the excited quantum eigenfunctions determining the SEP spectra. These results show that the computationally simple and pictorial classical mechanical techniques used here offer readily available means for extracting the dynamics from highly excited molecular spectra.
High-resolution spectroscopy of NaI around the atomic asymptotes Na (3p 2P1/2,3/2)+I ( 2P3/2)
Authors: Bluhm H. , Lindner J. , Tiemann E.
Published: July 2002

Excitation spectra of NaI were recorded in the wavelength range 239–256 nm with a frequency doubled pulsed dye laser. The assignment of 27 vibrational bands shows that the observed spectra belong to a new bound covalent state below the atomic asymptotes Na(2P1/2,3/2) + I(2P3/2) and above the predissociating state A0+ [S. H. Schaefer, D. Bender, and E. Tiemann, Chem. Phys. 89, 65(1984)]. The Dunham parameters and the potential curve for the new state are given. The potential curve is compared with that derived by the analysis of a fluorescence progression from a broadband excitation that was published recently by Bower et al. [J. Chem. Phys. 89, 4478 (1988)].
Coriolis interaction in polyatomic molecules: A classical coupled spin representation
Authors: Martens Craig C.
Published: July 2002

A coupled spin representation is introduced to describe the classical dynamics of two vibrational modes of a polyatomic molecule coupled by Coriolis interaction to overall molecular rotation. The mechanisms of Coriolis-induced periodic energy exchange between the vibrational modes, resonant vibration–rotation interaction, and chaotic energy transfer are described, and the dependence of the dynamics on rotational angular momentum, vibrational energy, total energy, and parameters in the Hamiltonian is explored using classical trajectory integrations and the surface of section method. The integrable motion occuring in the prolate limit is considered geometrically from the coupled spin representation, which provides clear insight into the dynamics of the system. In addition, this approach allows analytic results describing the effect of Coriolis coupling on vibration–rotation dynamics to be obtained.