bluuebunny/crossref_metadata_2025 huggingface dataset:
A photoionization study of the vinyl radical
Authors: Berkowitz J. , Mayhew C. A. , Ruščić B.
Published: July 2002

The photoionization spectrum of vinyl radical is reported, from its observed threshold to 1160 Å. Two methods of preparation have been employed; (a) the abstraction reaction of F atoms with C2H4, and (b) the pyrolysis of divinyl mercury at 1200 K. In both experiments, relatively sharp autoionization structure is observed, and interpreted as a Rydberg series converging to the excited 3A″ state of vinyl cation. The analysis leads to an adiabatic ionization energy of ∼10.7 eV for this state, with a structure similar to that of vinyl radical but with an increased C–C distance. The observed ionization threshold for the ground state of vinyl cation is 8.59±0.03 eV with the F atom reaction, and 8.43±0.03 eV with the pyrolysis method. The lower value in the latter experiment is interpreted as a hot band. The relatively low value of the photoionization cross section near threshold implies a large geometry change between vinyl radical and ground state vinyl cation. A progression in the in-plane C–H bending vibration is indicated in the photoionization spectrum; it is quite possible that the vibrational 0–0 transition lies one quantum lower than our detected limit. With this bracketed adiabatic ionization potential and the appearance potential of C2H+3 (C2H4), a C–H bond energy in ethylene of 107–110 kcal/mol (0 K) is deduced.
Energy dependence of infrared emission from azulene C–H stretching vibrations
Authors: Shi Jichun , Bernfeld Diane , Barker John R.
Published: July 2002

New data are described on the energy dependence of infrared fluorescence (IRF) from the C–H stretch modes (∼3050 cm−1) of vibrationally excited azulene (C10H8). An optoacoustic method was used to measure the absorbed laser energy and determine the number of excited molecules, while time-resolved infrared emission was recorded. The experimental uncertainties of ∼5% are much smaller than in previous work and the agreement between the experiments and the theory for the IRF intensity is excellent for total vibrational energies from 14 000 to 33 000 cm−1.
Laser catalysis and transition state spectra of the H+H2 exchange reaction
Authors: Seideman Tamar , Shapiro Moshe
Published: July 2002

Electronic excitation ‘‘transition-state’’ spectra of the H+H2 exchange reaction are computed by a uniform semiclassical approximation, at a number of collision energies. The spectra, which compare well with the coupled channels computations of Engel et al. [J. Chem. Phys. 82, 4844 (1985)] are shown to yield unique information pertaining to ultrashort time dynamics. The transition amplitudes are then incorporated in a general formulation by which the laser catalysis scheme, suggested recently by Shapiro and Zeiri [J. Chem. Phys. 85, 6449 (1986)] is treated exactly. According to this scheme, reaction barrier crossings can be achieved through resonant light scattering via a bound upper electronic state. The laser acts as a catalyst, since no net photons are absorbed or emitted. When the process is coherent, interference between ‘‘natural’’ (nonradiative) tunneling and the optical process is shown to lead to ‘‘Fano-type’’ dependence of the reactive probabilities on laser frequency: The reaction is stopped on the red side and enhanced on the blue side of the absorption line. For an ensemble of reactants with thermal-like distribution of kinetic energies, laser catalysis is shown to depend linearly on the laser power. For H+H2, the effect is most pronounced at threshold and subthreshold energies.
Photodissociation dynamics of NH3: Rovibronic absorption analysis of the Ã–X̃ transition and nuclear spin polarization in the NH2 (X̃ 2B1) product
Authors: Fuke Kiyokazu , Yamada Hiroyuki , Yoshida Yasushi , Kaya Koji
Published: July 2002

The Ã–X̃ electronic spectra of jet-cooled NH3, obtained by probing the LIF of the NH2 (X̃ 2B1) fragments, are presented. The homogeneous linewidths for the v′2 =0 and v2 =1 vibrational bands are estimated as 32±2 and 30±2 cm−1, respectively. A partial analysis of the excitation energy dependence of the rotational energy distribution in NH2 (X̃ 2B1) is also examined. The results clearly show that the photodissociation of the Ã state NH3 proceeds via a near planar structure in a nuclear spin conserved manner.
Collisional vibrational quenching of NO+(v) ions
Authors: Morris Robert A. , Viggiano A. A. , Dale F. , Paulson John F.
Published: July 2002

Vibrational quenching rate constants have been measured for NO+(v>0) ions with 15 neutral quenching molecules by the SIFDT-monitor ion technique. The temperature dependence of the quenching rate constants for the reactions of the neutrals N2, CO2, and CH4 has been investigated from 208 to 450 K. The dependence of the CH4 quenching rate constant on collision energy has been determined in the energy range 0.03–0.12 eV at 208 and 296 K. Also measured are rate constants for some of the reactions pertinent to the monitor ion technique.
A study of collisional disalignment of state-selected nitric oxide A 2Σ+ (J,M
J) using multiphoton ionization
Authors: Snow L. D. , Compton R. N. , Miller John C.
Published: July 2002

An optical–optical double resonance technique using a multiphoton ionization detection scheme is employed to study the disalignment (collisional redistribution of MJ levels) of state selected NO A 2Σ+ (v′=0, J′= 4 1/2) upon collisions with Xe, N2, NO, CO2, and CHF3. Cross sections for MJ -changing collisions are estimated from two-color polarization measurements as a function of the pressure of the perturbing gas.
Infrared (2 to 8 μm) fluorescence of the W 3Δu→B 3Πg and w 1Δu→a 1Πg systems of nitrogen
Authors: Fraser Mark E. , Rawlins Wilson T. , Miller Steven M.
Published: July 2002

Eleven transitions in the W 3Δu→B 3Πg (W–B) and w 1Δu→a1Πg (w–a) systems of nitrogen have been observed in the infrared including the previously unobserved (1,0) and (2,1) W–B features at 6.5 and 7.65 μm, respectively. The fluorescence spectra were observed in a cryogenic reaction chamber at pressures of ∼3 mTorr (0.4 Pa), following expansion of flowing N2/Ar mixtures excited by microwave discharges at ∼1 Torr. Einstein coefficients for the w–a system, calculated using a published transition moment function, predict the radiative lifetimes of the lower vibrational levels of the w 1Δu state to be a factor of 3 longer than earlier estimates. Using a spectral simulation and linear least-squares fitting technique, the published W–B and calculated w–a branching ratios are verified for the transitions observed across the 2 to 4 μm region. The observed vibrational/electronic state distributions are not characteristic of those expected for direct excitation, but appear to result from extensive collisional coupling among excited states of nitrogen which occurs in the high pressure region prior to expansion.
Two-bulb thermal separation factors of helium isotopes as a test of recent interatomic potentials
Authors: Taylor William L.
Published: July 2002

New measurements on the thermal diffusion factor of the 3He–4He system are reported. The two-bulb method was used to measure αT in the temperature range 2.45 to 50 K and employed an improved method to treat the experimentally determined separations. Five recently proposed intermolecular potentials for helium were examined for their ability to predict αT through quantum mechanical and classical calculations as compared to all available experimental values in the temperature range ∼2 to 900 K.
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o study of the two-photon bound–bound electronic transitions of t
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s-butadiene
Authors: Galasso V.
Published: July 2002

The two-photon absorption properties of trans-1,3-butadiene have been calculated at an ab initio level employing RPA vertical excitation energies and transition moments and taking the random molecular orientation into account. For single-beam two-photon absorption with every possible combination of photon polarization the transition probability coefficients and polarization ratios for the low-lying 1Ag and 1Bg final states are reported. The strongest two-photon absorptivity is predicted under excitation from linearly polarized photons for 1Ag states and from circularly polarized photons for 1Bg states. The polarization ratios take sizably different values for the various 1Ag states. The case of concerted absorption of two photons with various polarizations from two laser beams has also been investigated exhaustively as a function of the photon frequencies.
Shape resonance influence on the photoelectron angular distributions from O2 C 3Πg, v=0–3
Authors: Miller P. J. , Chupka W. A. , Winniczek J. , White M. G.
Published: July 2002

Vibrationally resolved, photoelectron angular distributions are reported for the (2+1) REMPI of the C 3Πg(3sσg), v′=0–3 Rydberg levels of O2. Ionization transitions leading to different vibrational final states (v+) of O+2 exhibit markedly different angular distributions, suggesting different ionization mechanisms. The experimental data are qualitatively reproduced by very recent ab initio calculations which attribute the non-Franck–Condon behavior to the presence of a kσu shape resonance imbedded in the ionization continuum. Much of the remaining anomalous behavior is attributed to a Fano-type resonance in the Πu channel.
Shape-resonance-induced non-Franck–Condon effects in (2+1) resonance enhanced multiphoton ionization of the C 3Πg state of O2
Authors: Stephens J. A. , Braunstein M. , McKoy V.
Published: July 2002

We show that strong non-Franck–Condon effects observed in (2+1) resonance enhanced multiphoton ionization of the C 3Πg state of O2 are due to the kσu shape resonance previously observed in single-photon studies of diatomic molecules. Calculated vibrational branching ratios for the v′=2,3 levels of the C 3Πg state are in reasonable agreement with experiment. Certain discrepancies remain in comparing theoretical results with the measured spectra, and possible electron-correlation effects which underly this are discussed.
A correlation potential for molecular systems from the single particle Green’s function
Authors: Holleboom L. J. , Snijders J. G. , Baerends E. J. , Buijse M. A.
Published: July 2002

It is well known that the correlated ground state energy can be derived from a knowledge of the single particle Green’s function of a system, even though the two density cannot be obtained from it. In this paper it is shown that the single particle Green’s function in fact contains more detailed information than the total energy alone, to the extent that the local Slater‘xnLöwdin correlation potential can be obtained from it. This potential can be used as a more detailed criterion to judge the quality of approximate Green’s functions than the total energy by itself. The formalism leads moreover to a natural partitioning of the correlation energy into terms depending on the correlation correction to the one density alone and a remaining ‘‘true’’ correlation contribution. In this paper we calculate the single particle Green’s function using the second order approximation to the self-energy for a series of small model systems (He, Be, H2, LiH, and H2O). The correlation potential and the partitioning of the correlation energy are used to analyze this approximation and to assess its accuracy in these systems.
Localization phase diagram for the energetically and substitutionally disordered Anderson/quantum percolation model
Authors: Root L. J. , Skinner J. L.
Published: July 2002

To study the localization of Frenkel excitons in binary systems, we consider a model that has features both of the Anderson model (diagonal disorder characterized by a probability distribution of width w) and of the quantum percolation model (substitutional disorder characterized by an occupational probability p for one of the components). With a finite-size scaling (phenomenological renormalization group) technique, and the concept of quantum connectivity, we calculate the position of the phase boundary separating localized from extended states in the w–p disorder plane. At the two endpoints of the boundary, we find that for the Anderson model the critical disorder is wc=15.95±0.25, and for the quantum percolation model the localization threshold is pq=0.477±0.011.
Near resonant V–R,T energy transfer in the relaxation of vibrationally excited HF by CO
Authors: Raybone D. , Wategaonkar S. J. , Setser D. W.
Published: July 2002

Rotationally state specific quenching of HF(v) by CO is studied using the fast flowing afterglow technique incorporating FTIR spectroscopic detection of HF(v) products. It is shown that HF(v=3) quenching by CO proceeds predominantly by near energy resonant V–R,T channels and not by a V–V process. The near resonant Δv=1 channel accounts for, at least, 50% of the total quenching of HF(v=3). The total removal rate for HF(v=3) quenching by CO was measured and a rate constant, k3(CO)=(2.77±0.22)×10−12 cm−3 molecule−1 s−1, was determined.
Adsorption kinetics of surfactant molecules at a liquid–air interface
Authors: Rasing Th. , Stehlin T. , Shen Y. R. , Kim M. W. , Valint P.
Published: July 2002

The adsorption of the surfactant sodium–dodecyl naphthalene–sulfonate from the bulk to the water–air interface was studied by optical second harmonic generation. An initial rapid Langmuir type of adsorption to about 80% of a monolayer is observed, followed by a much slower adsorption until a full monolayer is formed.
Nascent vibrational distribution of N2(B 3Πg) produced by the reaction of Kr(3P2) with N2
Authors: Tsuji Masaharu , Yamaguchi Kazuo , Nishimura Yukio
Published: July 2002

Krypton beam was expanded into a low pressure chamber through a nozzle. The target nitrogen gas was injected into the chamber. The emission spectrum was analyzed and the vibrational distribution of nitrogen determined.(AIP)
A reanalysis of the molecular beam electric resonance Stark effect data for the a 3Π state of carbon monoxide
Authors: Woods R. Claude , Saykally Richard J.
Published: July 2002

The highly precise measurements of Stark coefficients in the lambda doubling transitions of a 3Π CO obtained in previously published molecular beam electric resonance experiments have been reanalyzed. Certain perturbations between vibrational levels of the a 3Π and a′ 3Σ+ states that were neglected in the earlier treatment have been found to be important. An apparent error, by a factor of in the Stark matrix elements that were used in prior work, has also been corrected. The previously reported experimental determination of the dipole moment function of a 3Π CO is shown to be invalid. Thus, the relatively flat appearance of the dipole moment function found in previous ab initio studies may indeed be correct. The dipole moment in the a′ 3Σ+ state and the transition moment between the a 3Π and a′ 3Σ+ states are substantially revised to −0.79 and +0.64 D, respectively.
Statistical properties of the scanning simulation method for polymer chains
Authors: Meirovitch Hagai
Published: July 2002

The scanning method is a computer simulation technique for polymer chains, which is especially suitable to handle chains with finite interactions and chains that are subject to various geometrical constraints. A chain is constructed step by step with the help of transition probabilities, obtained by scanning the possible continuations of the chain in future steps (called future chains). We discuss in detail the efficiency of the method and for that we study certain autocorrelation functions for three lattice models: self-avoiding walks (SAWs) on a square lattice, a random walk model for polymer adsorption and trails with attractive interactions. We demonstrate that for SAWs the scanning method is significantly more efficient than the related method of Rosenbluth and Rosenbluth. We also develop and test a new procedure in which the transition probabilities are obtained, not by exact enumeration of all the future chains, but from a relatively small sample of future chains, generated by another scanning procedure. This ‘‘double scanning’’ process is expected to be useful for complex macromolecules such as polypeptides.
Monte Carlo simulation of the crystal structure of the rotator phase of n-paraffins. II. Effects of rotation and translation of the rigid molecules
Authors: Yamamoto Takashi
Published: July 2002

Structures and molecular motions in the rotator phases of n-paraffins are simulated by use of the Monte Carlo method—both rotational and translational degrees of freedom of the molecules being taken into account. Molecules of rigid trans-planar structure are assumed to be placed in a two-dimensional orthorhombic or orthohexagonal lattice with lattice parameters a and b. A model of the nearly continuous molecular motion is adopted in this work; each molecule is assumed to have 36 discrete orientations (in 10° intervals) and 10 discrete translations (in 0.128 Å intervals) around and along the molecular axis, respectively. Molecular packing modes in various lattice systems of different a/b values are examined ranging from that of the ordinary orthorhombic phase (a/b=1.50) to that of the rotator-II (R-II) phase (a/b=31/2). It is found that the crystals in the rotator phases are generally composed of many domains within which molecules are packed more or less in order. In the rotator-I (R-I) phase, fairly large structural changes are observed with the change in a/b value; the structure of the R-I phase reported in our previous MC work should be slightly modified. The structure of the R-II phase, on the other hand, is found to be essentially the same as that of our previous MC work. Modes of the molecular motion in the R-II phase are studied by monitoring the time evolution of the rotational and translational states of all molecules in the system. It is found that the molecules are making fairly active rotational oscillation around one of the six equivalent directions connecting nearest neighbor molecular pairs. The molecules are also making sporadic large rotations to the different directions. The translational motion of the molecules is found to be very active and evidently independent of the rotational motion. The overall molecular motions in the rotator phase are found to be described in terms of the movement of the boundaries of the ordered domains.
On the mechanism of desorption from surfaces induced by electronic transitions
Authors: Avouris Ph. , Kawai R. , Lang N. D. , Newns D. M.
Published: July 2002

We analyze the mechanism by which positive ions and neutral species desorb from surfaces as a result of photon- or electron-beam induced electronic transitions. The system fluorine on aluminum is used as a prototype. We first present results of ab initio density-functional calculations of the potential energy curves of several charge states of fluorine on aluminum. We find that fluorine adsorbs as F−(2s22p6) and is strongly bound in the ground state. Valence (i.e., 2s22p5) and core (e.g., 2s12p6)-ionized states are, however, repulsive. F+(2s22p4) ions form bound states on Al but their adsorption energies are much smaller and their equilibrium distance is further out from the surface than those of F− ions. The difference in the bonding of positive and negative ions is ascribed to differences in the corresponding screening mechanisms. Screening of negative ions proceeds only by an image mechanism, while screening of positive ions can proceed by both image and charge–transfer mechanisms in which charge from the metal occupies the large-radius 3s and 3p orbitals of F. The resulting partially neutralized positive-ion states have reduced image attraction and increased electron kinetic energy (Pauli) repulsion. Franck–Condon transitions from the ground state populate the repulsive part of F+ potential energy curve and lead to efficient F+ desorption. F++ states are strongly bound and do not desorb. The same conclusions are reached by a more general analysis of the desorption of electronegative atoms from any metallic substrate, based on the concepts of effective medium theory. Finally, we discuss the applicability of our conclusions regarding the desorption of neutral and ionic fluorine to desorption of molecular adsorbates and also desorption from nonmetallic substrates.
Atomic forces between noble gas atoms, alkali ions, and halogen ions for surface interactions
Authors: Wilson J. W. , Heinbockel J. H. , Outlaw R. A.
Published: July 2002

The components of the physical forces between noble gas atoms, alkali ions, and halogen ions are analyzed and a data base developed from analysis of the two-body potential data, the alkali–halide molecular data, and the noble gas crystal and salt crystal data. A satisfactory global fit to this molecular and crystal data is then reproduced by the model to within several percent. Surface potentials are evaluated for noble gas atoms on noble gas surfaces and salt crystal surfaces with surface tension neglected. Within this context, the noble gas surface potentials on noble gas and salt crystals are considered to be accurate to within several percent.
Optical luminescence excitation spectra of molecular oxygen in the soft x-ray region
Authors: Yang B. X. , Hanson D. M. , Tohji K.
Published: July 2002

The observation of an anomaly in the optical luminescence excitation spectrum of oxygen in the region of the oxygen K edge is reported. Dispersed luminescence spectra were obtained for x-ray excitation at the pi and sigma resonances, at the anomaly, and in the continuum. These spectra indicate enhanced production of O2+2 ions at both the sigma resonance and at the anomaly. The anomaly thus is attributed to a shake-up or shake-off state associated with an antibonding sigma molecular orbital of oxygen. This work also demonstrates that optical luminesence spectra provide state-specific information about the products of core hole excitation and relaxation.
Infrared diode laser spectra of MgH and MgD (X 2Σ+)
Authors: Lemoine B. , Demuynck C. , Destombes J. L. , Davies P. B.
Published: July 2002

Infrared absorption spectra of magnesium hydride in its 2Σ+ ground state have been measured at Doppler limited resolution in a dc discharge of hydrogen over magnesium. Most of the transitions arise from the fundamental bands of 24MgH and 24MgD, but, in addition, a few lines attributable to 25Mg and 26Mg species have been detected, as well as v=2←1 hot band transitions in 24MgH and 24MgD. Most of the lines are characterized by a doublet splitting due to the electron spin–rotation interaction (γN⋅S) which helps to distinguish them from the broader Rydberg transitions of atomic magnesium observed in the same spectral region. The spectra were analyzed in a combined fit with the FIR pure rotational transitions which leads to an accurate set of molecular parameters either for each vibrational level or as Dunham coefficients. The mass dependence of the molecular parameters is examined in terms of possible breakdown of the Born–Oppenheimer approximation.
Resonance transition probabilities by the complex Lanczos recursion method
Authors: Kolin Orit , Leforestier Claude , Moiseyev Nimrod
Published: July 2002

Resonance transition probabilities are obtained without the need to compute eigenvectors of the complex scaled Hamiltonian matrix. The new computational approach is a combination of the complex Lanczos recursion method, the complex coordinate method, and the extension of the Wyatt and Scott modified QL algorithm to complex matrices. To illustrate the method, the coupling strength between a resonance state and different scattering channels of HD from a flat Ag surface are calculated.
Position, momentum, and phase-space one-electron densities of H2, N2, and LiH
Authors: Anchell James L. , Harriman John E.
Published: July 2002

Electronic position and momentum densities are commonly used to study bonding. The Husimi function, a phase-space density, complements our understanding of these usual densities. Its value at position q and momentum k gives the probability for finding an electron in a Gaussian wave packet state centered at q, k. We have examined these functions for H2, LiH, and N2. We find that the Husimi function provides a useful physical decomposition of coordinate density differences into regions labeled by the momentum and of momentum density or density difference into contributions from different spatial regions.
State-selective photodissociation dynamics of NOCl: The influence of excited state bending and stretching vibrations
Authors: Qian C. X. W. , Ogai A. , Iwata L. , Reisler H.
Published: July 2002

The photodissociation of jet-cooled NOCl following excitation in the lowest, structured absorption band is reported. We find that NO(2∏3/2) is produced preferentially, and that the rotational distributions depend strongly on the number of bending quanta in the upper electronic state. In addition, the NO vibrational excitation depends on parent NO stretch excitation. NO(v″=0) is predominantly produced when v′1 =0 is excited, while NO(v″=1) is mainly formed following excitation of v1 =1.
The adsorption and photochemistry of Mo(CO)6 on Rh(100)
Authors: Germer Thomas A. , Ho W.
Published: July 2002

The adsorption and photochemistry of Mo(CO)6 on Rh(100) has been studied using thermal desorption spectroscopy (TDS), photoinduced desorption (PID), and electron energy loss spectroscopy (EELS). It is found that Mo(CO)6 adsorbs onto the surface at 80 K dissociatively as carbonyl fragments and CO and adsorbs molecularly after the first monolayer is filled. Photochemistry arising from charge–transfer electronic excitation of adsorbed Mo(CO)6 is observed to correlate directly with the presence of multilayer adsorption. Results from PID indicate that UV radiation is effective in removing at least two, but not all, of the CO, ligands from each physisorbed Mo(CO)6 molecule. The rate of deexcitation by the metal surface is not sufficiently fast to quench the observed photochemistry. The effects of coadsorption of Mo(CO)6 on a presaturated CO ordered overlayer on Rh(100) are also studied. It is found that Mo(CO)6 adsorbs molecularly without affecting the CO(sat) layer and undergoes photodissociation. The quenching rate of the excited states of Mo(CO)6 is much slower on the ordered CO surface than on the dissociated surface containing carbonyl fragments and CO.
Collision-induced dissociation processes of Nb+4 and Fe+4: Fission vs evaporation
Authors: Loh S. K. , Lian Li , Hales David A. , Armentrout P. B.
Published: July 2002

Collision-induced dissociation (CID) of Nb+4 and Fe+4 with Xe has been studied over an energy range of 0–20 eV with a new instrument designed to study cold cluster ions. The energy dependences of the cross sections are presented. It is found that the lowest energy pathway for Nb+4 CID is fragmentation into molecular ions, or fission. Analysis of this process yields D0(Nb+2–Nb2)=4.3±0.3 eV. Fe+4, on the other hand, fragments only by sequential loss of Fe. Analysis of the lowest energy process gives D0(Fe+3–Fe)=1.8±0.2 eV. No evidence for molecular Fen fragments is observed.
Orientation and relaxation of nonlinear elastic dumbbells in electric fields: Modeling transient electric birefringence
Authors: Nath Sunil , Bowers John S. , Prud’homme Robert K.
Published: July 2002

The orientation and extension of a finitely extensible nonlinearly elastic (FENE) dumbbell in an electric field is analyzed to model the nonlinear phenomena observed during orientation and relaxation of flexible polymer molecules in transient electric birefringence measurements. In the model the dumbbell interacts with the electric field via an induced dipole with a dipole moment proportional to the end-to-end separation of the dumbbell. The electric field produces a strong ‘‘elongational’’ deformation of the molecule which has analogies with the deformations produced by elongational flow. The steady state birefringence, as a function of field strength, is calculated in two ways: first using matched expansions around low and high field conditions, and then using Peterlin’s approximation of preaveraging the connector force law. Both techniques yield similar results: Kerr law behavior is observed at low field strengths and saturation of birefringence at high field strengths. Birefringence relaxation after the field is removed shows a strong field dependence on the initial rate of relaxation. Previously proposed means of analyzing birefringence relaxation data for flexible molecules are discussed in light of this observation.
Explicit solutions of normal form of driven oscillatory systems in entrainment bands
Authors: Tsarouhas George E. , Ross John
Published: July 2002

As in a prior article (Ref. 1), we consider an oscillatory dissipative system driven by external sinusoidal perturbations of given amplitude Q and frequency ω. The kinetic equations are transformed to normal form and solved for small Q near a Hopf bifurcation to oscillations in the autonomous system. Whereas before we chose irrational ratios of the frequency of the autonomous system ωn to ω, with quasiperiodic response of the system to the perturbation, we now choose rational coprime ratios, with periodic response (entrainment). The dissipative system has either two variables or is adequately described by two variables near the bifurcation. We obtain explicit solutions and develop these in detail for ωn/ω=1; 1:2; 2:1; 1:3; 3:1. We choose a specific dissipative model (Brusselator) and test the theory by comparison with full numerical solutions. The analytic solutions of the theory give an excellent approximation for the autonomous system near the bifurcation. The theoretically predicted and calculated entrainment bands agree very well for small Q in the vicinity of the bifurcation (small μ); deviations increase with increasing Q and μ. The theory is applicable to one or two external periodic perturbations.
Relativistic configuration interaction calculations for polyatomics: Applications to PbH2, SnH2, and GeH2
Authors: Balasubramanian K.
Published: July 2002

A relativistic configuration interaction scheme is described for polyatomic molecules containing heavy atoms. In this method first complete active space MCSCF followed by large scale configuration interaction calculations are carried out. The natural orbitals generated in the large scale CI are then used in the relativistic CI (RCI) calculations, which include the spin–orbit integrals. The spin–orbit integrals are obtained using large Gaussian basis sets, with the operator expressed as a difference of relativistic effective core potentials, and then transformed over the natural orbitals. The transformed integrals are included as one-electron matrix elements in the RCI. This procedure thus takes into account both electron correlation and spin–orbit effects. The method is applied to the spin–orbit states derived from the three low-lying states of PbH2, SnH2, and GeH2 (1A1, 3B1, and 1B1). The spin–orbit mixings of the 1A1 and 3B1(A1) states in the RCI wave functions of PbH2 and SnH2 were found to be quite significant. For PbH2 this mixing lowers the 1A1 state by 1308 cm−1 while the 3B1(A1) is raised by 1371 cm−1 with respect to the 3B1 state without the spin–orbit splitting. The dipole moments of all the three radicals reveal that PbH2 has the largest dipole moment of the three. The electronic states of GeH2 and SnH2 are similar [1A1(A1)−3B1(A1):Te=23.1 kcal/mol for GeH2, Te=23.8 kcal/mol for SnH2], but the electronic states of PbH2 differ [1A1(A1)–3B1(A1):Te=41 kcal/mol] from their lighter analogs in that the splittings of the excited states are considerably higher. The Mulliken population analyses of the CI natural orbitals reveal that this is primarily a result of the relativistic stabilization of the 6s orbital of the lead atom due to the mass–velocity contraction. The geometries of the A1 component of the 3B1 state are altered to a considerable extent by the spin–orbit term for both PbH2 and SnH2.
Fully numerical restricted Hartree–Fock calculations on open-shell hydrides: On the basis-set truncation error
Authors: Laaksonen Leif , Müller-Plathe Florian , Diercksen Geerd H. F.
Published: July 2002

Fully numerical Hartree–Fock calculations have been performed for the BeH(X2Σ+), BeH(A2Πr), CH(X2Πr), NH(X3Σ−), OH(X2 Πi), MgH(X2Σ+), and SiH(X2Πr) molecules. Total and orbital energies and multipole moments are reported for these molecules at the experimental bond distances. The dependence of the basis-set truncation error on the internuclear separation has been studied.
Chopped-laser two-photon photodissociation measurement of collisional relaxation in benzene ion
Authors: Ahmed M. Saber , Dunbar Robert C.
Published: July 2002

The collisional relaxation of photoexcited benzene ions was studied through two-photon photodissociation using the chopped-laser technique in the ion cyclotron resonance (ICR) ion trap. The number of collisions required for quenching varied from 1 for benzene quench gas to 95 for helium, with the quenching efficiency increasing with size and complexity of the neutral. A comparison with quenching of bromobenzene ion by many of the same quench gases showed energy transfer from benzene ion to be about a factor of 2.5 less efficient.
The effect of transverse cross relaxation on nuclear magnetic resonance dipolar spectra
Authors: Gan Zhe-Hong , Facelli Julio C. , Grant David M.
Published: July 2002

The 13C dipolar powder spectra of 13CH3 31PO(OH)2 at room temperature and 13CH3 19F at low temperature (T=25 K) are reported. An intense peak is observed at the center of the Pake doublet. This phenomenon is explained by the transverse cross relaxation mechanism between the 13C and either the 31P or 19F nuclei. This theory shows the importance of the nonsecular terms in the Redfield relaxation theory. The chemical shielding tensors and dipolar coupling constants of these two molecules are obtained by fitting the experimental spectra.
The Cs2 (3) 1Πu electronic state pumped by Ar+ laser lines
Authors: Amiot C.
Published: July 2002

The fluorescence spectra of the two electronic systems (3) 1Πu→X 1∑g appearing in the visible range and (3) 1Πu→(2) 1∑g in the near infrared has been observed using an argon ion laser to excite Cs2 molecules. Fluorescence lines were accurately measured with a high resolution Fourier transform interferometer. Accurate molecular constants and potential energy curves have been determined for the (3) 1Πu electronic state. The main constants are as follows: Te(cm−1) =20 684.669; we(cm−1) =30.398; Be(cm−1) =0.009 12; Re(Å) =5.2735. Possible correlation with the atomic limit 7 2P1/2+6 2S1/2 yields a dissociation energy De=4730±1 cm−1.
Excited state structure and femtosecond ring-opening dynamics of 1,3-cyclohexadiene from absolute resonance Raman intensities
Authors: Trulson Mark O. , Dollinger Gavin D. , Mathies Richard A.
Published: July 2002

Resonance Raman spectra of 1,3-cyclohexadiene have been obtained with excitation wavelengths ranging from 300 to 253 nm. The absolute excitation profiles were analyzed to determine the initial dynamics that occur following photoexcitation in the lowest energy 260 nm 1B2 absorption band. The initial atomic displacements parallel the Woodward–Hoffmann, symmetry-allowed, ring-opening pathway towards all-cis-hexatriene. The potential surface displacements in the optically prepared 1B2 excited state are, however, a small fraction of the total distance along the reaction coordinate. The low absolute scattering cross sections indicate an excited state dephasing time of 10−14 s, and the fluorescence quantum yield of 2×10−6 shows that the total dephasing rate is dominated by ultrafast internal conversion. The Gaussian homogeneous line shape indicates a nonexponential excited electronic state population decay. We attribute these observations to an allowed crossing of the 1B2 state with a second excited 2A1 state that is responsible for the subsequent excited-state ring-opening reaction.
Evidence for mode specific dynamics in the stimulated emission pumping spectra of HFCO
Authors: Choi Young S. , Bradley Moore C.
Published: July 2002

Stimulated emission pumping spectra of HFCO in a pulsed jet at 0.2 cm−1 resolution in the range of 13 000 to 23 000 cm−1 are reported. Above the dissociation threshold, ≊14 000 cm−1, linewidths vary from 0.15 to 2.0 cm−1 for S0 vibrational levels within an energy range of a few hundreds of cm−1 . Substantial vibrational mode dependence of intramolecular vibrational energy redistribution and/or dissociation rates is likely.
Time-resolved laser optogalvanic spectroscopy of iodine in a radio frequency discharge
Authors: Kumar D. , Clancy P. L. , McGlynn S. P.
Published: July 2002

Pulsed laser optogalvanic (LOG) spectra of iodine vapor in a ∼32 MHz rf discharge were excited at 14 900–17 100 cm−1. Two distinct, time-resolved components were observed: a fast component, synchronous with the laser pulse, width ∼1 μs, followed by a slow component, width ∼100 μs, delayed relative to the laser pulse. The fast component exhibits atomic transitions of I(I) and I(II). The slow component reproduces the B̃←X̃ photoacoustic (PA) spectrum of molecular I2. The signal delay of the slow component accords with the velocity of acoustic waves in iodine vapor. The rf electrode region is the ‘‘sensitive’’ region where the acoustic wave generates the slow LOG signal. Two mechanisms of signal generation and propagation are involved. The fast signal originates in a two-step laser photoionization of plasma-excited atoms, the first-step being resonant, and/or in changes of the atomic collisional ionization rates. These processes occur on time scales shorter than the laser pulse and generate an ‘‘instantaneous’’ LOG signal by creating additional electron–ion pairs. The delay of the molecular LOG signal, which is mediated by the PA effect, indicates that local heating produces a negligible perturbation of the discharge impedance. This is contrary to common belief. The polarity of the slow LOG signal depends on the direction of the PA wave, suggesting that the signal is generated by an actual physical movement of charged species by the pressure wave. Thermal effects are involved, but only as precursors to the PA wave.
Two-laser resonance-enhanced photoionization spectrum of benzene
Authors: Müller G. , Fan J. Y. , Lyman J. L. , Schmid W. E. , Kompa K. L.
Published: July 2002

This paper reports results of an experiment involving two-laser resonance-enhanced photoionization of benzene. The excitation sources were two frequency-doubled dye lasers. The first laser pumped the molecule to a selected vibronic level of its first excited singlet state (1B2u), from where it was ionized by a time-delayed pulse of the second laser. The ion yield depends on the intermediate vibronic state as well as on the wavelength of the ionizing laser. From the structures and intensities of the measured ion spectra we derived vibrational frequencies and molecular parameters of the ground electronic state of the ion to a remarkable accuracy. The contributions of autoionizing Rydberg levels to the ionization cross section can clearly be distinguished from direct ionization. Several resonance peaks were assigned to transitions to vibrational modes within these Rydberg states.
A perturbation–trajectory method for the study of gas–surface collision dynamics
Authors: Jezercak Michael , Agrawal Paras M. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

A perturbation–trajectory method for determining the dynamics of gas–surface collision processes is described. The method is based upon the assumption that the motions of Q-zone atoms are unaffected by the collision process at the lattice surface. This assumption leads to a P-zone Hamiltonian that incorporates the effects of Q-zone motion in terms of time-varying P-zone–Q-zone interactions. The collision dynamics of the P zone are determined from an ensemble of stoichastic trajectories using this coupled Hamiltonian. The method is applied to three systems: (1) collinear inelastic atomic collisions with a ten-atom chain, (2) the inelastic scattering and absorption of NO on a Ag(111) surface, and (3) the collision and subsequent surface reactions of SiH2 on a Si(111) surface. Comparison of the perturbation results with those obtained using the full system Hamiltonian shows that under certain conditions the perturbation procedure yields very accurate results with a significant reduction in computational requirements. In general, the accuracy of the perturbation calculations increases as the incident-to-lattice-atom mass ratio decreases. A decrease in the strength of the interaction between the incident molecule and the Q zone, the incident translational energy, or the lattice temperature also improves the accuracy of the perturbation treatment. The method is therefore best suited to the study of inelastic, light-molecule collisions with heavy-atom surfaces at low temperature. Comparisons with previously reported gas–surface studies that employ a Langevin approximation are also given.
Rh atom ejection from keV ion-bombarded p(2×2)O/Rh{111}: Adsorption site and coverage determination from angle-resolved desorption measurements
Authors: Reimann C. T. , El-Maazawi M. , Walzl K. , Garrison B. J. , Winograd N. , Deaven D. M.
Published: July 2002

Angular distributions of Rh atoms desorbed by energetic ion bombardment of an oxygen covered Rh{111} surface are measured accurately using a multiphoton resonance ionization (MPRI) detection technique. The results, in conjunction with molecular dynamics calculations of the ion impact event show that these distributions reflect the near-surface crystal structure. The molecular dynamics calculations were performed using a many-body embedded-atom potential to describe the dynamics of the Rh atoms and a pair-wise additive potential to describe the oxygen–Rh interactions. Several oxygen overlayer structures were considered for molecular dynamics modeling of the desorption process, including p(2×2) overlayers with a coverage of 0.25 monolayer (ML), and p(2×1) overlayers with a coverage of 0.50 ML, both of which are consistent with low energy electron diffraction (LEED) data. Three different adsorption sites were tested: threefold symmetric sites over second layer Rh atoms, threefold symmetric sites over third layer Rh atoms, and atop sites. The calculated azimuthal angular distributions of desorbed Rh atoms for each of these cases are unique, matching the experimental data best in the case of a p(2×1) overlayer with oxygen atoms adsorbed in threefold symmetric sites over third layer Rh atoms. The calculated Rh atom desorption yield (ejected atoms per incident ion) is sensitive to the oxygen coverage in the range 0.25–0.50 ML. These calculations are important in developing a surface bonding site and coverage consistent with LEED and our experiments. The peak in energy distribution of ejected Rh atoms from the oxygen covered surface is at a lower energy value than that of the clean metal. This indicates that collisional energy loss processes contribute to determining the peak position as well as the well known binding energy effect.
Improved calculations of rate constants for the H+H2 reaction and its isotopic analogs at low temperatures
Authors: Takayanagi Toshiyuki , Nakamura Kazutaka , Sato Shin
Published: July 2002

The rate constants for the H+H2 reaction and its isotopic analogs at very low temperatures have been recalculated using variational transition-state theory including one-dimensional tunneling corrections on the Marcus–Coltrin path. For the calculation of the tunneling correction, numerical integration was used and compared with the use of the Eckart fitting function. Using these calculated rate constants, the Wigner threshold law was reexamined and decay rates of H and D atoms in solid H2, D2, and HD were reestimated. The qualitative trend with isotopes did not change from that found in previous calculations.
Mass effects and channel coupling sensitivity in vibrational energy transfer
Authors: Kreutz Thomas G. , Eno Larry , Rabitz Herschel
Published: July 2002

This paper examines the mass dependences of vibrationally inelastic integral state-to-state cross sections for the helium-hydrogenic scattering system. The cross sections are calculated within the coupled states-exponential distorted wave (CS-EDW) approximation, and are analyzed along with the conceptually simpler distorted wave (CS-DW) cross sections. Quantitative links between the EDW and DW results are made by means of component (or channel coupling) sensitivity coefficients, which measure the sensitivity of a given EDW cross section to the potential matrix elements coupling any two states.
Third order a
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o calculations of amplitudes of two-photon absorption for ions across the lanthanide series
Authors: Smentek-Mielczarek Lidia , Hess B.Andes
Published: July 2002

The third-order electron-correlation theory of two-photon absorption in rare earth ions in crystals has been extended by the effective operators arising from doubly excited configurations. The approach defined in terms of one-particle effective operators has been applied to the Pr+3, Nd+3, Eu+3, Gd+3, Tb+3, and Tm+3 ions. The relative importance of third-order contributions and their properties across the lanthanide series are presented. The discussion is based on the numerical results of exact ab initio calculations performed within the perturbed-function approach.
Solvent dynamical effects in electron transfer: Predicted consequences of non-Debye relaxation processes and some comparisons with experimental kinetics
Authors: McManis George E. , Weaver Michael J.
Published: July 2002

The consequences of non-Debye solvent relaxation upon the barrier-crossing dynamics of adiabatic electron-transfer processes have been explored numerically using a rate formulation due to Hynes for several common forms of the dielectric response function Ê(s), with the objective of assessing the likely experimental importance of such effects. For the ‘‘multiple Debye’’ form of Ê(s), analytic expressions for the required time-correlation function can be obtained, whereas for the Davidson–Cole and Cole–Cole forms numerical solutions to the inverse Laplace transform were required. Illustrative numerical results are presented of the increases in the adiabatic barrier-crossing frequency, νn, predicted to be engendered by the presence of higher-frequency relaxation components for dielectric conditions of likely experimental relevance. Substantial (five- to ten fold) rate enhancements are often obtained, resulting from the disproportionately large influence upon νn predicted to arise from the higher-frequency components of Ê(s). Neither νn, nor the non-Debye influence upon νn, are found to be affected greatly by alterations in the shape of the barrier top caused by variations in the electronic coupling matrix element. Comparisons between these numerical predictions and corresponding experimental solvent-dependent νn values extracted from metallocene self-exchange kinetics indicate that the former can account for a substantial fraction of the νn accelerations observed in alcohols and other non-Debye solvents. Roughly concordant non-Debye effects are also predicted from some other, but not all, recent rate formulations. The desirability of utilizing subpicosecond dynamical solvation information from fluorescence Stokes shifts to predict non-Debye effects upon electron-transfer barrier-crossing frequencies is pointed out.