bluuebunny/crossref_metadata_2025 huggingface dataset:
The identification of a 6Φ–X 4Δ intercombination system in the FeH radical at 448 nm
Authors: Carter Robert T. , Steimle Timothy C. , Brown John M.
Published: July 2002

A new band system has been discovered in the electronic spectrum of the FeH radical at 448 nm. The spectrum has been recorded at Doppler resolution by laser excitation spectroscopy with a sample at ambient temperatures, produced by the gas phase reaction between H atoms and iron pentacarbonyl Fe(CO)5. The fluorescence excited when the laser is tuned to individual rotational lines has also been dispersed. These experiments show that the light is emitted almost wholly at longer wavelengths into the 492 nm system (the ‘‘blue’’ system). The 448 nm system has been assigned as a 6Φ–4Δ transition; the lower state is shown to be the X 4Δ state previously characterized by Phillips et al. Accurate term values are obtained for two spin components in the 6Φ state, designated the g 6Φ state. The dispersed fluorescence experiments show that the lower state of the 492 nm system is the a 6Δ state and that it lies about 2060 cm−1 above the X 4Δ state. A conclusion of this study is that the 492 nm system of FeH involves the g 6Φ–a 6Δ transition.
Methane rotation and the phase diagram of CH4/xenon
Authors: Prager M. , Asmussen B. , Langel W. , Carlile C. J. , Blank H.
Published: July 2002

(CH4)cXe1−c systems with c=0.008, 0.015, 0.025, 0.1, and 0.9 have been investigated with inelastic neutron scattering. Tunnel transitions show almost free rotation of isolated CH4 quantum rotors at low methane concentrations and the appearance of dimers and trimers with increasing concentration. The rotational potential seen by a CH4 defect is explained by a model on the basis of statistical distributions of local orientational potentials. Disorder in a nonequilibrium matrix leads to a broad distribution of rotational potentials. A perturbed partially ordered phase II at large concentrations extends to higher concentrations than in CH4/Ar. No miscibility gap similar to that in CH4/Ar is found within the accessible concentration range c≤0.1 and c≥0.9.
The conformational problem studied through the torsional Raman spectra of CH2DCH2D, CHD2CHD2, and CH2DCHD2
Authors: Fernández-Sánchez J. M. , Gómez P. C. , Montero S.
Published: July 2002

The torsional Raman spectra of CH2DCH2D, CHD2CHD2, and CH2DCHD2, recorded with an improved conventional Raman spectrometer, are interpreted here in the frame of an exact rotational–torsional Hamiltonian. The kinetic and potential operators of this Hamiltonian are discussed in detail, showing that the term responsible for the existence of two distinguishable conformers is of kinetic nature. For CH2DCH2D and CHD2CHD2 it is shown how to separate the rotational and the torsional motions by means of an appropriate choice of the molecular axis system. The wave numbers and intensities of both torsional Raman spectra are satisfactorily reproduced within the limits of the experiment with the following effective potential parameters: CH2DCH2D, V3=1002 cm−1, V6=6.0 cm−1; CHD2CHD2, V3=995 cm−1, V6=7.3 cm−1. For CH2DCHD2, only partially tractable in the present approach, an approximate estimate of V3=995 cm−1 has been obtained. Besides the quantitative interpretation of the torsional spectra, a discussion of the main facts of the conformational problem in connection to the spectroscopic observation is presented. In particular, the induced conformational transitions described here may contribute to the understanding of the elementary photoisomerization processes in molecules of the type AX2Y–AX2Y.
Resonance enhanced multiphoton ionization photoelectron spectroscopy on nanosecond and picosecond time scales of Rydberg states of methyl iodide
Authors: Dobber M. R. , Buma W. J. , de Lange C. A.
Published: July 2002

Rydberg states of methyl iodide have been investigated using resonance enhanced multiphoton ionization in combination with photoelectron spectroscopy with nanosecond and picosecond laser pulses. The study of the ns (6≤n≤10) Rydberg states in two-, three-, and four-photon excitations has resulted in an unambiguous identification of state [1] in the 7s and 8s Rydberg states. As a consequence, it is concluded that the transition to 6s[1] in two- and three-photon excitations is anomalously weak. The application of photoelectron spectroscopy to identify the electronic and vibrational nature of a resonance has led to a major reinterpretation of the excitation spectrum of the 6p Rydberg state in two-photon excitation. In many of the recorded photoelectron spectra anomalous electrons are observed, which derive from a one-photon ionization process. This process is suggested to find its origin in the mixing of 6p and 7s character into higher-lying Rydberg states. The major difference between resonance enhanced multiphoton ionization photoelectron spectroscopy with nanosecond and picosecond lasers is found in a less effective dissociation of the molecule in the picosecond experiments.
Electron transfer and back transfer in bichromophoric donor–acceptor systems
Authors: Grabowska J. , Hajzner A. , Choynowski A. , Sienicki K.
Published: July 2002

Theoretical investigations of forward and reverse photoelectron transfer in bichromophoric systems embedded in solid solution is presented. Respective equations for decay profiles of donor and radical pairs are presented. Assuming a specific Gaussian-like distribution function between donor and acceptor molecules, the decay profiles of donor and radical pairs were analyzed. The analysis of influence of excluded volume on photophysical observables is also presented. The results obtained show the importance of introduction of specific distribution functions in analysis of photoelectron transfer in bichromophoric systems.
Local mode of motions in amorphous solid. III. Contribution of hydrogen bonds in D-sorbitol
Authors: Naoki Motosuke , Ujita Koji
Published: July 2002

Dielectric properties of the local mode of motions (the β relaxation process) in two D-sorbitol glasses are presented as a function of temperature and pressure. The glasses were vitrified from the liquid by isobaric cooling at a rate of −0.2 K/min under 0.1 and 78.5 MPa. The following results are different from those observed in molecular and polymer glasses: the relaxation magnitude of the higher-density glass vitrified under 78.5 MPa is larger than that of the lower-density glass vitrified under the atmospheric pressure; the distribution of the relaxation time of the higher-density glass is narrower than that of the lower-density glass; and the relaxation magnitude of each glass appears to increase slightly with pressure. The results are interpreted by the difference in the distribution of intermolecular hydrogen bonds frozen-in at the vitrification points. It is also suggested that the hydrogen bonds in the glasses may be slightly decomposed and/or weakened by external pressure.
Quantum activated rate theory: Variational optimization of planar dividing surfaces
Authors: Messina Michael , Schenter Gregory K. , Garrett Bruce C.
Published: July 2002

A variational procedure is presented for finding the optimal planar dividing surface within a centroid-density based quantum rate theory for the model of a general reaction coordinate coupled to a harmonic bath. The approach described here is a limiting form of the method for choosing the best coordinate and momentum dependent dividing surfaces that was previously presented by the authors [J. Chem. Phys. 98, 8525 (1993)]. The present approach can also be considered a direct quantum mechanical generalization of the classical variational method of Berezhkovskii, Pollak, and Zitserman [J. Chem. Phys. 97, 2422 (1992)]. We also relate this method to the analytical approach of Voth [Chem. Phys. Lett. 170, 289 (1990)] that incorporates a transmission coefficient in the centroid-density based quantum rate theory. The variational procedure is also applicable to systems coupled to a continuum of oscillators, and it is shown that this procedure can be efficiently implemented for an arbitrary number of oscillators in the bath. Numerical results are presented for an Eckart barrier coupled to a bath of harmonic oscillators. Numerical results show that a strict variational optimization of the planar dividing surface offers some improvement for the rate constants relative to those of the analytic theory of Voth, thus justifying the extra work needed for the variational search.
An investigation of the 355 nm photodissociation of NO2 by state-resolved photofragment imaging
Authors: Hradil V. P. , Suzuki T. , Hewitt S. A. , Houston P. L. , Whitaker B. J.
Published: July 2002

The 355 nm photodissociation of NO2 cooled in a supersonic beam has been investigated by state-resolved photofragment imaging. The NO and O(3PJ) photofragments were state-selectively ionized and projected onto a two-dimensional, position-sensitive detector to obtain speed and angular distributions. The speed distribution of the O(3P2) fragment displays two peaks corresponding to oxygen produced in coincidence with NO(υ=0) and NO(υ=1). The angular distributions for the O(3P2) and for the NO in several vibrational and rotational levels can be characterized by an anisotropy parameter of β=1.2±0.3. This value, while higher than that measured previously, is consistent with a dissociation lifetime on the order of 200–400 fs and with the colder rotational temperature of the current beam experiment. The rotational distributions of the NO product are found to be in good agreement with other recent measurements.
Spectroscopy of the indium argon van der Waals complex: A high resolution study of the B 2Σ1/2←X2 2Π3/2 system
Authors: Hackett Peter A. , Balfour Walter J. , James Andrew M. , Fawzy Wafaa M. , Shetty Belvai J. , Simard Benoit
Published: July 2002

The InAr van der Waals complex has been characterized by high resolution laser induced fluorescence excitation spectroscopy. Six vibronic bands of the B 2Σ1/2←X2 2Π3/2 transition have been observed and five of these (v′,0), where v′=1–5, have been rotationally analyzed. Rydberg–Klein–Rees potential curves were constructed for the B 2Σ1/2 state using the rotational and vibrational constants determined from these spectra. Equilibrium bond lengths were determined for the B and X2 states and a dissociation energy was determined for the B state. The stronger bonding present in the B state is rationalized in terms of penetration of the argon atom into the diffuse 6s orbital of indium. Evidence is presented that the B state potential energy curve has a barrier at long range, due to Pauli repulsion, of ∼60 cm−1. An analysis of the hyperfine structure involving the 115In nucleus was made. It is concluded that the X2 state conforms to Hund’s coupling case aβ, whereas the B state conforms to case bβs. The extent of 6s–6p hybridization in the upper state was measured from hyperfine splittings and was used in conjunction with a simple electrostatic model to estimate the polarizability of the indium atom in the 6s 2S1/2 state. A value of 68(4) Å3 was obtained (1σ error).
An ab initio potential energy surface for the study of rotationally inelastic OH–H2 collisions
Authors: Offer Alison R. , van Hemert Marc C.
Published: July 2002

A new two-configuration self-consistent field (SCF) plus dispersion potential for the interaction of OH with H2 is presented. The ground state of OH is a 2Π state leading to a two component potential which is diagonal in the adiabatic electronic basis in which the quantum chemical calculations are done. The transformation between the adiabatic basis and the electronic basis used in collision calculations is described. The transformed potential matrix is Hermitian in this electronic basis when OH–H2 configurations with no plane of symmetry are considered. The orientation dependence of the elements of the potential matrix is given in the form of a spherical expansion and the results are compared with previous potential calculations. The comparison is discussed in terms of the dependence on the orientation of the H2 molecule.
Experimental study of the nature of the glass transition process in monohydroxy alcohols
Authors: Murthy S. S. N. , Nayak S. K.
Published: July 2002

The nature of the glass transition process in six monohydroxy alcohols, viz., the isomers of butanol, 2-propanol, and 3-methyl–1-butanol had been studied in detail using dielectric spectroscopy (frequency range 10+6–10−3 Hz) and differential scanning calorimetry (DSC). Measurements were made down to a temperature of 77 K in dielectric experiments and a temperature of 95 K in DSC. These results together with the published microwave data on these liquids reveal the existence of two glass transition processes—one due to the hydrogen bonded network which is very dominant and the other one due to the nonhydrogen bonded molecules, which is much weaker. Associated with the latter process, there is a secondary (or β-) relaxation process whose activation energy varies from 4.8 to 6.7 kcal/mol which is dependent on the shape and size of the liquid molecules. In addition to the β process, all the alcohols revealed yet another sub-Tg (γ) process whose activation energy varies from 2.1 to 2.6 kcal/mol and is found to be little dependent on the molecular shape. These results perhaps indicate that the β process is somehow connected to the relaxation of the free monomeric molecule and the γ process to that of hydroxyl group rotation, respectively.
Emergence of circularity at linear polarized excitation of molecules
Authors: Auzinsh M. P. , Ferber R. S.
Published: July 2002

General conditions causing partial transformation of optical alignment of excited state angular momenta J into orientation at weak linear polarized broad line excitation of molecules are examined. As is shown, the phenomenon takes place under the effect of an external perturbing factor leading to such magnetic sublevel M splitting ωMM′ when ωMM±1≠ω−M≠1−M and ωMM′ is of the order of molecular state relaxation rate. An analytical expression is obtained describing the appearing circularity of fluorescence for arbitrary J values. A numeric example is given for 1∑ state molecules with J=10 assuming a quadratic Stark effect in a homogeneous external electric field.
A molecular dynamics method for obtaining the vibrational spectra of macromolecules
Authors: Wozny C. E. , Sumpter B. G. , Noid D. W.
Published: July 2002

A molecular dynamics-based method is used to compute the vibrational spectra and dispersion curves of a polyethylene chain. The results are compared and contrasted with those of normal mode calculations, demonstrating the validity of the approach and providing fundamental insight into a number of dynamical processes. The strength of the method is its computational efficiency and that it represents an exact classical dynamics approach to calculating vibrational spectra, thereby eliminating many of the weaknesses of normal mode calculations (i.e., the harmonic approximation).
Spin–orbit configuration interaction study of the potential energy curves and radiative lifetimes of the low-lying states of bismuth hydride
Authors: Alekseyev Aleksey B. , Buenker Robert J. , Liebermann Heinz-Peter , Hirsch Gerhard
Published: July 2002

An ab initio configuration interaction (CI) study including the spin–orbit coupling interaction is carried out for the lowest 23 states of the bismuth hydride molecule by employing relativistic effective core potentials for the bismuth atom. The computed spectroscopic constants are in good agreement with corresponding experimental data, although there is a tendency to overestimate bond lengths by 0.05–0.10 Å and to underestimate the vibrational frequencies accordingly. The B0+ excited state is found to have no dissociation barrier, and its radiative lifetime is computed to be 4.3 μs, with parallel transitions to X10+ being significantly stronger than the perpendicular B–X21 species. The experimental E0+ state is assigned as the third root of this symmetry and its potential curve possesses a dissociation barrier of 1840 cm−1. This result explains the predissociation characteristics observed for this state and is also consistent with the failure to observe emission from it when attempts are made to form it in recombination processes. This barrier also needs to be taken into account in estimating the X10+ dissociation energy of this molecule from existing experimental data, and on this basis a De value of 2.28 eV is obtained which is in reasonably good agreement with the present directly computed result of 1.98±0.06 eV. A number of other bound states and avoided crossings are indicated in the calculations which may be of relevance in future experimental investigations of this system.
Quantum rates for nonadiabatic electron transfer
Authors: Egger Reinhold , Mak C. H. , Weiss Ulrich
Published: July 2002

We study a general asymmetric spin-boson model for electron transfer processes in the nonadiabatic regime. Our path-integral approach reduces to the conventional Marcus result in the high-temperature limit, whereas quantum effects lead to distinct deviations from classical behaviors at low temperatures. Adopting a simple but realistic model spectral density for the environmental modes, we show that for low temperatures, the rate becomes sensitive to the details of the spectral density. In particular, due to nuclear tunneling, the rate maximum does not correspond to the usual classical estimate for the activationless case and the rate is asymmetric around the maximum. A centroid factorization using a properly defined nuclear reaction coordinate provides new insight into the mechanism of nonadiabatic electron transfer.
Collision-induced state-to-state energy transfer in the ν2+3ν3 rovibrational manifold of gas-phase acetylene
Authors: Milce A. P. , Barth H.-D. , Orr B. J.
Published: July 2002

Time-resolved infrared-ultraviolet double resonance spectroscopy is used to measure collision-induced rovibrational energy transfer in the ν2+3ν3 region (∼11 600 cm−1) of gas-phase acetylene. Of particular interest is rotationally resolved V–V transfer between the Fermi-coupled 2133 and 11234351 levels, for which the rate is relatively high (approximately 13% of the Lennard-Jones collision rate) and the relevant rovibrational states markedly perturbed.
Picosecond vibrational dynamics of several S1 bands in jet-cooled p-difluorobenzene
Authors: Zhang Xu , Smith Jonathan M. , Knee J. L.
Published: July 2002

Picosecond pump–probe threshold photoelectron spectroscopy was used to investigate the vibrational dynamics of four vibrational bands in the S1 state of p-difluorobenzene in the range 2000 to 2900 cm−1. In this energy region the vibrations exhibited intermediate case restricted vibrational dynamics as evidenced by observed quantum beats as well as irreversible statistical limit decay. More complete coverage of the S1 state was prohibited by the wavelength restrictions on both the pump and probe laser wavelengths required in the experiment. The observed restricted vibrational redistribution is in contrast to the very rapid irreversible decays inferred from room temperature, high pressure chemical timing experiments on the same molecular bands. The differences are discussed in terms of the role of molecular rotations and the possible different experimental observations.
Structural measurements of hydrogen-bonded van der Waals dimers and trimers by rotational coherence spectroscopy: 2,5-diphenyloxadiazole with argon, methane, water, and alcohols
Authors: Troxler Thomas , Smith Philip G. , Stratton John R. , Topp Michael R.
Published: July 2002

Picosecond time-resolved polarized fluorescence experiments involving time-correlated single-photon counting have studied rotational coherence phenomena of hydrogen-bonded and other molecular aggregates in their lowest excited singlet states. The experiments are supported by detailed simulations. Using the molecule 2,5-diphenyl-1,3,4-oxadiazole (PPD) as a host species, experiments have compared van der Waals aggregates with Ar1,2, (CH4)1,2, (H2O)1,2, (CH3OH)1,2, (C2H5OH)1, and (1-C3H7OH)1. Bare PPD, and the argon and methane aggregates all exhibit prominent J-type recurrences. The resulting sums of rotational constants (B+C) are consistent with center-of-mass-bound, three-dimensional structures, having out-of-plane distances for the attached species of 3.3–3.5 Å. The 1:2 aggregates involving argon and methane exhibit additive spectral shifts and nearly additive rotational recurrence times. This shows that the sites for addition of consecutive species are equivalent.

Calculations of rotational constants confirm these findings. All except the Ar 1:2 cluster exist close to the prolate symmetric top limit. On the other hand, the excitation spectra of complexes involving hydrogen-bonding species all show small complexation shifts at the 1:1 level and disproportionately larger shifts at the 1:2 level. Similar nonadditive behavior is seen for the rotational recurrence transients. Hydrogen-bonded species differ from the nonpolar cases, since they show both prominent C-type and J-type transients. This shows that these species all differ significantly from prolate symmetric tops. Detailed simulations reveal that all of the hydrogen-bonding species produce aggregates that involve a single hydrogen bond to one of the PPD nitrogen atoms. This imposes a planar type of structure on the 1:1 water and methanol complexes. On the other hand, the aggregates methanol 1:2, ethanol 1:1, and propanol 1:1 all involve a distinct out-of-plane twist, consistent with the increasing influence of dispersive interactions.

Hydrogen bond distances (N...H–O) are found to be in the range 2.7–2.9 Å, and the hydrogen-bond angles (N–N...H–O), relative to the PPD long axis, range from 115° to 130°. In addition, the water and methanol 1:2 aggregates both contain hydrogen-bonded dimer units that resemble the free dimers of each species as identified by infrared and microwave techniques. For example, we find the (O...H–O) distance in the methanol dimer complex to be ≊2.7 Å.
Frequency-selective heteronuclear recoupling in rotating solids
Authors: Bennett Andrew E. , Becerra Lino R. , Griffin Robert G.
Published: July 2002

In solid state nuclear magnetic resonance spectroscopy, several methods have recently been introduced for the purpose of restoring dipolar couplings into magic angle spinning experiments. These powerful techniques are useful for the measurement of internuclear distances and, more generally, for filtering and correlation experiments in noncrystalline materials. In the case of heteronuclear spins, the rotational echo double resonance experiment provides an elegant and practical approach to the spectrally nonselective reintroduction of dipolar interactions. In this article, we describe an approach to restore heteronuclear couplings which is spectrally selective—in particular we recouple the observed spins only with those nonobserved spins lying at exact resonance (and at multiples of the spinning frequency) in the triple resonance experiment. We refer to the technique as frequency-selective dipolar recoupling.
Laser-induced gratings in free jets. III. Saturation-induced anharmonic gratings
Authors: Dunlop James R. , Rohlfing Eric A.
Published: July 2002

We apply two-color laser-induced grating spectroscopy (LIGS) to obtain excitation spectra of the S1(Ã 1B1)–S0(X̃ 1A1) system of jet-cooled azulene. The ground-state depletion grating created by excitation with crossed beams is probed via either the S2(B̃ 1A1)–S0(X̃ 1A1) or S4(D̃ 1A1)–S0(X̃ 1A1) transitions. First-order LIGS spectra of the S1–S0 origin band at several grating-laser intensities reveal saturation-induced line broadening and significant saturation dips at line center. We develop a simple model, based on the anharmonic grating created in the saturated regime, and use it to fit the first-order data. From these fits we determine a saturation parameter and the linewidth of the S1–S0 origin absorption band, which is homogeneously broadened due to rapid internal conversion (τ∼1 ps) in the S1 state. We also observe LIGS spectra of the azulene origin taken at the second Bragg order of the induced grating; the observed second-order lineshapes are also fit with the anharmonic-grating model.
Thermal electron attachment to SF4 and SF6
Authors: Miller Thomas M. , Miller Amy E. Stevens , Paulson John F. , Liu Xifan
Published: July 2002

Rate coefficients for electron attachment to SF4 and SF6 have been measured over the temperature range 300–550 K using a flowing-afterglow Langmuir-probe apparatus. The 300 K rate coefficient for SF4 is 2.5±0.6×10−8 cm3 s−1, a value 11 times smaller than the corresponding rate for SF6. The attachment rate coefficients for both SF4 and SF6 are nearly independent of temperature up to 500 K, and decline somewhat at still higher temperatures. SF−4 is the only ionic product of attachment to SF4 observed over the entire temperature range. SF−6 and SF−5 are products of attachment to SF6; an ‘‘activation energy’’ of 0.42±0.02 eV is inferred for SF−5 production.
Ab initio potential energy curves for low-lying states of carbon disulfide
Authors: Tseng D. C. , Poshusta R. D.
Published: July 2002

Potential energy surfaces for the ground singlet (S0), lowest excited singlet (S1), and lowest triplet (T0) electronic states of carbon disulfide are explored using ab initio Hartree–Fock (HF), generalized valence bond (GVB) and multiconfiguration self-consistent-field (MCSCF) methods using the 6-31G* basis. The lowest singlet hypersurface, S0, contains the linear (D∞h) equilibrium geometry ground state X 1Σ+g and the previously unreported bent (C2v) conformer 1A1. The calculated geometry of the linear state [re(C–S)=1.544 Å] compares favorably with experiment [r0(C–S)=1.556 Å]; there are no experimental values to compare with the calculated geometry of the bent state [re(C–S)=1.718 Å, <SCS=78.4°]. The transition state between the linear and bent conformers is determined to have C2v geometry [r(C–S)=1.691 Å, <SCS=103.4°] and a barrier height of 7.7 kcal/mol. We also explore the 1B2 surface, S1, and the 3B2 surface, T0, finding the equilibrium geometry, dipole moments, and vibrational frequencies of these states as well as those of the ground state and bent conformer.
Hyperfine structure in high spin multiplicity electronic states: Analysis of the B 4Π–X 4Σ− transition of gaseous NbO
Authors: Adam A. G. , Azuma Y. , Barry J. A. , Merer A. J. , Sassenberg U. , Schröder J. O. , Cheval G. , Féménias J. L.
Published: July 2002

The (0,0) band of the B 4Π–X 4Σ− transition of NbO, near 6600 Å, has been analyzed from spectra taken at sub-Doppler resolution. The transition is notable for the great width of its Nb nuclear hyperfine structure, which is caused principally by the unpaired 5sσ electron in the ground state interacting with the large magnetic moment of the 4193Nb nucleus (I=9/2). A fit to the ground-state combination differences, including four very precise microwave lines measured by Suenram et al. [J. Mol. Spectrosc. 148, 114 (1991)], has given a comprehensive set of rotational, spin, and hyperfine parameters. Prominent among these are the third-order spin–orbit distortions of the spin-rotation interaction and the Fermi contact interaction, which are large and well determined, reflecting different degrees of spin–orbit contamination of the the 4Σ1/2− and 4Σ3/2− components of the ground state.

The δ 2π B 4Π state was hard to fit, for a number of reasons. First, its spin–orbit structure is asymmetric, because of strong perturbations by a 2Π state which has been identified in this work, from among the various weak bands in the NbO spectrum near 7000 Å; the result is that many high order centrifugal distortion terms are needed in an effective Hamiltonian model for the rotation. Second, the hyperfine structure is perturbed, not only by this 2Π state, but by distant Σ and Δ states at higher energy. The δ 2σ* C 4Σ− state at 21 350 cm−1 appears to be one of these. The distant states generate large apparent nuclear spin-rotation interactions, both within and between the Λ components of the Π state, as a result of cross terms between matrix elements of the operators −2BJ⋅L and aI⋅L. Similar cross terms arising from the operators AL⋅S and aI⋅L produce corrections to the Fermi contact matrix elements and are responsible for the unexpected negative sign of the magnetic hyperfine parameter d. The ‘‘off-diagonal’’ quadrupole parameter e2Qq2 is very large, and causes some of the higher J line shapes of the B–X system to be noticeably asymmetric at Doppler limited resolution; its value is consistent with the electron configuration of the B 4Π state being δ 2π.
Solvent dynamics influence on chemical reaction dynamics
Authors: Berezhkovskii A. M. , Dudko S. A.
Published: July 2002

For chemical reactions going on in solvents any transformations among the reactants are accompanied by corresponding reorganization of the surrounding solvent. A theory of the solvent reorganization dynamics influence on the chemical reaction dynamics is constructed for heavy particle charge transfer reactions in polar solvents. It is shown that the solvent’s influence on the reaction dynamics can be taken into account by introducing a few additional coordinates—the effective solvation (media) coordinates—so that the reaction subsystem involves these coordinates along with the ‘‘chemical’’ one, intrinsically responsible for the chemical conversion. Equations of motion for the reaction subsystem are derived. All the parameters appearing in these equations are explicitly expressed in terms of quantities characterizing both the reaction and the solvent.
When is a potential accurate enough for structure prediction? Theory and application to a random heteropolymer model of protein folding
Authors: Bryngelson Joseph D.
Published: July 2002

Attempts to predict molecular structure often try to minimize some potential function over a set of structures. Much effort has gone into creating potential functions and into creating algorithms for minimizing these potential functions. This paper develops a formalism that addresses a complementary question: What are the accuracy requirements for a potential function that predicts molecular structure? The formalism is applied to a simple model of a protein structure potential. The results of this calculation suggest that high accuracy predictions (∼1 Å rms deviation in α-carbon positions) of protein structures require monomer–monomer interaction energies accurate to within 5% to 15%. The paper closes with a discussion of the implications of these results for practical structure prediction.
Toward the observation of silanone (H2SiO) and hydroxysilylene (HSiOH) via microwave spectroscopy
Authors: Ma Buyong , Schaefer Henry F.
Published: July 2002

Ab initio quantum mechanical methods were employed to study the H2SiO and HSiOH (both cis and trans) isomers, resulting in high-level theoretical predictions of the equilibrium geometries, rotational constants, dipole moments, relative energies, vibrational frequencies, transition state structures, and the activation energy for the isomerization between the cis- and trans-HSiOH isomers. Basis sets as large as triple zeta plus double polarization plus silicon and oxygen atom f and hydrogen atom d functions [TZ2P(f,d)] have been used with the self-consistent-field configuration interaction including all single and double excitations (CISD), and coupled cluster including all single and double substitutions (CCSD) methods, as well as CCSD with the effects of connected triple excitations added perturbatively [CCSD(T)]. Our predictions for the dipole moment components and geometry of silanone (H2SiO) were instrumental in its recent microwave spectroscopic identification (accompanying paper by Bogey and co-workers) and are in excellent agreement with the experimental results. The silanone (H2SiO) molecule is predicted to lie about 0.5 kcal mol−1 lower in energy than the HSiOH isomers. In contrast with previous theoretical work, cis-HSiOH may be slightly more stable than trans-HSiOH. The experimental IR spectrum for the HSiOH isomer, which may have been misassigned to trans-HSiOH, is closer to that for cis-HSiOH at the TZ2P(f,d) CISD level of theory. The isomerization between cis- and trans-HSiOH takes place along the torsional mode, and the activation energy is predicted to be 8.3 kcal mol−1.
From ionic aqueous solvation shell to bulk fluid: A structural-energetic stability problem
Authors: Degrève Léo , Quintale Clovis
Published: July 2002

The aqueous solvation shell structures for two ions, Li+ and F−, were obtained through Monte Carlo simulations of systems consisting of one ion immersed in water, using the four points transferable intermolecular potential (TIP4P). The clear definition of the tridimensional structure of the solvation shells of these ions, both constituted by four water molecules, permits the calculation of the solvent molecules’ energy in the immediate vicinity of the central ion. A pseudopotential of mean force and the mean energy per molecule were also calculated. The energy profiles are easily related to the formation and the stability of the interface ion–solvent molecules. The stability of the solvation shell is due to (i) an energy difference between the molecules in the solvation shell and in the bulk phase greater, in absolute value, than the thermal energy, and (ii) an energy barrier that separates the solvation shell from the bulk. In general, most of the cationic aqueous solvation shells are similar because the cations are small ions and because the water molecule dipole vector is oriented to the cation’s center. On the other hand, the anionic aqueous solvation shells are complex structures due to the larger size of the anions and due to the fact that the typical approximate collinearity anion–H–O perturbs strongly the solvent immediate neighborhood. These facts result in monolayer cationic and bilayers anionic solvation shells. These observations were confirmed by the structure of the solvation shells of several different ions and their energies.
A kinetic theory of suspensions. II. The steady flow of a hard-sphere suspension
Authors: Jorquera Hector , Dahler John S.
Published: July 2002

Calculations are reported of the steady-state shear and bulk viscosity coefficients of a suspension of freely rotating, hard spheres dispersed in a continuous Newtonian solvent. These calculations are based on a theory recently reported in this journal. Our theoretical predictions are compared with those of other theories, with the results of computer simulations, and with experimental data.
Reduction of degenerate four-wave mixing spectra to relative populations II. Strong-field limit
Authors: Williams Skip , Zare Richard N. , Rahn Larry A.
Published: July 2002

The degenerate four-wave-mixing (DFWM) signal is said to be saturated when the population difference of the two levels involved in the resonant transition oscillates with a rate (Rabi frequency) greater than the relaxation and dephasing rates. The field intensity at which this occurs is referred to as the DFWM saturation intensity. We find that the DFWM saturation behavior predicted by nondegenerate two-level models is in close agreement with the observed power dependence of (0,0) band transitions of the CH A 2Δ−X 2Π system. Furthermore, when the linear polarization states of the excitation fields are varied, the saturation intensity does not change significantly. In contrast, large differences in the DFWM signals are observed as a function of input field polarization and rotational branch. These differences are nearly independent of laser intensity. The DFWM signal differences are rationalized using the diagrammatic perturbation theory (DPT) expressions described in the preceding paper. We find that the DPT expressions are accurate to 10%–30% at saturating laser intensities. The important aspects of the reduction of saturated DFWM signals to relative internal-state distributions are outlined in environments where population relaxation and dephasing events are dominated by collisions, and a rotational temperature analysis is presented of the CH radical in an atmospheric-pressure oxyacetylene flame. Rotational temperatures determined using saturated DFWM are estimated to be accurate to 5%.
Direct measurements of rotation-specific, state-to-state vibrational energy transfer in highly vibrationally excited acetylene
Authors: Tobiason J. D. , Utz A. L. , Crim F. F.
Published: July 2002

Vibrational overtone excitation followed by laser-induced fluorescence detection allows the direct measurement of rotationally resolved vibrational energy transfer rates in highly vibrationally excited acetylene molecules. We detect transfer from the initial, even rotational states Ji=0–22 of 3ν3 (ν̃0=9640 cm−1) to the nearly isoenergetic final state Jf=4 of ν1+ν2+ν3+2ν4, l=0 (ν̃0=9668 cm−1). For these pathways, we observe changes in energy of up to ‖ΔE‖=530 cm−1 (≊2.5 kT) and in angular momentum quantum number of up to ‖ΔJ‖=18 in a single collision, and we measure state-to-state rate constants of about 0.1 μs−1Torr−1 (160 collisions). Measurements under single collision conditions ensure that the vibrational relaxation is free of any rotational equilibration. By applying detailed balance and summing the resulting reverse rate constants, we obtain a total rate constant of 1.3 μs−1Torr−1 (13 collisions) for transfer from ν1+ν2+ν3+2ν4, l=0, Jf=4 to all final rotational states in 3ν3. The energy transfer rate between two specific rovibrational states decreases exponentially with increasing energy difference. The vibrational relaxation does not have a strong angular momentum dependence in general, but transfer from the initial rotational states 3ν3, J=16, and J=20 is anomalously fast. The Fermi resonance of 3ν3 and ν1+ν2+ν3+2ν4, l=0 appears to enhance collisional transfer between the pair by a factor of 10 or more over that for uncoupled levels, and the anomalously fast transfer from initial states 3ν3, J=16 and 20 is probably due to their relatively strong, rotation-specific intramolecular coupling with other nearby, unobserved vibrational states.
First spectroscopic evidence for a muonium-containing molecule: NeMu* chemiluminescence
Authors: Baer Susan , Fleming Donald G. , Sloan James J. , Arseneau Donald J. , Kolbuszewski Marcin , Wright James , Senba Masayoshi , Pan James J. , Snooks Rodney
Published: July 2002

Evidence for the formation of NeMu*, an isotopic analog of the Rydberg molecule NeH*, has been obtained from the observation of chemiluminescent emission in the near-infrared region. This is the first spectroscopic detection of a muonium-containing molecule. NeMu* was formed by stopping a 4 MeV muon (μ+) beam in a target vessel containing 1–6 atm of Ne and ∼1 Torr Ar. The wavelength spectrum of the emission, from ∼680–1000 nm, was measured using a variable-wavelength filter, with a resolution of ±12.5 nm. Lower resolution spectra were also taken with a series of long pass filters. A complete histogram of photon events vs time was collected for each wavelength. Two strong transitions are observed, centered at 818 and 943 nm. Identification of NeMu* was made by a comparison of the experimental spectrum with a simulated spectrum based on detailed ab initio calculations, extended to higher excitation levels than had heretofore been reported. Both experimental and theoretical results are reported here. Although the mechanism by which the emitting states in NeMu* are formed remains unclear, radiolysis effects appear to play a dominant role, indicating that NeMu+ (the product of muon thermalization in Ne) undergoes charge exchange with metastable Ar* and/or is neutralized by a spur electron, both species produced during the slowing down of the high energy muon.
Hemiquantal study of the isotopic exchange reaction O18(3P) +O16O16(3Σ−g)→O18O16 (3Σ−g)+O16(3P)
Authors: Chajia M. , Jacon M.
Published: July 2002

A hemiquantal three-dimensional calculation of the isotopic exchange reaction O18(3P) +O16O16(3Σ−g)→O18O16 (3Σ−g) +O16(3P), which plays an important part in the atmospheric ozone photochemistry, is presented. The method gives an estimation of the lifetime of the activated complex which may be formed during the collision. Reaction scattering cross sections and rate constants are determined. A comparison with previous classical calculations and experimental results are made.
Femtosecond dynamics of I2(B 3Πu+) in liquids from resonance Raman spectra
Authors: Xu J. , Schwentner N. , Chergui M.
Published: July 2002

The intensity distributions in the overtones of resonance Raman spectra for I2 in liquid CCl4 and Xe are presented for several excitation energies in the bound part of the B 3Π0u+ state. In liquid CCl4, the decrease in intensity with v″ is monotonous and for all excitation energies similar to that of a free wave packet in the gas phase. A dephasing time shorter than the round trip time of 300 fs and a dephasing process which occurs beyond 3 Å is inferred, in accordance with predissociation to the a or a′ states earlier proposed. The intensity distributions in liquid Xe contain a fast decay and a plateau and the strength of both contributions varies with excitation energy. For low v′ in the B state a predissociation to the B″ 1Π1u state within 40 fs in the range of internuclear distances between 2.78 and 2.85 Å is observed which is less pronounced for large v′. The wave packet moves undistorted beyond the crossing with the B″ state up to 3 Å.
Variational transition state theory for the CI−+CH3Cl SN2 exchange reaction in water
Authors: Gershinsky Gidon , Pollak Eli
Published: July 2002

Variational transition state theory (VTST) is applied for the first time to a realistic simulation of a reaction in a liquid. Rate information is obtained from Monte Carlo thermal classical centroid averages of second derivatives of the full potential of interaction. A well defined friction function is computed and found to be in excellent agreement with previous approximate prescriptions leading to identical rate constants.
Control of a quantum dynamics by a locally optimized laser field. II. Application to a system with dissipation
Authors: Sugawara M. , Fujimura Y.
Published: July 2002

We present a theory for controlling the dynamics of a dissipative, quantum system with a laser field optimized locally in time. The theory is applicable to both weak and strong field control of the quantum dynamics. The theoretical groundwork is based on the equation of motion of the density matrix in Liouville space. Interactions between the molecules and the heat bath are taken into account within a Markov approximation. The derivation of the locally optimized laser field in a feedback form is based on the local optimization theory in the Hilbert space, proposed in a previous paper [M. Sugawara and Y. Fujimura, J. Chem. Phys. 100, 5646 (1994)]. The theory is applied to a simple, two-level quantum system with a dephasing constant. We present both the calculated time evolution of the off-diagonal density matrix element and that of the population of the states in the optimized laser field. These calculations show that the control of the system by the laser field is sufficient to avoid the dephasing effects. We discuss how the dephasing dynamics affects the optimization of the laser field.
Orientation dependence of 2H nuclear magnetic resonance spin-lattice relaxation in phospholipid and phospholipid:cholesterol systems
Authors: Morrison Clare , Bloom Myer
Published: July 2002

Structural and dynamic properties of phospholipids in the liquid crystalline phase were investigated using the general orientation dependence of spin-lattice relaxation for spin-1 nuclei, namely deuterium. Relaxation rates 1/T1Z and 1/T1Q for Zeeman and quadrupolar spin-lattice relaxation, respectively, can be expressed as a sum of Legendre polynomials up to order 4 for axially symmetric motions: 1/T1i= a0i+a2iP2(cos β)+a4iP4(cos β), i=Z, Q, where β is the angle between the axis of symmetry of the motion (in this case the bilayer normal) and the static magnetic field. The coefficients api are functions of order parameters and reduced spectral densities. We explore the structural and dynamical information that can be obtained from the orientation dependence of relaxation without invoking detailed models. Some conformational models, such as the rotational isomeric model for the acyl chains, were investigated. We studied the acyl chains of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, the influence of temperature and the addition of cholesterol, and two positions on the headgroup of dipalmitoyl-sn-glycerol-3-phosphocholine.
An imaginary energy method-based formulation of a quantum rate theory
Authors: Zhu Jianjun , Cukier R. I.
Published: July 2002

An imaginary energy method is used to derive a rate constant expression that leads to the WKB tunneling rate at low temperature and to the quantum transition state rate at high temperature. For weak coupling, the imaginary energy method also leads to Fermi’s Golden Rule formula. The rate expression is analyzed for the model of one-dimensional motion in a metastable potential profile with linear coupling to a bath of harmonic oscillators (Kramers’ problem). We recover the results of the Kramers, Grote-Hynes, and Wolynes (KGHW) theory from the rate expression. Our rate expression is given, in part, in terms of partition functions and is suited to a path integral treatment. The path integral evaluation of the rate constant also yields the KGHW formula. We use the same expression to analyze the low-temperature behavior of tunneling in a double-well potential for a system linearly coupled to a bath, to provide a new expression for electron transfer in the case of strong coupling of the electron to its two sites.
Comment on ‘‘Integral equation theory for charged liquids: The structure of macroions in solution and the inversion of experimental data’’ [J. Chem. Phys. 100, 2244 (1994)]
Authors: Kunz Werner , Calmettes Patrick , Cartailler Thierry , Turq Pierre
Published: July 2002

The question is discussed whether small-angle neutron scattering measurements and their interpretation with continuum-solvent models can yield reliable information about hydrophobic association of small organic ions in water.
Numerical Liouville approach: Formulation of third-order nonlinear optical susceptibilities
Authors: Nakano M. , Yamaguchi K. , Matsuzaki Y. , Tanaka K. , Yamabe T.
Published: July 2002

As a continuation of a previous paper [Phys. Rev. A 50, 2989 (1994)], this article develops and describes in detail the new nonperturbative approach to the third-order nonlinear optical susceptibilities χ(3)g(ω) in third-harmonic generation (THG), electric-field-induced second-harmonic generation (EFISH), degenerate four-wave mixing (DFWM), electric-field-induced optical rectification (EFIOR), and electric-field-induced Kerr effect (EFIKE) for N-state quantum systems interacting with intense electric fields. The present method utilizes the Fourier transformation of numerically exact solutions of the Liouville equation, so that it can provide both real and imaginary nonlinear optical spectra valid for arbitrary laser intensities, frequencies, and relaxation. As an application of the method, we investigate the characteristics of nonlinear optical spectra in THG, EFISH, and DFWM for a three-state model that mimics the electronic excited states of t-octatetraene obtained from a full configuration interaction (FCI) calculation using the Pariser–Parr–Pople (PPP) Hamiltonian. Comparisons between the present nonperturbative and the conventional perturbative results are also carried out. The magnitudes of the third-order nonlinear susceptibilities in the off-resonant region show the following trend: χ(3)(THG)≳χ(3)(EFISH)≳χ(3)(DFWM), with χ(3)(THG) exhibiting the largest frequency dispersion.