bluuebunny/crossref_metadata_2025_split huggingface dataset:
Neutron diffraction structural study of pyroelectric Sr(NO2)2⋅H2O at 294, 100, and 20 K
Authors: Lundgren J.-O. , Kvick Å , Liminga R. , Abrahams S. C. 
Published: July 2002

Strontium nitrite monohydrate [Sr(NO2)2⋅H2O] has been studied at 294, 100, and 20 K by neutron diffraction. Integrated intensities of 2958 (294 K), 4239 (100 K), and 2154 (20 K) reflections were measured at the Brookhaven National Laboratory high flux beam reactor, resulting in 2068 (294 K), 2030 (100 K), and 1934 (20 K) independent structure factors. Structural refinement in monoclinic space group P21 led to R(F)=0.029 (294 K), 0.019 (100 K), and 0.034 (20 K). Lattice constants at 295 K are a=12.5831(4), b=8.94941(15), c=4.4860(2) Å, β=99.111(4)°; at 100 K, a=12.539(2), b=8.909(1), c=4.475(1) Å, β=99.086(20)°; and at 20 K, a=12.542(2), b=8.901(1), c=4.472(1) Å, β=99.030(20)°.
The expansivity of all four lattice parameters is nonlinear, with α1=32.8×10−6 K−1, β1=76×10−9 K−2; α2=31.6×10−6 K−1, β2=44×10−9 K−2; α3=15.6×10−6 K−1, β3=15×10−9 K−2; α4=−2.8×10−6 K−1, β4=−21×10−9 K−2 for αTi =α295i [1+αiΔT+βi(ΔT)2], ΔT=T−295 K. The mean-square amplitude of Sr varies linearly with temperature between 20 and 294 K, and extrapolates to a zero-point value of 0.0010(2) Å2. A survey shows that atoms with masses greater than about 80 have similar zero-point amplitudes which increase toward the maximum exhibited by H, with its zero-point amplitude of about 0.023 Å2, as the atomic mass decreases. The coordination (9 and 10) of the two independent Sr2+ ions remains unchanged with temperature: the coordination shell radius decreases on average by 0.012 Å from 294 to 20 K. Comparison with previous x-ray structural results on Sr(NO2)2⋅H2O shows the latter are influenced by scattering from the nitrogen atom lone-electron pair. Small but significant differences in N–O distances among the four independent NO−2 ions are detected at 294 K (0.020 Å) and at 20 K (0.017 Å). The mean N–O distance at 20 K for all four ions is identical at 1.253–1.254 Å. The maximum atomic displacement between 294 and 20 K is that by a nitrite ion oxygen, of 0.045 Å.
Forward and inverse functional variations in rotationally inelastic scattering
Authors: Guzman Robert , Rabitz Herschel 
Published: July 2002

This paper considers the response of various rotational energy transfer processes to functional variations about an assumed model intermolecular potential. Attention is focused on the scattering of an atom and a linear rigid rotor. The collision dynamics are approximated by employing both the infinite order sudden (IOS) and exponential distorted wave (EDW) methods to describe Ar–N2 and He–H2, respectively. The following cross sections are considered: state-to-state differential and integral, final state summed differential and integral, and effective diffusion and viscosity cross sections. Attention is first given to the forward sensitivity densities δ0/δV(R,r) where 0 denotes any of the aforementioned cross sections, R is the intermolecular distance, and r is the internal coordinates. These forward sensitivity densities (functional derivatives) offer a quantitative measure of the importance of different regions of the potential surface to a chosen cross section. Via knowledge of the forward sensitivities
and a particular variation δV(R,r) the concomitant response δ0 is generated. It was found that locally a variation in the potential can give rise to a large response in the cross sections as measured by these forward densities. In contrast, a unit percent change in the overall potential produced a 1%–10% change in the cross sections studied indicating that the large + and − responses to local variations tend to cancel. In addition, inverse sensitivity densities δV(R,r)/δ0 are obtained. These inverse densities are of interest since they are the exact solution to the infinitesimal inverse scattering problem. Although the inverse sensitivity densities do not in themselves form an inversion algorithm, they do offer a quantitative measure of the importance of performing particular measurements for the ultimate purpose of inversion. Using a set of state-to-state integral cross sections we found that
the resultant responses from the infinitesimal inversion were typically small such that ‖δV(R,r)‖≪‖V(R,r)‖. From the viewpoint of an actual inversion, these results indicate that only through an extensive effort will significant knowledge of the potential be gained from the cross sections. All of these calculations serve to illustrate the methodology, and other observables as well as dynamical schemes could be explored as desired.
Electronic and structural characteristics of five poly membered heterocycles (polythiophene, polypyrrole): An ultraviolet and x-ray photoelectron spectroscopy study
Authors: Tourillon G. , Jugnet Y. 
Published: July 2002

The electronic and structural properties of electrochemically synthetized five poly membered heterocycles [polypyrrole (PP), poly-N-methylpyrrole (PNMeP), polythiophene (PT), polybithiophene (PBT), and poly-3-methylthiophene (PMeT)] in their doped conducting and undoped semiconducting states have been determined by x-ray photoelectron spectroscopy (XPS) and UV photoelectron spectroscopy (UPS). A systematic evolution of the π bonding orbitals is observed by going successively from thiophene to bithiophene and to terthiophene which leads finally to the formation of an intrinsic π bonding band for the undoped polythiophene, located at 1.05 eV below the Fermi level Ef. This π bonding band is also detected with PP, PBT, and PMeT in good agreement with theoretical band structure calculations and implies a long range order along the polymeric chains. In contrast, PNMeP valence spectra exhibit broad features connected with the existence of structural defects and/or torsion angle between the monomeric units. Shake-up satellites are generally observed on the high binding energy side of the C1s, S2p (or N1s) core levels related to π→π* transitions. The S/C (or N/C) ratio is always very close to the predicted theoretical value which means that the structural architecture of the polymeric backbone is very similar to the monomer one. Upon ClO−4 doping, the π bonding band is extended up to ∼0.2 eV below Ef.
Simultaneously, the bonding band intensity decreases, and the C1s, S2p, (or N1s) core level peaks become broad and asymmetric (PP, PMeT, PT). These evolutions are completely reversible during the doping–undoping processes, independent of the anion used (BF−4,SO3CF−3) and of the thickness of the film (20–1000 Å). These modifications are dramatically reduced with PNMeP although its doping level is as high as 30%. These UV photoelectron spectroscopy and x-ray photoelectron spectroscopy results combined with the near edge x-ray absorption fine structure (NEXAFS) characteristics give insight into the conduction mechanism inside these conducting polymers: Appearance of a metallic-like behavior due to first the extraction of electrons from the π bonding band, and second, an extension of the π and π* bands towards Ef narrowing the band gap from 2.1 eV in the undoped state to 0.3–0.4 eV in the doped form.
Diffuse vibrational structures in photoabsorption spectra: A comparison of CH3ONO and CH3SNO using two-dimensional a
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o potential energy surfaces
Authors: Schinke Reinhard , Hennig Steffen , Untch Agathe , Nonella Marco , Huber J. Robert 
Published: July 2002

We investigated the photodissociation of methyl nitrite (CH3 ONO) and methyl thionitrite (CH3 SNO) within the first absorption band (S1 ←S0 ). The calculations were based on a two-dimensional model including the O–NO/S–NO and N=O bond distances as active coordinates. The S1 -potential energy surfaces were calculated with quantum chemical methods and the dynamical calculations were performed exactly within the time-independent approach. The main emphasis is on the origin of diffuse vibrational structure in the photoabsorption spectrum of both molecules. A low potential barrier of 0.086 eV along the O–NO dissociation coordinate in CH3 ONO prevents immediate dissociation and leads to an initial state dependent lifetime for the excited complex of 100–250 fs corresponding to 3–8 NO vibrational periods. CH3 ONO decays nonadiabatically via vibrational predissociation. The absorption spectrum of CH3 ONO is dominated by narrow Feshbach-like scattering resonances which can be characterized by two quantum numbers, m and n*: m=0 and 1 specifies the quanta of excitation in the O–NO bond and n*=0,1,2,... specifies the excited vibrational level of the N=O bond. The potential barrier is absent in CH3 SNO and the dissociation is direct on the time scale of about 10 fs corresponding to only one third of a NO vibrational period. Nevertheless, the absorption spectrum exhibits diffuse vibrational structures. The shape of the individual absorption peaks is determined by the classical Franck–Condon reflection principle. The dissociation of CH3 SNO is primarily adiabatic which leads to a pronounced energy dependence of the final NO vibrational state distribution. The diffuse structures originate in both cases from excitation of the NO stretching vibration. In order to make contact with time-dependent theory we calculated the autocorrelation function of the time-dependent wave function by inverse Fourier transformation of the energy-dependent spectra. The agreement with available experimental data for both molecules is quite satisfactory. This includes the energy spacing of the vibrational structure, the overall shape of the absorption spectrum, and the
lifetime of the excited complex.
The evolution of vibrational phase space during the collisional relaxation of highly excited collinear CS2
Authors: Bruehl Margaret , Schatz George C. 
Published: July 2002

This paper presents a detailed theoretical analysis of the vibrational relaxation of highly excited CS2 (initially 32 640 cm−1) in collinear collisions with a thermal bath of He atoms. The relaxation is simulated by a classical molecular dynamics method in which CS2 undergoes successive collisions with thousands of He atoms. In most of our studies the CS2 coordinates and momenta at the end of one collision are used as input to the next collision, so it is possible to examine the detailed evolution of the CS2 vibrational phase space during the relaxation process. By restricting motion to being collinear, it is possible to characterize this evolution using surfaces of section and other methods. Comparisons of our collinear results with corresponding three-dimensional simulations indicates that the collinear restriction does not alter the relaxation process significantly.
Our phase space analysis indicates that individual relaxation sequences can evolve in a variety of different ways depending on the initial location in phase space and on the details of subsequent collisions. Much of the initial phase space is chaotic, and if a sequence starts in such a region then after usually less than 30 collisions, the CS2 has moved into a nonlinear resonance zone where the antisymmetric and symmetric stretch modes have frequency ratios of 5:2, 7:3, or 9:4. These nonlinear resonances do not greatly change the ensemble averaged energy transfer per collision 〈ΔE〉 compared to the chaotic regions, but they are collisionally stable relative to these regions. As a result, it takes an energetic collision to kick the molecule out of a nonlinear resonance. If kicked out, then usually within a few more collisions another nonlinear resonance (or perhaps the same) has been entered. As relaxation progresses molecules caught in nonlinear resonances eventually drop down to simple quasiperiodic regions where the frequency ratio is not constrained to be a ratio of integers. We do find a region of phase space that is quasiperiodic even at 32 640 cm−1, corresponding to a ‘‘hyperspherical mode’’ in which most of the vibrational energy is locked up in antisymmetric stretch motion. Molecules in this region of phase space relax much more slowly than in chaotic and resonant regions. In addition, molecules starting initially in a chaotic region can be kicked into this hyperspherical mode region, leading to an additional slowing of the relaxation as the molecule drops down the well. This additional slowing plays an important role in determining the dependence of 〈ΔE〉 on the molecular vibrational energy E.
In particular, we find that 〈ΔE〉 varies linearly with E if phase space undergoes forced randomization after each collision, but it shows a stronger than linear dependence when redistribution is not forced. This implies that deviations from linearity in the dependence of 〈ΔE〉 on E provide a measure of the division of phase space into regions that have very different relaxation characteristics.
Fluorescence quenching of the phenanthrene excimer on Al2O3(0001): Coverage and distance dependence
Authors: Haynes D. R. , Helwig K. R. , Tro N. J. , George S. M. 
Published: July 2002

The fluorescence from disordered phenanthrene adlayers on Al2O3(0001) was examined in ultrahigh vacuum at 20 K using laser-induced fluorescence techniques. The fluorescence spectra was consistent with emission from a phenanthrene excimer. The fluorescence lifetime of the phenanthrene excimer was studied versus phenanthrene coverage and versus distance from the Al2O3(0001) surface using xenon spacers. As a function of coverage on Al2O3(0001), the fluorescence lifetime decreased from τ=34±0.5 ns at phenanthrene coverages of Θ≥20 ML to τ=7±0.5 ns at Θ=1 ML. As a function of xenon spacer distance from the Al2O3(0001) surface, the fluorescence lifetime also decreased from τ=34±0.5 ns at distances of d&gt;100 Å to τ=7±0.5 ns at d=7 Å. Fluorescence measurements versus phenanthrene coverage on xenon, butane, acetonitrile and methanol multilayers revealed that the phenanthrene excimer fluorescence lifetime was constant at τ≊35 ns on the molecular multilayer surfaces. These results indicated that the reduction of the fluorescence lifetime was particular to the Al2O3(0001) surface.
The fluorescence lifetimes versus phenanthrene coverage and xenon spacer distance on Al2O3(0001) were equivalent when the phenanthrene coverage was converted to total adlayer thickness. This correspondence suggested that the excited electronic energy in disordered phenanthrene adlayers transfers rapidly to phenanthrene excimers at the phenanthrene–vacuum interface. Subsequently, competition occurs between fluorescence quenching by the Al2O3(0001) surface and phenanthrene excimer fluorescence. In addition, fluorescence lifetime measurements vs phenanthrene coverage on CaF2 thin films displayed similar fluorescence quenching. Fluorescence lifetimes versus phenanthrene coverage on O2 molecular multilayers also revealed fluorescence quenching that was attributed to a charge-transfer mechanism. The observed fluorescence quenching on Al2O3(0001) indicates that the surface states of this ionic crystal may be accessible for electronic energy transfer even though Al2O3 is a known insulator. A Förster electronic energy transfer mechanism was used to analyze the observed fluorescence lifetimes vs phenanthrene coverage or xenon spacer distance on Al2O3(0001).
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o potential-energy surfaces for Cd(1P)+H2=CdH(X 2Σ+)+H, HCdH(X 1Σ+g), Cd(3P)+H2, and Cd(1S)+H+H
Authors: Boatz Jerry A. , Gutowski Maciej , Simons Jack 
Published: July 2002

The outcome of laser excitation of the van der Waals CdH2 complex [which consists of a ground-state Cd(1S) atom bound to an H2 molecule] to its lowest-energy singlet state(s), which connects with Cd(1P)+H2, is examined by calculating ab initio potential-energy surfaces for the above excited singlet states, the corresponding underlying triplet states [connecting to Cd(3P)+H2 ], and the singlet ground state. It is necessary to study such a multitude of states because energetically accessible reaction products such as Cd(3P)+H2, CdH(X 2Σ+)+H, Cd+H+H, and HCdH(X 1Σ+g) connect to various surfaces. In carrying out these calculations, the lowest-energy surface of 1A1 symmetry was evaluated at nearly 1100 C2v geometries using complete-active-space self-consistent-field wave functions with a valence double-zeta and a double-zeta-plus-polarization basis set on cadmium and hydrogen, respectively. The excited singlet and triplet surfaces that correlate with 1P1 and 3PJ levels of Cd plus ground-state H2 were examined in detail along paths that flux prepared by laser excitation of the singlet surface(s) is most likely to follow. Among the photoexcited singlet states, the 1B2 surface is found to provide the most energetically attractive approach path for the excited CdH2 van der Waals complex to access geometries from which HCdH, Cd+H+H, and CdH+H can be formed via intersection and coupling with the lowest 1A1 surface.
The 1B1 surface is found to be weakly attractive, and the excited 1A1 surface to be repulsive at geometries characteristic of the nascent photoexcited species. The underlying triplet surfaces are found to be repulsive (3A1), weakly attractive (3B1), and slightly more attractive (3B2) at such geometries. The repulsive 3A1 surface intersects the 1B2 surface in the latter’s entrance-channel ‘‘streambed’’; the 3B1 state intersects the 1B2 surface closer to where the 1B2 and lowest 1A1 cross. The 3A1 and 1B1 surfaces also intersect in the entrance streambed of the latter. All such triplet–singlet crossings provide paths to Cd(3P)+H2.
Theory of vibrational energy relaxation in liquids: Vibrational–vibrational energy transfer
Authors: Adelman S. A. , Muralidhar R. , Stote R. H. 
Published: July 2002

A theoretical treatment of the vibrational–vibrational (VV) contribution to the vibrational energy relaxation time T1 of a solute normal mode in a molecular solvent, which extends a previous treatment [S. A. Adelman, R. H. Stote, and R. Muralidhar, J. Chem. Phys. 99, 1320 (1993), henceforth called Paper I] of the vibrational–translational–rotational (VTR) contribution to T1, is outlined and expressions for this VV contribution, valid for the infinitely dilute diatomic solution, are presented. The treatment is based on the formula T1=β−1(ωl), where β(ω) is the friction kernel of the relaxing solute mode and where ωl is its liquid phase frequency. β(ω) is evaluated as the cosine transform of the autocorrelation function 〈ℱ̃(t)ℱĩ〉0v of the fluctuating generalized force exerted by the vibrating solvent on the solute normal mode coordinate conditional that this coordinate is fixed at its equilibrium value. 〈ℱ̃(t)ℱ̃〉0v is expressed as a superposition of the rigid solvent autocorrelation function 〈ℱ̃(t)ℱ̃〉0 and a correction which accounts for solvent vibrational motion. For diatomic solvents one has 〈ℱ̃(t)ℱ̃〉0v= 〈ℱ̃(t)ℱ̃〉0+NSMD(t) cos ωDt F(ΩD), where NS=number of solvent molecules, MD(t) is the vibrational force gradient autocorrelation function, ωD and ΩD are solvent molecule liquid phase frequencies, and F(Ω)=1/2ℏΩ−1 coth[ℏΩ/2kBT].
The Gaussian model is assumed for 〈ℱ̃(t)ℱ̃〉0 and MD(t) yielding β(ω) as a superposition of a Gaussian centered at ω=0 which mediates VTR processes and a Gaussian centered at ω=ωD which mediates VV processes. Vector integral expressions for MD(t), ωD, and ΩD are presented which are similar to the expressions for ωl and 〈ℱ̃(t)ℱ̃〉0 given in Paper I. These expressions permit the evaluation of the VV contribution to T1 from the atomic masses, bondlengths, vibrational frequencies, and site–site interaction potentials of the solute and solvent molecules and from specified rigid solvent equilibrium site–site pair correlation functions of the liquid solution.
The validity of the rotational isomeric state model for short alkyl chains with large substituents: A maximum entropy analysis of the 1H dipolar couplings within the trimethylene group of a liquid crystal molecule
Authors: Drobny G. P. , Shilstone G. N. , Catalano D. , Veracini C. A. , Zimmermann H. 
Published: July 2002

Deuterium decoupled proton spectra of the chain region of a partially deuteriated liquid crystal have been taken. The liquid crystal is a trimethylene chain with perdeuteriated cyanobiphenyloxy groups on either end: 1,3-bis(4,4′-cyanobiphenyloxy-d8) propane (BCBO3-d16). The spectra have been analyzed to give the dipolar couplings within the propyl chain at various temperatures in the nematic phase of the liquid crystal. These couplings are the most sensitive measurements of molecular flexibility that have yet been taken of an alkyl chain in a pure liquid crystal molecule and give a unique insight into rotational potentials of the flexible component essential to a liquid crystal molecule. The couplings have been analyzed using a maximum entropy method designed for a two-rotor system; in our case the two C–C bonds in the propyl chain. The analysis was also performed by adding to the data set the deuterium quadrupolar splittings of the chain hydrogen atoms, previously measured on the same molecule but deuteriated in the chain. The method picks out the flattest distribution consistent with the data. This also happens to be the most likely distribution given only the data used in the analysis and no other constraints.
The results for BCBO3-d16 show a probability maximum for a partially eclipsed conformation (twisted approximately 86° in opposite directions about each C–C bond from the all-trans conformation), a smaller maximum for the all-trans conformation itself, and no significant probability for any of the gauche conformations. The possible motions, that are suggested by these results, therefore, are concerted librations (±86°) between the all-trans position and the partially eclipsed conformations, and full (360°) rotations. These results are in stark contrast to the assumptions made in the rotational isomeric state (RIS) model that the probability distribution can be well approximated by constraining the molecules to exist only in trans and/or gauche states with fast isomerization between such states. In fact the analysis rules out a rotational isomeric state distribution altogether, since our data could not be reproduced by restricting the distribution in this way. Instead it is shown that other distributions are not only possible, but also more likely than any kind of constrained trans–gauche model.