bluuebunny/crossref_metadata_2025_split huggingface dataset:
Square-well gas collision integrals and diffusion in a square-well Lorentz gas
Authors: Korlipara Ravi K. , Stell George , Karkheck John 
Published: July 2002

A representation for collision cross sections of a square-well dilute gas mixture is developed that reduces their computation to the evaluation of one-dimensional integrals with integrands of elementary closed form. Consequently, the computation of collision integrals and Chapman–Enskog and Kihara transport-coefficient approximations, which depend on the collision cross sections, is analytically and numerically simplified. These results are then applied to the limiting case of a Lorentz gas, i.e., a dilute gas of mobile particles diffusing through a bed of scatterers that can be regarded as fixed. In particular, our results are used to evaluate the exact diffusion coefficient DL of a Lorentz gas with square-well potential interactions as a function of both square-well width and temperature, i.e., square-well depth. (We treat here only the case in which the scatterers are dilute enough for the problem to be accurately described by a Boltzmann equation.) The exact DL is compared with its first and second Chapman–Enskog approximations, D1 and DCE2, as well as its second Kihara approximation, DK2. (The first Kihara approximation coincides with the first Chapman–Enskog approximation so that it is unnecessary to attach a superscript to D1 to distinguish between the two approximation schemes.) The DK2 is found to be of high accuracy over the whole range of parameters studied, suggesting that it represents the most accurate of these three approximations available for the binary diffusion coefficient of a dilute square-well gas. Finally, a simple approximation for the collision integrals linear in well depth (i.e., inverse temperature) as well as one that adds to the linear term all contributions from ‘‘soft’’ scattering are used to evaluate D1. For wide wells the latter yields an excellent approximation (which, in turn, is a good approximation to DL for such wells). This approximation is no longer useful for narrow wells. As the temperature goes to zero, DL approaches the diffusion coefficient of a hard-sphere Lorentz gas with interaction diameter equal to the hard-core diameter plus the square-well width. This represents a
special case of the general limiting coincidence of square-well and square-mound scattering in the infinite-strength limit.
Kinetics of liquid–liquid phase separation in a binary mixture with miscibility gap: Study of noncritical mixtures of 2,6-dimethyl pyridine/water near the lower critical point
Authors: Mayer W. , Woermann D. 
Published: July 2002

Temperature jump experiments with a characteristic heating time of 0.5 ms are used to study the kinetics of liquid/liquid phase separation in noncritical mixtures of 2,6-dimethyl pyridine and water (‖x-xc‖&gt;0.02; x, mole fraction of 2,6-dimethyl pyridine; xc, critical composition) as functions of composition and supersaturation. The response of the system to a temperature jump is monitored by measuring the intensity of light scattered by the sample at different scattering angles Θ (30°&lt;Θ&lt;90°) as a function of time (up to several seconds). Starting from a level of constant intensity reached with a time constant in the range of milliseconds after a temperature jump the scattered intensity increases with time. For small supersaturations consecutive maxima and minima of scattered intensity develop.
They reflect diffusion limited growth of droplets of the emerging second liquid phase (linear relationship between the square of the radius of growing droplets and time). The growth rate increases with increasing supersaturation. Practically no nucleation barrier is found for the onset of growth of droplets for small supersaturations and compositions (‖x−xc‖&gt;0.03). The number density N of growing droplets is obtained from simultaneous measurements of scattered intensity and turbidity. For small supersaturations N is independent of time but changes with supersaturation in a characteristic way (classical nucleation model). There exists an upper limit of overheating δT* above which the number density N increases drastically. The ratio (δT*/ΔTc,p) is independent of the reduced temperature ε̃ of phase separation (ε̃=ΔTc,p/Tc; ΔTc,p=Tc−Tp; Tc, critical temperature, Tp, temperature of phase separation). This is expected for mixtures of noncritical composition. The theory proposed by Langer and Schwartz to treat nucleation and growth of droplets in metastable near critical fluids does not describe the phenomena observed in this study. The composition of the samples used (‖x−xc‖&gt;0.02) appears to be too far away from the critical.
Orientational analysis of pelargonic acid at liquid–solid interfaces, drops, and ultrathin liquid films, by polarized Raman spectroscopy
Authors: Yahiaoui Bechir , Masson Michèle , Harrand Monique 
Published: July 2002

Polarized Raman spectra of pelargonic acid (PA), CH3–(CH2)7–CO2H, at liquid–solid interfaces were recorded, using the excitation of the surface plasmon of silver. In this device, the exciting electric field decreases exponentially (or quasi-exponentially) into the sample. Therefore, the molecules in the vicinity of the solid surface are preferentially illuminated. This study was performed on two types of systems: 1°—One interface: a drop of PA is deposited on a solid surface [silver, silica, cetyltrimethylammonium bromide (CTAB), or Langmuir–Blodgett (LB) layers of barium stearate]; 2°—two interfaces: liquid ultrathin films of PA are placed between silica or barium stearate LB layers; the thickness of the film (250 to 5 nm) being measured by an interferential method. The shape of the 2800–3000 cm−1 band, which is very sensitive to the chain conformation, was studied. Since in the convenient polarization, the spectra of the drops were different from the spectrum of the isotropic liquid (classical cell), we can assume that the PA molecules have an anisotropic distribution in the vicinity of a solid surface and that the molecules are, therefore, more or less uniaxially oriented.
The orientation is dependent on the nature and the smoothness of the solid surface, as is the distance over which the orientation spreads. (1°) If the distance of orientation is short relative to the penetration depth (≂100 nm) of the exciting wave, most of the illuminated molecules belong to an isotropic distribution: there is, therefore, little difference between the spectra of the drop and of the isotropic liquid, but, as in the ultrathin films only the oriented molecules are illuminated, spectra of films and liquid are very different. (2°) If the orientation distance is long (PA on barium stearate LB layers), the greater part of the illuminated molecules are oriented: the spectrum of the drop is therefore very different from the spectrum of the bulk (isotropic liquid); and the spectra of ultrathin films are similar to the spectrum of the drop. The differences observed in the band shapes are due to an increasing intensity of the peaks assigned to the symmetric vibrations of the CH2 and CH3 groups. These modifications can be explained by the comparison of the calculated scattering activities of uniaxially oriented molecules and random ones. The causes of the orientation and conformation change of the hydrocarbon chain have been discussed.
Unusual photofragmentation dynamics in the multiphoton ionization of Cr(CO)6 /methanol van der Waals heteroclusters
Authors: Peifer William R. , Garvey James F. 
Published: July 2002

Mixed van der Waals clusters containing Cr(CO)6 and methanol are generated in the free-jet expansion of a pulsed beam of seeded helium and subjected to 248 nm multiphoton ionization (MPI) at moderate laser fluence, and the product ions are analyzed by time-of-flight mass spectrometry. We find that the multiphoton dissociation and ionization dynamics of solvated Cr(CO)6 are strikingly different from those of the naked molecule. Two principal sequences of heterocluster ions are identified in the mass spectrum. A major sequence with the empirical formula SnCr(CO)+x (x=0,1,2), where S is a methanol molecule, first appears in the mass spectrum at n+x=6. A minor sequence with the empirical formula SnCr(CO)+x (x=5,6), first appears in the mass spectrum at n+x=7.
We discuss two possible dynamical schemes for MPI of Cr(CO)6 /methanol heteroclusters: One scheme, in which initial photoionization of neutral Cr(CO)6 -containing clusters gives rise to solvated Cr(CO)+6 primary photoions, which subsequently photodissociate to yield the observed coordinatively unsaturated daughter fragments; and an alternative scheme, in which initial photodissociation in the neutral manifold gives rise to one or more coordinatively unsaturated primary photoproducts, each of which subsequently undergoes photoionization. We consider, in a qualitative fashion, the mass spectral fragmentation patterns predicted by these two alternative schemes under conditions of extremely high laser fluence (where essentially all one-photon processes are saturated), and compare these predictions with the observed MPI mass spectra following high-fluence irradiation at both 248 and 350 nm. Assuming that any secondary photodissociation in the ionic manifold occurs statistically, our high-fluence results are inconsistent with a dynamical scheme in which MPI precedes photodissociation.
We suggest that the dynamics are correctly described by initial photodissociation in the neutral manifold, followed by MPI of the coordinatively unsaturated primary photoproduct clusters. Intracluster stabilization of nascent photoion internal energy, either through evaporative cooling or collisional V–V energy transfer, is suggested to strongly influence subsequent metal–ligand bond dissociation.
Measurements and calculations of oscillations and phase relations in the driven gas-phase combustion of acetaldehyde
Authors: Tsujimoto Kim K. , Hjelmfelt Allen , Ross John 
Published: July 2002

Oscillations in light emission and species concentrations, are measured as periodic perturbations are simultaneously applied to the input rates of acetaldehyde and oxygen in the gas-phase combustion of acetaldehyde in a continuous-flow stirred tank reactor for conditions where the autonomous reaction itself is oscillatory. The experimental results are compared with the predictions of a five-variable thermokinetic model. We measure periodic responses in the fundamental entrainment band (ratio of frequency of perturbation to frequency of response equal to unity) for four different values of phase shift between the acetaldehyde and oxygen perturbation wave forms as we vary the frequency and amplitude of the external periodic perturbations. Outside of the entrainment bands we find quasiperiodic response. We determine the phases of the light emission and six species concentrations, as measured with a mass spectrometer, with respect to the periodic perturbation, the variation of these phases across the fundamental entrainment band for different values of reactant phase shift and for different amplitudes of perturbation, and the effects of the phase shift between the two input perturbations on the light emission response of the system for different frequencies of perturbation. Both the experiments and calculations predict a widening of the entrainment band with an increase in perturbation amplitude, and the same variation in bandwidths for the four values of reactant phase shift studied. The experiments and calculations also predict the same general trends in light phase and species phases (difference between the light emission and species concentrations with respect to the perturbing wave form) as the band is traversed for different amplitudes of perturbation and for different values of reactant phase shift. Some discrepancies in the species phases exist; however, the similarities which exist between the experiments and the calculations provide further substantiation for the model. We find that changing the phase shift between reactant inflows from 0° to 360° produces a sharp change in light phase near 180°, in both the experiments and calculations.
In general, we also find that the light emission remains entrained near the total flux minimum of the perturbing wave form, and we attribute this correlation to the net heat loss which arises from the reactant flux through the reactor.
The Floquet theory of nuclear magnetic resonance spectroscopy of single spins and dipolar coupled spin pairs in rotating solids
Authors: Schmidt Asher , Vega Shimon 
Published: July 2002

The Floquet theory for the description of magic angle sample spinning (MASS) nuclear magnetic resonance (NMR) experiments is introduced. MASS NMR signals of single spin systems and homonuclear two-spin system in the presence of direct and indirect dipolar couplings are evaluated. The single spin system is utilized to develop our Floquet formalism for the MASS NMR experiments and the coupled spin system is discussed to demonstrate its methodology and its applicability. This theoretical approach enables the calculation of the positions, the amplitudes, and line shapes of MASS center and sideband. The diagonalization of the MASS Floquet matrices of the different spin systems provides eigenvalues and eigenvectors which in turn determine the frequencies and intensities of the various bands in the MASS spectra. Their frequency shifts can be explained in terms of anticrossing of Floquet states and their amplitudes can be calculated via the matrix elements of transition–amplitude Floquet operator. New Floquet operators are introduced and the commutation relations between them utilized to enable the evaluation of diagonalization matrices. The Floquet theory is applied to determine the rotational resonance conditions for homonuclear dipolar-coupled spin pairs. It is shown that significant band shifts are expected when degeneracies between Floquet states occur. These shifts endow the centerband and sidebands with powder line shapes from polycrystalline samples. Simulations of experimental line shapes can provide direct and indirect dipolar coupling constants, including their relative signs.
This approach is valid for all possible dipolar coupling strengths as well as for all sample spinning speeds and isotropic and anisotropic chemical shift parameters. The decay of zero-quantum coherences in the coupled spin-systems is also considered in terms of the Floquet approach. The sensitivity of the MASS line shapes close to the rotational resonance conditions with respect to the relative magnitudes of the chemical shift anisotropies of the spins and their direct and indirect coupling constants is examined by the calculation of various MASS spectra. To do so the interaction parameters of the different 13C nuclei in glycine and alanine are used. Experimental results of the weakly dipolar coupled spin pairs in uniformly 13C labeled alanine are also presented and compared with theoretical results.
Dynamics of the glass-forming liquid di-2-ethylhexyl phthalate (DOP) as studied by light scattering and neutron scattering
Authors: Floudas G. , Higgins J. S. , Fytas G. 
Published: July 2002

Dynamic light scattering (depolarized Rayleigh and polarized Rayleigh–Brillouin) and quasielastic neutron scattering are employed to study the dynamics of the glass-forming liquid di-2-ethylhexyl phthalate (DOP) (Tg=184 K). The depolarized Rayleigh scattering measurements were made in the temperature range from 303 to 433 K, the polarized Rayleigh–Brillouin measurements in the range from 263 to 433 K, and the quasielastic neutron-scattering measurements in the range from 37 to 312 K and in the Q range from 0.33 to 1.84 Å−1. The orientation times for DOP, obtained from a single Lorentzian fit to the experimental depolarized spectra at high T, are in good agreement with recent dielectric data for the primary (α) relaxation. However, at lower T, the viscosity increases more strongly than the orientation times and the Stokes–Einstein–Debye equation which can adequately describe the dynamics in the high-T range is insufficient at temperatures close to Tg. The relaxation time obtained from the Rayleigh–Brillouin experiment is about 1 order of magnitude faster than the orientation times. In the neutron-scattering experiment we find a strong decrease of the elastic intensity and a corresponding increase of the quasielastic intensity around Tg.
The data analysis with respect to the dynamics (from a two Lorentzian fit) revealed the existence of three processes affecting the high-frequency range: (i) a ‘‘fast’’ (τ2∼10 ps) Q-independent motion with weak T dependence (E2=1.54 kcal/mol), (ii) a ‘‘slow’’ Q-dependent motion, and (iii) a flat background increasing with T and Q. The fast process is discussed in terms of a very localized motion of the phenyl group (β relaxation) and, as such, as a precursor of the the primary (α) relaxation. The relaxation time of this process (τ2) was found to compare nicely with the time τmax from the Rayleigh–Brillouin (RB) experiment suggesting that the latter is caused by fast localized motions. The slow process is discussed in terms of the jump-diffusion model. The activation energy associated with the jump-diffusion times is 6.1 kcal/mol and it is associated with large-scale diffusional motion of the DOP molecule. The relaxation times obtained from this process are compared with the relaxation times obtained from the depolarized and dielectric techniques for the primary relaxation. Finally, the background can be identified with fast local motions and/or low-frequency excitations relaxing outside the energy window of our experiment.
Jet spectroscopy and excited state dynamics of SiH2 and SiD2
Authors: Fukushima Masaru , Mayama Shinya , Obi Kinichi 
Published: July 2002

Silylene radicals, SiH2 and SiD2, are generated in a supersonic free jet by ArF laser (193 nm) photolysis of phenylsilane and phenylsilane-α-d3, respectively. LIF excitation and dispersed fluorescence spectra are measured for the ν2 vibronic bands of the Ã 1B1−X̃ 1A1 transition. The heterogeneous predissociation to Si (3P)+H2 is proposed from the anomalous rotational structure in the excitation spectra; the rotational lines of the r(1) subbranch (K′a=0←K■a=1) have stronger intensity than those of the r(0) subbranch (K′a=1←K■a=0), though the latter is expected to be stronger due to the low temperature Boltzmann distribution in the jet. The time-resolved excitation spectra demonstrate shorter lifetime of K′a=1 rovibronic levels in the Ã 1B1 state.
The heterogeneous predissociation is interpreted with the second order perturbation: Ã 1B1 –(a-type Coriolis)→X̃ 1A1 -(spin–orbit)→ã 3B1→Si(3P)+H2. It is demonstrated experimentally that there is a potential barrier associated with the dissociation path of ã 3B1→Si(3P)+H2, the height of which is estimated to be 1540–2160 cm−1 from the bottom of the Ã 1B1 state. The electronic transition moment of the Ã 1B1–X̃ 1A1 transition is estimated to be ‖μe‖2=0.26e2a20 from the Einstein equation for spontaneous emission using measured fluorescence lifetimes for single rovibronic levels with Ka=0 and calculated Franck–Condon factors. The onset of a second predissociation channel, Ã 1B1→Si(1D)+H2, at the (0,7,0) vibronic level of SiH2 Ã 1B1 is manifested as a sharp decrease in the observed fluorescence lifetime for the v′2=7, J′=0 level relative to that predicted for a pure radiative lifetime.
Nascent product states in the photoinitiated reaction of O3 and H2O
Authors: Sauder Deborah G. , Stephenson John C. , King David S. , Casassa Michael P. 
Published: July 2002

The rotational, vibrational and fine-structure state distributions for the reaction 16O(1D)+ H218O →16OH +18OH, triggered by 266 nm photolysis of O3, have been measured under conditions where less than 1% of the nascent fragments experienced collisions prior to detection. The distributions are qualitatively different than those reported earlier for 266 nm photolysis, which were evidently affected by collisions. The rotational and vibrational state distributions are similar to recent 248 nm photolysis experiments, but with differences attributed to collisional and/or energetic effects in those experiments. The ‘‘new’’ 16OH is formed with vibrational populations in the ratio 0.39(v=0):0.29(v=1):0.3(v≥2). Gaussian rotational energy distributions peaked near N=12 give average rotational energies of 〈Erot〉 = 3440 and 2780 cm−1 for 16OH v=0 and v=1, respectively. The ‘‘old’’ 18OH is much colder with vibrational populations 0.94(v=0):0.06(v=1) and a 18OH v=0 Gaussian rotational energy distribution characterized by 〈Erot〉 = 1920 cm−1.
There is negligible rotational alignment of the 16OH fragments [βμJ′ = β20(02) = 5A(0)2/4 = 0.06± 0.09], which is significantly less than expected for fragment rotations aligned with respect to the O+H2O relative velocity vector. The spin-orbit propensities deviate slightly from the statistical expectation and are characterized by [F2,N]/[F1,N]=(0.89±0.06) ×N/(N + 1). The Λ-doublet distributions for 18OH (all N) and 16OH (low-to-moderate N) fragments conform to an unconstrained-dynamics prior distribution. A slight propensity for Π(A′) in excess of this expectation is seen for the 16OH (high N) fragments. These new results are discussed in terms of possible insertion and abstraction mechanisms for the reaction.
Large amplitude skeletal isomerization as a promoter of intramolecular vibrational relaxation in CH stretch excited hydrocarbons
Authors: McIlroy Andrew , Nesbitt David J. 
Published: July 2002

The high resolution, slit jet cooled infrared v=1←0 methyl asymmetric stretch spectra of trans-2-butene and 1-butene are reported. Both of these molecules are singly unsaturated butene chains, have 30 vibrational degrees of freedom, and yield nearly equivalent vibrational state densities (ρvib≊200 states/cm−1) at CH stretch levels of excitation. The key difference between these two molecules is the presence of a large amplitude C–C–C skeletal torsional coordinate in 1-butene corresponding to a low barrier, internal isomerization pathway which is completely absent in trans-2-butene. The trans-2-butene asymmetric CH stretch (ν16) spectrum is fully discrete at 0.002 cm−1 resolution, and the coarse structure readily assigned to zero order rovibrational transitions (J′K′aK′c ← J″K″aK″c) in an asymmetric top. Fragmentation of these zero order transitions into spectral ‘‘clumps’’ of fine structure provides direct evidence for coupling of the CH stretch to vibrational bath states, but no evidence for loss of Ka′ and Kc′ as good quantum labels in the spectrum.
The average density of coupled states is found directly from the spectrum to be 114 states/cm−1, i.e., on the order of 0.5 ρvib. In contrast to the behavior in trans-2-butene, the 1-butene v=1←0 methyl asymmetric stretch spectrum exhibits an essentially continuous absorption contour even at Trot=6 K and 0.002 cm−1 resolution. On closer inspection, the 1-butene spectral envelope exhibits reproducible, intramolecular vibrational relaxation (IVR) induced fine structure limited by apparatus resolution and characteristic of highly congested IVR coupling. Analysis of this fine structure indicates a density of coupled states on the order of 1 000–10 000 states/cm−1, i.e., 20–30-fold in excess of ρvib, and 1–2 orders of magnitude larger than observed in trans-2-butene. In order to model the degree of fine structure observed in the spectrum, this level of spectral congestion essentially requires complete mixing of all ρvib⋅(2J′+1) rovibrational states consistent with conservation of total energy and angular momentum. The qualitatively dramatic differences between 1-butene and trans-2-butene behavior support a simple model for strong vibration-rotation (V-R) coupling in the bath states due to large amplitude skeletal motion in the C–C–C torsional mode which greatly enhances the available state density for IVR. Hence, the presence of a low barrier, skeletal isomerization coordinate may prove to be a general, moiety specific promoter for IVR processes in CH stretch excited hydrocarbons.