bluuebunny/crossref_metadata_2025_split huggingface dataset:
Physical study of laser-produced plasma in confined geometry
Authors: Fabbro R. , Fournier J. , Ballard P. , Devaux D. , Virmont J. 
Published: July 2002

We study in this paper the different physical processes involved in laser-produced plasma in confined geometry. With this technique, a laser irradiates a target at an intensity of a few GW/cm2, and the produced plasma is confined by a transparent overlay to the laser which covers this target. This configuration has appeared necessary for example for metallurgical applications where, for a given laser energy, enhanced pressures must be realized in order to achieve high shock pressures. Therefore, a physical study of this method is useful in order to optimize this technique. We have first developed an analytical model which describes the different steps involved in this process, points out the interest of this technique, and compares it to the direct ablation regime. In the first stage, during the laser heating, the generated pressure is typically 4–10 times greater than the corresponding one obtained in direct ablation. The second step begins after the switch-off of the laser and is characterized by an adiabatic cooling of the plasma which maintains the applied pressure over a period which is about 2 times the laser-pulse duration. Finally, the third stage concerns also the adiabatic cooling of the recombined plasma, but during this period the exerted pressure is too small to realize a plastic deformation of the material.
We show that the impulse momentum given to the target is mainly generated during this step. This model allows us to also determine the velocities of thin foils accelerated with confined plasmas, and we show that very high hydrodynamic efficiencies can be achieved by this technique. Experimentally, we measured with quartz gauges, the pressures obtained in confined geometry, for 30-, 3-, and 0.6-ns laser-pulse duration. This study shows that short pulse durations are sensitive to the initial roughness of the interface, and such an effect should be suppressed by using a liquid confinement. Then, we conclude that a large fraction of the absorbed laser energy (80%–90%) is used for the ionization of the medium in these conditions of irradiation. Finally, we experimentally point out that the laser-induced breakdown of the confining medium is the main mechanism which limits the generated pressure and show the influence of the laser-pulse duration on this effect.
Complete basis set correlation energies. III. The total correlation energy of the neon atom
Authors: Petersson G. A. , Yee Arnold K. , Bennett Andrew 
Published: July 2002

Within the framework of many-body perturbation theory, the total correlation energy can be partitioned into: intraorbital pair energies, eii; interorbital pair energies, αβeij and ααeij; double-excitation pair-coupling terms eij,kl(D); and higher-excitation pair-coupling terms, eij,kl(S,T,Q,...). The asymptotic convergence of pair natural orbital expansions for each of these terms is determined for the model problem of n infinitely separated helium-like ions with infinite nuclear charge. For example, the asymptotic form of the basis set truncation error in an αβ-interorbital pair energy is LimitNij→∞Δαβeij =αβfij (𝒥μ=1Nij Cμij)2 ((−225/ 4608)) (Nij+δij)−1 , where Nij is the number of pair natural orbitals and Cμij is the coefficient of pair natural orbital configuration  μij. Numerical studies of the neon atom verify that this model behavior applies to real many-electron systems. The pair-coupling terms beyond third-order contribute less than 1% of the total correlation energy in a variety of atoms and molecules and can therefore be neglected. As a practical test of the use of the asymptotic forms to extrapolate the remaining terms, a double zeta plus polarization set of pair natural orbitals was used. Extrapolation of each of the neon pair energies to the value for a complete basis set yields an independent electron pair approximation equal to −0.4233 hartree, which is 108.6% of the experimental correlation energy (−0.3896±0.001 hartree). Including the third-order MP-MBPT pair-coupling terms and extrapolating to a complete basis set gives a total correlation energy equal to −0.3904 hartree, which is 100.2±0.2% of the experimental value. A similar
calculation on H2O gave equally good results (calc. −0.3706; expt. −0.370±0.003 hartree) indicating that this DZ+P CBS method is applicable to polyatomic potential energy surfaces.
Computational studies of SiH2+SiH2 recombination reaction dynamics on a global potential surface fitted to a
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o and experimental data
Authors: Agrawal Paras M. , Thompson Donald L. , Raff Lionel M. 
Published: July 2002

The recombination dynamics for the SiH2+SiH2→H2Si=SiH2 reaction are studied by quasiclassical trajectory methods using a global potential-energy surface fitted to the available experimental data and the results of various ab initio calculations. The potential surface is written as the sum of 18 many-body terms whose functional forms are motivated by chemical and physical considerations. The surface contains 41 parameters which are fitted to calculated geometries, fundamental vibrational frequencies, and energies for H2Si=SiH2, H2Si=SiH, H2Si=Si, HSi=Si, Si2, H2, and SiH2, and to various calculated and/or measured reaction barrier heights and activation energies. In general, the equilibrium bond lengths and angles given by the global surface are in agreement with ab
i n i t i o results to within 0.03 Å and 0.5°, respectively. The calculated exothermicities for various reactions involving silicon and hydrogen atoms are in excellent agreement with previous MP4 calculations and with experimental data. The average absolute error is 1.90 kcal/mol. The average absolute deviation of the predicted fundamental vibrational frequencies for H2Si=SiH2, H2Si=SiH, H2Si=Si, and SiH2 from the results reported by Ho et al. is 52.9 cm−1. The calculated barrier height for molecular hydrogen elimination from SiH2 is 34.27 kcal/mol with a backreaction barrier of 0.63 kcal/mol. The barrier for 1,2 elimination of H2 from H2Si=SiH2 is 115.3 kcal/mol with a backreaction barrier of 30.7 kcal/mol. The formation cross sections for H2Si=SiH2 decrease with both relative translational energy and internal SiH2 energy with translational energy being the more effective in reducing the cross sections. Thermally averaged formation cross sections vary from 66.3 Å2 at 300 K to 28.7 Å2 at 1500 K. The corresponding thermal rate coefficients lie in the range 2–4×1014 cm3/mol s over this temperature range and exhibit a maximum at an intermediate temperature. The trajectory details indicate that the reaction exothermicity is primarily partitioned into the Si–Si stretch and the H–Si–H bending modes upon formation of Si2H4. Energy transfer from the Si–Si stretch to the Si–H stretching modes is a relatively slow process occurring on a time scale of 10−12 s, which is about three to four times that previously computed for other polyatomic systems. Transfer from the Si–Si stretch to the H–Si–H bending modes is a faster process.
On the evaluation of non-Born–Oppenheimer interactions for Born–Oppenheimer wave functions. V. A body fixed frame approach. Applications to isotope effects on equilibrium geometries and the adiabatic correction for the X 1Σ+ state of LiH
Authors: Jensen James O. , Yarkony David R. 
Published: July 2002

The evaluation of the total second derivative nonadiabatic coupling matrix element H(J,I,R)=〈ψJ(r;R)‖∑i(−1/2 Mu)(∂2/∂R2i )ψI(r;R)〉r is considered. Here ψJ(r;R) is the adiabatic Born–Oppenheimer electronic wave function which in this work will be approximated by a large-scale CI wave function developed from an MCSCF reference space. For diatomic and triatomic systems the computational effort associated with the evaluation of H(J,I,R) can be reduced considerably by the use of a body fixed frame approach. In this approach costly evaluation of the derivative wave function with respect to noninternal degrees of freedom in the space fixed frame is replaced by the evaluation of matrix elements of many electron operators including the mass polarization operator (total electronic linear momentum squared) and the L2 operator (total electronic orbital angular momentum squared). The equivalence of the body fixed frame and space fixed frame results leads to valuable diagnostic equations which provide stringent tests of the derivative methodology used to evaluate the remaining second derivatives with respect to internal coordinates. The methods presented here are applied to the benchmark systems BeH+ and LiH. The Born–Oppenheimer diagonal correction or adiabatic correction (AC) is evaluated for the X 1∑+ state of these systems and used to consider the effect of isotopic substitution on equilibrium geometries. For the X 1∑+ state of LiH a troubling discrepancy exists between the AC determined by advanced theoretical and experimental techniques.
For R≲Re the AC determined directly with specialized CI wave functions and the experimental value inferred from a detailed spectroscopic analysis of the A→X emission agree. However, for R&gt;Re theory and experiment disagree qualitatively. For R≲Re our results are consistent with the previous work. For R&gt;Re our results are in accord with the experimentally derived AC.
Absolute state-selected and state-to-state total cross sections for the reaction Ar+(2P3/2,1/2)+O2
Authors: Flesch G. D. , Nourbakhsh S. , Ng C. Y. 
Published: July 2002

Absolute spin–orbit state-selected total cross sections for the reactions, Ar+(2P3/2,1/2)+O2→O+2+Ar [reaction (1)], O++O+Ar [reaction (2)], and ArO++O [reaction (3)], have been measured in the center-of-mass collision energy (Ec.m.) range of 0.044–133.3 eV. Absolute spin–orbit state transition total cross sections for the Ar+(2P3/2,1/2)+O2 reaction at Ec.m.=2.2–177.6 eV have also been examined. The appearance energies for the formation of O+ (Ec.m.=2.9±0.2 eV) and ArO+ (2.2±0.2 eV) are in agreement with the thermochemical thresholds for reactions (2) and (3), respectively. The cross sections for O+2, O+, and ArO+ depend strongly on Ec.m. and the spin–orbit states of Ar+, suggesting that reactions (1)–(3) are governed predominantly by couplings between electronic potential energy surfaces arising from the interactions of Ar+(2P3/2)+O2, Ar+(2P1/2)+O2, and O+2+Ar.
In the Ec.m. range of 6.7–22.2 eV, corresponding to the peak region of the O+ cross section curve, the cross sections for O+ are ≥50% of those for O+2. The production of O+ by reaction (2) is interpreted to be the result of predissociation of O+2 in excited states formed initially by reaction (1). The formation of charge transfer O+2(ã 4Πu) has been probed by the charge transfer reaction O+2(ã 4Πu)+Ar. The results indicate that in the Ec.m. range of 0.4–3.0 eV charge transfer product O+2 ions are formed mainly in the O+2(ã 4Πu) state. Experimental evidence is found supporting the conclusion that the vibrational distributions of O+2(ã 4Πu) formed in reaction (1) and by photoionization of O2 in the energy range between the O+2(ã 4Πu, v=0) and O+2(Ã 2Πu, v=0) thresholds are similar.
The population of O+(4S) formed by reaction (2) has also been measured by the reaction O+(4S)+N2→NO++N. In the Ec.m. range of 3–44 eV, product O+ ions of reaction (2) are shown to be dominantly in the O+(4S) ground state. At Ec.m.≥14 eV, the retarding potential energy analysis for O+2 shows that more than 98% of the charge transfer O+2 ions are slow ions formed mostly by the long-range electron jump mechanism. Product ArO+ ions are observed only in the Ec.m. range of 2.2–26.6 eV. At Ec.m. slightly above the thermochemical thresholds of reactions (2) and (3), the overwhelming majority of ArO+ and O+ ions are scattered backward and forward with respect to the c.m. velocity of reactant Ar+, respectively. This observation is rationalized by a charge transfer predissociation mechanism which involves the formation of ArO+ and O+ via nearly collinear Ar+–O–O collision configurations at Ec.m. near the thresholds of reactions (2) and (3).
Raman spectroscopic investigation of irreversibly compacted vitreous silica
Authors: Walrafen G. E. , Chu Y. C. , Hokmabadi M. S. 
Published: July 2002

Polarized, depolarized, isotropic, difference, and reduced Raman spectra were obtained from vitreous silica, irreversibly compacted at 600 °C and 50 kbar, to ambient densities as high as 2.73 g cm−3. The 60 cm−1 ‘‘Boson’’ peak; and, the intense optic, Si–O–Si bending peak at 440 cm−1 including its low-frequency shoulders at ≊210–240 and ≊340 cm−1; were observed to move strongly upward in position, but at two different rates with density rise. The two peak frequencies behave experimentally like spherical transverse acoustic (TA) or shear (S), and spherical longitudinal acoustic (LA) or pressure (P) stress waves of an isotropic elastic solid. In contrast, downward frequency shifts were observed for the transverse optic (TO) 1060 cm−1 and longitudinal optic (LO) 1200 cm−1 modes in the Si–O stretching region, but at absolute rates equal, respectively, to those of the 60 and 440 cm−1 peak frequencies thus linking optic as well as acoustic modes to the elastic modulii.
The TO and LO splitting was shown from a phonon coupling model to result from very strong interaction between the optic modes and the low-frequency S and P modes. The frequency decreases of the TO and LO modes indicate a decrease in the Si–O stretching force constants, related to a decrease in the mean Si–O–Si bridging angle, from ≊144° to ≊120°, inferred from frequency increases at 440 and ≊800 cm−1. A decrease in the mean O–2nd-O distance also occurs, but severe distortion of the SiO4 tetrahedra does not seem probable to densities of 2.73 g cm−3. The dilatational P-mode wavelength ≥5 Å corresponds to volume-related distances, e.g., Si–2nd-O, O–2nd-O, or Si–2nd-Si. The shear mode wavelength is larger (≊13 Å) and similar to the structural correlation length, estimated roughly from x-ray data, where G(r)≊1. Both S and P modes involve small coherence volumes, and both may have negligible group velocities, i.e., standing waves. The low-frequency S scattering is weakly to completely depolarized, but the higher-frequency P scattering may be strongly polarized, falling near or just below (BeF2, B2O3, GeO2, SiO2, ZBLAL) the first very strong minimum in the Raman depolarization ratio, located above the S mode.
This new criterion locates the actual P mode of fused silica within the ≊200–350 cm−1 shoulder region. This region is intense in the density of states, determined by neutron inelastic scattering (NIS). S and P modes are resolved in the Raman spectra of several glasses, but their relation to the low-order spherical acoustic stress waves of isotropic media has, heretofore, not been widely recognized.
Para phenylenediamine radical cation structure studied by resonance Raman and molecular orbital methodsa)
Authors: Chipman Daniel M. , Sun Q. , Tripathi G. N. R. 
Published: July 2002

The vibrational and electronic structures of p-phenylenediamine radical cation have been studied using time-resolved resonance Raman spectroscopy and molecular orbital calculations. The Raman spectra in aqueous solution show striking variations in intensity profile when excitation is varied from 340 to 480 nm. Excitation in resonance with the 460–480 nm absorption shows enhancement of only totally symmetric vibrations. Most prominent are the δNH2 scissor mode at 1658 cm−1 and the Wilson modes ν8a (CC stretch) at 1644 cm−1, ν7a (CN stretch) at 1423 cm−1, and ν9a (CH bend) at 1184 cm−1. Also observed are ν1 (ring breathe) at 840 cm−1 and ν6a (CCC bend) at 467 cm−1. With excitation in the 340–410 nm region the ν1 band becomes relatively stronger and an additional band at 1524 cm−1 appears. This latter band, which dominates the spectrum at 360 nm, is assigned to the nontotally symmetric vibration ν8b (CC stretch) having b3g symmetry that gains intensity through vibronic coupling. Raman spectra of ring- and amine-deuterated radicals allow distinction of overtone and combination bands enhanced by Fermi resonance from fundamentals due to δNH2 and ν8a (CC stretch) modes in the 1600–1700 cm−1 region. The theoretical calculations show reasonable agreement with the observed vibrational frequencies and lead to detailed information on the bond structure and normal modes of the radical. The calculated CN bond lengths of 1.33 Å are intermediate between lengths typical of single and double bonds in aromatic amines. The calculations also allow interpretation of the experimental absorption spectrum. The broad medium intensity absorption peaked at 460 nm with a shoulder at 480 nm is assigned to a 2B3u ←2B2g transition, the very weak 360 nm absorption to 2Au ← 2B2g and the very strong ∼325 nm absorption to a higher 2B3u ← 2B2g.
Calculations with the simple relation Ik∝ (V′k)2/ωk, which requires evaluation only of vertical excited state gradients, are found to satisfactorily describe the dramatic changes in Raman intensity profile that are observed with different excitation wavelengths. The 460 and 480 nm Raman spectra are governed by scattering from the lowest 2B3u state. Spectra taken in the 410–340 nm range are governed mainly by preresonance scattering from the higher 2B3u state, which is vibronically coupled to the weak 2Au state via the b3g symmetry ν8b (CC stretch) vibration. The relative intensities of totally symmetric modes calculated for excitation into the two 2B3u excited states agree well with those in the two distinct patterns observed experimentally.
Spectroscopic interrogation of heterocluster isomerization. I. Simulations of nuclear dynamics and electronic spectroscopy
Authors: Ben-Horin Narda , Even Uzi , Jortner Joshua 
Published: July 2002

In this paper we explore the structure, energetics, nuclear dynamics, isomerization, and electronic-vibrational spectroscopy of large finite systems by constant energy molecular dynamics (MD) simulations of heteroclusters of 9,10-dichloroanthracene (DCA) with rare-gas atoms, i.e., DCA⋅Arn (n=1–34) and DCA⋅Krn (n=1–18). We consider the spectroscopic implications of heterocluster size effects, isomer-specificity, nuclear dynamics, and isomerization as manifested by the spectral shifts (δν) and homogeneous linewidths (Δ) for the S0→S1 transition, the ionization potential shifts (δI) from S0, and the shifts (Δτr) of the pure radiative lifetimes of S1 for these heteroclusters, with δν, δI, and Δτr being given relative to the corresponding observable for the bare DCA molecule. MD simulations of the energetics, rigid and nonrigid structures, fluctuations in structural parameters, and transport properties over a broad temperature region (8–70 K) allowed for the identification of several isomerization phenomena, i.e., correlated surface motion, surface melting, side crossing, wetting–nonwetting, and nearly rigid–nonrigid transitions. The size dependence of the temperature onsets for various isomerization processes was established. The hierarchical occurrence of several isomerization processes for a single heterocluster composition, together with the sequential occurrence of distinct isomerization processes with increasing the cluster size, was documented.
In the size domain explored herein the temperature onsets of surface melting, side crossing, and wetting–nonwetting transitions tend to decrease with increasing the heterocluster size, in contradiction to the rule that the cluster melting temperature increases with increasing its size, reflecting finite microsurface effects. The spectroscopic observables were simulated using quantum mechanical expressions for δν, δI, and Δτr at a fixed nuclear configuration, and were summed over the entire MD trajectory. The heterocluster isomerization phenomena could not be identified by the temperature and/or size dependence of δν, Δ, and δI, while Δτr data exhibit a marked structural sensitivity, providing a promising approach for the characterization of heterocluster isomerization. The MD simulations elucidate the size dependence and isomer specificity of spectral shifts, homogeneous linewidths, ionization potential shifts, and pure radiative lifetimes of heteroclusters. The intriguing phenomenon of spectral line narrowing with increasing the cluster size for these heteroclusters, is attributed to the size dependent reduction of inhomogeneous broadening which originates from the cumulative contribution of the isomer-specific spectral shifts and of homogeneous broadening effects.
Critical thinking and the academic vocational divide
Authors: Halliday John 
Published: July 2002

ABSTRACTThere is considerable interest in unifying post‐compulsory education and training systems through the common teaching of core or key skills. Critical thinking appears to be a very attractive example of such a skill but this article argues against the idea that critical thinking is a core skill that should be widely taught to bridge what is termed ‘the academic vocational divide’. Some of the attractions of this idea are acknowledged but it is argued that the very meanings of critical, academic and vocational depend upon the context and purpose of thinking and it is misleading to suggest that thinking is any kind of a skill. On the basis of this argument and an empirical study it is concluded that when the idea informs curricular policy and practice, far from bridging an academic vocational divide, the idea will actually widen it. If this conclusion is correct for what appears to be a most plausible candidate for a core or key skill, then policies designed to bridge an academic vocational divide that are based on the core or key skills idea should be abandoned.
Pragmatic Comprehension In Secondary School‐Aged Students With Specific Developmental Language Disorder
Authors: Rinaldi Wendy 
Published: July 2002

AbstractThis study explores the hypothesis that there may be particular difficulties for secondary school students with specific developmental language disorder (SDLD) in understanding contextual, pragmatic meaning. Sixty‐four SDLD students aged 11+ to 14+ years are compared with chronological‐age‐matched and language‐age‐matched non‐impaired students. New procedures are used to examine comprehension of two types of ambiguity where the context determines speaker intention: inconsistent messages of emotion and multiple meanings in context. These types of ambiguity are evident in a range of communicative intent, e.g. sarcasm, idiomatic expression, deceit and humour. Preliminary study into adolescent language suggests that at this age there is a growing expectation for students to understand these kinds of communication, both in the classroom and socially. The study finds that the SDLD students were less able than both comparison groups to use context to understand implied meanings. Nonimpaired children were also more able to rule out literal interpretations when they did not know the non‐literal meaning. These findings were statistically significant. The implications for research and practice are discussed, including those of diagnostic assessment, in the light of the literature survey revealing that many currently available do not assess pragmatic meaning comprehension. There is a challenge to the view that disorders in the semantic and pragmatic domains necessarily co‐occur, as suggested by the diagnostic category semanticpragmatic disorder.