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Two semiclassical, initial value representation (IVR) treatments are presented for the correlation function <ψ[sup (f )]|e[sup -iH⁁t/h]|ψ[sup (i)]>, where ψ[sup (i)] and ψ[sup (f )], are energy eigenfunctions of a “zero-order” Hamiltonian describing an arbitrary, integrable, vibrational system. These wave functions are treated semiclassically so that quantum calculations and numerical integrations over these states are unnecessary. While one of the new approximations describes the correlation function as an integral over all phase space variables of the system, in a manner similar to most existing IVR treatments, the second approximation describes the correlation function as an integral over only half of the phase space variables (i.e., the angle variables for the initial system). The relationship of these treatments to the conventional Herman-Kluk approximation for correlation functions is discussed. The accuracy and convergence of these treatments are tested by calculations of absorption spectra for model systems having up to 18 degrees of freedom, using Monte Carlo techniques to perform the multidimensional phase space integrations. Both treatments are found to be capable of producing spectra of excited, anharmonic states that agree well with quantum results. Although generally less accurate than full phase space or Herman-Kluk treatments, the half phase space method is found to require far fewer trajectories to achieve convergence. In addition, this number is observed to increase much more slowly with the system size than it does for the former methods, making the half-phase space technique a very promising method for the treatment of large systems. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Two-color photon echo peak shift spectroscopy was used to study electronic coupling in a phthalocyanine homodimer. Two optical parametric amplifiers were used to produce pulses to excite the split lower states of LuPc[sub 2][sup -]. The existence of a two-color peak shift indicates the existence of correlation between these two dipole-allowed states. The nature of this correlation is discussed based on theoretical predictions of the interactions between exciton and charge resonance states. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The role of fluctuations in both the density functional theory (DFT) and the field theory (FT) of nanosystems is studied. It turns out that although fluctuations are rigorously incorporated into the general formalism of DFT, they are often omitted in the choice of an approximate free energy functional that must be constructed in order to solve the basic integral equation appearing in DFT. Aside from the analytical discussion, it is demonstrated, in connection with a particular system (fluid in a nanopore) that the effects of fluctuation are missing when one of the most common functionals for this system is used. The demonstration involves a comparison of the results of Monte Carlo simulation with the predictions of DFT when this free energy functional is used. The applicability of FT and DFT in the context of the theory of nucleation is also discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this article we consider the electronic diabatic presentation of a two-state system with the aim of earning insight regarding the distribution of conical intersections in a given region. In this process we revealed explicit relationship between the diabatic potentials and the locations of conical intersections. The study is accompanied with numerical examples as worked out for a model and ab initio potential energy surfaces of the Na+H[sub 2] system. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Triple-resonance excitation and high-resolution photoelectron spectroscopy are combined to characterize the mode selectivity of vibrational autoionization of the high Rydberg states of NO[sub 2]. Photoelectron spectra and vibrational branching fractions are reported for autoionizing Rydberg states converging to the NO[sub 2][sup +] X [sup 1]Σ[sub g][sup +](110) state, that is, with one quantum in the symmetric stretch, ν[sub 1], and one quantum in the bending vibration, ν[sub 2]. These results indicate that autoionization proceeds most efficiently through the loss of one quantum from the symmetric stretch rather than from the bending vibration. The implications of this result are discussed in terms of the autoionization mechanism. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Coupled-cluster theory with connected single and double excitation operators (CCSD) and related approximations, such as linearized CCSD, quadratic configuration interaction with single and double excitation operators, coupled-cluster with connected double excitation operator (CCD), linearized CCD, approximate CCD, and second- and third-order many-body perturbation theories, are formulated and implemented for infinitely extended one-dimensional systems (polymers), on the basis of the periodic boundary conditions and distance-based screening of integrals, density matrix elements, and excitation amplitudes. The variation of correlation energies with the truncation radii of short- and long-range lattice sums and with the number of wave vector sampling points in the first Brillouin zone is examined for polyethylene, polyacetylene, and polyyne, and is shown to be a function of the degree of π-electron conjugation or the fundamental band gaps. The t[sub 2] and t[sub 1] amplitudes in the atomic orbital (AO) basis are obtained by first computing the t amplitudes in the Bloch-orbital basis and subsequently back-transforming them into the AO basis. The plot of these AO-based t amplitudes as a function of unit cells also indicates that the t[sub 2] amplitudes of polyacetylene and polyyne exhibit appreciably slower decay than those of polyethylene, although the asymptotic decay behavior is invariably 1/r[sup 3]. The AO-based t[sub 1] amplitudes appear to correlate strongly with the electronic structure, and they decay seemingly exponentially for polyethylene whereas they stay at a constant magnitude across the seventh nearest neighbors of polyacetylene and polyyne, which attests to far reaching effects of nondynamical electron correlation mediated by orbital rotation. Nonetheless, the unit cell contributions to the correlation energies taper below 10[sup -6] hartree after 15 Å for all three polymers. The basis set dependence of the decay behavior of t[sub 2] amplitudes is also examined for linear hydrogen fluoride polymer (HF)[sub ∞] and linear beryllium polymer (Be)[sub ∞] employing the STO-3G, 6-31G, and 6-31G[sup *] basis sets, and proves to be rather small. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Recently, experimental and theoretical studies on the water system are very active and noticeable. A transferable intermolecular potential seven points approach including fluctuation charges and flexible body (ABEEM-7P) based on a combination of the atom-bond electronegativity equalization and molecular mechanics (ABEEM/MM), and its application to small water clusters are explored and tested in this paper. The consistent combination of ABEEM and molecular mechanics (MM) is to take the ABEEM charges of atoms, bonds, and lone-pair electrons into the intermolecular electrostatic interaction term in molecular mechanics. To examine the charge transfer we have used two models coming from the charge constraint types: one is a charge neutrality constraint on whole water system and the other is on each water molecule. Compared with previous water force fields, the ABEEM-7P model has two characters: (1) the ABEEM-7P model not only presents the electrostatic interaction of atoms, bonds and lone-pair electrons and their changing in respond to different ambient environment but also introduces “the hydrogen bond interaction region” in which a new parameter k[sub lp,H](R[sub lp,H]) is used to describe the electrostatic interaction of the lone-pair electron and the hydrogen atom which can form the hydrogen bond; (2) nonrigid but flexible water body permitting the vibration of the bond length and angle is allowed due to the combination of ABEEM and molecular mechanics, and for van der Waals interaction the ABEEM-7P model takes an all atom–atom interaction, i.e., oxygen–oxygen, hydrogen–hydrogen, oxygen–hydrogen interaction into account. The ABEEM-7P model based on ABEEM/MM gives quite accurate predictions for gas-phase state properties of the small water clusters (H[sub 2]O)[sub n] (n=2–6), such as optimized geometries, monomer dipole moments, vibrational frequencies, and cluster interaction energies. Due to its explicit description of charges and the hydrogen bond, the ABEEM-7P model will be applied to discuss properties of liquid water, ice, aqueous solutions, and biological systems. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A method of calculating analytical energy gradients of the singlet and triplet excited states, ionized states, electron-attached states, and high-spin states from quartet to septet states by the symmetry-adapted-cluster configuration-interaction general-R method is developed and implemented. This method is a powerful tool in the studies of geometries, dynamics, and properties of the states of molecules in which not only one-electron processes but also two- and multielectron processes are involved. The performance of the present method was confirmed by calculating the geometries and the spectroscopic constants of the diatomic and polyatomic molecules in various electronic states involving the ground state and the one- to three-electron excited states. The accurate descriptions were obtained for the equilibrium geometries, vibrational frequencies, and adiabatic excitation energies, which show the potential usefulness of the present method. The particularly interesting applications were to the C[sup ′] [sup 1]A[sub g] state of acetylene, the A [sup 2]Δ[sub u] and B [sup 2]Σ[sub u][sup +] states of CNC and the [sup 4]B[sub 1] and a [sup 4]Π[sub u] states of N[sub 3] radical. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A method is proposed for improving the accuracy and efficiency of free energy simulations. The essential idea is that the convergence of the relevant measure (e.g., the free energy derivative in thermodynamic integration) is monitored in the reverse direction starting from the last frame of the trajectory, instead of the usual approach, which begins with the first frame and goes in the forward direction. This simple change in the use of the simulation data makes it straightforward to eliminate the contamination of the averages by contributions from the equilibrating region. A statistical criterion is introduced for distinguishing the equilibrated (production) region from the equilibrating region. The proposed method, called reverse cumulative averaging, is illustrated by its application to the well-studied case of the alchemical free energy simulation of ethane to methanol. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

According to the definition in the text, the correlation energy of 1s[sub C][sup 2] of carbon atoms, the primary and secondary C–H bonding electron pairs in some CH[sub 3], CH[sub 2] fragments and CH[sub 3](CH[sub 2])[sub m]CH[sub 3] (m=1–5) linear alkane molecules are calculated and analyzed. The transferability of the correlation energies of these electron pairs in the linear alkanes is investigated. The results indicate that the correlation energy of 1s[sub C][sup 2] is perfectly transferable in the respective methyl and methylene groups, while the correlation energies of the primary and secondary C–H bonding electron pairs are approximately transferable in methyl and methylene groups. The analysis of the results of group correlation energy shows that both of the correlation energies of methyl and methylene groups are transferable in these linear alkanes. The correlation energies of methylene group in CH[sub 3](CH[sub 2])[sub m]CH[sub 3] (m=1–5) molecules are slightly decreasing showing a converging trend to a “standard” methylene group in linear alkanes. The excellent fitting relationship between the total correlation energy and the number of methylene groups of the linear alkanes shows that the total correlation energy is a linear function of the number of methylene groups, which means that the total correlation energies of large linear alkanes can be reproduced and predicted by counting the numbers of methylene groups. In this way, total correlation energy of large linear alkane molecule can be approximately calculated using this simple group additive scheme with substantial saving in computational time. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A method is presented for expressing the occupied self-consistent-field (SCF) orbitals of a molecule exactly in terms of chemically deformed atomic minimal-basis-set orbitals that deviate as little as possible from free-atom SCF minimal-basis orbitals. The molecular orbitals referred to are the exact SCF orbitals, the free-atom orbitals referred to are the exact atomic SCF orbitals, and the formulation of the deformed “quasiatomic minimal-basis-sets” is independent of the calculational atomic orbital basis used. The resulting resolution of molecular orbitals in terms of quasiatomic minimal basis set orbitals is therefore intrinsic to the exact molecular wave functions. The deformations are analyzed in terms of interatomic contributions. The Mulliken population analysis is formulated in terms of the quasiatomic minimal-basis orbitals. In the virtual SCF orbital space the method leads to a quantitative ab initio formulation of the qualitative model of virtual valence orbitals, which are useful for calculating electron correlation and the interpretation of reactions. The method is applicable to Kohn–Sham density functional theory orbitals and is easily generalized to valence MCSCF orbitals. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

While the determination of free-energy differences by MD simulation has become a standard procedure for which many techniques have been developed, total entropies and entropy differences are still hardly ever computed. An overview of techniques to determine entropy differences is given, and the accuracy and convergence behavior of five methods based on thermodynamic integration and perturbation techniques was evaluated using liquid water as a test system. Reasonably accurate entropy differences are obtained through thermodynamic integration in which many copies of a solute are desolvated. When only one solute molecule is involved, only two methods seem to yield useful results, the calculation of solute–solvent entropy through thermodynamic integration, and the calculation of solvation entropy through the temperature derivative of the corresponding free-energy difference. One-step perturbation methods seem unsuitable to obtain entropy estimates. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In a discharged supersonic jet of acetonitrile and carbon disulfide, we have for the first time observed an electronic transition of the NC[sub 3]S radical using laser-induced fluorescence (LIF) spectroscopy. A progression originating from the C–S stretching mode of the upper electronic state appears in the excitation spectrum. Each band of the progression has a polyad structure due to anharmonic resonances with even overtones of bending modes. Rotationally resolved spectra have been observed by high-resolution laser scans, and the electronic transition is assigned to à [sup 2]Π[sub i]–X [sup 2]Π[sub i]. For the vibronic origin band, the position and the effective rotational constant of the upper level have been determined to be 21 553.874(1) and 0.046 689(4) cm-1, respectively. The dispersed fluorescence spectrum from the zero vibrational level of à [sup 2]Π[sub 3/2] has also been observed; its vibrational structure is similar to that of the LIF excitation spectrum, showing a prominent C–S stretching progression with polyad structures. The vibrational frequencies of the C–S stretching mode in the ground and excited electronic states are determined to be 550 and 520 cm-1, respectively. Fluorescence decay profiles have been measured for several vibronic levels of the à state. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Transitions of two different stereoisomers of the He...ICl(X,v[sup ″]=0) weakly bound complex, one with a T-shaped orientation and another that is most likely linear, have been observed in laser-induced fluorescence experiments performed in the ICl B–X region [Bradke and Loomis, J. Chem. Phys. 118, 7233 (2003)]. Here we present experimental and theoretical results aimed at confirming the previous assignments and at gaining additional insights into the He+ICl interactions. High resolution action spectra were recorded in the same region to identify those features that could be attributed to transitions of the He...I[sup 35]Cl(X,v[sup ″]=0) isomers and not to higher-order complexes, He[sub n]...I[sup 35]Cl, where n>=2, or I[sup 37]Cl containing species. Calculations of the rovibronic spectra of the He...I[sup 35]Cl complexes in the ICl B–X, 2–0 and 3–0 regions were performed using an ab initio potential energy surface for the He+ICl(X,v[sup ″]=0) ground state [Prosmiti et al., J. Chem. Phys. 117, 7017 (2002)] and two different pairwise additive potentials for the He+ICl(B,v[sup ′]=2,3) excited states [Waterland et al., J. Chem. Phys. 89, 7277 (1988); Gray and Wozny, J. Chem. Phys. 94, 2817 (1991)]. The rotation-vibration energies and wave functions for the He...I[sup 35]Cl complexes were obtained for all bound states with total angular momentum J<10 using both of these potentials. Electronic spectra were generated using these results, assuming that the transition moment lies along the ICl bond and is not perturbed by the presence of the helium atom. The calculations qualitatively reproduce the He...I[sup 35]Cl action spectrum and strongly support the previous assignments. The calculations also indicate that some of the spectral congestion observed near the linear band may be attributed to transitions of the linear isomer to multiple intermolecular levels in the excited state. Coriolis coupling strongly mixes He...ICl(B,v[sup ′]) states with rotational excitation, making simulations and assignments of the linear band observed in the experimental spectrum difficult. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The method, introduced in the preceding paper, for recasting molecular self-consistent field (SCF) or density functional theory (DFT) orbitals in terms of intrinsic minimal bases of quasiatomic orbitals, which differ only little from the optimal free-atom minimal-basis orbitals, is used to elucidate the bonding in several silicon clusters. The applications show that the quasiatomic orbitals deviate from the minimal-basis SCF orbitals of the free atoms by only very small deformations and that the latter arise mainly from bonded neighbor atoms. The Mulliken population analysis in terms of the quasiatomic minimal-basis orbitals leads to a quantum mechanical interpretation of small-ring strain in terms of antibonding encroachments of localized molecular-orbitals and identifies the origin of the bond-stretch isomerization in Si[sub 4]H[sub 6]. In the virtual SCF/DFT orbital space, the method places the qualitative notion of virtual valence orbitals on a firm basis and provides an unambiguous ab initio identification of the frontier orbitals. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Spin-locking of half-integer quadrupolar nuclei, such as [sup 23]Na (I=3/2) and [sup 27]Al (I=5/2), is of renewed interest owing to the development of variants of the multiple-quantum and satellite-transition magic angle spinning (MAS) nuclear magnetic resonance experiments that either utilize spin-locking directly or offer the possibility that spin-locked states may arise. However, the large magnitude and, under MAS, the time dependence of the quadrupolar interaction often result in complex spin-locking phenomena that are not widely understood. Here we show that, following the application of a spin-locking pulse, a variety of coherence transfer processes occur on a time scale of ∼1/ω[sub Q] before the spin system settles down into a spin-locked state which may itself be time dependent if MAS is performed. We show theoretically for both spin I=3/2 and 5/2 nuclei that the spin-locked state created by this initial rapid dephasing typically consists of a variety of single- and multiple-quantum coherences and nonequilibrium population states and we discuss the subsequent evolution of these under MAS. In contrast to previous work, we consider spin-locking using a wide range of radio frequency field strengths, i.e., a range that covers both the “strong-field” (ω[sub 1]>ω[sub Q][sup PAS]) and “weak-field” (ω1<ω[sub Q][sup PAS]) limits. Single- and multiple-quantum filtered spin-locking experiments on NaNO[sub 2], NaNO[sub 3], and Al(acac)[sub 3], under both static and MAS conditions, are used to illustrate and confirm the results of the theoretical discussion. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using a newly constructed three sector field mass spectrometer (resulting in a BE1E2 field configuration) we have measured the kinetic energy release distributions of neon, argon, and krypton cluster ions. In the present study we used the first two sectors, B and E1, constituting a high resolution mass spectrometer, to select the parent ions in terms of mass, charge, and energy, and studied the decay of those ions in the third field free region. Due to the improved mass resolution we were able to extend earlier studies carried out with a two sector field machine, where an upper size limit arose from the fact that several isotopomers contribute to a decaying parent ion beam when the cluster size exceeds a certain value. Furthermore we developed a new data analysis. It allows us to model also fragment ion peaks that are a superposition of different decay reactions and thus we can determine the average kinetic energy release for all decay reactions of a given cluster ion. In a further step we used these results to determine the binding energies of cluster ions Rg[sub n] (n>=10) by applying finite heat bath theory. The smaller sizes have not been included in this analysis, because the validity of finite heat bath theory becomes questionable below n≈10. The present average kinetic energy releases and binding energies are compared with other experiments and various calculations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The utility of recording Raman spectroscopy under liquid nitrogen, a technique we call Raman under nitrogen (RUN), is demonstrated for ferrocene, uranocene, and thorocene. Using RUN, low-temperature (liquid nitrogen cooled) Raman spectra for these compounds exhibit higher resolution than previous studies, and new vibrational features are reported. The first Raman spectra of crystalline uranocene at 77 K are reported using excitation from argon (5145 Å) and krypton (6764 Å) ion lasers. The spectra obtained showed bands corresponding to vibrational transitions at 212, 236, 259, 379, 753, 897, 1500, and 3042 cm-1, assigned to ring–metal–ring stretching, ring–metal tilting, out-of-plane CCC bending, in-plane CCC bending, ring-breathing, C–H bending, CC stretching and CH stretching, respectively. The assigned vibrational bands are compared to those of uranocene in THF, (COT)[sup 2-], and thorocene. All vibrational frequencies of the ligands, except the 259 cm-1 out-of-plane CCC bending mode, were found to increase upon coordination. A broad, polarizable band centered about ∼460 cm-1 was also observed. The 460 cm-1 band is greatly enhanced relative to the vibrational Raman transitions with excitations from the krypton ion laser, which is indicative of an electronic resonance Raman process as has been shown previously. The electronic resonance Raman band is observed to split into three distinct bands at 450, 461, and 474 cm-1 with 6764 Å excitation. Relativistic density functional theory is used to provide theoretical interpretations of the measured spectra. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A quantum-mechanical model is designed for the calculation of termolecular association reaction rate coefficients in the low-pressure fall-off regime. The dynamics is set up within the energy transfer mechanism and the kinetic scheme is the steady-state approximation. We applied this model to the formation of ozone O+O[sub 2]+M→O[sub 3]+M for M=Ar, making use of semiquantitative potential energy surfaces. The stabilization process is treated by means of the vibrational close-coupling infinite order sudden scattering theory. Major approximations include the neglect of the O[sub 3] vibrational bending mode and rovibrational couplings. We calculated individual isotope-specific rate constants and rate constant ratios over the temperature range 10–1000 K and the pressure fall-off region 10[sup -7]–10[sup 2] bar. The present results show a qualitative and semiquantitative agreement with available experiments, particularly in the temperature region of atmospheric interest. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The nascent CaH product in the reaction Ca(4s4p [sup 1]P[sub 1])+H[sub 2]→CaH(X [sup 2]Σ[sup +])+H is obtained using a pump–probe technique. The CaH(v=0,1) distributions, with a population ratio of CaH(v=0)/CaH(v=1)=2.7±0.2, may be characterized by low Boltzmann rotational temperature. According to Arrhenius theory, the temperature dependence measurement yields a potential barrier of 3820±480 cm[sup -1] for the current reaction. As a result of the potential energy surfaces (PES) calculations, the reaction pathway favors a Ca insertion into the H[sub 2] bond along a (near) C[sub 2v] geometric approach. As the H[sub 2] bond is elongated, the configurational mixing between the orbital components of the 4p and nearby low-lying 3d state with the same symmetry makes significant the nonadiabatic transition between the 5A[sup ′] and 2A[sup ′] surface in the repulsive limbs. Therefore, the collision species are anticipated to track along the 5A[sup ′] surface, then undergo nonadiabatic transition to the inner limb of the 2A[sup ′] surface, and finally cross to the reactive 1A[sup ′] surface. The observed energy barrier probably accounts for the energy requirement to surmount the repulsive hill in the entrance. The findings of the nascent CaH product distributions may be reasonably interpreted from the nature of the intermediate structure and lifetime after the 2A[sup ′]–1A[sup ′] surface transition. The distinct product distributions between the Ca(4 [sup 1]P[sub 1]) and Mg(3 [sup 1]P[sub 1]) reactions with H[sub 2] may also be realized with the aid of the PES calculations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We investigate recently published methods for extending density functional theory to the description of long-range dispersive interactions. In all schemes an empirical correction consisting of a C[sub 6]r[sup -6] term is introduced that is damped at short range. The coefficient C[sub 6] is calculated either from average molecular or atomic polarizabilities. We calculate geometry-dependent interaction energy profiles for the water benzene cluster and compare the results with second-order Møller–Plesset calculations. Our results indicate that the use of the B3LYP functional in combination with an appropriate mixing rule and damping function is recommended for the interaction of water with aromatics. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The rotationally resolved LIF spectra of four different isotopomers of the phenol-nitrogen cluster have been measured to elucidate the structural parameters of the cluster in ground and electronically excited (S[sub 1]) state. The fit of the rotational constants has been performed by a genetic algorithm and by an assigned fit to the line frequencies. The results of both methods are compared. The intermolecular structures are fit to the inertial parameters and are compared to the results of ab initio calculations for both states. This fit was performed under the restriction that the geometry of the monomer moieties do not change upon complexation. Of the remaining five intermolecular parameters two dihedral angles were fixed due to the planarity of the complex, which was inferred from the inertial defects of all isotopomers. The distance of the nearest nitrogen atom to the hydrogen atom of the phenolic hydroxy group is found to decrease upon electronic excitation of the chromophore considerably more than predicted from ab initio calculations. This deviation between theory and experiment can be traced back to the absence of electron–electron correlation in the performed complete active space self-consistent field calculations. The shortening of the OH...NN “hydrogen” bond upon electronic excitation is in agreement with the increased dipole moment of phenol in the S[sub 1]-state. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photoinduced reactions of the complexes Mg[sup +]–SCNC[sub 2]H[sub 5] and Mg[sup +]–NCSC[sub 2]H[sub 5] are studied comparatively in the spectral range of 230–440 nm. One-photon excitation of the complexes through the Mg[sup +] chromophore (3 [sup 2]P←3 [sup 2]S) gives rise to the evaporative fragment as well as the molecular activation and charge transfer products. The action spectra of the complexes consist of three broad peaks for Mg[sup +]–SCNC[sub 2]H[sub 5] and two for Mg[sup +]–NCSC[sub 2]H[sub 5], which accord with the structures obtained from quantum mechanics calculations. These calculations reveal two association isomers for Mg[sup +]–SCNC[sub 2]H[sub 5]: one is with Mg[sup +] being linked to the S atom and the other to the N atom. The former is more stable than the latter by only 0.23 eV. Both of the isomers have been shown to exist in the complex source employed in our experiments. On the other hand, only one stable structure is found for the complex Mg[sup +]–NCSC[sub 2]H[sub 5] characterized by the Mg[sup +]–N linkage. In general, the photofragments are dominated by Mg[sup +] at λ>400 nm, which decreases with decreasing wavelength accompanied by the increase in other photoproducts. In addition, the branching ratios of Mg[sup +] to other photoproducts are nearly constant in the short wavelength region but decrease with decreasing wavelength. The observed photoreactions have been reasonably explained. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) calculations are performed to study Cu[sub 2]O[sub x] (x=1–4) clusters in their neutral, anionic and cationic states. The ground state structures are obtained and found to exhibit linear or near linear structures, which are different from the two- or three-dimensional ones suggested by the previous theoretical calculations. The calculated electron affinities of the clusters are in good agreement with the experimental ones. The low-lying excited states for the clusters are calculated using time-dependent DFT and used to assign the features in the photoelectron spectra. Our results compare well with the available experimental data. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The interdiffusion of a solvent into a polymer melt has been studied using large scale molecular dynamics and Monte Carlo simulation techniques. The solvent concentration profile and weight gain by the polymer have been measured as a function of time. The weight gain is found to scale as t[sup 1/2], which is expected for Fickian diffusion. The concentration profiles are fit very well assuming Fick’s second law with a constant diffusivity. The diffusivity found from fitting Fick’s second law is found to be independent of time and equal to the self-diffusion constant in the dilute solvent limit. We separately calculated the diffusivity as a function of concentration using the Darken equation and found that the diffusivity is essentially constant for the concentration range relevant for interdiffusion. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The spectrum of water can be considered as the juxtaposition of the spectra of two molecules, with different total nuclear spin: ortho–H[sub 2]O, and para–H[sub 2]O. No transitions have ever been observed between the two different nuclear-spin isotopomers. The interconversion time is unknown and it is widely assumed that interconversion is forbidden without some other intervention. However, weak nuclear spin–rotation interaction occurs and can drive ortho to para transitions. Ab initio calculations show that the hyperfine nuclear spin–rotational coupling constants are about 30 kHz. These constants are used to explore the whole vibration–rotation spectrum with special emphasis on the coupling between nearby levels. Predictions are made for different spectral regions where the strongest transitions between ortho and para levels of water could be experimentally observed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Electric-field-induced electronic state g/u mixing of nearly isoenergetic rovibrational levels of the E0[sub g][sup +]([sup 3]P[sub 2]) and D0[sub u][sup +]([sup 3]P[sub 2]) ion-pair states of I[sub 2] has been observed using optical triple resonance combined with resonance ionization. Detectable mixing with applied fields of 1 kV/cm occurs over a range of energy level separations of ≤0.3 cm-1. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A double-quantum homonuclear correlation nuclear magnetic resonance experiment for dipolar-coupled half-integer quadrupolar nuclei in solids is presented. The experiment is based on rotary resonance dipolar recoupling and uses bracketed spin-lock pulses to excite double-quantum coherence and later to convert it to the zero-quantum one. A central-transition-selective π pulse at the beginning of the t[sub 1] evolution period differentiates coherence transfer pathways of double-quantum coherences arising from coupled spins and from a single spin, so that the latter can be efficiently filtered out by phase cycling. The experiment was tested on an aluminophosphate molecular sieve AlPO[sub 4]-14, a material with a variety of aluminum quadrupolar coupling constants, isotropic chemical shifts and homonuclear distances. In a two-dimensional spectrum aluminum dipolar couplings with internuclear distances between 2.9 and 5.5 Å were resolved. Although the experiment requires an application of weak radio-frequency fields, frequency offsets did not affect its performance crucially. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Metropolis Monte Carlo simulations are used to investigate the wetting of chemically nanopatterned surfaces, for the case of hexagonal disk patterns where liquid wishes to wet high-energy circular patches but not wet the background surface. We calculate the density profiles of saturated liquid adsorbed on a variety of such substrates, spanning the nanoscale to atomic scale patterns. In addition, statistical mechanical sum rules are used to obtain interfacial order parameters and interfacial free energies. We observe that Cassie’s law is typically obeyed, together with an associated breakdown of the mechanical interpretation of Young’s equation, for pattern wavelengths greater than 15 molecular diameters. Here, the adsorbed fluid exists as an array of hemi-drops. At about half this wavelength, the breakdown of Cassie’s law lies within realistic energy scales and is associated with the unbending of the outer surface of adsorbed films. For atomic scale patterns, the usual interpretation of Young’s equation is restored for films thicker than one monolayer. At high chemical contrast, when the monolayer in contact with high-energy regions would prefer to be crystalline, we observe a variety of exotic interfacial phenomena that may have technological significance. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a nonadiabatic calculation, within the hyperspherical adiabatic approach, for the ground state energy of the alkali–metal negative ions. An application to the sodium negative ion (Na[sup -]) is considered. This system is treated as a two-electron problem in which a model potential is used for the interaction between the Na[sup +] core and the valence electrons. Potential curves and nonadiabatic couplings are obtained by a direct numerical calculation, as well as the channel functions. An analysis of convergence is made and comparisons of the electron affinity with results of prior work of other authors are given.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

As it was shown from molecular dynamics of two-dimensional hard needles [Phys. Rev. E 66, 012201 (2002)], the uniaxial velocity autocorrelation function (VACF) of this system exhibits a two time scale character. This corresponds to the symmetry of the particles. In this paper we provide a theory of the Enskog type that corroborates the idea that the VACF can be successfully described as a sum of two single decays. From the comparison between molecular dynamics and theoretical results, we show that the Enskog approach is a satisfactory kinetic theory, which functions as well in the nematic as in the isotropic phase. Different properties of VACFs have been investigated, in particular, the dependence on the orientational order, temperature, and particles’ inertness. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Protein crystallization conditions are usually identified by empirical screening methods because of the complexity of the process, such as the existence of nonequilibrium phases and the different crystal forms that may result from changes in solution conditions. Here the crystallization of a model protein is studied using computer simulation. The model consists of spheres that have both an isotropic interaction of short range and anisotropic interactions between patch–antipatch pairs. The free energy of a protein crystal is calculated using expanded ensemble simulations of the Einstein crystal, and NpT–Monte Carlo simulations with histogram reweighting are used to determine the fluid–solid coexistence. The histogram reweighting method is also used to trace out the complete coexistence curve, including multiple crystal phases, with varying reduced temperature, which corresponds to changing solution conditions. At a patch–antipatch interaction strength five times that of the isotropic interaction, the protein molecules form a stable simple cubic structure near room temperature, whereas an orientationally disordered face-centered-cubic structure is favored at higher temperatures. The anisotropic attractions also lead to a weak first-order transition between orientationally disordered and ordered face-centered-cubic structures at low temperature, although this transition is metastable. A complete phase diagram, including a fluid phase, three solid phases, and two triple points, is found for the six-patch protein model. A 12-patch protein model, consistent with the face-centered-cubic structure, leads to greater thermodynamic stability of the ordered phase. Metastable liquid–liquid phase equilibria for isotropic models with varying attraction tails are also predicted from Gibbs ensemble simulations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Unimolecular evaporation of weakly bound clusters made of rigid molecules is considered from the points of view of statistical theories and molecular dynamics simulations. We explicitly work out expressions for the kinetic energy released and product angular momentum distributions within the sphere+sphere and sphere+linear rigid body assumptions of phase space theory (PST). Various approximations are investigated, including the shape of the interaction potential between the two fragments and the anharmonicity of the vibrational density of states. The comparison between phase space theory and simulation for nitrogen and methane clusters shows a quantitative agreement, thereby suggesting that PST is accurate in predicting statistical observables in a wide range of systems under various physical conditions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The stress–strain relations and the yield behavior of a model glass (a 80:20 binary Lennard-Jones mixture) [W. Kob and H. C. Andersen, Phys. Rev. E 52, 4134 (1995)] is studied by means of molecular dynamics simulations. In a previous paper [F. Varnik, L. Bocquet, J.-L. Barrat, and L. Berthier, Phys. Rev. Lett. 90, 095702 (2003)] it was shown that, at temperatures below the glass transition temperature, T[sub g], the model exhibits shear banding under imposed shear. It was also suggested that this behavior is closely related to the existence of a (static) yield stress (under applied stress, the system does not flow until the stress σ exceeds a threshold value σ[sub y]). A thorough analysis of the static yield stress is presented via simulations under imposed stress. Furthermore, using steady shear simulations, the effect of physical aging, shear rate and temperature on the stress–strain relation is investigated. In particular, we find that the stress at the yield point (the “peak”-value of the stress–strain curve) exhibits a logarithmic dependence both on the imposed shear rate and on the “age” of the system in qualitative agreement with experiments on amorphous polymers [C. Ho Huu and T. Vu-Khanh, Theoretical and Applied Fracture Mechanics 40, 75 (2003); L. E. Govaert, H. G. H. van Melick, and H. E. H. Meijer, Polymer 42, 1271 (2001)] and on metallic glasses [W. L. Johnson, J. Lu, and M. D. Demetriou, Intermetallics 10, 1039 (2002)]. In addition to the very observation of the yield stress which is an important feature seen in experiments on complex systems like pastes, dense colloidal suspensions [F. Da Cruz, F. Chevoir, D. Bonn, and P. Coussot, Phys. Rev. E 66, 051305 (2002)] and foams [G. Debrégeas, H. Tabuteau, and J.-M. di Meglio, Phys. Rev. Lett. 87, 178305 (2001)], further links between our model and soft glassy materials are found. An example is the existence of hysteresis loops in the system response to a varying imposed stress. Finally, we measure the static yield stress for our model and study its dependence on temperature. We find that for temperatures far below the mode coupling critical temperature of the model (T[sub c]=0.435 in Lennard-Jones units), σ[sub y] decreases slowly upon heating followed by a stronger decrease as T[sub c] is approached. We discuss the reliability of results on the static yield stress and give a criterion for its validity in terms of the time scales relevant to the problem. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new approach for the analysis of intermolecular interactions in a solution is proposed. The changes in the interaction energy components due to the solvent effects are estimated on the basis of the interaction energy calculated in the presence of the electric field induced in a polarizable medium, or in the field of the effective fragment potentials. Obtained results indicate a significant increase in stabilization resulting from electrostatic interactions as a result of the cooperative interactions between interacting subsystems and solvent molecules. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Fluctuations of pressure and internal energy in a low-temperature liquid, generated by structure fluctuations, are related in a general and straightforward way to the differences between the long- and short-time thermodynamic susceptibilities. The relations are similar to, but not identical with, those between thermodynamic fluctuations and susceptibilities in classical statistical mechanics. The derivation is general and straightforward, and based on the assumption that the local structure is a slow-relaxing mode in low-temperature liquids and may be characterized by an order parameter. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a dynamical description of slow relaxation processes based on the extension of Onsager’s fluctuation theory to systems in local quasi-equilibrium. A non-Markovian Fokker–Planck equation for the conditional probability density is derived, and from it we obtain the relaxation equation for the moments. We show that the fluctuation-dissipation theorem can be formulated in terms of the temperature of the system at local quasi-equilibrium which is related to that of the bath by means of a scaling factor revealing lack of thermal equilibrium. Our theory may be applied to a wide variety of systems undergoing slow relaxation. We discuss in particular slow dynamics in glassy systems and Brownian motion in a granular gas. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photoinduced electron transfer (ET) between coumarin dyes and aromatic amines has been investigated in Triton-X-100 micellar solutions and the results have been compared with those observed earlier in homogeneous medium. Significant static quenching of the coumarin fluorescence due to the presence of high concentration of amines around the coumarin fluorophore in the micelles has been observed in steady-state fluorescence studies. Time-resolved studies with nanosecond resolutions mostly show the dynamic part of the quenching for the excited coumarin dyes by the amine quenchers. A correlation of the quenching rate constants, estimated from the time-resolved measurements, with the free energy changes (ΔG[sup 0]) of the ET reactions shows the typical bell shaped curve as predicted by Marcus outer-sphere ET theory. The inversion in the ET rates for the present systems occurs at an exergonicity (-ΔG[sup 0]) of ∼0.7–0.8 eV, which is unusually low considering the polarity of the Palisade layer of the micelles where the reactants reside. Present results have been rationalized on the basis of the two dimensional ET model assuming that the solvent relaxation in micellar media is much slower than the rate of the ET process. Detailed analysis of the experimental data shows that the diffusional model of the bimolecular quenching kinetics is not applicable for the ET reactions in the micellar solutions. In the present systems, the reactions can be better visualized as equivalent to intramolecular electron transfer processes, with statistical distribution of the donors and acceptors in the micelles. A low electron coupling (V[sub el]) parameter is estimated from the correlation of the experimentally observed and the theoretically calculated ET rates, which indicates that the average donor–acceptor separation in the micellar ET reactions is substantially larger than for the donor–acceptor contact distance. Comparison of the V[sub el] values in the micellar solution and in the donor–acceptor close contact suggests that there is an intervention of a surfactant chain between the interacting donor and acceptor in the micellar ET reaction. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The phase equilibrium of mixtures of Yukawa and charged Yukawa particles is studied by means of Gibbs ensemble Monte Carlo (GEMC) simulation method and the mean spherical approximation (MSA). The strength of the Coulomb energy compared to that of the Yukawa attraction is characterized by a coupling constant. For low coupling constants a classical vapor–liquid phase separation appears with a good agreement between GEMC and the MSA. For high coupling constant, a phase separation between a salt poor and a salt rich phase occurs that resembles the phase equilibrium behavior of the solvent primitive model. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A joint action of the Glauber single-spin-flip and the Kawasaki spin-exchange mechanisms upon the processes of phase transformation is examined in the framework of the one-dimensional kinetic Ising model. It is shown that the addition of the Kawasaki dynamics to that of Glauber accelerates the process of phase transformation in the initial stage, but slows it down in later stages. For the truncated form of Glauber dynamics, which excludes the processes of splitting and coagulation of clusters, the addition of the Kawasaki dynamics always accelerates the phase transformation process. Acting alone, the Kawasaki mechanism provides a cluster growth proportional to t[sup 1/2] (where t is the time) in the initial stage and proportional to t[sup 1/3] (Lifshitz–Slyozov–Wagner law) in the intermediate stage. In the final stage, a cluster size approaches exponentially its equilibrium value. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ab initio calculations of the melting properties of copper in the pressure range 0–100 GPa are reported. The ab initio total energies and ionic forces of systems representing solid and liquid copper are calculated using the projector augmented wave implementation of density functional theory with the generalized gradient approximation for exchange-correlation energy. An initial approximation to the melting curve is obtained using an empirical reference system based on the embedded-atom model, points on the curve being determined by simulations in which solid and liquid coexist. The approximate melting curve so obtained is corrected using calculated free energy differences between the reference and ab initio system. It is shown that for system-size errors to be rendered negligible in this scheme, careful tuning of the reference system to reproduce ab initio energies is essential. The final melting curve is in satisfactory agreement with extrapolated experimental data available up to 20 GPa, and supports the validity of previous calculations of the melting curve up to 100 GPa. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Determination of absolute surface coverage with sub-monolayer sensitivity is demonstrated using evanescent-wave cavity ring-down spectroscopy (EW-CRDS) and conventional CRDS by employing conservation of the absolute integrated absorption intensity between gas and adsorbed phases. The first C–H stretching overtones of trichloroethylene (TCE), cis-dichloroethylene, and trans-dichloroethylene are probed using the idler of a seeded optical parametric amplifier having a 0.075 cm-1 line width. Polarized absolute adsorbate spectra are obtained by EW-CRDS using a fused-silica monolithic folded resonator having a finesse of 28 500 at 6050 cm-1, while absolute absorption cross sections for the gas-phase species are determined by conventional CRDS. A measure of the average transition moment orientation on the surface, which is utilized for the coverage determination, is derived from the polarization anisotropy of the surface spectra. Coverage measurement by EW-CRDS is compared to a mass-spectrometer-based surface-uptake technique, which we also employ for coverage measurements of TCE on thermally grown SiO[sub 2] surfaces. To assess the potential for environmental sensing, we also compare EW-CRDS to optical waveguide techniques developed previously for TCE detection. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this paper we investigate the influence of self-affine roughness on the friction coefficient μ[sub f] of a rubber body under incomplete contact onto a solid surface. The roughness is characterized by the rms amplitude w, the correlation length ξ, and the roughness exponent H. It is shown that with increasing surface roughening at short and/or long length scales (decreasing H and/or increasing ratio w/ξ, respectively), the maximum of the friction coefficient μ[sub f] shifts to lower sliding velocities. The latter occurs only for conditions of incomplete contact for small contact length scales λ (<ξ). In all cases, the friction coefficient μ[sub f] increases monotonically with decreasing roughness exponent H and/or increasing roughness ratio w/ξ and attains its maximum value for sufficiently large contact length scales (>ξ). © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Argon adsorption (77 K) in atomistic silica nanopores of various sizes and shapes has been studied by means of grand canonical Monte Carlo simulations (GCMC). We discuss the effects of confinement (pore size), pore morphology (ellipsoidal, hexagonal, constricted pore), and surface texture (rough/smooth) on the thickness variation of the adsorbed film with pressure onto the disordered inner surface of porous materials (usually called t-plot or t-curve). We show that no confinement effect occurs when the diameter of the regular cylindrical pore is larger than 10 nm. For pores smaller than 6 nm, we find that the film thickness increases as the pore size decreases. We show that the adsorption isotherm in the rough pore can be described as the sum of an adsorbed amount similar to that found for a smooth pore (of the same radius) and a constant contribution due to atoms “trapped” in the infractuosities of the rough surface which act as a microporous texture. Simulation snapshots for Ar adsorption in hexagonal and ellipsoidal smooth pores indicate that at low pressures the gas/adsorbate interface retains memory of the pore shape and becomes cylindrical prior to the capillary condensation of the fluid in the pore. The film thickness in the hexagonal pore is close to that obtained for a cylindrical pore having a similar dimension. By contrast, we find that the film thickness for an ellipsoidal pore is always larger than that for an equivalent cylindrical pore (having the same length and volume but a circular section). We show that this effect strengthens as the pore size decreases and/or the pore asymmetry increases. Ar adsorption in a cylindrical constricted pore shows that the presence of the narrower part considerably modifies the adsorption mechanism. Finally, we report GCMC simulations of Ar adsorption (77 K) on a plane silica reference substrate for different intermolecular potentials. We discuss the effect of the interaction on the shape of the adsorption isotherm and compare our results with experiments. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The planar interface between two liquids having two degrees of affinity to mix has been studied by molecular dynamics simulations. The surface tension is calculated from the normal, P[sub N], and transverse, P[sub T], components of the pressure tensor P for a wide range of temperatures. An unusual increase in surface tension with increasing temperature is attributed to a pressure induced void transfer mechanism that is justified by basic thermodynamic arguments. This effect is diminished on the addition of a modest attractive potential between the two species, and there is a turnover point at higher temperatures beyond which the surface tension decreases with increasing temperature. An order parameter is identified as the gradient of the mole fraction distribution through the interfacial region. An additional effect is the dramatic inversion of the kinetic and potential contributions to the P[sub N] profile as the temperature is varied. It is found that a commonly used approximation for P, the Irving–Kirkwood 1 or IK1 method, results in a relatively modest unphysical variability in P[sub N] that weakly violates the condition of local mechanical stability. However, this artifact does not prevent the IK1 method from producing an interfacial tension which is nearly identical to that derived from the complete IK formula with no additional approximations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

High-dimensional, wave packet calculations have been carried out to model the surface temperature dependence of rovibrationally inelastic scattering and dissociation of hydrogen molecules from the Cu(111) surface. Both the molecule and the vibrating surface are treated fully quantum-mechanically. It is found, in agreement with experimental data, that the surface temperature dependence of a variety of dynamical processes has an Arrhenius form with an activation energy dependent on molecular translational energy and on the initial and final molecular states. The activation energy increases linearly with decreasing translational energy below the threshold energy. Above threshold the behavior is more complex. A quasianalytical model is proposed that faithfully reproduces the Arrhenius law and the translational energy dependence of the activation energy. In this model, it is essential to include quantized energy transfer between the surface and the molecule. It further predicts that for any process characterized by a large energy barrier and multiphonon excitation, the linear change in activation energy up to threshold has slope-1. This explains successfully the universal nature of the unit slope found experimentally for H[sub 2] and D[sub 2] dissociation on Cu. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The equilibrium and fluctuation methods for determining the surface tension, σ, and bending modulus, κ, of a bilayer membrane with a fixed projected area are discussed. In the fluctuation method the elastic coefficients σ and κ are measured from the amplitude of thermal fluctuations of the planar membrane, while in the equilibrium method the free energy required to deform the membrane is considered. The latter approach is used to derive new expressions for σ and κ (as well as for the saddle-splay modulus), which relate them to the pair-interactions between the amphiphiles forming the membrane. We use linear response theory to argue that the two routes lead to similar values for σ and κ. This argument is confirmed by Monte Carlo simulations of a model membrane whose elastic coefficients are calculated using both methods. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Point polarizable molecules at fixed spatial positions have solvable electrostatic properties in classical approximation, the most familiar being the Clausius–Mossotti (CM) formula. This paper generalizes the model and imagines various applications to nanosystems. The behavior is worked out for a sequence of octahedral fragments of simple cubic crystals, and the crossover to the bulk CM law is found. Some relations to fixed moment systems are discussed and exploited. The one-dimensional dipole stack is introduced as an important model system. The energy of interaction of parallel stacks is worked out, and clarifies the diverse behavior found in different crystal structures. It also suggests patterns of self-organization which polar molecules in solution might adopt. A sum rule on the stack interaction is found and tested. Stability of polarized states under thermal fluctuations is discussed, using the one-dimensional domain wall as an example. Possible structures for polar hard ellipsoids are considered. An idea is formulated for enhancing polarity of nanosystems by intentionally adding metallic coatings. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

NaCl hillocks have been grown on the NaCl(100) monocrystalline surface by simply making the tip of an atomic force microscope cantilever interact with the surface. A quantitative discussion about the hillock dissolution process as well as physical stability of these ionic surfaces has been made. Molecular dynamics simulations helped us to interpret the experimental data. An explanation for the hillock formation and dissolution phenomena is also discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Swelling and deswelling kinetics was investigated for three types of cylindrical poly(N-isopropylacrylamide) (PNIPA) gels differing in crosslink density. The temperature dependence curves of the volume of the gel specimens were different from one another. One of the gel specimens was considered as a critical gel showing the continuous volume phase transition. The volume change process of the specimens after a temperature jump was examined. In the deswelling processes with temperature jumps to temperatures higher than 35 °C, a phase separation was observed in the gel specimens and the volume change slowed down due to the homogenization after the phase separation. The value of the diffusion constant obtained without the phase separation decreased rapidly as temperature approaches the transition temperature. The rapid decrease for the critical gel indicates the emergence of the critical slowing-down. The value of the critical exponent for the correlation length suggests that the universality class for the volume phase transition of the critical PNIPA gel belongs to the class for the classical theory. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]