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Controlling molecular reactivity by shaped laser pulses is a long-standing goal in chemistry. Here, we suggest a direct optimal control approach that combines external pulse optimization with other control parameters arising in the upcoming field of vibro-polaritonic chemistry for enhanced controllability. The direct optimal control approach is characterized by a simultaneous simulation and optimization paradigm, meaning that the equations of motion are discretized and converted into a set of holonomic constraints for a nonlinear optimization problem given by the control functional. Compared with indirect optimal control, this procedure offers great flexibility, such as final time or Hamiltonian parameter optimization. A simultaneous direct optimal control theory will be applied to a model system describing H-atom transfer in a lossy Fabry–Pérot cavity under vibrational strong coupling conditions. Specifically, optimization of the cavity coupling strength and, thus, of the control landscape will be demonstrated. [ABSTRACT FROM AUTHOR]

We present a quantum-mechanical tier model for vibrational relaxation of low-lying excited states of an adsorbate vibrational mode (system), coupled to surface phonons (bath), at zero temperature. The tier model, widely used in studies of intramolecular vibrational energy redistribution in polyatomics, is adapted here to adsorbate-surface systems with the help of an embedded cluster approach, using orthogonal coordinates for the system and bath modes, and a phononic expansion of their interaction. The key idea of the model is to organize the system-bath zeroth-order vibrational space into a hierarchical structure of vibrational tiers and keep therein only vibrational states that are sequentially generated from the system-bath initial vibrational state. Each tier is generated from the previous one by means of a successor operator, derived from the system-bath interaction Hamiltonian. This sequential procedure leads to a drastic reduction of the dimension of the system-bath vibrational space. We notably show that for harmonic vibrational motion of the system and linear system-bath couplings in the system coordinate, the dimension of the tier-model vibrational basis scales as ∼Nl×υ. Here, N is the number of bath modes, l is the highest-order of the phononic expansion, and υ is the size of the system vibrational basis. This polynomial scaling is computationally far superior to the exponential scaling of the original zeroth-order vibrational basis, ∼MN, with M being the number of basis functions per bath mode. In addition, since each tier is coupled only to its adjacent neighbors, the matrix representation of the system-bath Hamiltonian in this new vibrational basis has a symmetric block-tridiagonal form, with each block being very sparse. This favors the combination of the tier-model with iterative Krylov techniques, such as the Lanczos algorithm, to solve the time-dependent Schrödinger equation for the full Hamiltonian. To illustrate the method, we study vibrational relaxation of a D–Si bending mode, coupled via two- and (mainly) one-phonon interactions to a fully D-covered Si(100)-(2 × 1) surface, using a recent first-principles system-bath Hamiltonian. The results of the tier model are compared with those obtained by the Lindblad formalism of the reduced density matrix. We find that the tier model provides much more information and insight into mechanisms of vibration-phonon couplings at surfaces, and gives more reliable estimates of the adsorbate vibrational lifetimes. Moreover, the tier model might also serve as a benchmark for other approximate quantum-dynamics methods, such as multiconfiguration wavefunction approaches. [ABSTRACT FROM AUTHOR]

Based on a concise review of the experimental data, a theory of the structural, kinetic, and dynamic features of the Fischer cluster is put forward. This theory is based on the idea of heterophase fluctuations of glass-forming liquids. According to these ideas in a one-component liquid the solidlike and fluidlike transient associations of molecules (fluctuons) are formed. The fluctuons differ in the short-range order and, consequently, in the free energy. They form a random free energy landscape. The random field free energy functional of the Ginzburg–Landau type, which accounts for the free energy landscape, is deduced. Based on this, the phase equilibrium, critical behavior, and ordering phenomena of the heterophase liquid are considered. It is shown that in the critical region an effective attractive interaction of the fluctuons results in their gravitation and formation of correlated fractal aggregations. The aggregation consists of the fluctuons of similar short-range order. The fractal aggregation formation is a special structural state of the liquid above the glass transition temperature. The condition at which the aggregations are formed is formulated. Thus it is shown that the Fischer cluster is an array of fractal aggregations of fluctuons. The growth kinetics and dynamics of the Fischer cluster are described. The experimental data are analyzed and discussed using the theory that is put forward. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The high frequency part of the depolarized light scattering susceptibility has been measured for ortho-terphenyl (OTP) in a broad temperature and pressure range and analyzed using the mode-coupling theory (MCT). We found that the time-temperature-pressure superposition does not work for OTP in the frequency range of the susceptibility minimum and both a and b exponents of the MCT are temperature and pressure dependent. The pressure induced shift of the position of the minimum is much weaker than that of the α-peak resulting in dT[SUBc]/dP = 8.5 K/kbar while dT[SUBg]/dP = 26 K/kbar. Thus any universal scaling of the entire susceptibility including both the α-peak and the MCT minimum is not possible. These results are in contradiction to the previously reported conclusions obtained from the analysis of incoherent neutron scattering data [A. Tölle et al., Phys. Rev. Lett. 80, 2374 (1998)]. [ABSTRACT FROM AUTHOR]

A microscopic theory based upon the model of molecular defect volume fluctuations is developed to provide an interpretation of the ultraslow mode and excess static light scattering intensity observed in supercooled glass-forming liquids. By including the defect volume as a variable in the set of slow variables comprising of the number density and momentum density, it is shown that in an one component viscoelastic fluid, a diffusive mode associated with the fluctuations of defect volume is present, in addition to the usual q independent structural relaxation mode associated with density fluctuations. Fluctuations of defect volume arise from the configurational rearrangement of molecules. A two fluid model previously proposed for interpreting the anomalous quasielastic light scattering results in an one component fluid follows naturally from the present model. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have studied the temperature dependence of dielectric relaxation times in terms of the peak frequency fmax(T) of dielectric loss ε″(ω) and the dc-conductivity σdc(T) of several glass-forming liquids, covering 12 decades in the peak frequency fmax and 9 decades in σdc. Although dc-conductivity samples the mobility of ionic tracers, its variation with temperature is similar to that of fmax(T). The fmax(T) and σdc(T) are analyzed using the temperature-derivative method and compared to the viscosity data η-1(T). While most liquids reveal a common Vogel–Fulcher–Tammann (VFT) behavior for fmax, σdc and η-1 in an extended temperature range T≥Tm, some liquids deviate from this behavior by displaying a crossover at T=TA to an Arrhenius regime. In these cases the quantity fmax(T) decouples from the common curves for σdc(T) and η-1(T) and attains activation energies in excess (∼40% for alcohols) of those related to translational processes. For many samples a departure from the VFT behavior occurs at lower temperatures TB

We have measured the dielectric relaxation times of the α process of phenyl salicylate (salol) covering 11 decades in frequency. Being representative for the class of low molecular weight organic glass forming materials, the highly resolved temperature dependence of the dynamics in salol does not follow a particular function like the Vogel–Fulcher–Tammann (VFT) law over the entire accessible range of temperatures. In order to conduct a detailed and unambiguous analysis of the temperature dependence, we take advantage of the derivatives of the experimental log(fmax) values with respect to temperature, which allow us to either linearize the frequently used temperature laws or to resolve subtle changes in fmax(T) by decreasing the number of free parameters. In this manner we observe that none of the common routes for rationalizing the dynamics like Arrhenius, VFT, Souletie scaling, and idealized mode-coupling theory account for the experimental findings properly. However, we do observe a VFT behavior within the limits 265 K≤T≤320 K, i.e., for temperatures ranging from significantly above the glass transition at Tg=220 K to far above the melting point. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In polymer blends the Flory χF parameter, describing the short range interactions between chain segments, is frequently represented by a structureless point-like interaction. However, in the mean field approximation, the influence of this parameter is directly enhanced by the degree of polymerization of the chains. This large enhancement is also effective with regard to the short range spatial structure of χF(r) and makes it possible to detect these structural details from a small angle neutron scattering experiment. We present a new way of representing the neutron data so as to explicitly determine these details and we demonstrate the method for the system polytetramethyl carbonate/deuterated polystyrene. The temperature dependence displayed by this system appears to be anomalous within the mean field approximation. However, we show that it can be successfully interpreted in terms of a structural model for χF(r) which consists of spatially separated positive and negative contributions within a 1–10 Å range. This form is supported by examining the relation of χF to the pseudopotential interactions between the segments. In the experimental part of this work we also compare our model with SANS data from two other systems (a) polyvinylmethyl ether/deuterated polystyrene, and (b) polyparamethyl styrene/deuterated polystyrene. The last two show a ‘‘normal’’ behavior and are consistent with a relatively structureless χF parameter. [ABSTRACT FROM AUTHOR]

We report light scattering measurements of the static structure factor and mutual diffusion coefficient for the compatible polymer mixture poly (phenylmethylsiloxane)/poly(styrene). The measurements were made for a series of molecular weights of the poly(styrene) of 2000, 4000, and 9000 for different volume fractions of the poly(styrene) over a range of temperatures from 350 to 520 K. The principal aim was to try and distinguish between the various theoretical approaches which have appeared in the literature and which give radically different results for the concentration dependence of the mutual diffusion coefficient in a binary polymer mixture. Since the static structure factor was measured in the same light scattering experiment as the diffusion coefficient we have also been able to obtain the concentration, molecular weight, and temperature dependence of the Onsager coefficient directly from experiment. A concentration-dependent glass transition temperature complicates the comparison with theory and we propose a method for the data analysis to deal with this problem. Our results are consistent with the expression for diffusion coefficient based on the dynamic random phase approximation. [ABSTRACT FROM AUTHOR]

Correct relations between the dynamic light scattering spectrum and the mechanical creep compliance and longitudinal stress modulus of a viscoelastic liquid are provided. Analysis of the effect of temperature fluctuations on the longitudinal stress modulus is also given. The result provides a basis for determination of mechanical relaxation spectra using the dynamic light scattering technique. [ABSTRACT FROM AUTHOR]

Depolarized Rayleigh scattering (DRS) and dielectric spectroscopy (DS) are employed to study the solvent and polymer dynamics in the polystyrene (PS)/toluene system and for PS concentration up to 0.6 g/ml. The DRS measurements were made in the temperature range from 255 to 373 K with different interferometers making possible the separation of the ‘‘fast’’ solvent (in the ps time scale) from the ‘‘slow’’ polymer dynamics (in the ns time scale). The DS measurements were made in the temperature range from 128 to 310 K and two processes could also be observed. With DRS and DS we investigate the very different mobilities in the PS/toluene solutions over a frequency range of ∼10 decades. The prominent features of the experimental data are (i) the existence of two primary (α-) relaxations in the macroscopically homogeneous solutions which have been analyzed using both techniques and (ii) that these relaxations reflect the polymer and solvent dynamics and display the usual Vogel–Fulcher–Tammann behavior, however, with a broad distribution of relaxation times, especially at low temperatures. The existence of two time scales in the solutions is discussed in terms of concentration heterogeneities composed of PS-rich and toluene-rich domains, large enough to exhibit two glass transitions. As a result of the strong temperature dependence for the fast solvent reorientation times the rate of change of solvent friction with polymer concentration becomes temperature dependent at low temperatures. [ABSTRACT FROM AUTHOR]

We report static and dynamic light scattering experiments of an almost symmetric polymer mixture made up from poly(dimethylsiloxane) (PDMS), N=260, and poly(ethylmethylsiloxane) (PEMS), N=340, with N being the degree of polymerization, both below the entanglement molecular weights. The mixture exhibits an upper critical solution temperature Tc[bar_over_tilde:_approx._equal_to]57 °C. The measurements were performed at the critical composition [lowercase_phi_synonym]c,PEMS = 0.465 in a broad temperature range in the one phase region above the spinodal point. The main results for the static case are: the temperature dependence of the static structure factor S(q=0) can be described by a mean field behavior. For T close to Tc, a crossover to an Ising behavior is observed according to a modified Ginzburg criterion. From the angular dependence of S(q), the static correlation length ξ is determined via an Ornstein–Zernike plot. Our experimentally determined values for limT→∞S(0) and limT→∞ ξ, respectively, are in agreement with theoretical predictions. For the dynamic case, the main results are summarized as follows: as expected, the mutual diffusion coefficient D, accessible by quasielastic light scattering, shows a critical slowing down for T→Tc.For qξ≥1, we observe that the q scaling of the Rayleigh linewidth Γ changes from a q2 to a q3 behavior, which is in agreement with mode coupled expressions. This occurs in a relatively broad temperature range, due mainly to the fact that polymer mixtures exhibit a larger ξ0 ∝[Square_Root]N, on the contrary to any other systems known, which allows us therefore to reach the region qξ≥1 even with light scattering easily. From the separation of the measured Rayleigh linewidth into a critical part and a background part, we have estimated the crossover between mode coupled to nonmode coupled dynamics.... [ABSTRACT FROM AUTHOR]

Depolarized Rayleigh spectra of ortho-terphenyl (OTP) were measured in the temperature range from Tg to Tg+190 K. Two samples prepared with and without ‘‘clusters’’ were used for the measurement. Four different Fabry–Perot interferometers covering the time range from about 0.5 ps to 100 ns were employed. Two relaxation modes were observed: a slow mode with all its characteristics of the α process, and a fast mode with a constant, temperature independent relaxation time of about 3 ps. The fast mode has not been reported heretofore. The intensity of the fast mode vanishes at about the Vogel–Fulcher–Tamman temperature T0. The relaxation times of these two processes are found to be identical for both OTP with and without clusters. The temperature dependence of the relaxation time of the α process is Arrhenius at high temperatures, but shows a Williams–Landel–Ferry (WLF) behavior in the range from Tg to Tg+80 K for both samples with and without the long-range density fluctuations (i.e., cluster). [ABSTRACT FROM AUTHOR]

This report presents measurements of the frequency dependent dielectric loss ε‘(ω) and quasielastic light scattering of thermal density fluctuations of simple organic glassforming van der Waals liquids BCDE (bis-phenol-C-dimethylether) and BKDE (bis-kresol-C-dimethylether) having glass transition temperatures of Tg=240 K and Tg=261 K, respectively. The dielectric measurements of BCDE were carried out in a temperature range between 250 and 316 K, those of BKDE between 270 and 384 K covering a frequency range from 10-1 to ∼109 Hz in each case. Quasielastic light scattering data were taken between the glass transition temperatures and T[bar_over_tilde:_approx._equal_to]Tg+30 K in a correlation time window of 10-6 s up to 102 s. As main results we have found the following: The relaxation process in the BCDE melt can be described using a Kohlrausch–Williams–Watts (KWW) function with a temperature independent β parameter. Its value was found to be identical either from the light scattering result or from the stretched exponential fit to the Fourier transform of the dielectric loss data.The temperature dependence of the mean relaxation times of the BCDE melt was fitted with a Williams–Landel–Ferry (WLF) equation, or Vogel–Fulcher–Tammann equation, leading to WLF constants close to the universal ones. The BKDE system, however, behaves differently, where the dynamic response in the bulk amorphous state could only be described using a temperature dependent β parameter. This finding was confirmed by either the light scattering method as well as the dielectric relaxation method. Additionally we were able to measure a second dielectrically active process in BCDE only, arising in the high frequency region with an Arrhenius type of activation energy. In the BKDE melt, however, the superposition of different processes causes a temperature dependence of the β parameter when the... [ABSTRACT FROM AUTHOR]

In this paper we report quasielastic light-scattering experiments of the compatible binary mixture of simple viscoelastic liquids made up of o-terphenyl as one component and newly synthesized model materials, 1,1-di(paramethoxyphenyl)-cyclo-hexan and 1,1-di(paramethoxyphenyl, metamethyl)-cyclohexan, as the other components. The measurements were done in a wide temperature range above Tg ([bar_over_tilde:_approx._equal_to]Tg+100 K) of the mixtures and in a volume fraction range up to [lowercase_phi_synonym]=0, 5 of bis-cresol-cyclohexane-dimethylether and bis-phenol-cyclohexane-dimethylether. We have found that the measured dynamics is governed by three effects: At short times we observe the dynamics of the density fluctuations of the system characterized by a broad distribution of relaxation times, which can be scaled to a masterplot taking the Tg of the mixtures into account. At longer times we observe two translational diffusion processes whose decay is single exponential with a characteristic time proportional to q-2, q being the scattering vector. The faster one is attributed to the interdiffusion coefficient of the system. The second, slower mode is related to the motion of dynamic clusters which have their origin in a spatial inhomogeneity of the samples. The mean cluster diameter is about 1000 Å, thus in any case the occurrence of the slow mode is accompanied by a strong increase of the elastically scattered intensity towards small scattering angles. This mode also appears in pure components. We relate its intensity to the fluctuations of an order parameter, usually introduced to describe the glassy state. [ABSTRACT FROM AUTHOR]

Using the Kramers–Kronig relation, we have provided a correlation between the longitudinal compliance and the time correlation function of density fluctuations that can be directly measured in a light scattering experiment. Photon correlation spectroscopic experiments have been carried out to test the theory. Good agreement between the dynamic light scattering data and the mechanical dynamic compressibility data has been obtained. [ABSTRACT FROM AUTHOR]

Presents a study which described the design and operation of a high-temperature measuring cell for SQUID magnetometers to determine the magnetic susceptibility and order parameter of nematic liquid crystals. Suitability of the device for precision measurements of magnetic susceptibility; Influence of molecular biaxiality on the determination of the order parameter; Elements of SQUID magnetometry.