Showing papers on "Relaxation (NMR) published in 1992"

Dynamic coupling between stress and composition in polymer solutions and blends

Abstract: Phenomenological hydrodynamic equations are proposed for entangled polymer blends as generalization of those for polymer solutions. They can describe coupling between macroscopic flow and relative diffusion. The key concept we use is the "tube velocity" introduced by Brochard in the problem of mutual diffusion in polymer blends. As applications, (I) we give a general expression for the time-correlation function of the polymer concentration around equilibrium and examine its relaxation in some typical cases. It can be strongly influenced by the viscoelastic effect when the two polymers have different lengths. Our expression can also be used for gelling solutions and explains previous dynamic light scattering experiments at the sol-gel transition. (ii) Detailed calculations are performed for the case of a single rheological relaxation time (the Maxwell model). The steady state structure factor is obtained to Iinear order in macroscopic flow. (iii) We predict that composition inhomogeneity is created in mixtures oflong and short polymers undergoing nonuniform flow. Its origin is that the longer chains support stress more than the shorter ones and the resultant imbalance of stress causes relative motion of the two polymers. These results are applicable both to solutions and blends.

Mapping of the spectral densities of N-H bond motions in eglin c using heteronuclear relaxation experiments.

Abstract: A new strategy is used for studying the internal motions of proteins based on measurements of NMR relaxation parameters. The strategy yields values of the so-called spectral density functions J(omega) for N-H bond vectors. The spectral density functions are related to the distribution of frequencies contained in the rotational (overall and internal) motions of these NH bond vectors. No a priori model assumptions about the dynamics are required in this approach. The method involves measurements of six relaxation parameters consisting of 15N longitudinal relaxation rates, transverse relaxation rates of in-phase and antiphase coherence, the relaxation rates of heteronuclear 1H-15N two-spin order, the heteronuclear 1H-15N nuclear Overhauser effects, and longitudinal relaxation rates of the amide protons. The values of the spectral density functions at the five frequencies 0, omega N, omega H + omega N, omega H, and omega H - omega N are determined from the relaxation parameters using analytical relations derived previously [Peng & Wagner (1992) J. Magn. Reson. 98, 308-332]. Here, the method is applied to characterize the backbone dynamics of the 15N-enriched proteinase inhibitor eglin c, a protein of 70 residues. The values for J(0) and J(omega N = 50 MHz) vary significantly with the amino acid sequence, whereas the spectral densities at higher frequencies, J(450 MHz), J(500 MHz), and J(550 MHz), are typically much smaller and show no significant variation with the sequence. The collective behavior of the J(omega) values indicate greater internal motion for the proteinase binding loop residues and the first eight N-terminal residues. The additional internal motion in these regions is in the rate range below 450 MHz. The values of J(omega) are also compared with root mean square deviations (rmsds) of backbone atoms as obtained in NMR structure determinations. Low values of J(0) and J(omega N) are correlated with high rmsds. Spectral densities at higher frequencies, J(450 MHz), J(500 MHz), and J(550 MHz), are small and show no correlation with rmsds. A comparison with the spectral density functions obtained by fitting the experimental data to the functional dependence of the Lipari and Szabo formalism [Lipari & Szabo (1982a) J. Am. Chem. Soc. 104, 4546-4559] is made.

Interfacial tension between polymer melts measured by shear oscillations of their blends

Abstract: When shear oscillations are performed with melts of polystyrene (PS)/poly(methylmethacrylate) (PMMA) blends the total deformation has a large elastic portion at low frequencies. This finding is caused by the interfacial tension acting between the two phases of the blends. A simple model allows one to describe the influence of the interfacial tension on the storage and the loss moduli in a broad frequency range for all mixing ratios of the blend components. The weighted relaxation spectra τH(τ) of such blends show an additional peak with a characteristic relaxation time that is correlated with the interfacial tension. From this characteristic relaxation time the interfacial tension α between the melts of PS and PMMA can be determined. The resulting α=(1.9±0.3)×10−3 N/m at 170 °C is in good agreement with the value obtained from recovery following melt elongation of the blends.

Vibrational energy transfer of CO/Cu(100): Nonadiabatic vibration/electron coupling

Abstract: Vibrational energy relaxation of the internal C–O stretching mode of carbon monoxide in the c(2×2) overlayer on the Cu(100) surface at 120 K is measured by picosecond pump–probe spectroscopy. A resonant 1.5 ps infrared pulse at ν=2085 cm−1 pumps the C–O stretching mode. The energy relaxation is monitored by sum frequency generation from a delayed pair of 1.5 ps infrared and visible pulses. A single component decay, with a decay time of 2.0 ±0.5 ps, is reported. Uncertainties in the actual excited state lifetime are discussed, and the actual lifetime is estimated to be 2.0 ±1.0 ps. This lifetime is close to the lower limit of 1.2 ps set by the observed vibrational linewidth of 4.5 cm−1. The energy relaxation process is interpreted to occur by nonadiabatic energy transfer to the electrons (electron‐hole pair excitations) of the copper substrate, and the measurement supports previous assertions that the nonadiabatic energy transfer rate for this system is very rapid. The nonadiabatic energy transfer lifetime of this mode has previously been estimated by density‐functional calculations [T. T. Rantala and A. Rosen, Phys. Rev. B 34, 837 (1986)], and has recently been calculated by extrapolation of ab initio Hartree–Fock electronic structure calculations for CO on copper clusters [M. Head‐Gordon and J. Tully, preceding paper, J. Chem. Phys. 96, 3939 (1992)]. The calculated lifetimes in both cases are in the 1–3 ps range, in good agreement with the experimentally measured value.

Vibrational relaxation on metal surfaces : molecular-orbital theory and application to CO/Cu(100)

Abstract: A nonempirical theory of vibrational relaxation at metal surfaces via nonadiabatic coupling to conduction electrons is presented. Using a single determinant Hartree–Fock (HF) description of the electronic states of the system, an expression for the lifetime of an excited vibration is obtained. Under certain additional assumptions, all the quantities necessary to calculate the lifetime can be obtained from the results of ab initio HF calculations on cluster models of the adsorbate‐metal system. As a practical test of this procedure, the lifetime of the excited v=1 vibrational state of CO on Cu(100) is calculated using clusters of 6, 10, and 14 copper atoms. Results ranging between 1.1 and 3.5 ps are obtained, with our preferred procedure yielding 1.7 ps for the largest cluster, in good agreement with experiment. Extensions of this approach may also be valuable for treating other nonadiabatic phenomena at metal surfaces.

Flux-creep crossover and relaxation over surface barriers in Bi2Sr2CaCu2O8 crystals.

Abstract: Flux creep in Bi 2 Sr 2 CaCu 2 O 8 crystals exhibits two different regimes as a function of time, as well as of temperature and magnetic field. The short-time, low-temperature regime has a peak in current density versus field, which is enhanced by irradiation defects. The long-time, high-temperature regime has a monotonic and sharp falloff (step) in current density versus magnetic field, which is suppressed by irradiation defects. The former is identified with bulk pinning, and the latter with a surface barrier

Molecular dynamics of lipid bilayers studied by incoherent quasi-elastic neutron scattering

Abstract: Molecular motions in highly oriented multilayers of dipalmitoylphosphatidylcholine were studied as a function of temperature and hydration using incoherent quasi-elastic neutron scattering (QENS). The short range diffusive motions of the lipid molecules and the chain/headgroup dynamics were evaluated : 1) by measurement of the dependence of the elastic incoherent structure factor (EISF), the line-width Γ and the dynamic structure factors on the scattering vector Q for two orientations of the sample. The orientations were chosen such that the scattering vecto Q was either predominantly perpendicular or parallel to the membrane normal ; 2) by comparing data from protonated and chain deuterated lipids and 3) by the use of instruments of different energy resolution (i.e. time-of-flight and backscattering spectrometers exploring time regimes of 10-13 s to 10-11 s and 10-11 s to 10-9 s respectively). In the fluid phase the time-of-flight spectra revealed a restricted isotropic in-plane and out-of-plane diffusion of the hydrocarbon chain and headgroup protons. The mean displacements range from ≈0.6 A for methylene protons near the glycerol backbone to 7 A for protons near the chain ends. These values are obtained for a water content of 23 wt%. The values are somewhat increased at 30wt% of water. Measurements of the temperature variation of the EISF and the line-width Γ revealed a remarkably high degree of chain dynamics in the gel (Lβ')-phase. The total elastic intensity as observed with the backscattering instrument showed that Lα-Lβ'-phase transition is only well expressed at Q-values around 1 A-1, while the number and mobility of the chain defects characterized at Q≈2 A-1 (possibly gtg-kinks) increase continuously between 2 °C and 70 °C. In the time regime explored by the backscattering instrument, motions of the whole lipid molecules are also seen. It was interpreted in terms of a superposition of local in-plane and out-of-plane diffusion and lateral diffusional jumps between adjacent sites as predicted by the free volume model. For a sample containing 12 wt% of water at 60 °C the diffusion coefficient for the out-of-plane motion is D∥=6×10-6 cm2/s with an amplitude of 2.25 A. In-plane the diffusion coefficients range from Dmin ⊥=1.5×10-7 cm2/s to Dmax ⊥=6×10-6 cm2/s. The lateral diffusion coefficient is Dlat=9.7×10-8 cm2/s in reasonable agreement with FRAP measurements. The strong increase of the lateral mobility with increasing water content yielded an exponential law for the variation of the diffusion coefficient with excess area per lipid (i.e. hydration) in agreement with the free volume model. The out-of-plane motion is characterized by an amplitude of about 0.5 A in the time-of-flight time regime and of 2-3 A in the backscattering time regime. The origin of this discrepancy could be the thermally excited membrane undulations since their relaxation times of ≈3×10-9 s (obtained in a separate spin-echo study) agree roughly with the reciprocal line-width of 2.5×10-9 s for the backscattering instrument at Q↦0. The time-of-flight result of 0.5 A can be attributed to a dynamic surface roughness.

Correlation between non-Debye behavior and Q behavior of the α relaxation in glass-forming polymeric systems

Abstract: By means of quasielastic neutron scattering we have shown that the dynamics of the α relaxation on three different polymeric glasses shows a clear non-Debye behavior on a mesoscopic time scale. The spectral shape is found to be independent of temperature and momentum transfer (Q). The characteristic relaxation time follows a power-law Q dependence, τ(Q)∞Q -n (n>2), n being dependent on the system. We found that the Q behavior and the non-Debye behavior are directly correlated. These results have important implications concerning the physical mechanisms behind the dynamics of the α relaxation

Conditions for Bose-Einstein condensation in magnetically trapped atomic cesium

Abstract: We study conditions necessary for the observation of Bose-Einstein condensation in a magnetically trapped sample of atomic Cs gas. These conditions are associated with the value of the elastic scattering length, the rate of elastic scattering events, and the lifetime for decay of the density due to both magnetic dipole relaxation in two-body collisions, as well as recombination to Cs2 in three-body collisions. We find that, on the basis of these conditions, the prospects for observing Bose-Einstein condensation are favorable for a gas of ground-state Cs atoms in the highest state of the lowest hyperfine manifold. In all calculated elastic and inelastic two-body rates we find a pronounced resonance structure, which can be understood in terms of the interplay between the singlet-triplet interaction and the hyperfine, Zeeman, and magnetic dipole interactions. The experimental observation of these resonances may help to eliminate present uncertainties about interaction potentials.

Study on dynamical structure in water and heavy water by low-frequency Raman spectroscopy

Abstract: Depolarized Raman spectra below 250 cm−1 in water and heavy water were measured and analyzed from 373 K to the supercooled region The spectral feature of the central component below 20 cm−1 is stressed in the present work The spectra below 250 cm−1 in water and heavy water are interpreted as a superposition of one Debye‐type relaxation mode and two damped harmonic oscillators The damped harmonic oscillators (broadbands around 60 cm−1 and 190 cm−1) are interpreted as the restricted translational modes Analyzing the temperature dependence of the relaxation mode, the reciprocal relaxation time τ−1 in heavy water changes linearly with τ−1 ∝ (T − 240 K) in the whole temperature range On the other hand, the temperature dependence of the reciprocal relaxation time in water above 303 K deviates from a straight line which holds below 298 K as τ−1 ∝ (T − 225 K)

Optical modification of a single impurity molecule in a solid

Abstract: THE possibility of obtaining information about solids on a truly microscopic scale has stimulated several recent advances in the optical detection and spectroscopy of single impurity centres in solids. For the system composed of pentacene impurity molecules in the crystal p-terphenyl, absorption1and fluorescence excitation2 studies at liquid-helium temperatures have led to direct observations of the lifetime-limited homogeneous linewidth of a single pentacene molecule3, as well as the surprising observation of spontaneous spectral diffusion in a crystal4. Spectral diffusion—changes in the resonance frequency of an impurity molecule with time as a result of structural relaxation processes in the molecular environment—is generally expected in amorphous hosts. We report here the observation of optical spectra of single perylene impurity molecules in a polymeric (polyethylene) host. At 1.5 K, individual perylene molecules show the expected spectral diffusion; further-more, we observe light-induced changes in resonance frequency (that is, persistent spectral hole-burning5) for certain single molecules. These observations suggest the possibility of optical storage at the single-molecule level.

The relaxation time spectrum of nearly monodisperse polybutadiene melts

Abstract: The relaxation behavior of polymers with long linear flexible chains of uniform length has been investigated by means of dynamic mechanical analysis. The relaxation time spectrum (H(λ)) follows a scaling relationship with two self-similar regions, one for the entanglement and terminal zone, and a second one for the transition to the glass. This can be described in its most general form (termed “BSW spectrum”) as H(λ) = H e λne + H g λ− n g for λ < λmax and H(λ) = 0 for λmax < λ, where H e , H g , n e , n g are material constants and λmax is the molecular weight dependent cut-off of the self-similar behavior. In this study, the dynamic mechanical response has been measured and analyzed for four highly entangled, nearly monodisperse polybutadienes with molecular weights from 20000 to 200000. The data are well represented by the BSW spectrum with scaling exponents of n e = 0.23 and n g = 0.67. The values of the exponents obtained in this work are about the same as those found for polystyrene samples in a previous study. This suggests that the two types of polymers have a similar relaxation pattern. However, at this point further refinement of the experiments is needed before being able to draw definite conclusions about the universality of the exponents.

Magnetization and relaxation curves of fast relaxing high-Tc superconductors

Abstract: The magnetization curves of rapidly relaxing type-II superconductors are calculated by means of Monte Carlo simulations and numerical solutions of the partial differential equation for thermally activated flux-creep. Several models for the current dependence of the activation energy ( U ( j )= U c (1- j / j c ), U ( j )=( U c / μ )(( j c / j ) μ -1) and U ( j )= U c ( j c / j )) are considered. The calcu lated curves reproduce all the features exhibited by experimental magnetization curves, even when the critical current is assumed to be field-independent. This remarkable result shows explicitly that strong flux relaxation effects can lead to spurious field-dependent critical currents. The characteristic features of the magnetization curves are related to the relaxation behaviour of the corresponding flux density profiles. The dependence of hysteresis loops on the magnetic field sweep rate is investigated in detail and is shown to contain basically the same information as the time dependence of the magnetization during relaxation.

Theory of the semiconductor photon echo.

Abstract: The semiconductor Bloch equations are solved numerically for a two-pulse photon-echo configuration. The time-dependent diffracted signal is computed and the significance of many-body effects, carrier relaxation, and dephasing is investigated in detail. Assuming femtosecond-pulse excitation at various intensities and frequencies, distinctly different results are obtained if the exciton or the continuum electron-hole-pair states are excited. It is shown that pure exciton excitation produces a free-induction decay signal and no photon echo. An echo signal is obtained only if continuum states are excited either directly by choosing the central pulse frequencies appropriately or if the band-gap renormalization is sufficiently strong to shift continuum states into resonance. A continuous transition between free-induction decay and photon-echo signal is obtained with increasing excitation amplitude. A perturbative analytical analysis of the equations allows one to identify the role of the many-body effects in producing the different features.

High pressure nuclear magnetic resonance measurement of spin–lattice relaxation and self‐diffusion in carbon dioxide

Abstract: Spin–lattice relaxation time and self‐diffusion coefficient in 13CO2 have been measured on the four isotherms 0, 25, 50, and 75 °C at pressures ranging from 10 to 500 bar. The governing relaxation mechanism in this range is shown to be spin–rotation relaxation. Low pressure T1 data are adequately described by Gordon’s theory, while high pressure T1 data agree semiquantitatively with the diffusion model proposed by Hubbard. The low density self‐diffusion coefficient behavior is in agreement with the kinetic theory of the dilute gas phase. Strong divergence from Enskog theory is observed in the dense gas and liquid phases. The hard‐sphere theory predicts the self‐diffusion coefficient within ±2% at densities above 1.5 ρc. The simple hydrodynamic theory predicts the self‐diffusion coefficient within ±5% at densities above 1.5 ρc.

Relaxation of Molecules with Chemically Significant Amounts of Vibrational Energy: The Dawn of the Quantum State Resolved Era

Abstract: The simplest model for chemical reactions that proceed by unimolecular decomposition in the gas phase is the Lindemann mechanism, in which a substrate S is excited by collisions to a level S* with energy sufficient to cause bond breaking or molecular rearrangement ( 1-3). For large molecules, the time scale for decomposition of S* is sufficiently long that further collisions with the bath molecules can cause deactivation of the excited substrate, thus quenching the reaction process. The overall mech­ anism can be summarized by the following equations:

Dynamic light scattering study of concentrated microgel solutions as mesoscopic model of the glass transition in quasiatomic fluids

Abstract: This paper presents a light scattering study of the dynamics of concentrated solutions of nearly monodisperse (σ≊0.16) spherical micronetwork particles consisting of highly cross‐linked polystyrene dissolved in carbon disulfide, i.e., a ‘‘good’’ solvent. Above volume fractions of φ=0.50 the intermediate scattering function, measured over a time window of 10−7 to 103 s using the ALV5000 correlator, decays in two steps and shows indications of nonergodic behavior for φ≥0.64. Such behavior is typical for glass forming systems and has recently been found close to the glass transition of a hard sphere colloidal system [W. van Megen and P. N. Pusey, Phys. Rev. A 43, 5429 (1991)]. Thus the introduced system can be used for modeling the glass transition of atoms on a mesoscopic scale. The traditional analysis of structural relaxation in terms of a Kohlrausch–Williams–Watts distribution yields a mean relaxation time which follows the empirical Mooney equation as a function of concentration and thus corresponds to Vogel–Fulcher–Tammann behavior. However, the necessity to add an unspecified ‘‘intermediate’’ process between the short and long time KWW decays demonstrates the limitations of this ‘‘pragmatic’’ approach. The mode coupling theory of the glass transition interprets the intermediate scattering function consistently over nearly seven decades in time, the intermediate region corresponding to the crossover from β to α relaxation (von Schweidler law). The critical volume fraction of 0.636 derived by this analysis corresponds to a value of 0.59 for an ideal monodisperse system which is well in accord with other experimental and computer simulation studies of the glass transition of atomic systems.

The molecular dynamics around the glass transition and in the glassy state of molecular organic systems: A 2H−nuclear magnetic resonance study

Abstract: 2H‐nuclear magnetic resonance(NMR)‐spin–lattice relaxation experiments have been performed for studying the crossover from viscous (α process) to secondary (β processes) dynamics in the van der Waals liquid orthoterphenyl and the H‐bridged network glycerol. The essential and general features, observed in both systems, are the following: (a) a dominating α process in the liquid and viscous regime; (b) a change from exponential to nonexponential spin–lattice relaxation as the temperature is lowered below a characteristic temperature above Tg; (c) the existence of a slow (>10−9 s) secondary reorientational process in the highly viscous regime; and (d) the existence of a fast (∼10−12 s) local process in the glassy state. Whereas the slower process is shown to be the one known from dielectric studies, we attribute the fast mode to a β process found in quasielastic neutron scattering.

Steady state in magnetic resonance pulse experiments.

Abstract: Relaxation during multiple-pulse magnetic resonance experiments is treated by an average Liouvillian technique. The finite lattice temperature is taken into account by means of phenomenological correction terms. Both the transient and long-term response of the spin system are readily treated. We predict and verify a novel steady state of correlated spin polarizations in a spin-pair system under a periodic sequence of strong π pulses

Dynamics of ion solvation in a Stockmayer fluid

Abstract: Molecular dynamics computer simulations were performed to study the dynamics of the ionic solvation in a Stockmayer fluid. The simulations show that the solvent relaxation proceeds in two time regimes. Most of the relaxation occurs in a short time period during which the relaxation process can be described by a Gaussian function. The long time regime can be described by an exponential relaxation. The decay exponent of the relaxation function in this regime is the same as the exponent describing the decay of the single dipole correlation function. In addition, the contribution of the rotational and translational modes of the solvent to the energy relaxation was investigated. It was found that when the rotational mode is the dominant mode of the solvent motion the relaxation occurs from the outside–in, in accordance with the Onsager ‘‘snowball’’ picture. When the influence of the translational mode is increased the Onsager picture breaks down.

The structure of water determined by microwave dielectric study on water mixtures with glucose, polysaccharides, and L-ascorbic acid

Abstract: Dielectric relaxation measurements over an extremely wide frequency region from 1 MHz to 20 GHz were performed on water mixtures with glucose, polysaccharides, and L‐xylo ascorbic acid by the use of time domain reflectometry. For mixtures of polysaccharides bigger than maltotriose, two relaxation peaks were definitely observed. The high frequency relaxation is the water relaxation and the low frequency one is due to orientation of polysaccharide molecules. In the case of glucose, only one relaxation peak could be observed. It is shown that a hexagonal cluster in the lattice of ice can be replaced easily by the glucose molecule, where the lattice is distorted slightly, but stabilized by several hydrogen bonds between the glucose molecule and the lattice. Although the cluster can be replaced by the L‐ascorbic acid molecule too, the lattice cannot be kept stable. Its water mixture shows two relaxation peaks clearly. It is suggested that water has a structure of the distorted lattice of ice. Fluctuation of th...

Molecular Dynamics Analysis of NMR Relaxation in a Zinc-Finger Peptide

Abstract: The dynamical behavior of the 25-residue «zinc-finger» peptide xfin31 1 has been modeled through molecular dynamics simulations in vacuum and in water, and by normal mode and by Langevin mode analyses. The effects of internal motion on dipolar nuclear magnetic relaxation of C-H, N-H, and H-H spin pairs have been calculated, and the results for C-H pairs are compared to experimental results. Calculated internal correlation functions for directly bonded C-H and N-H spin pairs and for H-H spin pairs show rapid, subpicosecond initial decays followed by slower transitions to nearly constant «plateau» values

Deuteron nuclear magnetic resonance study of the dynamic organization of phospholipid/cholesterol bilayer membranes: molecular properties and viscoelastic behavior.

Abstract: The influence of cholesterol on the dynamic organization of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers was studied by deuteron nuclear magnetic resonance (2H NMR) using unoriented and macroscopically aligned samples. Analysis of the various temperature- and orientation-dependent experiments were performed using a comprehensive NMR model based on the stochastic Liouville equation. Computer simulations of the relaxation data obtained from phospholipids deuterated at the 6-, 13- and 14-position of the sn-2 chain and cholesterol labeled at the 3 alpha-position of the rigid steroid ring system allowed the unambiguous assignment of the various motional modes and types of molecular order present in the system. Above the phospholipid gel-to-liquid-crystalline phase transition, TM, 40 mol % cholesterol was found to significantly increase the orientational and conformational order of the phospholipid with substantially increased trans populations even at the terminal sn-2 acyl chain segments. Lowering the temperature continuously increases both inter- and intramolecular ordering, yet indicates less ordered chains than found for the pure phospholipid in its paracrystalline gel phase. Trans-gauche isomerization rates on all phospholipid alkyl chain segments are slowed down by incorporated cholesterol to values characteristic of gel-state lipid. However, intermolecular dynamics remain fast on the NMR time scale up to 30 K below TM, with rotational correlation times tau R parallel for DMPC ranging from 10 to 100 ns and an activation energy of ER = 35 kJ/mol. Below 273 K a continuous noncooperative condensation of both phospholipid and cholesterol is observed in the mixed membranes, and at about 253 K only a motionally restricted component is left, exhibiting slow fluctuations with correlation times of tau R perpendicular greater than 1 microsecond. In the high-temperature region (T greater than TM), order director fluctuations are found to constitute the dominant transverse relaxation process. Analysis of these collective lipid motions provides the viscoelastic parameters of the membranes. The results (T = 318 K) show that cholesterol significantly reduces the density of the cooperative motions by increasing the average elastic constant of the membrane from K = 1 x 10(-11) N for the pure phospholipid bilayers to K = 3.5 x 10(-11) N for the mixed system.

Ultrasonic and hypersonic relaxations of monohydric alcohol/water mixtures

Abstract: Ultrasonic absorption spectra have been measured between 150 kHz and 1.6 GHz for mixtures of water with ethanol, the propanol isomers, and all isomers of butanol at 25 {degrees}C. With the exception of the systems exhibiting a miscibility gap (n-butanol isobutyl alcohol, sec-butyl alcohol), the measurements have been performed over the complete composition range. Various relaxation functions have been fitted to the measured spectra, among them such reflecting stoichiometrically well-defined chemical reactions and, alternatively, functions based on conception of fluctuations in the composition of the binary liquids. The frequency dependent excess absorption of the complete set of mixtures can be well represented assuming a sum of a Debye-type term with discrete relaxation time and a Fixman-Kawasaki term. The latter reflects a distribution of relaxation times as resulting from concentration fluctuations including critical behavior. The parameters of these relaxation terms are discussed to yield information on the underlying molecular mechanisms. One result is the finding of the fluctuation correlation length in the completely miscible systems to not exceed some molecular diameters. This result is important for our ideal of the molecular behavior of these liquids. It is also relevant to our discussion of frequency-dependent measurements of nuclear magnetic relaxation rates andmore » also of dielectric spectroscopy. 90 refs., 10 figs., 6 tabs.« less