Measuring the change in the intermolecular Raman spectrum during dipolar solvation

We demonstrate a method to directly measure the change in the spectrum of intermolecular solvent fluctuations as a function of time after electronic excitation of a solute, and this method is applied to the dye Coumarin 102 (C102) in acetonitrile. The complete intermolecular response is captured following resonant excitation with time domain third-order Raman spectroscopy. In a previous report, we introduced this method and used it to probe one point in the intermolecular response as a function of time after solute excitation (Underwood, D. F., Blank, D. A. J. Phys. Chem. A 2003, 107 (7), 956). Here we extend this approach to recover the change in the entire intermolecular response as a function of time. To our knowledge the results provide the first direct measurement of the difference in the equilibrated intermolecular response after excitation of a solute and its evolution during a dipolar solvation event. Excitation of C102 results in a significant increase in the solvent-solute interaction due to a large increase in the dipole moment. The observed change in the intermolecular response is consistent with a rapid change in local solvent density, with intermolecular kinetic energy transfer changing the response on longer time scales. Evolution of the response exhibits a strong frequency dependence and suggests changes over longer distances at longer delay times. The measured change in the spectrum of solvent fluctuations represents a direct experimental confirmation of the breakdown of linear response and confirms predictions from molecular dynamics simulations.     

Field-free three dimensional molecular axis alignment

We investigate strategies for field-free three dimensional molecular axis alignment using strong nonresonant laser fields under experimentally realistic conditions. Using the polarizabilites and rotational constants of an asymmetric top rotor molecule (ethene, C2H4), we consider three different methods for axis alignment of a Boltzmann distribution of rotors at 4 K. Specifically, we compare the use of impulsive kick laser pulses having both linear and elliptical polarization to the use of elliptically polarized switched laser pulses. We show that an enhanced degree of field-free three dimensional alignment of ground vibronic state molecules obtains from the use of two orthogonally polarized, time-separated laser pulses.     

Residual-stress measurement using surface displacements around an indentation

An experimental method is described which can measure the direction and magnitude of residual and applied stress in metals. The method uses optical interference to measure the permanent surface deformation around a shallow spherical indentation in a polished area on the metal specimen. The deviation from circularly symmetrical surface deformations is measured at known values of applied stress in calibration specimens. This deviation from symmetry can then be used to determine the direction and magnitude of tensile residual stress in specimens of the same material. Determination of compressive residual stress is more limited. A model of the indentation process is offered which qualitatively describes experimental results in 4340 steel for both tensile and compressive stress. The model assumes that the deformation around an indentation os controlled by stresses analogous to those around a hole in an elastic plate. Various conditions are discussed which affect the indentation process and its use to measure stress, including (a) the rigidity of support of the indentor and specimen, (b) the size and depth of the indentation, (c) the uniaxial stress-strain behavior of the specimen material.     

Communication: Length scale dependent oil-water energy fluctuations

Interfacial fluctuations in the cohesive (van der Waals) interaction energy of spherical oil-drops with water provide evidence of a length scale dependent transition from linear to non-linear response behavior. For sub-nanometer oil-drop sizes, energy fluctuations are found to be independent of the van der Waals coupling strength, while nanometer (and larger) size oil drops experience highly non-linear energy fluctuations. The latter behavior is linked to enhanced hydrophobic density fluctuations and the emergence of entropic contributions to oil-water cohesive interaction free energies.     

Temporal dynamics of longitudinal and transverse velocity structure functions retrieved from the data of acoustic sounding

Temporal dynamics of vertical profiles of the longitudinal and transverse velocity structure functions and characteristics retrieved using an original procedure from regular measurements of vertical profiles of the wind velocity vector with a minisodar operating at a frequency of 4900 Hz is discussed. It is established that the structure functions first increase with the separation of the observation points and then are saturated; moreover, in agreement with the data available from the literature, the longitudinal velocity structure function is much greater than the transverse one. The altitude behavior of the velocity structure characteristic is well described by a z ^−2/3 dependence predicted theoretically. In this case, the structure function calculated from one-point measurements of the vertical wind velocity component exceeds the corresponding structure function retrieved for the vertical separation of the observation points. This is also in agreement with generalizations from the theory of isotropic turbulence.     

Detection of milk oligosaccharides in plasma of infants

Human milk oligosaccharides (HMO) are one of the major components of human milk. HMO are non-digestible by the human gut, where they are known to play important functions as prebiotics and decoys for binding pathogens. Moreover, it has been proposed that HMO may provide sialic acids to the infant that are important in brain development, however this would require absorption of HMO into the bloodstream. HMO have consistently been found in the urine of humans and other mammals, suggesting systemic absorption. Here, we present a procedure for the profiling of milk oligosaccharides (MO) in plasma samples obtained from 13 term infants hospitalized for surgery for congenital heart disease. The method comprises protein denaturation, oligosaccharide reduction, and porous graphitized carbon solid phase extraction for purification followed by analysis using nHPLC-PGC-chip-TOF-MS. Approximately 15 free MO were typically observed in the plasma of human infants, including LNT, LDFP, LNFT, 3′SL, 6′SL, 3′SLN, and 6′SLN, of which the presence was confirmed using fragmentation studies. A novel third isomer of SLN, not found in human or bovine milk was also consistently detected. Differences in the free MO profiles were observed between infants that were totally formula-fed and infants that received at least some part breast milk. Our results indicate that free MO similar in structure to those found in human milk and urine are present in the blood of infants. The method and results presented here will facilitate further research toward the possible roles of free MO in the development of the infant.     

Refractive index variation in nonaqueous sterically stabilized copolymer particles

We present procedures for preparing sterically stabilized polymer particles whose refractive index can be controlled over a range of a few percent. Particle sphericity and size distribution are such that suspensions crystallize at high concentrations. This at least ensures that Brownian motion dominates over particle settling and that the polydispersity is no more than about 10%. Of particular interest are new particles comprising poly(methylmethacrylate-co-trifluoroethylacrylate) that can be optically matched in single solvent, namely cis-decalin.