ALBERT

Notes: Two models of Langmuir turbulence, the nonlinear Schrödinger equation and the Zakharov equations, are solved numerically for an initial value problem in which the electric field evolves from an almost flat initial condition via the modulational instability and finally saturates into a set of solitons. The two models agree well with each other only when the initial dimensionless electric field has an amplitude less than unity. An analytic soliton gas model consisting of equal-amplitude, randomly spaced, zero-speed solitons is remarkably good at reproducing the time-averaged Fourier spectra in both cases.

Notes: The cubically nonlinear Schrödinger equation is an important model of nonlinear phenomena in fluids and plasmas. Numerical solutions in a spatially periodic system commonly involve truncation to a finite number of Fourier modes. These solutions are found to be stochastic in the sense that the largest Lyapunov exponent is positive. As the number of modes is increased, the size of this exponent appears to converge to zero, in agreement with the recent demonstration of the integrability of the spatially periodic case.

Notes: The analytic theory of weak Langmuir turbulence is well known, but very little has previously been done to compare its predictions with numerical solutions of the basic dynamical evolution equations. In this paper, numerical solutions of the statistical weak turbulence theory are compared with numerical solutions of the Zakharov model of Langmuir turbulence, and good agreement in certain regimes of very weak field strength is found.

Notes: We introduce a novel spectroscopic technique which utilizes a two-pulse sequence of femtosecond duration phase-locked optical laser pulses to resonantly excite vibronic transitions of a molecule. In contrast with other ultrafast pump–probe methods, in this experiment a definite optical phase angle between the pulses is maintained while varying the interpulse delay with interferometric precision. For the cases of in-phase, in-quadrature, and out-of-phase pulse pairs, respectively, the optical delay is controlled to positions that are integer, integer plus one quarter, and integer plus one half multiples of the wavelength of a selected Fourier component. In analogy with a double slit optical interference experiment, the two the two pulse experiments reported herein involve the preparation and quantum interference of two nuclear wave packet amplitudes state of a molecule.These experiments are designed to be sensitive to the total phase evolution of the wave packet prepared by the initial pulse. The direct determination of wave packet phase evolution is possible because phase locking effectively transforms the interferogram to a frame which is referenced to the optical carrier frequency, thereby eliminating the high (optical) frequency modulations. This has the effect of isolating the rovibrational molecular dynamics. The phase locking scheme is demonstrated for molecular iodine. The excited state population following the passage of both pulses is detected as the resultant two-beam dependent fluorescence emission from the B state. The observed signals have periodically recurring features that result from the vibrational dynamics of the molecule on the electronically excited potential energy surface. In addition, coherent interference effects cause the magnitude and sign of the periodic features to be strongly modulated. The two-pulse phase-locked interferograms are interpreted herein by use of a simple analytic model, by first order perturbation theory and by quantum mechanical wave packet calculations. We find the form of the interferogram to be determined by the ground state level from which the amplitude originates, the deviation from impulsive preparation of the wave packet due to nonzero pulse duration, the frequency and anharmonicity of the target vibrational levels in the B state, and the detuning of the phase-locked frequency from resonance. The dependence of the interferogram on the phase-locked frequency and phase angle is investigated in detail.

Notes: Vacuum UV laser-induced fluorescence has been used to detect the atomic products in the photofragmentation of H2CO, D2CO, and HDCO. The dissociation is shown to occur on both the ground (S0) and the first triplet (T1) potential surfaces. The T1 exit barrier height (2.9–6.0 kcal/mol) and the S1→T1 intersystem crossing rate near the T1 barrier top (4×107 s−1) have been determined experimentally. The threshold energy for the radical dissociation channel on the S0 surface, 86.57±0.16 kcal/mol, has also been measured accurately by rovibronically resolved photolysis. The best values for the heat of formation (9.99±0.19 kcal/mol) and the C–H bond enthalpy (15.69±0.19 kcal/mol) of HCO radical have been derived from the measured threshold energies as well.

Notes: A method called RHODIUM (removal of homonuclear dipolar coupling and the use of off-magic-angle spinning) can be applied effectively to study the carbon-13 NMR of liquid crystals in natural abundance. In this method, the sample is spun rapidly (∼1 kHz) at an angle (θ) slightly less than the magic angle, so that the nematic director aligns along the spinning axis and the dipolar couplings are reduced by a factor of (3 cos2 θ−1)/2. Proton–proton dipolar couplings are removed by a special decoupling sequence (BLEW-48) so that carbon–proton dipolar couplings can be readily identified. The coupling constants (DC–H) of individual carbons are obtained by using a two-dimensional technique of separated local field spectroscopy. The RHODIUM method has been used to study the molecular ordering in the nematic phase of 4-n-alkyl-4'-cyanobiphenyls (kCB). Three homologs (5CB, 6CB, and 7CB) were investigated at TNI−T =4 K. Values of DC–H of the aliphatic carbons give rich information on the energetics of chain conformations and those of the aromatic carbons can be used to calculate order parameters of the phenyl rings. It was found that the values of DC–H for the α, β, and γ carbons in 5CB, 6CB, and 7CB are almost the same, and so are the values of Szz and Sxx−Syy of the rings. The results for 5CB are in good agreement with those obtained from deuterium and proton NMR studies of specially deuterated compounds.

Notes: Dispersed fluorescence spectra following excitation of the DCCl A˜-X˜ vibronic bands between 570 and 620 nm were successfully recorded for the first time using a combination of direct current discharge and supersonic free jet expansion techniques to produce the radical. This is the first experimental study on the DCCl X˜1A′ state vibrational structure. Analysis of the dispersed fluorescence spectra reveals details of the vibrational structure of the DCCl X˜1A′ state. Emission bands to all three vibrational modes were observed and the fundamental vibrational frequencies were determined: 2078 cm−1 (C–D stretch), 893 cm−1 (bend), and 801 cm−1 (C–Cl stretch). Vibrational parameters including anharmonicities and the bend-stretch coupling constant for the DCCl X˜1A′ state were also determined using a least-square fit. In addition, based on our results, the triplet-singlet energy gap (the zero-point level energy gap) could be estimated to be approximately 11±2 kcal/mol. © 2002 American Institute of Physics.

Notes: Electric-field-induced molecular reorientation dynamics in polymer-dispersed liquid crystal (PDLC) films are characterized in detail using near-field scanning optical microscopy (NSOM) methods developed previously [Mei and Higgins, J. Phys. Chem. A 102, 7558 (1998)]. In these experiments, a modulated electric field is applied between the aluminum-coated NSOM probe and an indium-tin-oxide (ITO) substrate. The field causes reorientation of the liquid crystal within the ITO-supported PDLC film. The reorientation process is observed by near-field optical means. In this paper, it is conclusively shown that under appropriate conditions the dynamics observed occur in extremely small volumes, and are substantially confined within the near-field optical regime. The volume in which the dynamics are probed may be controlled by varying the experimental parameters (i.e., field strength and modulation frequency) employed. Conclusive evidence for confinement is obtained from both theoretical arguments and experimental results. Calculations of the electric fields in a model dielectric medium show that the largest fields occur very near the NSOM probe. Experimental observation of spatial variations in the threshold (i.e., the "Frederiks transition") for liquid crystal reorientation provide further evidence. The most direct evidence is provided by the observation of sub-diffraction-limited resolution in dynamics images of (approximate) 1 μm thick samples. Spatial variations in the observed dynamics are interpreted to reflect the energetics of local liquid crystal organization, the details of the reorientation process, and also polymer/liquid-crystal interfacial interactions. Finally, important information on the local rotational viscosity and elastic force constants within individual liquid-crystal droplets is obtained. © 2000 American Institute of Physics.

Notes: We present a continuum model for the optical response of dilute solutions of polymer molecules. The solvent is represented as a uniform dielectric continuum. Polymer molecules are modeled as flexible cylinders of constant cross section. The optical properties of a dissolved polymer molecule are represented by two different refractive indices in inner and outer concentric regions about the cylinder axis. The model is thus designed to take into account the effects of solvation on the solution optical response. The molecular conformation is represented by the worm-like chain model. Isotropic and depolarized light scattering intensities due to dissolved polymer molecules are evaluated approximately to quadratic order in the scattering wave vector. The refractive index increment of the solution is evaluated as well. The effects on these quantities of molecular shape, size, stiffness, and relative sizes of concentric regions and refractive indices within them are studied. We compare the results of this model to those of several other optical response models of polymer solutions. We discuss how the results of the present model differ from those obtained using the independent scatterer approximation and suggest experiments which would reveal these differences.

Notes: By pumping the 4–0 and 5–0 C–H overtones of cis-1, 3, 5-hexatriene, the rates of 1, 3-cyclohexadiene formation are measured using a Stern–Volmer analysis. While the rate for 5–0 C–H excitation is faster than 4–0 C–H excitation, the latter is about one order of magnitude larger than that calculated from an RRKM treatment. Moreover, nonmonotonic variation in rates are found as a function of excitation wavelength within each overtone region. These observations cannot be explained by assuming pure homogeneous broadening of each overtone transition, but can be rationalized by including inhomogeneous contributions, which may originate from hot bonds in the initial state and/or from a rotational dependence of this electrocyclic reaction rate near threshold.