Binary and recoil collisions in strong field double ionization of helium

We have investigated the correlated momentum distribution of both electrons from nonsequential double ionization of helium in a 800 nm, 4.5 x 10(14 W/cm2 laser field. Using very high resolution coincidence techniques, we find a so-far unobserved fingerlike structure in the correlated electron momentum distribution. The structure can be interpreted as a signature of the microscopic dynamics in the recollision process. We identify features corresponding to the binary and recoil lobe in field-free (e,2e) collisions. This interpretation is supported by analyzing ab initio solutions of a fully correlated three-dimensional helium model.     

Research on the vesicle-micelle transition by 1H NMR relaxation measurement

We have investigated how the dynamics of surfactant molecules changes with the vesicle-micelle transition by (1)H NMR relaxation studies on the sodium decyl sulfate (SDeS)-decyltrimethylammonium bromide (DeTAB)-deuterium oxide system. The study has been planned with reference to the phase diagram of the SDeS-DeTAB-water system deduced from thermodynamic analysis of the surface tension data. The spin-lattice relaxation time (T(1)) and the spin-spin relaxation time (T(2)) are measured at 90 and 400 MHz at various total molalities, m, and compositions, X(2), of the surfactants. The data were analyzed according to the "two-step" model developed by Wennerstr?m et al. and molecular dynamics of the surfactant is discussed from the viewpoint of correlation time tau(f) associated with the local fast motion of the surfactant molecule, correlation time tau(s) associated with the slow overall motions of the aggregate and surfactant molecules within it, and local order parameter S. We find tau(s) of vesicles is an order of magnitude larger than that of micelles signifying that the tumbling of vesicle particles and surfactant diffusion over the vesicle are much slower than those for micelle. Tau(f) and S for vesicles are also larger than those for micelles. Molecular environments of the surfactant are also discussed from the dependence of the chemical shifts on m at constant X(2) or from that on X(2) at constant m. When the chemical shifts in vesicle and micelle are compared at constant m, the chemical shifts in vesicle are displaced to a lower magnetic field than those in micelle, which implies that the surfactant molecules are arranged more closely to each other in the vesicle than in the micelle.     

DSC and high resolution X-ray diffraction coupling

Adaptive or smart hybrid composites consisting of a polymer matrix reinforced by aramid fibres and incorporating pre-strained Shape Memory Alloy (SMA) wires are able to tune some of their properties, such as their shape, the natural vibration frequency or the damping coefficient, in response to an external stimulus. The functional properties of these systems are directly related to the reversible martensitic transformation in the SMA elements. In this work the transformational behaviour of both free SMA wires and SMA wires embedded in polymer matrix is investigated by means of DSC. The martensitic transformation of the constrained wires is impeded by the polymer matrix, while the interface integrity plays a crucial role.     

Ruthenium-catalyzed [2+2] cycloadditions of bicyclic alkenes and ynamides

Ruthenium-catalyzed [2+2] cycloadditions between bicyclic alkenes and ynamides were investigated. The ynamide moiety was found to be compatible with the ruthenium-catalyzed cycloaddition conditions giving the corresponding cyclobutene cycloadducts in moderate to good yields (up to 97%). Diastereoselective cycloaddition utilizing chiral cyclic ynamides were also examined and a low to moderate level of asymmetric induction was observed.     

Few cycle dynamics of multiphoton double ionization

In intense field ionization, an electron removed from the atomic core oscillates in the combined fields of the laser and the parent ion. This oscillation forces repeated revivals of its spatial correlation with the bound electrons. The total probability of double ionization depends on the number of returns and therefore on the number of optical periods in the laser pulse. We observed the yield of Ne(2+) relative to Ne(+) with 12 fs pulses to be clearly less compared to 50 fs pulses in qualitative agreement with our theoretical model.     

Photosensitivity of As2S3 chalcogenide thin films at 1.5 microm

Chalcogenide glasses are promising candidates for all-optical switching and various nonlinear applications. However, we show that As2S3 thin films are photosensitive at wavelengths in the 1.5-microm telecommunication window. This sensitivity is evidenced by the formation of self-written waveguides in slabs, where channels as narrow as 1 microm are created. We also show the detrimental effects of such photosensitivity in ridge waveguides. This photosensitivity seems to occur only in thin-film form and not in bulk samples or fibers.     

Controlling vibrational wave packet motion with intense modulated laser fields

Intense, nonresonant laser fields produce Stark shifts that strongly modify the potential energy surfaces of a molecule. A vibrational wave packet can be guided by this Stark shift if the laser field is appropriately modulated during the wave packet motion. We modulated a 70 fs laser pulse with a period on the time scale of the vibrational motion (approximately 10 fs) by mixing the signal and idler of an optical parametric amplifier. We used ionization of H2 or D2 to launch a vibrational wave packet on the ground state of H2(+) or D2(+). If the laser intensity was high as the wave packet reached its outer turning point, the Stark shift allowed the molecule to dissociate through bond softening. On the other hand, if the field was small at this critical time, little dissociation was measured. By changing the modulation period, we achieved control of the dissociation yield with a contrast of 90%.     

A computer simulation of trapping electrophoresis

The gel electrophoretic migration of streptavidin-DNA complexes is severely altered by the phenomenon known as “trapping electrophoresis.” We present a first computer simulation study of this process. Our simulations use the very efficient biased reptation algorithm. The steady state is characterized by a large increase in band broadening and interband separation. However, we also find that for a narrow range of molecular sizes, the separation power of gel electrophoresis is greatly increased. We discuss the implications of our findings for the possible improvement of DNA sequencing technologies. © 1992 John Wiley & Sons, Inc.