Laser optogalvanic spectroscopy of argon in the wavelength region 605–740 nm

Two-photon optogalvanic transitions in Ar glow discharge with Nd: YAG laser pumped dye laser excitation in the frequency range 13520–16520 cm⁻¹ has been studied using linear and circular polarization. The intensities of two-photon optogalvanic transitions are very sensitive to changes in the incident laser power which is not the case with one-photon transitions. Intensity ratio for circular and linear polarized light for two photon transitions 6s′[1/2]°1←4s[3/2]°2, 6s′[1/2]°0←4s[3/2]°2, and 5d[1/2]°0←4s[3/2]°2, 5d[1/2]°1←4s[3/2]°2 are quite different from the other two-photon transitions. This has been explained as due to near one photon resonance of 4p′[3/2]1 level for the first pair and 4p′[1/2]1 for the second pair of transitions. The ratio of optogalvanic intensity for circular to linear polarized light has been theoretically estimated and compared with the observed results.     

Distinguishability ofSU(2)×U(1)×U′(1) andSO(10) gauge models from the Weinberg-Salam model

The predictions ofSU(2)×U(1)×U′(1) andSO(10) gauge models for the asymmetry parametersA-,B-,C _L andC _R in the deep inelastic scattering of polarized electrons and positrons by unpolarized protons and deuterons are compared with those calculated in the Weinberg Salam model for different values ofy. The model based on,SU(2)×U(1)×U′(1) group has been found almost indistinguishable from the Weinberg Salam model with regard to the parametersA-,B- andC _L (except forB- in the region 0≦y≦0.2) althoughC _R exhibits marked distinguishability. TheSO(10) model, for certain choice of its model parameters, can be distinguished from the Weinberg Salam model through measurement of the asymmetry parameters for different values ofy.     

Effects of liquid crystal environment on the spectroscopic and photophysical properties of well-known reacting systems 2,3-dimethylindole (DMI) and 9-cyanoanthracene (9CNA)

Electrochemical and steady-state and time-resolved spectroscopic studies on a disubstituted indole, 2,3-dimethylindole (DMI), and well-known electron acceptor 9-cyanoanthracene (9CNA) in liquid crystal (LC) 4-(n- pentyl)-4'-cyanobiphenyl (5CB) environment demonstrate entirely different spectroscopic and photophysical behaviors from those observed earlier by our research group with the same reacting systems in isotropic media n-heptane and acetonitrile (ACN). From the UV-vis absorption spectral measurements of the donor DMI in the presence of the acceptor 9CNA in liquid crystal medium (in 5CB) in various temperatures above the nematic-isotropic phase transition from 308 to 313 K (pseudo-ordered domain), it was observed that the lower energy lying absorption band of DMI situates in a longer wavelength region than the corresponding band observed in isotropic medium n-heptane or ACN. The possibility of the photochromic effect is discussed. In this band, the degree of mixing of the two closely spaced electronic states (1)L(a) (S(2)) and (1)L(b) (S(1)) of DMI was very prominent in the ordered LC environment (5CB) whereas in isotropic medium the dominant contribution for the formation of the lower energy band system primarily originates from the (1)Lb (S(1)) state, as evidenced from the steady-state polarization measurements. Both steady-state fluorescence quenching and time-resolved fluorescence studies clearly demonstrate in favor of the presence of only the static mode in LC environment. The situation differs in isotropic media where the dynamic process possesses the key role in the quenching mechanism. Expectedly, the transient absorption measurements by the nanosecond laser flash photolysis technique show a lack of formation of transient ionic species in the pseudo-ordered domain of 5CB. On the contrary, in isotropic solvents n-heptane and ACN, the transient absorption spectra measured by the same nanosecond laser flash photolysis technique exhibit the broad band of 9CNA radical anion at around 560 nm (9CNA-) and the band of neutral radical of DMI at 540 nm. It is inferred that the charge-separation reactions occurring within the present intermolecular systems could be stopped significantly by changing the nature of the environment from the isotropic to the LC's pseudo-ordered domain which situates closely above the nematic (N)-isotropic (I) phase transition temperature. From the steady-state and time-resolved investigations, it is revealed that, due to the hysteresis phenomenon, the nematic phase properties persist over a wide temperature range well within pseudo-ordered domain to some extent into the isotropic phases. The investigations with the different donor-acceptor inter- and intramolecular systems in 5CB and some other LC's environment are underway.     

Mesoscopic nanoshells: geometry-dependent plasmon resonances beyond the quasistatic limit

The plasmon response of a spherical metallic shell becomes significantly more complex as its size is increased beyond the quasistatic limit. With increasing size and decreasing aspect ratio (r1/r2), higher order multipolar modes contribute in a more dominant manner, and two distinct core-shell geometries exist that provide the same dipole plasmon resonance, with differing relative multipolar contributions in their overall spectral response. With further increase in particle size, the geometric tunability of the core-shell structure disappears, and in the infinite radius limit the plasmon response is consistent with that of a thin metallic film.     

Slow drying of a spray of nanoparticles dispersion. In situ SAXS investigation

Nanoparticles confined in droplets of less than a picoliter are forced to organize in submicronic dry grains through solvent evaporation. The evolution of structures of the grains and the constituent nanoparticles during the slow drying process are investigated in situ with small-angle X-ray scattering (SAXS) for the first time. The scattering results have been explained on the basis of the equilibrium thermodynamics of the droplets in the drying tube. We demonstrate that this technique is really efficient in describing the internal arrangement of the nanoparticles inside the drying droplets. Distinction between an almost homogeneous repartition of the nanoparticles in droplets and formation of core shell like particles even in strongly polydispersed droplets can be made using SAXS.     

Total synthesis of potential antitumor agent, (-)-dictyostatin

[structure: see text] The potential antitumor agent (-)-dictyostatin has been synthesized utilizing Brown crotylboration to achieve eight of the eleven chiral centers. The yield for the 26-step longest sequence is approximately 4%. The C9-C10 coupling is achieved via a stereoselective vinylzincate addition.