Time-resolved spectroscopy of laser emission from dye-doped droplets

Micrometer-sized droplets of Rhodamine 6G solution in water and ethanol are irradiated by high-intensity nanosecond pulses from a frequency-doubled Nd:YAG laser. Coupling of the spontaneous fluorescence emission with natural resonant modes of the spherical droplets results in stimulated emission, with each droplet behaving like a laser cavity. Spectral observations suggest that droplet lasing emission is supported by resonances of a single mode order. The emission exhibits faster rise times and is shorter lived than corresponding bulk-liquid fluorescence. Lasing in droplets is generally initiated almost simultaneously with elastic scattering, unlike stimulated Raman scattering, which is significantly delayed.

Stimulated Raman scattering in micrometer-sized droplets: measurements of angular scattering characteristics

Measurements of the angular scattering characteristics of elastic and stimulated Raman scattering (SRS) in single (nominal 20-microm-radius) water, ethanol, and CC1(4) droplets irradiated with 0.532-microm-wavelength radiation from a pulsed laser demonstrate that SRS is more isotropic than elastic scattering yet qualitatively mimics angularly smoothed elastic scattering patterns. The angular fine structure characteristic of the coherent elastic scattering process is lacking in SRS, regardless of whether the SRS derives from multiple orders of Stokes shifts, multiple resonances within a single Stokes shift, or single resonances within a single Stokes shift.

Omega-conotoxin GVIA binding to a high-affinity receptor in brain: characterization, calcium sensitivity, and solubilization

We describe unique, high-affinity binding sites for omega[125I]conotoxin GVIA in membranes from rat brain and rabbit sympathetic ganglia which appear to be primarily associated with N-type voltage-dependent calcium channels. The dissociation constant (KD) for the toxin in rat brain membranes is 60 pM. Physiologic extracellular concentrations of calcium inhibit toxin binding noncompetitively (IC50 = 0.2 mM). The regional distribution of the binding sites in rat brain differs markedly from that of dihydropyridine calcium antagonist receptors associated with L-type calcium channels. In detergent-solubilized brain membranes, toxin binding retains the same affinity, specificity, and ionic sensitivity as in particulate preparations.

Irradiance and laser wavelength dependence of plasma spectra from single levitated aerosol droplets

Plasma spectra initiated on single, optically levitated, micron-sized droplets of a glycerine-saturated brine solution are presented. The temporal development of the droplet plasma is monitored by observing neutral and once-ionized carbon and neutral sodium emission, and that of the air plasma by once-ionized nitrogen emission. The dependence of the plasma on laser parameters, such as wavelength and irradiance, is discussed.

Stimulated Raman scattering in micrometer-sized droplets: time-resolved measurements

Time-resolved measurements of elastic scattering and stimulated Raman scattering (SRS) in micrometer-sized water and carbon tetrachloride droplets irradiated with a pulsed, frequency-doubled Nd:YAG laser (pulse width 8 nsec, lambda = 532 nm, peak intensity ~1 GW cm(-2)) are reported. Elastic scattering of light is instantaneous within our measurement error, estimated to be <+/-3 nsec. On the other hand, the first Stokes shift in water and multiple-order (through ninth-order) Stokes shifts in carbon tetrachloride are delayed from the elastically scattered light by 5-7 nsec. The delay in SRS is apparently a consequence of structure resonances within the droplet, which acts as an optical cavity with relatively high Q. Quasi-periodic peaks in SRS spectra of water droplets are shown to be associated with elastic-scattering structure resonances having the same mode order.

Aerosol-induced laser breakdown thresholds: wavelength dependence

Aerosol-induced loser breakdown thresholds have been measured for liquid droplets at wavelengths lambda= 1.064, 0.532, 0.355, 0.266 microm using a Nd:YAG laser with 5-10-ns pulse duration. Breakdown thresholds are 2-3 orders of magnitude below those for clean air and range from 4 x 10(7) to 3 x 10(9) W cm(-2) for nominal 50-microm diam droplets, depending on laser wavelength and droplet composition. Thresholds decrease with decreasing wavelength; they also decrease for droplets having a higher real refractive index. For water droplets the breakdown threshold intensity varies approximately as lambda(0.5). The wavelength dependence of breakdown thresholds can be qualitatively explained by considering (1) the effect of enhancement of internal fields and energy density within and near droplets and (2) the increasing importance of multiphoton absorption processes at shorter wavelengths. Laser transmission losses through the breakdown plasma and observations of the suppression of stimulated Raman scattering by the addition of small quantitites of absorbing material to water and carbon tetrachloride droplets are also reported.

Time-resolved spectroscopy of plasmas initiated on single, levitated aerosol droplets

Plasma spectra resulting from the irradiation of single, optically levitated glycerine droplets by an intense pulse from a Nd:YAG laser are presented. The temporal development of the plasma is studied by observing emission lines of neutral and once-ionized carbon and of once-ionized nitrogen. Stark shifts of the neutral-carbon lines are used to compute the plasma electron density. The droplet data are compared with similar data for laser-induced breakdown on a graphite surface.

Enzymatic reactions on thin-layer chromatographic plates. II. Phospholipase A2 hydrolysis of phosphatidylcholine and separation of the products on a single plate

A procedure for the phospholipase A2 hydrolysis of phosphatidylcholine on a thin-layer chromatographic plate and subsequent separation of the products on the same plate is described. A 0.2-0.8-mg amount of Russell's viper venom (phospholipase A2) in 0.2 ml of 0.005 M calcium chloride solution was applied on a 0.5-mm silica gel G plate as a band over which 2-5 mg of egg phosphatidylcholine in 0.2 ml of diethyl ether containing 5% of methanol was evenly applied. After the reaction had proceeded for 15-20 min in a diethyl ether-saturated chamber at 25 degrees, the plate was developed with chloroform-methanol-water (65:25:4). The bands were identified and their contents extracted. The extent of hydrolysis under different reaction conditions was evaluated from the amount of lysophosphatidylcholine formed. Approximately 74.6% (maximum) conversion was obtained within 15 min at 25 degrees using a substrate to enzyme ratio of 4:1. The acyl group distributions in the 1- and 2-positions of hen egg phosphatidylcholine obtained from the gas-liquid chromatographic analysis of the methyl ester corresponding to the lyso and free fatty acid band agreed with those obtained by the method of Wells and Hanahan. The method is also applicable to phosphatidylethanolamine.

A method for the estimation of free and esterified cholesterol involving thin-layer chromatography

A method for the estimation of both free and bound cholesterol in serum and plasma involving thin-layer chromatography has been developed. The serum or plasma is applied as a band directly on the plate. Extraction of lipid material from this band and the chromatographic separation of the above components are carried out on the same plate. The components are extracted from the adsorbent directly with acetic acid and measured colorimetrically using iron(III) chloride-sulphuric acid reagent.