The computation of Q-switched laser pulses

Two methods for computing the single pulse of three- and four-level lasers with instantaneous Q-switching are described. The first method makes it possible to calculate the energy, power, and duration of the pulse. The second method, based on the approximation of the population inversion by means of suitable functions, yields its shape. The analytical solutions are compared with those obtained on an electronic digital computer. The optimal losses and limits of applicability of the formulas derived in the case of noninstantaneous Q-switching are determined.The authors are deeply grateful to B. I. Stepanov for his valuable suggestions.     

Coherent excitation of molecular vibrational modes by high-power ultrashort infrared laser pulses

Coherent dynamics of multiphoton excitation of molecular vibrational modes by subpicosecond IR laser pulses differs greatly from that of picosecond pulse excitation. The resonance response of a molecule is primarily determined by the power broadening rather than the laser carrier frequency. Selective excitation of high vibrational levels is possible with the use of subpicosecond pulses.     

Spectral methods for the equations of classical density-functional theory: relaxation dynamics of microscopic films

We propose a numerical scheme based on the Chebyshev pseudo-spectral collocation method for solving the integral and integro-differential equations of the density-functional theory and its dynamic extension. We demonstrate the exponential convergence of our scheme, which typically requires much fewer discretization points to achieve the same accuracy compared to conventional methods. This discretization scheme can also incorporate the asymptotic behavior of the density, which can be of interest in the investigation of open systems. Our scheme is complemented with a numerical continuation algorithm and an appropriate time stepping algorithm, thus constituting a complete tool for an efficient and accurate calculation of phase diagrams and dynamic phenomena. To illustrate the numerical methodology, we consider an argon-like fluid adsorbed on a Lennard-Jones planar wall. First, we obtain a set of phase diagrams corresponding to the equilibrium adsorption and compare our results obtained from different approximations to the hard sphere part of the free energy functional. Using principles from the theory of sub-critical dynamic phase field models, we formulate the time-dependent equations which describe the evolution of the adsorbed film. Through dynamic considerations we interpret the phase diagrams in terms of their stability. Simulations of various wetting and drying scenarios allow us to rationalize the dynamic behavior of the system and its relation to the equilibrium properties of wetting and drying.     

Symmetrisation of β-alkoxyalkylmercury(II) acetates

β-Alkoxyalkylmercury(II) acetates have been symmetrised in situ with alkaline sodium stannite to afford good yields of bis(β-alkoxyalkyl)mercurials, [R¹R²C(OR)CHR³]₂ Hg.     

Correlation of the physicochemical properties of symmetric 1,3-dialkoylamidopropane-based cationic lipids containing single primary and tertiary amine polar head groups with in vitro transfection activity

The physicochemical properties of a novel series of symmetric 1,3-dialkylamidopropane-based cationic amphiphiles [M. Sheikh, J. Feig, B. Gee, S. Li, M. Savva, In vitro lipofection with novel series of symmetric 1,3-dialkoylamidopropane-based cationic surfactants containing single primary and tertiary amine polar head groups, Chem. Phys. Lipids 124 (2003) 49-61] were studied by several techniques, in an effort to correlate cationic lipid structure with transfection efficacy. It was found that only the unsubstituted amine and tertiary amine dioleoyl derivatives 1,3lmp5 and 1,3lmt5, respectively, mediated in vitro transfection activity in the absence of helper lipids. This activity pattern was consistent with ethidium bromide fluorescence quenching studies, which indicated that only these two derivatives bound to and efficiently condense plasmid DNA at physiological pH. Dynamic light scattering indicated that lipoplexes made by these two cationic lipids were relatively small particles below 1 microm, in sharp contrast to lipoplexes bigger than 3 microm composed of saturated cationic derivatives. Transmission electron microscopy studies clearly indicated that cationic lipid dispersions made by saturated derivatives form multilamellar tubules at physiological pH. Calorimetric studies showed that cationic amphiphiles with saturated acyl chains longer than 12 carbons exhibit solid-to-liquid crystalline phase transitions above 37 degrees C. In agreement with the microscopy and calorimetry studies, Langmuir film balance experiments indicated that saturated derivatives with hydrophobic chains longer that 12 carbons are not well hydrated and exist at a chain-ordered state at ambient temperature. Calculation of compressibility moduli from monolayer compression isotherms at 23 degrees C suggested that monolayers made by cationic lipids bearing saturated acyl chains are less compressible relative to those of the dioleoyl derivatives 1,3lmp5 and 1,3lmt5. In conclusion, high hydration, increased fluidity and high elasticity of cationic lipid assemblies in isolation, all correlate with high in vitro transfection activity.     

Kinetic and thermodynamic analysis of 9‐nitrocamptothecin hydrolysis at physiological pH in the presence and absence of human serum albumin

Kinetic and thermodynamic analysis of the 9‐nitrocamptothecin (9NC) hydrolysis reaction in the presence and absence of human serum albumin (HSA) in phosphate‐buffered saline (PBS) of pH 7.4 was carried out by first derivative absorption spectroscopy. The thermodynamic parameters determined in these studies provided a mechanistic explanation toward the endothermic but yet thermodynamically favorable hydrolysis of 9NC at physiological temperature and pH. In the presence of HSA, the apparent rate constant of 9NC hydrolysis was 3–3.5 times higher than in 9NC solutions alone, whereas the apparent equilibrium constant of 9NC hydrolysis was found to increase at a higher extent in the presence of HSA than in PBS with increasing temperature, reaching almost complete hydrolysis of the 9NC to the 9NC‐carboxylate at 315.15 K. Importantly, the E_a of the 9NC hydrolysis reaction in the presence of HSA was determined to be on average 17 kJ mol^?1 lower than the E_a determined in plain PBS. Moreover, analysis of binding isotherms constructed for the HSA interaction with 9NC, using infinitely cooperative and independent binding models, demonstrated an incredibly higher binding constant for the 9NC‐carboxylate form as compared to the very weak and concentration‐dependent binding for the 9NC‐lactone species at 310.15 K. Taken together, the preferential association of the carboxylate form with HSA and the lower E_a of 9NC hydrolysis in the presence of HSA provide a mechanistic explanation for the shift of lactone–carboxylate equilibria toward the carboxylate product under physiological conditions of pH and ionic strength. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 693–703, 2010