Nonlinear optical studies of surfaces

A brief overview on second-harmonic generation and sum-frequency generation as surface analytical tools is given with a discussion on the intrinsic limitations of the techniques often encountered in the applications. The possibly of using nonlinear optical processes for surface studies has attracted increasing attention in recent years. Optical Second-Harmonic Generation (SHG) and Sum-Frequency Generation (SFG), in particular, have been well accepted as viable surface probes [1]. They have many advantages over the conventional techniques. By nature, they are highly surface-specific and has a submonolayer sensitivity. As coherent optical processes, they are capable of in-situ probing of surface in hostile environment as well as applicable to all interfaces accessible by light. With ultrafast pump-laser pulses, they can be employed to study surface-dynamic processes with a subpicosecond time resolution. These advantages have opened the door to many exciting research opportunities in surface science and technology. This paper gives a brief overview of this fast-growing new area of research.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Asymptotic analysis of Daubechies polynomials

To study wavelets and filter banks of high order, we begin with the zeros of . This is the binomial series for , truncated after terms. Its zeros give the zeros of the Daubechies filter inside the unit circle, by . The filter has additional zeros at , and this construction makes it orthogonal and maximally flat. The dilation equation leads to orthogonal wavelets with vanishing moments. Symmetric biorthogonal wavelets (generally better in image compression) come similarly from a subset of the zeros of . We study the asymptotic behavior of these zeros. Matlab shows a remarkable plot for . The zeros approach a limiting curve in the complex plane, which is the circle . All zeros have , and the rightmost zeros approach (corresponding to ) with speed . The curve gives a very accurate approximation for finite . The wide dynamic range in the coefficients of makes the zeros difficult to compute for large . Rescaling by allows us to reach by standard codes.

     

End-functionalized polyethylene oxide coated silica particles for packed capillary column supercritical fluid chromatography

Summary Polyethylene oxide (PEO)-based polymers with hydroxy, methoxy, and aminopropoxy terminal groups were coated on diol functionalized and hexamethyldisilazane end-capped silica particles. Proton-donor and proton-acceptor test solutes, including carboxylic acids, hydroxy-containing compounds, arylamines, and alkylamines were used to evaluate the chromatographic performances of these polymer coated particles under SFC conditions with neat CO₂ as mobile phase. It was found that the particles coated with hydroxy-terminated PEO were suitable for the separation of proton-donor compounds such as hydroxy-containing compounds and carboxylic acids, and the particles coated with aminopropoxy-terminated PEO could be used for the separation of amines. That is, the proton-accepting stationary phase is suitable for the separation of proton accepting solutes, including strong basic alkylamines (pK_b4), using neat CO₂ as mobile phase, while the protondonating stationary phase is suitable for the separation of proton-donating compounds such as carboxylic acids (pK_a4). Hydrogen bond basicity was found to be a critical factor for the chromatography of basic amines. Low volatility acidic and basic drugs were chromatographed using the new stationary phases. The stability of the PEO coated particles was determined by measuring the loss of organic carbon under SFC conditions. It was found that approximately 18 % of the coating (average molecular weight of 15,000) was washed out of the particles by supercritical CO₂ after 7 h at 350 atm and 50°C     

Total Synthesis of d,l-Isospongiadiol: An Intramolecular Radical Cascade Approach to Furanoditerpenes

A stereoselective oxidative free-radical cyclization of beta-keto ester polyenes 7 and 19 has been accomplished as a one-step entry to the tricarbocyclic synthons 8and 21 which contain five and six stereogenic centers, respectively. These key synthons possessing an axial carboethoxy group at C-4 were ultimately converted to the spongian skeleton (8--> 14 and 21 --> 25 -->14). The synthesis of d,l-isospongiadiol (3) from the common intermediate 14 was realized after introduction of the 2alpha-hydroxy group in the spongian A-ring via epoxidation of silyl enol ether 28 and subsequent desilylation.