Ablation thresholds and morphological changes of poly-l-lactic acid for pulse durations in the femtosecond-to-picosecond regime

Biodegradable polymers such as poly-l -lactic acid (PLLA) are essential tools for a wide range of medical applications because of their mechanical robustness and bio-affinity. Laser processing is commonly used to construct various structures from biodegradable polymers. However, in general, polymers deteriorate rapidly owing to laser-induced heating effects. In this study, we investigate the optimum irradiation conditions for PLLA processing, using an ultrafast laser and finely controlling the pulse duration over the femtosecond-to-picosecond range. We report on the morphological characteristics of the craters generated by single-shot and multiple-shot laser irradiation under pulse duration control. Our results show that crater morphology and damage threshold in the single-shot regime depend on pulse duration and that the degree of crater unevenness is insensitive to pulse duration in the multishot regime.     

Intensity enhancement of weak O2 a1Δg → X3Σg(-) emission at 1270 nm by collisions with foreign gases

Collision-induced near-IR emission of O(2) a(1)Δ(g) was investigated in O(2)/M (M = Ar, Kr, Xe, N(2), or CO(2)) gas mixtures, where the total pressure ranged from 10 to 100 atm, and gaseous O(2) dimol was excited with a pulsed dye laser at 630 nm through the simultaneous two-electron transition to prepare O(2) in the a(1)Δ(g) state. The a(1)Δ(g) → X(3)Σ(g)(-) emission intensity around 1270 nm increased with the number density of foreign gas (M) under constant O(2) number density. Emission enhancement efficiencies were in the order Xe > CO(2) > O(2) > Kr > N(2) > Ar; they are controlled by collisional enhancement during the near-IR emission at 1270 nm but not during photoabsorption at 630 nm. Efficiencies were converted into bimolecular rate constants to enhance the radiative a → X transition for the added gases. The rate constants were estimated as quadratically dependent on the molar refraction (or polarizability) of collision gas. The self-quenching rate constant was determined from the Stern-Volmer plot of the emission lifetimes measured in pure O(2).     

3C-SiC(001)-3 × 2 reconstructed surface analyzed by high-resolution medium energy ion scattering

The atomic structure of the 3C-SiC(001)-3 × 2 reconstructed surface was analyzed precisely by high-resolution medium energy ion scattering (MEIS). The present MEIS analysis unambiguously shows that the (3 × 2) surface consists of Si adatoms (1/3 ML, 1 ML = 1.05 × 10¹⁵ atoms/cm²) on top and underlying Si adlayer (2/3 ML) on the bulk truncated Si plane. As the result, the most probable structure is focused on the Two Adlayer Asymmetric Dimer Model predicted by ab initio calculations and the modified versions with alternating long and short dimers in the 2nd adlayer proposed by photoelectron diffraction (PED) and by grazing incidence X-ray diffraction (GIXRD) analyses. Observed MEIS spectra are well reproduced by the structure relatively close to that determined by PED rather than GIXRD. Interestingly, the first principle calculations using VASP (Vienna ab initio simulation package) prefer symmetric dimers in the second Si adlayer and non-relaxed interplanar distance between the top Si and 2nd C plane of the bulk-truncated surface, which are, however, unable to reproduce the observed MEIS spectra. The distorted 2nd adlayer (asymmetric dimers) may correlate with the compressed interplanar distance between the underlying Si and C planes.     

Short-step synthesis and structure-activity relationship of cortistatin A analogs

An improved method for synthesizing structurally simplified analogs of cortistatin A (1), a novel anti-angiogenic steroidal alkaloid from a marine sponge, was developed. In contrast to previous methods, step- and redox-economical synthesis was achieved using a known α-bromoketone as the starting material. The structure-activity relationship study revealed that the isoquinoline portion was strictly recognized by the target molecule. Surprisingly, the introduction of the acetamide moiety on the A-ring structure dramatically enhanced the selective antiproliferative activity against endothelial cells. This new method can be easily applied to gram-scale synthesis and enabled us to prepare various analogs, which were focused on the participation of the side chain and A-ring structure.     

The electromagnetic and hadronic diffractive dissociation of O ions

Events which satisfy the kinematics of p(¹⁶O,CHep), with low excitation energy in the (CHe) system and a low-energy recoil proton, have been identified in the interactions of 200 GeV/nucleon ¹⁶O ions with nuclear emulsion. An eikonal DWIA estimate of the target A-dependence of strong-interaction diffractive dissociation suggests that, on the basis of these hydrogen data, most of the (CHe) final states, previously ascribed to electromagnetic dissociation on heavy nuclei, might rather be hadronic in character. This contamination is very much less important in the dominant (NH) channel.     

Synthesis of the naphthalenecarboxylic acid derivative obtained from neocarzinostatin (NCS): A structure revision

The structure of the naphthalenecarboxylic acid derivative obtained from neocarzinostatin (NCS) was revised as methyl 2-hydroxy-7-methoxy-5-methyl-1-naphthalenecarboxylate (2a) by its synthesis.