Protonation of aqueous alanine by photoionization of water.

Hydronium ions produced by photolysis of water are used to study the protonation dynamics of alanine zwitterions in water. The measurements are done by UV-VIS and UV-IR femtosecond transient absorption spectroscopy on alanine in H2O and D2O. It is estimated that the reaction rate constant for the deuteration of alanine zwitterions is 4 × 1010 M-1 s-1, while the reverse process has a rate constant of 2 × 108 s-1. In addition to hydronium ions the photolysis of water yields hydrogen atoms and hydrated electrons together with hydroxyl radicals and hydroxyl ions. However, no other products resulting from reactions between aqueous alanine and the photolysis products of water are positively identified during the first 530 ps after the photolysis. Potential secondary reactions that are not observed experimentally are discussed and an upper limit is set for their yield where possible.

A 158 fs 5.3 nJ fiber-laser system at 1 microm using photonic bandgap fibers for dispersion control and pulse compression.

We demonstrate a 158 fs 5.3 nJ mode-locked laser system based on a fiber oscillator, fiber amplifier and fiber compressor. Dispersion compensation in the fiber oscillator was obtained with a solid-core photonic bandgap (SC-PBG) fiber spliced to standard fibers, and external compression is obtained with a hollow-core photonic bandgap (HC-PBG) fiber.

Far-infrared properties of DAST.

Using terahertz time-domain spectroscopy, we have measured the index of refraction and the absorption coefficient of the organic ionic salt 4-N, N-dimethylamino-4?-N?-methyl-stilbazolium tosylate (DAST). This promising organic electro-optic material exhibits strong absorption and dispersion for frequencies above 1 THz at both room temperature and 83 K. No reduction in the absorption strength is observed when DAST is cooled, suggesting a single-phonon origin of the resonances. A simple vibration of the anion and cation of the salt is suggested as the origin of the exceptionally strong far-infrared absorption and the high-frequency electro-optic properties of DAST.

Radiation patterns from lens-coupled terahertz antennas.

We report on investigations of the angular distribution of the radiation emitted from a terahertz antenna system equipped with a truncated spherical silicon lens. The pattern is calculated by wide-angle interference principles and Huygens-Fresnel diffraction theory. Experimental determination of the radiation pattern is performed by spatially resolved terahertz time-domain spectroscopy. Good agreement between theory and experiment is obtained, and we find that the terahertz beam can be represented by a Gaussian beam emitted from a circular aperture equal to the diameter of the lens.

Measurements of the phase shift and reshaping of terahertz pulses due to total internal reflection.

Using time-domain spectroscopic techniques with freely propagating electromagnetic beams of terahertz pulses, we have measured the phase shift and reshaping of subpicosecond pulses due to total internal reflection from a crystalline quartz prism. Our measured value of the phase shift is in excellent agreement with the theoretical prediction.