Jorge Codnia

Formation and Kinetics of Self-Assembled Structures of Magnetic Microparticles in Rotating Fields

We study the formation of self-assembled structures of magnetic microparticles subjected to the action of a biaxial alternating field. The used device comprises a set of two nested orthogonal Helmholtz coils that can produce rotating fields in the horizontal plane by applying proper alternating currents to the coils. In this way, it is possible to induce a planar organization in a set of magnetic micrometer-size Fe and Ni particles dispersed in a liquid. We investigate, by means of video-microscopy image analysis performed in the MATLAB environment, the time evolution of topological parameters, as the number of objects detected. We also investigate the circularity of the dominant object at different stages of the structure formation and the region of power-law behavior of the time evolution. We compare the characteristic times for pattern formation under different field configurations.

Infrared absorption of O3 under CO2 laser excitation

A homemade TEA CO2 multimode line-tunable laser was used as a radiation source to study the IR absorption of O3 in pure samples and in O3 – M mixtures (M=O2, N2, and air) at the initial steps of the excitation by direct calorimetric measurements. The fluence and pressure dependence of the O3 absorption cross section and the number of photons absorbed n were determined for all the available rotational lines P(4) through P(58) of the 9.5-µm emission band. The dependence of n on added nonreactive gas pressure over the range 0 to 750 Torr was studied for three different fluences of the P(30) line.

Laser flash photolysis with real-time mass spectrometry detection

The laser flash photolysis technique in a flow system with real time mass spectrometry detection is a suitable tool monitor the time evolution of chemical reactions. IR and UV lasers have been used to generate radicals to initiate a chemical reaction in a gas mixture. The appropriate selection of the pressure regimes (μtorr - 760 torr), the laser pulse repetition rate and the sample residence time in the photoreactor make possible the selective detection of final and intermediate reaction products. The selection of the temporal scales enables the independent study of the different stages of a process through the elimination of certain radical reactions. The rate constants associated to the corresponding kinetic scheme can be therefore determined and the reaction channels can be established. This technique was implemented for the study of oxidation reactions of chlorofluorocarbons of atmospheric interest.

CCI2 reactions with HCI, NO2 and O2

The IR Multiphoton Dissociation (IRMPD) of CDCl3 different gases was studied with the flash photolysis technique using a tunable CO2 TEA laser tuned to the 10P(48) line. CDCl3 samples were irradiated in the presence of HCl to analyze a possible recombination of the primary dissociation fragments CCl2 and DCl. The IRMPD of CDCl3 was also studied in mixtures of HCl/Ar, O2/Ar and NO2/Ar. In the experiences with added HCl, CHCl3 was obtained as major product evidencing the existence of the CCl2 and DCl recombination reaction. The dissociation probabilities per pulse obtained with the addition of different gases were compared.