A. González Ureña

Quantitative analysis of trace metal ions in ice using laser-induced breakdown spectroscopy

Abstract This work reports on a simple, quick-freeze method for the quantitative analysis of trace metal ions in liquids applying the laser-induced breakdown spectroscopy (LIBS) technique. Using this procedure with calibrated samples, well-characterized linear working curves were determined for Na and Al water solutions over the 0.01–1% concentration range. This allowed detection limits of the order of ppm. In addition, optimum experimental conditions were found that allow the analysis to be carried out in a fast and very easy manner, without the limitations and difficulties found with liquid samples. The advantages of this simple and direct method, developed and patented by the Instituto Pluridisciplinar–UCM, are discussed, and potential applications for industrial analysis are also suggested.

Dynamics of reactive collisions by optical methods

Abstract This paper reviews recent developments in the study of reactive collisions using optical methods. Although the basic approach is from the experimental viewpoint, attention is paid to the conceptual and theoretical aspects of the physics underlying modern reaction dynamics. After a brief resume of basic concepts and definitions on both scalar and vectorial quantities characterizing the chemical reaction, a significant body of this paper describes the recent achievements using laser techniques, mainly via laser-induced fluorescence, and chemiluminescence. Both high-resolution crossed-beam and high-resolution bulb studies are presented in a complementary fashion, as they provide a detailed picture of reaction dynamics through the measurement of quantum state specific differential cross-sections. Specific examples include the use of Doppler resolved laser-induced fluorescence, multiphoton ionization or Cars studies. Some examples are also included based on the use of product imaging techniques, the n...

Dynamical model for the “translational excitation features” in the atom—diatom reaction cross section

Abstract A collision model for the kinetic energy dependence of the reaction cross section has been obtained by considering total angular momentum limitation at the entrance (reactants) or exit (products) reactive channel in the atom—diatom scheme. The model provides a satisfactory representation of the main “translational cross section features” (i.e., Arrhenius like behaviour near threshold, maximum, post-maximum decline and minimum) found in the reaction cross section by molecular beam experiments. Maximum and minimum explanations, including some predictions as well as a comparison with previous theoretical treatments are reported. Also an approximate method to obtain dissociation energies of the product diatom from the cross-section data is presented with relative success.

Simple cross-section model for elementary reactions

Abstract A simple reaction cross-section model has been obtained. It consists of hard spheres where the mass of the products has been introduced via angular momentum conservation. For exothermic reactions the cross section shows a maximum, providing that a threshold energy is present. The model has been applied to the alkali-atom—alkyl-oxide reactions, giving a simple understanding of the systematic trend of the cross section and the energy dependence of its maximum.

Resonances in the photodepletion spectrum of the Ba…FCH3 weakly bound complex

Fano type resonances were observed in the photodepletion action spectrum of the Ba…FCH3 complex. They can be interpreted as a result of a significant coupling between the bound excited potential of the complex and the (open channel) ionic potential leading to BaF*+CH3 reaction products.

On the collision energy dependence of the reaction cross section

A microcanonical transition state model calculation is presented to account for the kinetic energy dependence of the reaction cross section σR(E T). An atom-diatom reaction scheme was used to obtain different expressions for the reaction cross section as a function of the internal density of states of the transition state and, on the other hand, of the adiabatic character of the rotational and vibrational motion of the diatomic. The model was applied to the M + RX → MX + R, M + X 20 → M0 + X 2, etc., where M = alkali, X = halogen or nitrogen and R = alkyl group. The model seems to explain satisfactorily the shape of these excitation functions and shows that the shape evolution of σR with E T is a direct consequence of the collision energy dependence of the transition state location. Indeed, it is shown that for several exoergic reactions the maximum in σR(E T) can be caused by the shift of the transition state from an early location (at the entrance channel) towards the product valley as E T increases.

Collision energy dependence of the K+C2H5Br→BrK+C2H5 reaction

Relative values of the total reaction cross section for the crossed molecular beams reaction K+C2H5Br→KBr+C2H5 have been measured over the collision energy (E T) range of 0·1–0·5 eV. The line of centres model as well as other post-threshold reaction cross section models fit the data satisfactorily, with a threshold of E 0=0·12+0·02 eV, in the studied energy zone. The present excitation function is compared to those of the closely related RX+M→MX+R (R=CH3, C2H5; X=I, Br ; M=K, Rb, Xe*) reactions and to RX dissociative electron attachment data. Definite family trends are observed for the different halogens, alkyl groups and attacking atoms. These trends are discussed in relation to the energy requirements and dynamics of the electron jump involved in such collisional processes.

REACTION DYNAMICS OF TRANSLATIONAL AND ELECTRONIC EXCITATION IN Ca + SF6 COLLISIONS

Relative values of the total chemiluminescence cross.section for the crossed molecular beam Ca∗

Reaction cross sections by time profile measurements under crossed‐beam conditions: The Ca(3P,1D)+N2O→CaO*+N2 reaction

The collision energy dependence of the Ca(3P,1D)+N2O→CaO*+N2 reaction has been determined by time profile measurements under crossed‐beam conditions. A narrow velocity distribution N2O beam collides with a pulsed beam of excited metastable Ca(3P,1D) atoms, produced by low‐voltage discharge. Time‐of‐flight spectra were used to monitor both the reagent translational and the chemiluminescence yield of the CaO* product (green arc band). The chemiluminescence cross section for excitation of the CaO green arc bands decreases with increasing translational energy with 1D reagent, but increases for the 3P state. The combination of a narrow velocity distribution of the supersonic N2O beam with short pulses of the Ca* beam leads to collision energy resolution of the order of magnitude of a few meV. This could be important not only for determining high resolution reaction thresholds, but perhaps for the onset of quantum effect in the total reaction cross section.

Reaction cross-section and product polarization in the Ca(1D2) + HBr → CaBr(A,B) + H reaction☆

Abstract The Ca ( 1 D 2 ) + HBr → CaBr(A,B) + H reaction has been studied by measuring its chemiluminescence at an average collision energy of ET = 0.16 eV under beam-gas conditions. Absolute values of the reaction cross-section for both chemiluminescence channels (e.g., A and B electronic states) were found to be σ A = 2.2 ± 0.9 A 2 and σ B = 0.52 ± 0.24 A 2 . In addition, the product polarization of the chemiluminescence emission was also measured, indicating a strong product rotational angular momentum alignment. The results are discussed in the light of a recent stereodynamical model for these, heavy + heavy-light, kinematically constrained reactions.