ANNUAL REPORT 2011 - Instituto de Estructura de la Materia

Rideal mechanism, with a preference for reaction of the adsorbed atoms with gas phase D atoms. Concerning ions,the triatomic H 3 + , H 2 D + , HD 2 + and D 3 + , produced primarily in reactions of diatomic ions with molecules, aredominant.The kinetics in low pressure (0.8–8 Pa) plasmas of H 2 + 10% N 2 mixtures has been investigated too, using theformer diagnostic techniques. The data analysis with a kinetic model has allowed identifying the main mechanismsresponsible for the observed distributions of neutrals and ions, and their evolution with pressure. The chemistry ofneutrals is dominated by NH 3 formation (up to 70% of remaining N 2 ) at the metallic walls of the reactor throughsuccessive hydrogenation of N and nitrogen containing radicals. Eley–Rideal and Langmuir–Hinshelwoodmechanisms are needed to account for the observed NH 3 concentrations. The plasma ionic composition, entirely dueto gas-phase processes, results from a competition between direct electron impact ionization, which depends onelectron temperatures, and ion–molecule chemistry. At 0.8 Pa, H 3 + , N 2 H + and NH 4+produced from protonation ofthe precursor molecules, and H 2 + and NH 3 + , coming from direct ionization, are found in comparable amounts. At 8Pa, collisions of H 3 + , NH 3 + and N 2 H + with NH 3 are to a great extent responsible for the final prevalence of NH 4 + .Besides, an overview of the main chemical processes which take place in cold plasmas generated in glowdischarges, both in the gas phase and at the surfaces in contact with the plasma has been done. Illustrative exampleshave been provided. Most common plasma characterization techniques have been also briefly reviewed.Dynamics and kinetics of chemical reactions and laser induced processesThe study of the the dynamics of proton (or deuteron) exchange D + + H 2 and H + + D 2 reactions has been pursuedwith special emphasis on the simulation of the whole set of measurements on cross sections and rate coefficientsgathered for these systems since the eighties. The results of this comprehensive comparison will be presented in thenear future. Rate coefficients for the mass extreme isotopologues of the H+H 2 reaction, namely Mu+H 2 and Heμ+H 2 , where Mu is muonium and Heμ is is a He atom in which one of the electrons has been replaced by a negativemuon, have been calculated using both accurate quantum mechnical (QM) and quasiclassical trajectory (QCT)methods and compared to recent experimental measurements and to calculations by Fleming et al. (Science 331,448, 20211). The QCT method can reproduce reasonably well the observations if the trajectories are weighted with aGaussian function favoring their proximity to the right quantal vibrational action. The analysis of the results showsthat the large zero point energy of the MuH product is the key factor in the large kinetic isotope effect observed.These works have been carried out within the framework of our “Unidad Asociada de Química Física Molecular”,which includes groups from the “Universidad Complutense” and the “Instituto de Química Física Rocasolano”.Laser induced processes have also been investigated in the context of this collaboration. The generation of highorder armonics from metal plasmas was studied using 1 kHz laser pulses. The dynamics of the H atom transferreaction in the photodissociation of pyrrole-ammonia clusters was investigated using the technique of velocity mapimaging. The results were found to favor a mechanism based on the direct NH bond rupture. A combinedexperimental (slice imaging) and theoretical (multisurface wave packet) study of the 304 nm photodissociatoion ofCH 3 I was also carried out. The analysis of the results evinced a concurrence of adiabatic and non-adiabatic channelsin the dissociation process.More details about this line of research and the researchers and supporting personnel that work in it, can be found inour webpage: http://www.iem.cfmac.csic.es/departamentos/fismol/fmap/main.htmMOLECULAR FLUID DYNAMICSThe main goal of this research line is the study of fluid flows in the interphase between the microscopicdescription,essentially molecular and quantum, and the macroscopic one, governed by the continuum fluidmechanics. Althoughboth limits are well developed as independent fields, their link is a sort of no-man's landlacking experimental data,sufficiently rigorous theoretical models, and efficient calculation methods. From the experimental point of view,undercooled liquid jets are an ideal medium for studying the homogeneous solidification, free from container walleffects orimpurities. On the other hand, supersonicgas jets are a very fruitful research media, where the study ofinelastic collisions, the fundamental mechanism ofenergy transfer between molecules, can be afforded.Along the last eighteen years, we have built at the Laboratory of Molecular Fluid Dynamics twocompleteinstruments for jet diagnostics, whose performance and flexibility are unique worldwide. One of them (A)is devoted to the study of cohesive collisions and liquid jets, and the other one (B) to inelastic collisions. Someimprovements have been carried out along 2011 on these instruments to suit the experiments. In this regard, a newsystem of three motorized microactuators to move the nozzle along an arbitrarily tilted path, with a resolution betterthan 100 nm, has been installed on instrument (A). This is of great help for the Raman spectroscopic study ofundercooled liquid microfilaments produced from glass capillary nozzles cooled by liquid He, which are rarelycollinear with the nozzle. On the other hand, we have acquired a special vacuum group for instrument (B), to pump68