/npsi/ctrl?lang=en http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?lang=fr Access and use of this website and the material on it are subject to the Terms and Conditions set forth at http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/jsp/nparc_cp.jsp?lang=en NRC Publications Archive Archives des publications du CNRCThis publication could be one of several versions: author's original, accepted manuscript or the publisher's version. / La version de cette publication peut être l'une des suivantes : la version prépublication de l'auteur, la version acceptée du manuscrit ou la version de l'éditeur. For the publisher's version, please access the DOI link below./ Pour consulter la version de l'éditeur, utilisez le lien DOI ci-dessous.http://dx.doi.org/10.1103/PhysRevA.85.011402Physical Review, 85, 1, pp. 011402-1-011402-4, 2012-01-11 Routes to formation of highly excited neutral atoms in the breakup of strongly driven H2 Emmanouilidou, A.; Lazarou, C.; Staudte, A.; Eichmann, U. We present a theoretical quasiclassical treatment of the formation, during Coulomb explosion, of highly excited neutral H atoms (H * ) for strongly driven H 2 . This process, where after the laser field is turned off, one electron escapes to the continuum while the other occupies a Rydberg state, was recently reported in an experimental study [B. Manschwetus et al., Phys.Rev .Lett.102, 113002 (2009)]. We find that two-electron effects are important in order to correctly account for all pathways leading to H * formation. We identify two pathways where the electron that escapes to the continuum does so either very quickly or after remaining bound for a few periods of the laser field. These two pathways of H * formation have distinct traces in the probability distribution of the escaping electron momentum components.

A validated numerical model developed for the study of helium barrier discharges in the presence of dry air impurities is presented in this paper. The model was used to numerically investigate the influence of air traces on the evolution of the helium dielectric barrier discharge (DBD). The level of dry air used as impurity was in the range from 0 to 1500 ppm, which corresponds to the most commonly encountered range in atmospheric pressure discharge experiments. The results presented in this study clearly show that the plasma chemistry and consequently the discharge evolution is highly affected by the concentration level of impurities in the mixture. In particular, it was observed that air traces assist the discharge ignition at low concentration levels (~55 ppm), while on the other hand, they increase the burning voltage at higher concentration levels (~1000 ppm). Furthermore, it was found that the discharge symmetry during the voltage cycle highly depends on the concentration of air. For the interpretation of the results, a detailed analysis of the processes that occur in the discharge gap is performed and the main reaction pathways of ion production are described. Thanks to this approach, useful insight into the physics behind the evolution of the discharge is obtained.

We study the formation of highly excited neutral atoms during the break-up of strongly-driven molecules. Past work on this significant phenomenon has shown that during the formation of highly excited neutral atoms (H * ) during the break-up of H2 in a linear laser field the electron that escapes does so either very quickly or after remaining bound for a few periods of the laser field. Here, we address the electron-nuclear dynamics in H * formation in elliptical laser fields, through Coulomb explosion. We show that with increasing ellipticity two-electron effects are effectively "switchedoff". We perform these studies using a toolkit we have developed for semiclassical computations for strongly-driven multi-center molecules. This toolkit includes the formulation of the probabilities of strong-field phenomena in a transparent way. This allows us to identify the shortcomings of currently used initial phase space distributions for the electronic degrees of freedom. In addition, it includes a 3-dimensional method for time-propagation that fully accounts for the Coulomb singularity. This technique has been previously developed in the context of celestial mechanics and we currently adopt it to strongly-driven systems. Moreover, we allow for tunneling during the time-propagation. We find that this is necessary in order to accurately describe the fragmentation of strongly-driven molecules.

In this paper a validated 2D axisymmetric plasma fluid model was used to study the influence of the level of nitrogen impurities on the processes that occur in a helium parallel plate dielectric barrier discharge. The level of nitrogen impurities was varied in the range 0.1-500 ppm. It was observed that the nitrogen impurities significantly affect the dominant ion species at breakdown and the discharge characteristics. Specifically, three different dominant ions were found, which are strongly dependent on the level of nitrogen impurities. These are: + He 2 (0.1-35 ppm), + N 2 (35-150 ppm) and + N 4 (150-500 ppm). In addition, the results show that the discharge characteristics are dependent on the dominant ion species at breakdown.

Atmospheric pressure plasma jet (APPJ) can be generated in capillary tubes flowing with pure helium and with admixtures of oxygen into the pure helium. The jet exiting the tube can be used for a variety of applications through surface interaction. In this study, a twodimensional axi-symmetric model has been developed to provide insight into the evolution of capillary helium plasma jet with and without the presence of oxygen admixtures and its interaction with a dielectric surface placed normal to the jet axis. The model considers the gas mixing of helium and ambient air and the analytical chemistry between helium, nitrogen and oxygen species. Experiments were performed in similar conditions as the simulations in order to get qualitative agreement between them. The numerical and experimental results show that the evolution of the helium plasma jet is highly affected by the introduction of oxygen admixtures. In particular, it was observed that the addition of oxygen admixtures in the helium gas promotes plasma bullet propagation on the axis of symmetry of the tube (instead off axis propagation for the pure helium plasma jet). On the other hand, the presence of the dielectric surface (the slab placed in front of the tube exit) forces the plasma bullet to spread radially. Furthermore, the plasma bullet speed decreases when the helium plasma jet is operated in the presence of oxygen admixtures. The numerical results also showed that He/O2 plasma jets induced much higher electric fields on the dielectric surface in comparison to the pure helium plasma jet.