Bulletin of the American Physical Society
68th Annual Gaseous Electronics Conference/9th International Conference on Reactive Plasmas/33rd Symposium on Plasma Processing
Volume 60, Number 9
Monday–Friday, October 12–16, 2015; Honolulu, Hawaii
Session UF4: Heavy-Particle Collisions and Swarms |
Hide Abstracts |
Chair: Michael Schulz, Missouri University of Science and Technology Room: 303 AB |
Friday, October 16, 2015 1:30PM - 2:00PM |
UF4.00001: Biomolecular Ionization and Fragmentation Dynamics after Interaction with keV Ions and Energetic Photons Invited Speaker: Thomas Schlathoelter |
Friday, October 16, 2015 2:00PM - 2:30PM |
UF4.00002: An independent-atom-model description of ion-molecule collisions including geometric screening corrections Invited Speaker: Tom Kirchner A simple way of calculating charged-particle-induced electron removal cross sections for molecular targets consists in adding up atomic cross sections for all the atoms that make up the molecule. This procedure is commonly referred to as Bragg's additivity rule (AR) and is based on the independent-atom model, in which the molecule is viewed as a collection of (undistorted) atoms. The AR works well at sufficiently high collision energies, where the atomic cross sections tend to be small. For electron-molecule collisions several extensions of the AR have been proposed to make it applicable to lower impact energies, where the atomic cross sections are large and the AR results in an overestimation of the experimental data. One such extension is the so-called screening-corrected additivity rule (SCAR) [1]. For each atomic cross section $\sigma_A$ in the AR a weight factor $0\le s_A \le 1$ is introduced to account for the partial screening of the atoms due to the geometrical overlap of the $\sigma_A$'s when viewed from the incident electron. The weight factors are determined heuristically and are interpreted as orientation-averaged screening coefficients. In this contribution, we propose a similar model for net ionization and electron transfer in heavy-particle collisions, but in contrast to the SCAR model the weight factors do depend on the orientation of the molecule relative to the projectile beam direction. For a given geometry we construct a space-filling-like model of the molecule by surrounding each atom $A$ by a sphere of radius $r_A = \sqrt{\sigma_A/\pi}$. The weight factors in the SCAR-like cross section formula are determined as those fractions of the $\sigma_A$'s that are visible for an observer that moves with the impinging projectile. The procedure is repeated for a number of molecular orientations, and total cross sections that can be compared with experimental data are obtained by averaging over all orientations. The atomic cross sections are calculated by using the two-center basis generator method [2], while the molecular geometry information that enters the calculation of the screening coefficients is taken from the literature. In my talk, I will explain the model in more detail and will present total cross section results for proton collisions from a number of targets ranging from diatomic molecules such as H$_2$ and CO to intermediate-size hydrocarbons such as butane. \\[4pt] [1] F. Blanco \textit{et al.}, Phys. Lett. A {\bf 374}, 4420 (2010).\\[0pt] [2] M. Zapukhlyak \textit{et al.}, J. Phys. B {\bf 38}, 2353 (2005). [Preview Abstract] |
Friday, October 16, 2015 2:30PM - 2:45PM |
UF4.00003: Electron swarm from very low to intermediate $E/N$ in homonuclear diatomic molecules H$_{2}$, O$_{2}$ and N$_{2}$ M.A. Ridenti, L.L. Alves, V. Guerra, J. Amorim In this work the homogeneous Boltzmann equation is solved in order to describe the electron swarm in N$_{2}$, O$_{2}$ and H$_{2}$ within the interval $10^{-4} - 10$~Td. Elastic, rotational and vibrational collisions are taken into account and it is shown how each of these channels contributes to the electron energy balance as a function of $E/N$. Three different approaches are adopted to account for the rotational collisions. The first one, which gives the most accurate results, consists of computing the discrete inelastic / superelastic collisional operator, written for a number of rotational levels that depends on the molecular gas and the specific rotational cross sections considered. The second approach is the continuous approximation for rotations, as proposed by the classical work of Frost and and Phelps (Phys. Rev. 1962). The last approach is a modified version of the continuous approximation for rotations, including a Chapman-Cowling corrective term proportional to the gas temperature, which is deduced here. Results from this last approach show that it may be used to bridge the gap between the discrete and the continuous descriptions at low/intermediate $E/N$. The calculations are compared with measurements for the available swarm parameters. [Preview Abstract] |
Friday, October 16, 2015 2:45PM - 3:00PM |
UF4.00004: Measurement of negative ion mobility varying with a little amount of H$_{2}$O in O$_{2}$ Yui Okuyama, Susumu Suzuki, Haruo Itoh A study of transport properties such as a mobility of charged particles is importance in understanding discharge plasmas. These fundamental data have been collected in some databases. The authors have been investigated the effects of impurities such as H$_{2}$O, CO$_{2}$, N$_{2}$ on the negative ion mobility in O$_{2}$ at high pressures including atmospheric pressure [1, 2]. Especially, the effect of a trace amount of H$_{2}$O in O$_{2}$ could not to be avoided for the mobility measurement due to formation of cluster ions O$_{2}^-$ $\cdot$ (H$_{2}$O)$_{\mathrm{n}}$ (n $=$ 1, 2, 3, ...). In this study, the mobility of negative ions was measured in O$_{2}$ varying with the H$_{2}$O concentration from 100 to 17000 ppb. The H$_{2}$O concentration was monitored with a trace moisture analyzer whose operation was based on a photoabsorption method. As the results, a constant mobility of 2.39 cm$^{2}$/V$\cdot$s was observed in ultrahigh-purity O$_{2}$ (99.99995{\%}, purified with a gas purifier), in which the H$_{2}$O concentration was monitored to be between 15 and 100 ppb. This value was good agreement with the mobility of O$_{4}^-$ in previous report [1]. Then, a small amount of H$_{2}$O from 2000 to 17000 ppb was added to the ultrahigh-purity O$_{2}$. Two kinds of mobilities 2.31 and 2.21 cm$^{2}$/V$\cdot$s were observed in H$_{2}$O concentration ranges of 2000 - 4600 and 4600 - 17000 ppb, respectively. Former one is good agreement with the mobility observed in high-purity O$_{2}$ (99.9999{\%}) [2]. These mobilities are considered to be those of O$_{2}^-$ $\cdot$ (H$_{2}$O)$_{\mathrm{n}}$ (n $=$ 1, 2). [1] Y. Okuyama et al, 66th Annual Gaseous Electronic Conference, 57, 8, MR1.00091 (2013). [2] Y. Okuyama et al, J. Phys. D: Appl. Phys., 45, 195202 (2012). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700