Bulletin of the American Physical Society
2006 59th Annual Gaseous Electronics Conference
Tuesday–Friday, October 10–13, 2006; Columbus, Ohio
Session WF2: Heavy Particle Collisions, Attachment, and Recombination |
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Chair: Zoran Petrovic, Institute of Physics, Belgrade Room: Holiday Inn Salon B |
Friday, October 13, 2006 10:00AM - 10:30AM |
WF2.00001: Detailed Studies of Cold and Ultracold Ions in RF Traps Invited Speaker: The physics and chemistry determining the composition of the interstellar medium is quite different from other chemical systems since low densities ($<$~10$^{7}$~cm$^{{\-}3})$ and low temperatures prevail. For example, modeling the early stages of protostellar collapse requires a detailed understanding of ultracold hydrogen plasmas. This contribution concentrates on recent results in the field of cold gas phase ion chemistry, producing, modifying or destroying C$_{m}$H$_{n}^{+}$ ions and their deuterated variants (m$\ge $0, n$\ge $0). All experimental activities with charged particles are based on \textit{inhomogeneous, time dependent electric fields}, created with suitable electrode arrangements. Rather complex machines have been constructed by combining for example temperature variable multi-electrode ion traps (e.g. a 22-pole trap) with atomic or molecular beams. In addition analytical tools such as laser induced reactions are used in order to understand the low energy dynamics on a state specific level. It is emphasized that reactions (in the trap or in space) often do not reach thermodynamical equilibrium, especially if traces of ortho-hydrogen are present. [Preview Abstract] |
Friday, October 13, 2006 10:30AM - 10:45AM |
WF2.00002: Kinetic Energy Dependence of Endothermic Charge Transfer (KEDECT) in Xenon Ion -- Hydrocarbon Collisions Charles Jiao, Biswa Ganguly, Alan Garscadden KEDECT has been studied by Fisher and Armentrout (ref 1) who showed clearly the strong influence of modest rare gas ion kinetic energy in opening up dissociative charge transfer channels in silane. Charge transfers of fast xenon ions with CH$_{4}$, C$_{2}$H$_{4}$ and C$_{3}$H$_{8}$ are studied using xenon plasma expansion through a small nozzle into low-pressure hydrocarbon gas. Branching ratios and relative reaction rates creating the parent ion and smaller molecular ions are measured under selected power levels of an rf coupled discharge. Xe$^{+}$ reacts with CH$_{4}$ to produce CH$_{4}^{+}$ (I) and CH$_{3}^{+}$ (II) in the ratio 1: 0.56 and an estimated rate coefficient 2.6 x 10$^{-10}$ cm$^{3}$/s. The Xe$^{+}(^{2}$P$_{1/2})$ reaction I with CH$_{4}$ is exothermic by 0.83 eV while reaction II is endothermic by 0.88 eV. The Xe$^{+}$ ($^{2}$P$_{3/2})$ reactions with CH$_{4}$ are endothermic by 0.48 and 2.19 eV respectively, however mass-spectrometry of the interaction region shows that the reactions do occur. The xenon plasma expansion can add up to 3 KTe kinetic energy to the ion, permitting the reactions to occur. Results for the other collision partners C$_{2}$H$_{4}$ and C$_{3}$H$_{8}$ are discussed. \newline \newline [1] Ellen R. Fisher and P.B. Armentrout, J. Chem. Phys., 93, 4858 (1990). [Preview Abstract] |
Friday, October 13, 2006 10:45AM - 11:00AM |
WF2.00003: Identification of modes of vibration in a HeNe* temporary molecule and interference effects in slow He-Ne collisions Cristian Bahrim, Joseph Hunt The model potential for electrostatic interaction between He(1s$^{2})$ and Ne$^{\ast }$(2p$^{5}$3p) atoms developed by Bahrim \textit{et al.} (\textit{Physical Review A }\textbf{56}, 1305 (1997)) leads to 36 adiabatic electronic potentials, which were successfully tested in calculations for experiments in atomic crossed beams and discharge cells. The existence of deep potential wells below 6 a$_{o}$ suggests that modes of vibration could form within these wells during a He-Ne collision. We identify several modes of vibration by using a Morse potential which best fits the electronic potential wells. Further, a set of transitions between vibrational-electronic states is proposed. For experimental testing of our results an IR laser spectroscopy technique is proposed. The abundance of Ne*(2p$_{i})$ atoms after collision and successful absorption of IR photons is discussed. Also, this paper explains the oscillations observed in quantum probabilities for intermultiplet transitions between $2p_{i}$ states of the Ne*(2p$^{5}$3p) atoms induced by collisions with He(1s$^{2})$ atoms, which were reported in \textit{Physical Review} $A$\textbf{ 56}, 1305 (1997). Our semi-classical model explains the formation of the quantum oscillations as being the result of the interference between matter waves associated to two collisional channels near the avoided crossing region between these channels. [Preview Abstract] |
Friday, October 13, 2006 11:00AM - 11:15AM |
WF2.00004: Study of Vibration-Vibration and Vibration-Electronic Energy Transfer in Nitric Oxide Allen White, Igor Adamovich, J. WIlliam Rich The v=1 vibrational level of nitric oxide is populated via resonant absorption of a single line carbon monoxide laser. Higher vibrational and electronic levels of nitric oxide are populated by v-v and v-e energy transfer mechanisms, respectively. Infrared overtone emission spectra are observed via time resolved step-scan Fourier transform infrared measurements and time resolved gamma band and beta band ultraviolet emissions are also measured. Additional measurements were taken to detect associative ionization in V-V pumped NO, as well as the electron production rate. The v-v energy transfer rates are inferred by comparing time-resolved experimental results to computational models. Results indicate that current NO kinetic rate models must be modified to describe experimentally observed population rise times presented here. [Preview Abstract] |
Friday, October 13, 2006 11:15AM - 11:30AM |
WF2.00005: Dissociative Electron Attachment to Acetylene Ann Orel, Slim Chourou Experimental studies of electron impact on acetylene show the presence of a $\Pi$$^{*}$ resonance at 2.6 eV which leads to C$_{2}$H$^{-}$ + H. These fragements both have $\Sigma$ symmetry (C$_{2}$H$^{-}$, $^{1}$$\Sigma$; H, $^{1}$S), therefore, there must exist a curve crossing at bent geometries to explain these fragements. We performed electron scattering calculations using the complex Kohn variational method to determine the resonance parameters of this system. We discuss the mecanisms leading to dissociation into the product channels and report the computed cross sections. The results are then compared to available experimental findings. Work supported by NSF PHY-05-55401. [Preview Abstract] |
Friday, October 13, 2006 11:30AM - 11:45AM |
WF2.00006: State-selected predissociation of H$_3$ Valery Ngassam, Ann E. Orel Experimental studies of the predissociation of well-defined Rydberg states of H$_3$ have produced a complex three-body fragmentation pattern that is highly dependent on the initial state and show dramatic isotope effects. We present results of theoretical investigations for the fragmentation of selected Rydberg states of H$_3$ into three ground state hydrogen atoms as well as two-body predissociation into H + H$_2^+$(v,j). The non-adiabatic couplings and the surfaces are taken from previous studies. The dynamics are carried out using a wave packet propagation method in full-dimensionality including the effects of the Jahn-Teller interaction. Work supported by the NSF PHY-02-44911. [Preview Abstract] |
Friday, October 13, 2006 11:45AM - 12:00PM |
WF2.00007: Yield of electronically excited CN molecules from the dissociative recombination of HNC$^{+}$ ion with electrons Rainer Johnsen, Richard Rosati, Daphne Pappas, Michael Golde We report flowing-afterglow measurements of the CN(B-X) and CN(A-X) emissions from the dissociative recombination (DR) of HNC$^{+}$ ions. A separate drift-tube study showed that the reaction Ar$^{+}$+ HCN, the precursor reaction used in the flow-tube experiment, produces mainly HNC$^{+}$ rather than its HCN$^{+}$ isomer. Recombining HNC$^{+}$ afterglows showed emissions of CN (B-X) and CN(B-A) but some arise from excitation transfer of metastable argon, Ar* + HCN. By adding xenon, Ar* atoms were removed and the pure recombination spectrum was recovered. Models simulating the ion-chemical processes, diffusion and gas mixing, were fitted to observed position-dependent CN band intensities. Absolute yields of CN (B) and CN (A) were inferred by comparing band intensities to those of CO bands from DR of CO$_{2}^{+}$ ions. We conclude that the 300 K recombination coefficient of HNC$^{+}$ is close to 2$\times $10$^{-7}$ cm$^{3}$/s, that CN(B) is formed with a yield of $\sim $20{\%} and CN(A) with a yield of $\sim $12{\%}. The rotational temperature of CN(B) is around 2500 K, and CN(B) and CN(A) are far more vibrationally excited than predicted by the ``impulse model'' of Bates. This finding suggests that the recombination may involve a multistep mechanism. [Preview Abstract] |
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