Direct evidence for channel-coupling effects in molecules: electron impact excitation of the a″¹Σ⁺_gstate of N₂

Abstract

New measurements of differential cross-sections for excitation of the a 1 + g (v = 0, 1) state in molecular nitrogen reveal a cusp-like angular distribution. This feature is distinctly observed for the first time in the present electron energy-loss experiment as a result of finer scattering angle grid and impact energy coverage than previous measurements. This feature is similar to that observed in atomic targets such as He, Hg and Ba. The observed phenomenon suggests an interference effect related to configuration-interaction coupling between lower and excited states that are of the same symmetry. It is hoped that the present work will stimulate theoretical investigations into the physics that governs this cusp-like behaviour.(Some figures in this article are in colour only in the electronic version)Electron impact excitation of atomic and molecular targets is of continuing importance in our understanding of a host of natural and man-made phenomena, including aurora [1], airglow [2], plasma sterilization [3] and lighting technologies [4,5]. Considerable effort has been made in the past to both theoretically and experimentally determine electron scattering crosssections for these targets. Whereas significant progress has been made for atomic targets [5][6][7], particularly through benchmarking against distinguishing experimental features such as resonances [8,9] and electron impact coherence parameters [10,11], even homonuclear diatomic molecular targets (e.g., N 2 ) remain a significant challenge to theory. This difficulty is largely on account of their reduced symmetry and additional rotational and vibrational degrees of freedom in nuclear motion, which couple into the electronic motion. Distinct molecular features observed experimentally are necessary to test and refine different theoretical approaches to better understand and quantify electron interference effects. Moreover, both

Division Xii / Commission 14 / Working Group Collision Processes

“…For molecules, new elastic scattering references have appeared as follows: C 3 (Munjal & Baluga 2006); CH 4 (Cho et al 2008); H 2 CO (Kaur & Baluja 2005); C 6 H 6 , C 6 F 6 , C 6 H 12 , C 6 H 14 , C 6 F 14 , C 8 H 16 , C 8 H 18 , and C 8 F 18 (Shi et al 2008); SF 4 (Szmytkowski et al 2005); SO 2 Cl 2 (Szmytkowski et al 2006); SiN 2 , SiCO, and CSiO (Fujimoto et al 2007); pyrazine (Winstead 2007); propane (Bettega, da Costa, & Lima 2008); methanol and ethanol (Khakoo et al 2008); propene and cyclopropane (Makochekanwa et al 2008); and NeF (Kaur et al 2008 ); CF 4 (Irrera & Gianturco 2005); N 2 (Tashiro & Morokuma 2007, Khakoo et al 2007, ethene (Allan, Winstead, & McKoy 2008); and NeF (Kaur et al 2008). …”

Threshold effects in positron scattering from isoelectronic series of atoms and molecules

“…Wigner cusps [3], first articulated in the context of nuclear scattering, represent a classic example of such channel coupling. They have also been observed in low-energy electron scattering from atoms and molecules [4][5][6]. Another, slightly different but very common, example of channel coupling occurs in electron scattering from both atoms and molecules and involves the formation of transient negative ions, or resonances.…”

Benchmarking electronic-state excitation cross sections for electron-N2collisions

“…It is therefore not surprising that significant effort, particularly at the Jet Propulsion Laboratory (JPL) and also more recently at the Fullerton campus of the University of California [7][8][9][10][11][12][13][14][15], has gone into measuring differential cross sections (DCSs) and integral cross sections for the electron impact excitation of the electronic states in N 2 . A summary of the early results can be found in Ref.…”

“…A summary of the early results can be found in Ref. [16], while those from the more recent measurements are detailed in the JPL/Fullerton papers [11][12][13][14][15]. Unfortunately, despite all these endeavors, if we were to characterize the level of agreement between these studies, over the common energy, angular range, or both, for the 17 lower-lying N 2 electronic states, then we could only conclude that it remains "patchy" at best.…”

“…We therefore report results from a very specific series of DCS measurements, which in no way attempt to mimic the comprehensive studies already available in the literature [7][8][9][11][12][13][14][15]17,21]. Rather, here we target various topical kinematic conditions in order to try and shed new light on the discrepancies between the available DCS data for the a 1 + g states.…”