Low - energy elastic electron scattering by acetylene

Acetylene, C2H2, is a prototypical molecule in several respects. As a readily available gas and one of the smallest and most symmetric polyatomic molecules, it is amenable to both experimental and computational study. It is the smallest member of the important family of unsaturated hydrocarbons and is isoelectronic with N2, having in common with those molecules both a bonding πu orbital and an empty πg antibonding (π∗) orbital that strongly influences photoabsorption, photoionization, and electron-collision spectra. Electron collisions are important in the chemistry of acetylene plasmas [1], and acetylene has been detected in natural environments where free electrons may occur, including the interstellar medium [2] and the atmospheres of Jupiter [3], Saturn and its moon Titan [4], Neptune [5,6], and Uranus [6–8]. Previous experimental and theoretical studies have treated several aspects of electron-acetylene collisions. Karwasz and coworkers [9] have reviewed the body of work up to 2001. Total scattering cross sections were measured at low energies by Brüche [10] and by Sueoka and Mori [11], and, more recently, at intermediate and high energies by Xing and coworkers [12], Ariyasinghe and Powers [13], and Iga and coworkers [14]. Total cross sections above 10 eV have been calculated using a number of simplified models [15–18]. Recently, Vinodkumar and coworkers [19] have computed the total cross section for electron-acetylene scattering from 1 to 5000 eV using a hybrid approach: below 15 eV, they apply theR-matrix method, while above 15 eV they use local potentials. Kochem and coworkers [20] studied the resonant vibrational excitation of acetylene via the 2 g resonance near 2.5 eV, while Andrić and Hall [21] measured vibrational excitation via both the 2 g resonance and a 2 g resonance at 6 eV, as well as higher-lying Feshbach resonances. Dissociative attachment mediated by the 2 g resonance has been the subject of several experimental studies [22–26], the most recent and detailed being that of May and coworkers [26], while Chorou and Orel [27,28] have carried out corresponding ab initio calculations in which the results agree well with the measurements of May and coworkers. At very low energies, information on the momentum-transfer and vibrationalexcitation cross sections has been inferred from drift-tube measurements, most recently by Nakamura [29]. Differential cross sections (DCSs) for elastic scattering of electrons by acetylene have been measured by several groups. In the intermediate-energy range, DCSs were obtained by Fink and coworkers [30] from 100 to 1000 eV and by Iga and coworkers [14] from 50 to 500 eV. At lower energies, the first measurements appear to be those of Hughes and McMillen [31], who obtained relative elastic DCSs from 10 to 100 eV. The study of Kochem and coworkers [20], though primarily focused on vibrational excitation, also produced elastic-scattering data in the low-energy range, including the DCS at 2 eV as a function of scattering angle and the DCS at a fixed angle of 90◦ as a function of energy between 0 and 3.5 eV. Further results from Kochem and coworkers were quoted in the later paper of Jain [32]. Elastic and vibrationally inelastic DCS measurements from 5 to 100 eV were reported by one of us, Khakoo, and coworkers [33]. However, discrepancies remain between existing measurements at all energies, motivating the present study using improved experimental techniques. There have been several computational studies of lowenergy elastic electron-acetylene scattering. Using local potentials to model the electron-molecule interaction, Thirumalai and coworkers [34] computed DCSs at 10 eV for rotationally elastic and inelastic scattering, as well as the rotationally summed DCS that is directly comparable to typical measurements, in which rotational structure is not resolved. Tossell [35] used a different local-potential approach, namely, the multiple-scattering Xα method, to compute the integral cross section in the vicinity of the 2 g resonance, as well as other properties. Khurana and Jain [36] and Jain [32] likewise applied local model potentials to the problem, reporting results up to 20 eV, while Gianturco and Stoecklin [37] used a similar approach to obtain results up to 50 eV. Using all-electron, ab initio methods, Krumbach and coworkers [38] focused on computing cross sections for resonant vibrational excitation, but in the process they obtained results for the energy and width of the 2 g resonance as a function of the C–C bond distance. We also note studies by Venkatnathan and Mishra [39], using an electron propagator method, and recently by Ghosh and coworkers [40], using coupled-cluster techniques, in which the energy and width of the 2 g resonance were computed at the equilibrium geometry. Mu-Tao and coworkers [41] used an all-electron formulation, i.e., the iterative Schwinger method,