Session GT1: Electron and Photon Collisions - Excitation

Quantitative measurements of electron collision cross sections and their database related to plasma modeling

The electron excitation cross sections of fluorine containing molecules are very relevant to plasma-assisted fabrication of large scaled-integrated circuits, semiconductor manufacturing and other technological applications. However, despite their importance, few cross sections have been reported except for major molecules such as tetrafluoromethanes. In the present study, therefore, we measured the differential and integral cross sections (DCSs and ICSs) of difluoromethane, CH₂F₂.

The cross-section measurements were performed using a crossed-beam apparatus in the incident electron energies of 4.0 - 15 eV and over scattering angle range of 10^o - 130^o. Angular distributions of elastically and inelastically scattered electrons were normalized to the well-known absolute elastic DCS of helium by the relative flow technique. Elastic and inelastic ICSs were also obtained by extrapolating the DCSs measured over the angular range from 0^o to 180^o.

The ICSs for vibrational excitations to the fundamental modes and their combinations were extracted in addition to the elastic DCSs reported in the literature. The cross-section dataset of CH₂F₂ molecules has been updated by adding the present vibrational ICSs for each vibrational mode.   

Calculation of electron scattering from tin atoms

The relativistic convergent close-coupling (RCCC) method has been applied to study electron scattering from the atomic tin. Tin is of interest in fusion research as it may be used to monitor the erosion of fusion reactor wall tiles. It is also of interest to the nano-lithography industry, particularly with respect to the production of extreme ultra-violet light (EUV) which is generated using a plasma of tin ions. Both these applications require comprehensive collision data sets to allow accurate modelling of plasmas involving tin. We obtained integrated and momentum transfer cross sections for elastic scattering alongside integrated and differential cross sections for excitations to the 5p², 5p6s, 5p5d and 5p6p manifolds from the ground and first four excited states for projectile energies ranging from 0.1 to 500 eV. Total ionisation cross sections were obtained for the ground and first four excited states, including contributions from direct ionisation of the valence 5p shell and the closed 5s shell, and contribution from excitation auto-ionization. The tin atom is described by a model with two 5p-electrons above a frozen [Kr]4d¹⁰5s² core. Phenomenological one- and two-electron polarization potentials were used to best fit the calculated energy levels and optical oscillator strengths to experiment. Convergence was tested using several models with the number of target states included in the calculations varying from 33 to 399. For total ionisation cross sections, we find good agreement with experiment and other theories while for excitation cross sections the agreement is mixed.   

Maximum angular momentum transfer in electron-atom collisions

The electron–atom collision process can be fully described by determining complex scattering amplitudes, which depend on collision energy and scattering angle. In the case of electronic excitation from the S to P atomic state, such data can be obtained using the electron-photon coincidence technique. The results of such experiments are usually presented as Electron Impact Coherence Parameters (EICP), which describe the angular distribution of the electron charge cloud of an excited P state. They are the alignment angle of the electron charge cloud (γ), its shape parameter (P_L), angular momentum transfer (L_perp) and coherence parameter (P⁺)[1].

 

Our experimental EICP data analysis for zinc and cadmium atoms allowed us to find some helical structures in gamma function in energy and scattering angle domains. They correspond to the special conditions in which only one of two scattering channels is available. Therefore the electron charge cloud is rotationally symmetric (P_L=0), the alignment angle parameter is undefined, and the angular momentum transfer reaches the maximum value (+/-1). Due to their local and narrow character, such structures can be an exact tool for verifying theoretical models with experimental results.

    

electron impact excitation cross sections of neutral molybdenum : cross sections of interest in plasma modeling

Molybdenum plays a vital role in plasma-mediated nuclear fusion devices such as International Thermonuclear Experimental Reactor (ITER), as it can be used to make first mirrors in the optical diagnostic systems. The present work calculates the electron impact excitation (EIE) cross sections for the fine structure resolved transitions in the Molybdenum (Mo) atom from the manifolds, 4d⁵5s, 4d⁴5s² , 4d⁶ and 4d⁵5p to the manifolds 4d⁵5s, 4d⁴5s² , 4d⁶ , 4d⁵5p, 4d⁴5s5p, 4d⁵5d, 4d⁵6s, 4d⁴5s6s, and 4d⁵7s. The calculations are performed using the multi-configurational Dirac-Fock (MCDF) wavefunctions and the relativistic distorted wave (RDW) approximation. The calculated oscillator strengths and transition probabilities are compared with the NIST atomic database for validating the reliability of the obtained cross sections. And EIE cross sections are compared with the available calculations and measurements reported by Badnell et al.,(J. Phys. B29,3683(1996)), Bartschat et al.,(J. Phys. B35,2899(2002)) and Smyth et al., (Phys.Rev.A96,042713(2017)). The cross sections are reported for the projectile electron energy ranging from threshold to 500 eV. The present consistent cross sections are required for developing collisional radiative models and thereby performing the spectroscopic characterization of non-equilibrium plasmas. The detailed results along with methodology and discussion will be presented at the conference.