350 likes | 453 Views
Department of Experimental Physics, Comenius University Bratislava , Slovakia. Formation of positive ions by electron impact: Temperature effects Š. Matejčík. Department of Experimental Physics. Division of plasma physics l ow temperature plasma
E N D
Department of Experimental Physics, Comenius University Bratislava, Slovakia Formation of positive ions by electron impact: Temperature effects Š. Matejčík
Department of Experimental Physics Division of plasma physics • low temperature plasma technological plasmas, glow discharges ... • high pressure discharges corona discharge, barrier discharge ... • reactions in the plasma electron impact ionization, electron attachment to the molecules ...
Reactions in plasma • Electron interactions • Photon induced reactions • Ion –molecule reactions • Neutral reaction • Reactions on the surfaces
Electron interactions in plasma • Electron impact ionization • Electron attachment • Excitation • Dissociation • Electron reaction on the surfaces
Comenius university • Electron attachment • Electron impact ionization • Electron induced reactions on surfaces Preparation • Electron impact excitation
Electron impact ionization: e + AB → AB+ + 2e Dissociative electron impact ionization: e + AB → A+ + B + 2e Ion pair formation: e + AB → A+ + B- + e Fusion related research Molecules: CH4, CH3D, CD4, C2H6, C3H8
We have studied • appearance energies (AE) of the positive ions • gas temperature effects on the AE • isotopic effects on AE Theory: Ionization energies (OVGF, CCSD(T)) Appearance energies (G3, G3B3 and CBS-APNO, CBS-Q)
Experimental setup: E = 0 – 150 eV FWHM ~50-140 meV Ie~20-100nA T = 293K 693K
high Tg Electron impact ionization low Tg σ E
Rate coefficient f(E,Te) high Tg low Tg σ E
VIE AE AIE A+ IE A
AE2 E f ΔAE = AE2-AE1= = Ei(T2)-Ei(T1) Ei(T)=Ev(T)+Er(T) AE1
Sensitivity in present experiment: σ ~10-26 - 10-25 m2 σ E
Threshold behaviour of the cross sections for EII Atoms: Wannier theory: w(E,AE,A,d) = 0 E<AE A(E-IE)d E>AE d=1.127 Fitting function: w(E,AE,A,d) f(E,U) - EEDF
Threshold behaviour of the cross sections d=1.127±0.01
Threshold behaviour of the cross sections for EII Molecules - empirical formula: w(E,AE1,AE2,d1,d2A) = 0 E<AE1 A(E-AE1)d1 E>AE1 A(E-AE2)d2 + A(E-AE1)d1E>AE2 ...
Temperature effects on Appearance energies of the ions Predicted by: Rosenstock – review article Plessis&Marmet – in CH4 and C2H6 papers H. M. Rosenstock, Int. J. Mass Spectr. Ion Phys., 20 (1976)139 P. Plessis et al., Can. J. Chem. 65 (1987) 1424, J. Phys. B, 16 (1983) 1770
CH4 e + CH4→ CH4+ + 2e CH3+ + H + 2e CH3+ + H- + 2e CH2+ + H2 + 2e CH+ + H2 + H+ 2e C+ + 2H2 + 2e ...
293K CH4+/CH4 693K
Ei(T) - rotational and vibrational energy of the molecule at T Ei =Ei(693) – Ei(293 K) ~ 0.12 eV Stano et al., J. Phys. B: At. Mol. Opt. Phys. , 36 (2003) 261
CH3+/CH4 293K 693K
AE1=12.75 eV AE1=12.62 eV CH3D+/CH3D 293K 693K
C2H6+/C2H6 293K 693K
C3H8+/C3H8 293K 693K
Summary Temperatures 293 and 693 K
Isotopic effect on AE Changes in vibrational energies in isotopomers Heavy isotopomer – lower zero point energy => larger AE Comparison: CH4, CD4, CH3D
Isotopic shift a) F. P. Lossing, A. W. Tickner, W. A. Bryce, J. Chem. Phys.19, 1254 (1951). b) V. H. Dibeler, M. Krauss, R. M. Reese, F. N. Harlee J. Chem. Phys.42, 3791 (1965).
Conclusions experimentally observed dependence of AE´s on the temperature magnitude of the shift ~vibrational and rotational excitation of the molecule confirmed isotopic shift in the appearance energies of the molecules
Contribution to new CRP • Experiment: • The temperature effects: • estimation of the partial cross sections in C3H8 for particular fragment ions • - evaluation of the rate coefficients • Theory - QM: • - BenHm - electronic structure, binding energies, volatility, ionization potentials and electron affinities • - H2 reactions with Ben
Coworkers Comenius University Bratislava M. Stano, E. Vašeková, J.D. Skalný, D. Kubala, J. Kočíšek Theorists:I. Hubac, J. Urban, P. Mach S. Denifl, T.D. Märk University Innsbruck Support VW Stiftung, EU (Euroatom-FUSION), IAEA, APVT