Electron Impact Ionization of selected hydrocarbon molecules Štefan Matejčík Comenius University Bratislava, Slovakia

Coworkers Dušan Kubala Jaroslav Kočíšek Peter Papp Pavel Mach Jan Urban Stephan Denifl Fabio Zappa Univ. Innsbruck

Electron impact ionisation - EII e + M → M+ + 2e e + M → X+ + (M-X) + 2e Appearance energies Partial cross sections for EII

Crossed beams apparatus

Electron impact ionization high Tg low Tg σ The EII is characterised from point of view of the kinetics by the cross section The cross section has a monotonic character with maximum at about 100 eV The EII is endothermic reaction with a threshold, called appearance energy of the ions In present experimet we have focused on the estimation of the threshold behaviour of the CS, estimation of the AE At elevated temperatures there are changes in the ion yield in the vicinity of the threshold E

Molecules studied in 2007 and 2008 Experiment C2D6 CH3D CD4 C3H8 C3H6D2 Theory BenHm

Ar and Ne cross sections R. Rejoub, B.G. Lindsay, R.F. Stebbings, Phys. Rev. A, 65 (2002) 042713.

Cross section EII C2D6 low temperature

Cross section EII C2D6 high temperature

Comparison total cs C2D6

Comparison partial cs C2D6

Comparison partial cs C2D6

Comparison partial cs C2D6

Comparison partial cs C2D6

Comparison partial cs C2D6

Threshold cross sections C2D6

Threshold cross sections C2D6

Ion Mass (amu) AE (100°C) AE (420°C) C2D6+ 36 11.72 11.48 C2D5+ 34 12.64 12.33 C2D4+ 32 12.00 11.54 C2D3+ 30 14.92 14.94 C2D2+ 28 14.95 14.42 CD+ 16 22.25 22.17 C2D5++ 17 35.71 34.12 CD2+ 18 15.69 15.66 C2+ 24 23.54 28.54 23.17 28.11 C2D+ 26 25.22 24.84

e + C2H6 Experiment Theory AE [eV] 293 K Thermochemistry [eV] G3B3 AIE [eV] C2H6+ 11.46±0.04 11.55 C2H5+ + H- C2H5+ + H 12.06±0.06 12.7±0.1 11.72 12.47±0.03 12.03 12.79 C2H4+ + H2 11.90±0.04 11.92±0.0006 11.93 C2H3+ + H + H2 15.02±0.1 14.47±0.46 14.83 C2H2++2H2 14.61±0.01 14.61 C2H++2H2+H C2H++H2+3H 25.7±0.3 20.5±0.15 25.1±0.15 21.65 26.12 C2+ + 3H2 22.6±0.3 20.95±0.3 21.73

EII to CD4 cross sections Tarnovsky et al. J. Phys. B: At. Mol. Opt. Phys. 29 (1996) 139

CD4+/CD4

CD4+/CD4

CD3+/CD4

CD2+/CD4

CD+/CD4

Tarnovsky et al. J. Phys. B: At. Mol. Opt. Phys. 29 (1996) 139

EII to CH3D cs

EII to C3H8

EII to C3H8

EII to C3H6D2

VIE VIE Now I would like to explain some basic concepts, probably trivial for most of the audience, concerning ionization In case of atoms we have simple situation, the atom in the ground state and the ground state of the ion. The ionization energy is defined as a difference between this two states. I the molecules is the situation more difficuls. We have to work with potential curves or potential energy surfaces and thus the IE depends on the initial geometry of the molecules. For moelcule usually two ionization anrgies are given, vertical IE and adiabatic IE. Vertical IE is defined as a energy necessary to ionise a molecule in the equilibrium geometry Adiabativ IE is defined as a energy difference between the energies os the ion and molecule in its ground states The experiments are sensitive on the lowest energy necessary to ionize molecule and this is colled appearance energy. This corresponds to the ionization energy at low distancies. As the geometry changes in the moelcule and molecular ion are only moderate, offen is the AE very close to the AIE.

EII to C3H6D2 e + M → C3H8 AE (eV) 290 K C3H6D2 (C3H8)+ v (C3H6D2)+ 10.51 ± 0.05 10.96 ± 0.05 10.81 ± 0.05 11.13 ± 0.05

Ionization cross sections calculations Kim-s BEB (binary encounter Bethe) theory [W. Hwang, Y.-K. Kim, M. E. Rudd, J. Chem. Phys. 104, 2956 (1996)], HF/6-31++G(2df,2pd) orbital kinetic energies OVGF/6-31++G(2df,2pd) ionization potentials for ionization from valence orbitals For core orbitals IP the HF values were used (their contribution to total cross section in studied region is very small).

Ionization potential of Be5Hn clusters as a function of “n”. Most stable cluster of given stechiometry. Method: MP4(SDTQ)/6-31++G(2df,2pd) Geometry: MP2/6-31++G(2df,2pd) Graf je pre najstabilnejsi konformer o danej stechiometrii, ktory sa nasiel. Zatial sme sli pre neradikalove klastre od n=10 po n=2, radikalove klastre (n=1,3) uz su spracovane, Na dalsich (n=5,7,9) sa pracuje… Aspon na tych najmensich je vidiet, ze radikalove a neradikalove sa hodne lisia, radikalove maju nizsi IP (pochopitelne, idu na closed-shell kation)

Total ionization cross sections for Be5Hn clusters Increasing hydrogen content in mixed clusters increases IP and decrease cross section for ionization

Conclusions Measured partial cs for EII to Appearance energy – C3H6D2 CD4 C2D6 C3H8 Appearance energy – C3H6D2 Calculated ionization energies Be5Hn EII total cross sections Be5Hn