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Dissociation of Multiply Ionized Molecules through Electron Impact. Pengqian Wang, C. R. Vidal Max-Planck-Institut f ü r extraterrestrische Physik Garching, Germany. Dissociation of Multiply Ionized Molecules through Electron Impact. Introduction Experimental setup Mass spectrometer
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Dissociation of Multiply Ionized Molecules through Electron Impact Pengqian Wang, C. R. Vidal Max-Planck-Institut für extraterrestrische Physik Garching, Germany
Dissociation of Multiply Ionized Molecules through Electron Impact • Introduction • Experimental setup • Mass spectrometer • Data Processing • Covariance mapping • 4. Results • Dissociation dynamics, cross sections • 5. Summary
1. Introduction • 1.Why electron impact ionization? • 1) Plasma processing in semiconductor manufacturing, • ion-molecule reaction in interstellar chemistry • 2) Electron gun: easy construction, better tunability • Problem • How much does each ionization stage contribute to a specific ion product? • 3. Purposes of this work • 1) Measuring the cross section for each dissociation channel, at 200 eV • 2) Studying dissociation dynamics • Molecules studied • Alkanes from methane to butane, OCS, HNCO (isocyanic acid), COFCl
2. Experimental setup 2.1 Experimental apparatus 2.2 A focusing time-of-flight mass spectrometer 2.3 Ion trajectory simulation
2.1 Experimental apparatusfor electron impact dissociative ionization
shield plates ion lens 2.2 A focusing time-of-flight mass spectrometer
2.3 Ion-trajectory simulation SIMION, protons, 25 eV
3. Data Processing 3.1 Principle of covariance mapping 3.2 Covariance mapping: example 3.3 Derivation of cross sections
true ion-ion coincidence 3.1 Principle of covariance mapping* Single pulse TOF spectrum: Xn(i)=0, 1 (nth pulse,ith time-bin) Second order covariance function: * Frasinski, Science 246, 1029 (1989).
3.3 Derivation of cross sections Dissociation of up to triply ionized molecules ________________________________________________ Parent ion Dissociation channel Cross section ________________________________________________ M+ X+ + n* s1(X+) M2+ X2+ + n s2(X2+) X+ + Y+ + n s2 (X+, Y+) M3+ X2+ + Y+ + n s3 (X2+, Y+) X+ + Y+ + Z+ + n s3 (X+, Y+, Z+) ________________________________________________ * "n" = neutrals (if exist).
4. Results 4.1 Multi-dimensional mass spectra 4.2 Dissociation dynamics 4.3 Cross sections
4.1 Multi-dimensional mass spectra(1) Singles mass spectrum HNCO
4.1 Multi-dimensional mass spectra(2) Covariance map C4H102+→ CHi+ + C3Hj+ + n n-butane
4.1 Multi-dimensional mass spectra(3) Covariance volume OCS3+→ C+ + O+ + S+
4.2 Dissociation dynamics(1) Sequential three-body dissociation: example-1 Second step out-of Coulomb efficient field 1)HNCO2+ HN+ + CO+ CO+ C+ + O -arctg(CO/C)= -66.8 (Exp. -66.33) 2)HNCO2+ HNC+ + O+ HNC+ C+ + n -arctg(HNC/C)= -66.0 (Exp. -66.73)
4.2 Dissociation dynamics(2) Sequential three-body dissociation: example-2 Second step inside Coulomb field 1)COFCl2+ Cl+ + COF+ COF+ CO+ + F -arctg(COF/CO)= -59.2 (Exp. -48.81.5) 2)COFCl2+ Cl+ + COF+ COF+ CF+ + O -arctg(COF/CF)= -56.6 (Exp. -49.51.5)
4.2 Dissociation dynamics(3) Metastable decay 1) Dissociation in the acceleration region (short life time) HNCO2+ → H+ + NCO+ 2) Dissociation in the drift tube (long life time) HNCO2+ → HCO+ + N q1= -arctg(N/HCO) q2= -arctg(HCO/N)
4.2 Dissociation dynamics(4) Metastable decay: observed ____________________________________________________________________ Molecule Tail of V-shape at Identification ____________________________________________________________________ C2H6 H + + C2H3+ C2H42+ H + + C2H3+ C3H8 CH3+ + C2H2+ C3H52+ C3H52+ CH3+ + C2H2+ H + + C3H3+ C3H42+ H ++ C3H3+ C4H10 CH3++ C3H+ C4H42+ CH3+ + C3H+ * C4H10+C4H10+ C3H7+ + CH3 * C4H9+C4H9+ C3H6+ + CH3 * OCS CO+ + S+ OCS2+OCS2+ CO+ + S+ * HNCO H + + NCO+ HNCO2+ H+ + NCO+ * HNCO+ HNCO+ HCO+ + N ____________________________________________________________________ * newly discovered.
4.3 Cross sections(1) Overview Total single to triple ionization cross sections and the destinations of these molecular ions
4.3 Cross sections(2) Cross sections for the different dissociation channels HNCO2+ → X+ +Y+ + n HNCO3+ → X+ +Y+ +Z+ + n
4.3 Cross sections(3) Comparison with other works 1)Total single ionizationcompared with theory 2)C3H8+ CmHn+ + n compared with other experiment -●-Present experiment -○- Ne + C3H8+ (Kupriyanov, 1965)
5. Summary • The absolute cross sections for the various dissociation channels of up to triply ionized molecules have been measured. • Molecules: small alkanes, some heteronuclear polyatomic molecules. • Ionization: electron impact at 200 eV. • Data Processing: two- and three-dimensional covariance mapping. • Information on dissociation dynamics can be retrieved from the covariance maps. New metastable decays have been discovered.
* Dissociation dynamics(*) Sequential three-body dissociation 1) Deferred charge separation 1) XYZ2+→ XY2+ + Z 2) XY2+→ X+ + Y+ 2)Initial charge separation 1)XYZ2+→ XY+ + Z+E1 2) XY+→ X+ + YE2 E1 Orientation angle q = arctg (pz/px) Second step far from Z+: q= -arctg (XY/X) Second step close to Z+: q= -45
* Dissociation dynamics(*) Three-body dissociation • Spontaneous dissociation • Sequential dissociation • B1. Deferred charge separation • 1) XYZ2+→ XY2+ + Z • 2) XY2+→ X+ + Y+ • B2. Initial charge separation • 1)XYZ2+→ XY+ + Z+E1 • 2) XY+→ X+ + YE2 E1 • Orientation angle q = arctg (pz/px) • Second step far from Z+: q= -arctg (XY/X). • Second step close to Z+: q= -45.
(*) Cross sections(*) Interesting facts-1 • Ionic product distribution from the dissociation of alkane dications • Clear similarities can be seen.
* Dissociation dynamics(*) Two-body separation Island orientation = arctg(qxpy/qypx)
