1. 1 Pengqian Wang Department of Physics Western Illinois University March 4, 2013

2. 2 Garching, GERMANY

3. 3 Manhattan, KANSAS

4. 4 Motivations for studying the dissociation of molecular ions Exploring the structure and energy levels of molecules and molecular ions, which is a main goal of atomic and molecular physics.Exploring the structure and energy levels of molecules and molecular ions, which is a main goal of atomic and molecular physics. Understanding the dynamics of molecules when interacting with electron beams or intense lasers.Understanding the dynamics of molecules when interacting with electron beams or intense lasers.

5. 5 In this talk Introduction 1) Dissociation of molecular ions 2) Challenge: Pathway-specific measurementIntroduction 1) Dissociation of molecular ions 2) Challenge: Pathway-specific measurement Methods 1) Coincidence mass spectroscopy 2) Three-dimensional momentum imagingMethods 1) Coincidence mass spectroscopy 2) Three-dimensional momentum imaging Results 1) Electron impact dissociative ionization 2) Molecules interact with intense lasersResults 1) Electron impact dissociative ionization 2) Molecules interact with intense lasers

6. 6 Dissociation of molecular ions: Molecular orbitals and potential energy curves Light or electron beams Excitation Momentum and kinetic energy release Dissociation Ionization Dissociative ionization

7. 7 Dissociation channels of molecular ions Example: Dissociative ionization of CO _________________________________ Parent ionDissociation channel _________________________________ CO  CO +  CO +  C + + O  C + O +  CO 2+  CO 2+  C 2+ + O  C + O 2+  C + + O +  CO 3+  … _________________________________

8. 8 Dissociation of molecular ions: Pathway-specific measurements are required Challenge 1: Distinguishing different channels. CO +  C + + O CO 2+  C + + O + CO 3+  C + + O 2+ Solution: Coincidence measurement 1) ~1 molecular dissociation per pulse. 2) Record the fragments produced in each single pulse. Challenge 2: Pathway-specific measurements: Identifying different pathways. CO 2+  C + + O + + E k1 CO 2+  C + + O + + E k2 Solution: Momentum imaging Measure the three-dimensional momentum of each fragment.

9. 9 Experimental Methods

10. 10 Time-of-flight mass spectrometer Detector Acceleration region Flight tube

11. 11 Conventional non-coincidence time-of-flight mass spectrum: Example

12. 12 Coincidence time-of-flight mass spectrum: Principle Single pulse mass spectra Corresponding raw coincidence mass spectra Accumulated raw coincidence mass spectra

13. 13 Refining the data: Covariance mapping False True Covariance mapping: = – RawFalseTrue – = Raw HNCO Time-of-flight

14. 14 Delay-line anode Resolution: 0.5 ns, 250 m Three-dimensional momentum imaging: Time- and position-sensitive detectors Microchannel plate

15. 15 Experimental Results

16. 16 n-butane Distinguishing the dissociation channels n-C 4 H 10

17. 17 Measuring the cross section data for different dissociation channels HNCO 2+ → X + +Y + + n HNCO 3+ → X + +Y + +Z + + n Example: Electron impact ionization of isocyanic acid

18. 18 Determining the dissociation sequence 1)HNCO 2+  HN + + CO + CO +  C + + O -arctan(CO/C)= -66.8  (Exp. -66.3  3  ) 2)HNCO 2+  HNC + + O + HNC +  C + + NH -arctan(HNC/C)= -66.0  (Exp. -66.7  3  ) Example: Tree-body sequential dissociation p - p A+A+ B +C+C A+A+ A+A+ B + C C B+B+  HNCO

19. 19 Three-dimensional momentum imaging Example: H 2 + + nh  790nm  H + + H Laser field (x 1, y 1, t 1, x 2, y 2, t 2 )  (p 1x, p 1y, p 1z, p 2x, p 2y, p 2z ) p 1 + p 2 = 0 C  laser field p1p1 p2p2 x

20. 20 Dissociation from different vibrational levels Example: H 2 + + nh H + + H 1s  g Laser field Momentum distributionKinetic energy distribution

21. 21 Summary Coincidence mass spectroscopy distinguishes different dissociation channels and pathways.Coincidence mass spectroscopy distinguishes different dissociation channels and pathways. We measured the cross section data and studied the dissociation sequences of electron impact dissociative ionization of molecules.We measured the cross section data and studied the dissociation sequences of electron impact dissociative ionization of molecules. We did three-dimensional momentum imaging for the dissociation of molecular ions in intense laser fields.We did three-dimensional momentum imaging for the dissociation of molecular ions in intense laser fields.