Using COLTRIMS for Pump-Probe Studies of Molecular Dynamics

1. Using COLTRIMS for pump-probe studies of molecular dynamics I. Bocharova L. Cocke, I. Litvinyuk, A. Alnaser, C. Maharjan, D. Ray

2. Outline • Motivation • Coulomb explosion imaging. • Experiment requirements. • Experimental setup. • H2 and D2 experiments. • N2 and O2 experiments. • C2H2 experiment. • Future plans.

3. Motivation To study the structure and its time evolution of different gas molecules, using Coulomb explosion imaging

4. Laser Coulomb Explosion Imaging Accelerator Coulomb Explosion Imaging Originally developed for investigation of a static structure: collision of molecular ions beam and thin foil Now using laser short pulses interacting with molecules in gas phase Coulomb explosion imaging

5. Direct method which allows for best time resolution : can use short pulses  Possible to observe molecules with fast dynamics such as D2 D2 Theory: 4 fs Exp. 8fs Exp. >40fs Why Coulomb explosion imaging?

6. Requirements • Laser impulse shorter than vibration period of molecule. • High intensity to produce highly charged states, so explosion potential can be approximated by Coulomb potential. • Minimize the thickness of molecular target beam, so that interaction volume is minimal.

7. Gas jet y Piezoelectric slit z x Laser pulse Recoil detector M Recoil side of spectrometer Experimental setup

8. Looking for explosion fragments in coincidence Energy, eV Magnitude of vector sum of all fragments momenta (a.u.)

9. Pump-probe

10. d d Pump pulse Probe pulse θ Pump-probe setup d d pump-pulse probe-pulse d d pump-pulse θ x probe-pulse

11. Diatomic molecule: double well potential. Picture is asymmetric in laser field. R0 is an interatomic distance for neutral molecule. Distance R between two centers increases. At some critical distance Rc enhanced ionization occurs. (CR)EI – (Charge Resonance) Enhanced Ionization e- R0 Rc

12. S(E,t) (2) D++D+ probe Dt Dt D2+(X2Sg+) (1) pump D2(X1Sg+) D2 experment KER at fixed delays

13. D2 KER vs Delay spectrum R, a.u. Long pulse (30fs) CREI counts KER, eV Laser parameters: pump 8fs 3x1014 W/cm2 probe 8fs 9x1014 W/cm2.

14. D2: theory and experiment Theoretical calculation: Xiao-Min Tong, C.D. Lin

15. 20 20 KER (eV) 10 10 0 0 0 50 100 0 50 100 DELAY (fs) H2 experiment KER (eV) DELAY (fs)

16. PIPICO 3000 O+O+ TOF2(ns) O2+O+ 2000 O2+O2+ O3+O2+ O3+O3+ O2+5 1000 2000 1000 2800 O2+4 3000 O2+3 N+N+ O2+2 2000 O2+ N2+N+ N2+N2+ O2 N3+N+ N3+N2+ N3+N3+ N4+N3+ N4+N2+ 1000 1000 2000 2800 TOF 1 N2 and O2 experiment 1000 100 TOF 2 10 1

17. O3+ + O2+ Pair O2++ O2+ Pair 80 100 1000 600 600 KER (eV) 50 400 40 200 200 0 0 0 0 80 150 80 150 DELAY (fs) DELAY (fs) KER Spectra for Oxygen

18. N2++N2+ pair N3++N2+ pair PIPICO 80 70 3000 1000 N+N+ 40 40 KER (eV) 100 2000 TOF 2 N2+N+ N2+N2+ 10 N3+N+ N3+N2+ 0 0 N3+N3+ 60 120 60 120 N4+N3+ N4+N2+ 1000 1 DELAY (fs) DELAY (fs) 1000 2000 2800 TOF 1 KER Spectra for Nitrogen

19. C C C C H H H H C2H2 : polyatomic molecule C2H2 : isomerization of acetylene to vinylidene Time scale? the upper limit established is 60 fs1 H-CC-H  [H-CC-H]2+ CH+ + CH+  C+ + CH2+ Idea: With short pulses pump-probe technique can be applied to follow the dynamics of isomerization process. acetylene vinylidene 1 T. Osipov, C. L. Cocke, M. H. Prior, A. Landers, Th. Weber, O. Jagutzki, L. Schmidt, H. Schmidt-Böcking, and R. Dörner, Phys. Rev. Lett. 90, 233002 (2003).

20. CH+ + CH+ C2H+ + H+ C2+ + H+ CH2+ + C+ C2+ + C+ px (a.u.) TOF2 C2+ + C2+ pz (a.u.) TOF1 C2H2 acetylene and vinylidene channels separation Momentum-imaging investigations of the dissociation of D2+ and the isomerization of acetylene to vinylidene by intense short laser pulses. A. S. Alnaser, I. Litvinyuk, T. Osipov, B. Ulrich, A. Landers, E.Wells, C. M. Maharjan, P.Ranitovic, I. Bocharova, D.Ray and C.L.Cocke. Journal of Physics B: Atomic, Molecular & Optical Physics. (accepted)

21. Future plans • C2H2 experiment. • Continue experiments with N2 and O2. • CO2: triatomic molecule.

22. Thank you!