1. Thomas J. Preston, Maitreya Dutta, Brian J. Esselman, Michael A. Shaloski, Robert J. M C Mahon, and F. Fleming Crim UW-Madison Aimable Kalume, Lisa George, and Scott A. Reid University of Marquette Isomerization Between CH 2 ClI and CH 2 Cl-I in Cryogenic Matrices Studied on Ultrafast Timescales

2. Isomerization of Halomethanes First identified in solid matrices with frequency-domain spectroscopy* Return to parent following electronic excitation Stable in cryogenic solids Transient in liquids *Maier…Hess, JACS, 112, 5117 (1990).

3. Experiment Frequency DomainTime Domain 500:1 Pulsed deposition Tunable, Nd:YAG nanosecond dye laser Static, difference spectroscopy 220:1 Continuous deposition Ultrafast, Ti:Sapphire laser Transient absorption spectroscopy

4. Potential Energy Sketch

5. Transient Absorption - Photolysis

6. Transient Absorption - Probe

7. Transient Absorption - Recovery

8. Spectroscopy of Parent and Isomer

10. Pump and Probe Wavelengths

11. Probe Vibrational Energy with Electronic Transition

14. Isomer Absorption In Ar, N 2, CH 4

15. Transient Absorption: CH 4

18. Rise time,  1 / ps 1.1(2) 1.4(1) 2.3(3) 3.1(3)

19. Transient Absorption: CH 4 Rise or fall time,  2 / ps 42(7) 276(122) 49(13) 37(5)

20. Potential – 1D

21. Isomer Formation and Relaxation  1 : 1 ps  2 : 40 ps MP2/Sadlej-pVTZ

22. Trends Same in CH 4, N 2, and Ar

23. Complete Picture Vibrationally excited isomer forms in about 1 ps Vibrational relaxation occurs in about 40 ps Both of these timescales are similar in liquid and super-critical fluids

24. Unanswered Questions What is the detailed process of formation (  1 )? Formation time is 2x faster in N 2 and CH 4 than in Ar What is the detailed process of cooling (  2 )? Rates are indiscriminate of matrix complexity Rates similar among solids, liquids, super-critical fluids Opportunity for MD simulations

25. Acknowledgements Scott Reid and group at Marquette Static spectroscopy Calculations Bob McMahon and group at UW-Madison Matrix isolation apparatus Calculations Fleming Crim and group at UW-Madison

26. End of slide show, click to exit.

29. Thomas J. Preston, Maitreya Dutta, Brian J. Esselman, Michael A. Shaloski, Robert J. McMahon, and F. Fleming Crim UW-Madison Aimable Kalume, Lisa George, and Scott A. Reid University of Marquette Isomerization of CH 2 Cl-I to CH 2 ClI in Cryogenic Matrices: A Study on Ultrafast Timescale

30. Recap: Formation of the Isomer

31. Explore Potential Energy Surface a

32. Setup Probe Preparation Pump Destruction Wall-Clock Time Isomer Population

33. Probe Isomer or Parent?

34. Prep. Pump Probe

35. Destruction of Isomer pump = 400 nm probe = 400 nm pump = 800 nm probe = 400 nm

36. IR Pump – C-H Stretch Overtone Loss to Parent Vibrational Relaxation 6000 cm -1

37. IR Pump – C-H Stretch Fundamental 3000 cm -1 Vibrational Relaxation

38. Vibrational Relaxation

41. Excite C-H stretch Energy flows into Franck-Condon mode (  1 ) Energy flows into surroundings (  2 )

42. Infrared Excitation 100  m-thick samples 10-20% conversion to isomer Small absorption at C-H overtone Exceptionally large cross section at C-H fundamental

43. Acknowledgements Scott Reid and group at Marquette Static spectroscopy Calculations Bob McMahon and group at UW-Madison Matrix isolation apparatus Calculations Fleming Crim and group at UW-Madison