Intramolecular Energy Redistribution in C 60 M. Boyle, Max Born Institute

2 Outline 1.) Single pulse measurements revisited 2.) Pump - Probe Measurements Fragmentation and Ionization  Intramolecular Energy Redistribution

3 Complex many body system High symmetry, thus still treatable Still gives surprises Recent Experimental Highlights at MBI Excitation of Rydberg Series of C 60 PRL 87 (2001) From above threshold ionization to statistical electron emission: The pulse duration dependence of C 60 photoelectron spectra PRL 84 (2000) 2128 Ionization and fragmentation of C 60 with sub-50 fs laser pulses J. Chem. Phys. 112 (2000) Sequential ionization of C 60 with femtosecond laser pulses J. Chem. Phys. 114 (2001) Experimental System : C 60

4 Electron TOF e-e- Ion + C 60 -Oven Double µ-Metal Shielding Wiley-McLaren Reflectron TOF x y z Experimental Method : Time of Flight

5 Single Pulse Measurements Revisited  P   el-ph  P >>  el-ph  P <<  el-ph Ionization Fragmentation Energy redistribution Laser interaction with C 60

6 Competition of ionization and fragmentation: Two extreme cases 25 fs: highly charged C 60 no fragmentation of C 60 + at high fluence =800nm 5 ps: fragmentation and delayed ionization no highly charged ions (25 J/cm 2 ) (35 J/cm 2 )

7 Ion spectra: Competition of ionization and fragmentation 25 fs 110 fs 500 fs 5 ps Laser pulse duration C at 800nm and constant pulse intensity (3 x W/cm 2 )

8 Counts electron kinetic energy / eV Photo electron spectra: Pulse Duration Dependence = 795 nm a) - d): 8 x W/cm 2 e): 5 x W/cm 2 a b c d e  p = 25 fs  = 110 fs  = 500 fs  = 5 ps  = 70 fs Transition from ATI to statistical electron emission

9 ATI and thermal energy redistribution statistical electron emission: K K electron kinetic energy / eV electron signal /log. units  I = const = 3 x W/cm 2 F =const = 1 J/cm 2 I = 2 x W/cm 2 T = K I = 8 x W/cm 2 T = K 25 fs 110 fs 500 fs Electron Temperatures

10 Single Pulse Measurements of C 60 timescalePESphotoions process in operation t < 100 fsATI peaksmultiply charged C 60 Excitation energy redistribution t < 1 ps (500 fs  200 fs) hot electrons with high KE multiply charged C 60 and fragments e-phonon coupling statistical electron emission t > 1 pscooler electrons ‘typical’ bimodal distribution phonon-phonon coupling ps, ns, µs thermionic e-emision delayed ionization, fragmentation sequential C 2, radiative cooling

11 Time Resolved Measurements of C 60 Motivation: *Mechanisms and time scales of energy redistribution are of considerable interest for understanding and perhaps controlling molecules. (i.e. control over the fragmentation pattern) *Pump-probe allows for an controlled input of energy *Limited range of pulse duration with single pulse

12 Delayed ionization at varying pulse widths (constant fluence) time of flight /channels fs1ps 1000fs3ps 500fs2ps 1500fs5ps C 60 + C 58 + C 56 + ** delayed log ion intensity E. E. B. Campbell, K. Hoffmann and I. V. Hertel; Eur. Phys. J. D 16 (2001) 345 The transition from direct to delayed ionisation of C 60

13 Time Resolved Measurements of C 60 - One Color pump-probe measurements - Wavelength – 800 nm - Pulse Duration of ~100 fs - Unequal pulse intensity (3:1 ratio) - Co-linear Michelson arrangement (interference fringes) + time - time t=0 Stronger pulse leads weaker Weaker pulse leads stronger

14 Fragmentation and Ionization Dynamics Fragmentation Ionization t < 500 fs electronic to select vibrational modes

15 Total Signal Strong competition of fragmentation and ionization

16 Fragmentation and Ionization Dynamics

17 Ratio between metastable and direct fragmentation Indication of internal energy related to the energy absorbed

18 Direct Fragmentation - Double Charged Not Normalized

19 Fit of Decay (excitation in neutral system)

20 Direct Fragmentation - Triple Charged

21 Fit of Decay (excitation in neutral system)

22 Direct Fragmentation - Single Charged

23 Fit of Decay (excitation in neutral system)

24 Cold C 60 Source Aggregation chamber Liquid N 2 Walls He Buffer Gas Electron TOF Ion RETOF

25 Hot vs. Cold C 60  C 60-2n ++ C 60 ++

26 Hot vs. Cold C 60 HOT COLD

27 Summary of pump-probe results time scaleprocessin system (-1) - (-10) psenergy redistributionneutral < 500 fs e  phonon increased absorption + bottleneck ionic system up to 15 psenergy remains in bottleneck statesionic system psnon-exponential decayionic system much laterfragmentation actually occurs ionic system

28 Conclusions Pump-probe measurements have indicated the time scales for energy relaxation Difference between vibrationally hot and cold C 60 sources easier coupling of vibrational modes when hot The results agree and build upon single pulse measurements

29 Thanks Max Born Institute Dr. C.P.Schulz Prof. I.V.Hertel Göteborg University and Chalmers University of Technology M.Hedén - cold source Prof. E.E.B. Campbell $$$$ Support through DFG:SfB 450 (TP A2) and European Large Scale Facilities

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