1. VIBRATIONAL ENERGY RELAXATION OF BENZENE DIMER STUDIED BY PICOSECOND TIME-RESOLVED INFRARED-ULTRAVIOLET PUMP-PROBE SPECTROSCOPY R. KUSAKA and T. EBATA Hiroshima University

2. 3860038800 (1+1’)REMPI hole-burning(isomer1) hole-burning(isomer2) C 6 H 6 6 0 1 Structure of benzene dimer h(Top) isomer 1 isomer 2 h = C 6 H 6, d = C 6 D 6 h(Top) h(Stem) (C 6 H 6 -C 6 D 6 ) + C 6 D 6 6 0 1 d(Top) d(Stem)

3. Vibrational energy relaxation Top Stem How fast is vibrational energy relaxation ? IR S0S0 k IVR initial bath IR S0S0 k Top k Stem initial bath k Stem k Top Are anddifferent ?

4. Site-specified excitation isomer 1 isomer 2 h(Stem) h(Top) h(Stem) h(Top) d(Top) d(Stem) h(Stem) d(Top) h(Top) d(Stem) homodimer heterodimer vibrational energy Bh(Stem) - Ah(Top) Ah(Stem) + Bh(Top) delocalized vibrationlocalized vibration We can investigate pure site-specified relaxation by using heterodimer. isomer 1 isomer 2 h(Stem) h(Top) site-specified excitation EE

5. IR-UV pump-probe spectroscopy IR k IVR S0S0 monomer IR spectra in CH stretching region, obtained by ns laser system. h(Stem) h(Top) UV  t IP S1S1 real time measurement of population decay

6. Experimental setup Energy and time resolutions are 5 cm -1 and 12 ps. (monitor hd + mass)

7. Excitation scheme h*(Stem) h*(Top) Two isomers can be distinguished by choosing UV frequency. 38600 h(Top) h(Stem) 38700 (1+1’)REMPI hole-burning Electronic spectra obtained by ns laser system C 6 H 6 6 0 1 ps laser ns laser IR spectra of heterodimer ns laser Excitation scheme by picosecond pulses

8. Site-specified vibrational relaxation 1 2 15 ps 95 ps 670 ps 365003640036300 IR=3077 cm -1 UV wavenumber / cm -1 h*(Stem) h*(Top) 1 2 12008004000 = 500 ps 1000600 200 2 1 = 100 ps The lifetimes are quite different, in spite of similar IR excitation energy. h*(Stem) h*(Top) 19 1 Transient UV spectra obtained by ps laser system

9. Why is the relaxation faster in Stem site ? H H H H H H 12008004000 = 500 ps 1000600 2 1 = 100 ps Coupling with inter molecular vibration Stem > Top Reason parallel perpendicular

10. monomer h(Top) Why is the relaxation faster in Stem site ? Additional reasons Vibrational frequencies of Stem benzene are more red-shifted.    Symmetry of Stem benzene is lower.       are IR-inactive CH stretching vibrations in monomer and Top benzene. obtained by ns laser system

11. Relaxation process IVR IR S 0 initial bath UV  t IP S 1 UV hd + broad transition from bath state x 5 transition from initial level VP IR S 0 initial VP case 1 case 2 Binding energy ~800 cm -1 Excitation energy 3077 cm -1 < Vibrational predissociation (VP) occurs.

12. Lifetimes of vibrational predissociation 100050001500 Time profile of the broad UV transition IR=3077 cm -1 Intensity broad (  t) k Stem, IVR k Stem, IVR – k Stem, VP {exp(–k Stem, VP  t) – exp(–k Stem, IVR  t)} k Top, IVR k Top, IVR – k Top, VP {exp(–k Top, VP  t) – exp(–k Top, IVR  t)} = A + B+ B (A / B = 1) k IVR IR S 0 initial bath k VP  IVR / ps  VP / ps Stem Top 100 500 decay rise Stem Top Stem Top UV S 1 UV hd +  = 1/k) 100 ps 500 ps 900 ps 500 900

13. Process and lifetimes of vibrational relaxation H H H H H H H H H H H H H H H H H H IVR VP 100 ps500 ps 900 ps Stem Top IR=3077 cm -1 statistically redistributed  (E*) Stem =  (E*) Top [density of states] Coupling between initial and bath states is 5 times stronger in Stem than in Top. initialbathdissociation x 5 x 1.8

14. Summary Vibrational energy relaxation of benzene dimer was studied, in real time, by picosecond time-resolved pump-probe spectroscopy. hd initial hd bath h + d IVR IR S 0 VP initial bath dimer monomer S 0  IVR / ps  VP / ps Stem Top 100500 900 Stem Top By using deuterated heterodimers, process and lifetimes of pure Stem and Top relaxation were investigated.

16. RI-MP2(Full)/VDZ with BSSE correction. global minimum J. Phys. Chem. A. 2007, 111, 3446

17. J. Chem. Phys. 1999, 111, 572

18.             monomer

19. C 6 H 6 -C 6 H 6

20. 40200 Population time evolution 60 Benzene monomer IR S 0 IVR UV  t IP S 1 D 6h CH

21. Process and rate of vibrational relaxation 160012008004000 [hd bath ] = k IVR k - k VP {exp(-  t/  VP ) - exp(-  t/  IVR )} step function IVR IR UV  t IP S 1 S 0 h UV hd + VP initial bath IVR = 110 ps VP = 530 ps

22. 1.larger clusters 2.six-zero-one transition of Stem and Top 3.mode dependence 4.homodimer 5.the reasons of kIVR