Ultrafast memory loss and energy redistribution in the hydrogen bond network of liquid H 2 O

1. Ultrafast memory loss and energy redistribution in the hydrogen bond network of liquid H2O M. L. Cowan et al., Nature, 434, 119‐202 (2005) MIYASAKA Laboratory Akiko NAGAFUJI

2. Contents • Introduction ・Anomaly of water ・Vibrational modes of H2O ・OH stretching vibration • Experimental ・Nanofluidic sample cell ・Principle of photon echo & transient grating ・Experimental set-up • Results & Discussion • Summary

3. Introduction Water The density of ice … smaller than that of liquid water Melting point & boiling point … anomalously high temperature Masakazu Matsumoto et al., Nature, 416, 409 (2002) Because of hydrogen bond network!!

4. Introduction Vibrational modes of H2O Asymmetric stretch mode (liquid H2O … 3430 cm-1) Symmetric stretch mode (liquid H2O … 3210 cm-1) Bending mode (liquid H2O … 1650cm-1)

5. The transition frequency of an individual OH stretching oscillator in hydrogen bond network The local environment!! Introduction OH stretching vibration • Free OH group…(a) Very sharp!! • Hydrogen-bonded OH group…(b) Inhomogeneous broadening!! Erik T. J. Nibbering et al., Chem. Rev., 104, 1887 (2004)

6. Experimental Sample cell Before… • The high optical density of the OH stretching mode in pure H2O • Parasitic window signals in conventional samples Hinder attempts to access the fastest relaxation processes of liquid water in pure H2O Nanofluidic sample cell! Windows & water layer Extremely thin!!

7. Experimental What is photon echo ? Pulse 1⇒Excitation (coherent polarization) Pulse 2⇒Input of phase information Pulse 3 ⇒Read of the information

8. Experimental Transient grating (τ=0) （過渡回折格子） Pulse 1 & Pulse 2 arrive simultaneously… Interference pattern Incidence of Pulse 3… Bragg diffraction!!

9. Experimental Stimulated photon echo The coherent polarization occurred by Pulse 1 is preserved during the delay time τ… Interference can take place between the coherent polarization and the electric field of Pulse 2 !! Photon echo signal is generated by Pulse 3!!

10. 1 2 3 Sig. Ref. τ T T τ -tRS Experimental Experimental set-up τ…the lower part of roof mirror T…translation stage tRS…pulse 1, 2, 3 ⇒compensating plate • Three pulses…～1 mJ, 70 fs, 3350 cm-1 (the center of the OH stretching band) • Photon echo signal…heterodyne detection by spectral interferometry

11. The strong anharmonicity of the OH stretching oscillator Erik T. J. Nibbering et al., Chem. Rev., 104, 1887 (2004) Results & Discussion • The positive peak⇒reduced absorption (bleaching) & stimulated emission • The negative peak ⇒excited state absorption At T=0 The absorption below 3000 cm-1 & the bleaching above 3500 cm-1…very fast decay!! The bleaching between 3170 and 3400 cm-1…slower decay & picosecond timescale rise!!

12. …(1) Results & Discussion Anisotropy （偏光異方性解消） Electric field of the pump light Selective excitation!! Rotational diffusion of the selectively excited molecules

13. parallel perpendicular anisotropy Results & Discussion ◎In the case of H2O… • Intermolecular transfer of the OH stretch excitation (faster than 100 fs) • The orientational diffusion of the OH groups (4.0±0.4 ps) (Sander Woutersen et al., Nature, 402, 507 (1999)) ◎Parallel & perpendicularpolarization conditions… • Fast decay (95 fs) • Slower rise (1.3 ps) The decay of the anisotropy …very fast!! (75 fs) The fast energy transfer!!

14. …(2) τor : the orientational diffusion time [OH] : the concentration of molecules containing an OH group r0 : the Förster radius T1 : the lifetime of the excited state Results & Discussion “Resonant intermolecular transfer of vibrational energy in liquid water” (Sander Woutersen et al., Nature, 402, 507 (1999)) τor=4.0±0.4 ps ro=2.10±0.05Å T1=740 fs [OH]=111M ◎In the HDO : D2O… The decay of anisotropy ⇒ equation (2) ◎In liquid H2O… Instantaneous decay!!

15. Results & Discussion Energy transfer The decay of the anisotropy is very fast. ⇒ 75 fs!! The transfer process… dipole-dipole interaction (Förster transfer mechanism) But!! The detail mechanism … not yet analyzed!! Quadrupole-quadrupole interaction?? Anharmonic coupling ?? （quadrupole：四極子）

16. Results & Discussion ◎Fast component… The 95 fs decay ⇒ depopulation of the ν=1 state ◎Slow component… The 1.3 ps rise ⇒ heating effects associated with vibrational relaxation ◎Anisotropy… The 75 fs decay ⇒ energy transfer among the OH stretching vibrations Shorter than the 700 fs time constant in a diluted 6:1 D2O:H2O mixture The high concentration of OH stretching oscillator ⇒rapid intermolecular transfer of vibrational energy!! Sander Woutersen et al., Nature, 402, 507 (1999)

17. Results & Discussion ◎On-diagonal (bleaching & stimulated emission) • At T=0 fs… • Peak is stretched along the diagonal, indicating inhomogeneous broadening. • At T=50 fs… • This inhomogeneity is almost entirely lost. • By 100 fs… • It is completely gone. ◎Off-diagonal (excited state absorption) Peak decays on the same timescale. Ultrafast structural variation!!

18. Summary • The fastest relaxation processes of liquid water in pure H2O can be observed by using a specially designed, ultrathin sample cell. • This work demonstrates the ultrafast energy transfer process and structural changes in the hydrogen bond network of liquid H2O.