Anomalous Slowing Down of the Vibrational Relaxation of Liquid Water upon Nanoscale Confinement Adriaan M. Dokter, Sander Woutersen, and Huib J. Bakker FOM Institute for Atomic and Molecular Physics [AMOLF], Kruislaan 407 Amsterdam, The Netherlands (Received 12 January 2005; published 5 May 2005) We study the vibrational dynamics of nanodroplets of liquid water with femtosecond two-color midinfrared pump-probe spectroscopy. For the smallest nanodroplet, containing 10–15 water molecules, the lifetime T 1 of the O-H stretch vibrations is equal to 0:85  0:1ps, which is more than 3 times as long as in bulk liquid water. We find that the truncation of the hydrogen-bond network of water leads to a dramatic change of the relaxation mechanism. DOI: 10.1103/PhysRevLett.94.178301 PACS numbers: 82.53.Uv, 78.47.+p, 81.07.Nb, 82.30.Rs An excellent model system to study the properties of nanoconfined water is formed by so-called reverse mi- celles: small droplets of water covered by a surfactant and contained in an apolar solvent matrix. The water molecules close to the surfactant layer were found to be much more structured and much less mobile than the molecules in bulk liquid water [1]. The water in the core, i.e., a few molecular layers away from the interface, is much more similar to bulk liquid water [2], although in infrared spectroscopic studies it was shown that even for very large micelles with a diameter >6nm, the hydrogen bonds between the water molecules are still weaker than in bulk liquid water [3,4]. In a study of the vibrational dynamics of pseudohalide ions ( N 3  , NCO  , and NCS  ) in aqueous solution [5,6], it was found that the vibrational lifetime time increases when the size of the micelle is decreased. In this Letter, we present a systematic study of the effect of nanoconfinement on the vibrational dynamics of water itself. We find that the confinement to the nanometer scale leads to an increase of the lifetime of the O-H stretch vibrations of the water molecules that is much stronger than can be expected from the change in vibrational frequency. The investigated samples were mixtures of the surfac- tant AOT (sodium di-2-ethylhexylsulfosuccinate, Aldrich, 99%), n octane (Aldrich, anhydrous grade 99  %), and water H 2 O (Aldrich, HPLC grade). The molar water-to- AOT ratio w 0 was set to 1, 2, 4, 7, and 12, respectively, creating reverse micelles containing a water pool consist- ing of 10 to 10:000 water molecules. The radius R in nm of the micelle can be approximated from R  0:15  w 0 , with w 0 H 2 O=AOT [7]. In the pump-probe experiments, an intense femtosecond midinfrared pulse (pump) is used to excite a fraction of the water molecules to the first excited vibrational state of the O-H stretch vibration. The time-dependent absorption changes that result from this excitation are probed with a second, much weaker pulse (probe) with a polarization at the magic angle (54:7  ) with respect to the polarization of the pump. The transmitted probe beams are dispersed in a monochromator, and detected by a 3  32 MCT (mercury- cadmium-telluride) array detector. The frequency resolu- tion is 16 cm 1 per pixel of the array. The midinfra- red pump and probe pulses are independently tunable be- tween 2.6 and 3:3 m  3000–3800 cm 1 , have pulse durations of 150 fs, and energies of 10 and 0:1 J, respec- tively [8]. Figure 1 shows the linear absorption spectrum of the OH-stretching band of water confined in reverse micelles of various sizes. The amplitude of the low-frequency part of the OH absorption band strongly decreases as the water content is reduced. Figure 2 displays transient spectra for a reverse micelle with w 0  4. At early delays the transient spectra reveal a bleaching at the v  0 ! 1 transition around 3500 cm 1 and an induced absorption at the v  1 ! 2 transition extending from 3300 cm 1 to below 3000 cm 1 . The measurable spectral range was limited on the low-frequency side by the strong C-H absorption of AOT and the octane solvent. At longer delays the FIG. 1. Linear infrared absorption spectra of the O-H stretch vibrations of water in AOT/water/octane reverse micelles of various sizes: w 0  1, 2, 4, 7, and 12. The spectra are normalized for comparison. The absorption at the low-frequency side con- tains a strong contribution of the high-frequency wing of the absorption of the C-H stretch vibrations of AOT and n octane. PRL 94, 178301 (2005) PHYSICAL REVIEW LETTERS week ending 6 MAY 2005 0031-9007= 05=94(17)=178301(4)$23.00 178301-1  2005 The American Physical Society