Reorientation-induced relaxation of free OH at the air/water interface revealed by ultrafast heterodyne-detected nonlinear spectroscopy

• Published in: Nature communications 2020-10, Vol.11 (1), p.5344-5344, Article 5344
• Main Authors: Inoue, Ken-ichi, Ahmed, Mohammed, Nihonyanagi, Satoshi, Tahara, Tahei
• Format: Article
• Language: English
• Subjects: 140/125; 639/638/440; 639/766/94; Air-water interface; Energy transfer; Humanities and Social Sciences; Hydrogen; Hydrogen bonding; Hydrophobicity; Interfaces; Isotope effect; multidisciplinary; Science; Science (multidisciplinary); Singular value decomposition; Spectroscopy; Spectrum analysis; Water chemistry
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-020-19143-8

The uniqueness of water originates from its three-dimensional hydrogen-bond network, but this hydrogen-bond network is suddenly truncated at the interface and non-hydrogen-bonded OH (free OH) appears. Although this free OH is the most characteristic feature of interfacial water, the molecular-level understanding of its dynamic property is still limited due to the technical difficulty. We study ultrafast vibrational relaxation dynamics of the free OH at the air/water interface using time-resolved heterodyne-detected vibrational sum frequency generation (TR-HD-VSFG) spectroscopy. With the use of singular value decomposition (SVD) analysis, the vibrational relaxation ( T 1 ) times of the free OH at the neat H 2 O and isotopically-diluted water interfaces are determined to be 0.87 ± 0.06 ps (neat H 2 O), 0.84 ± 0.09 ps (H 2 O/HOD/D 2 O = 1/2/1), and 0.88 ± 0.16 ps (H 2 O/HOD/D 2 O = 1/8/16). The absence of the isotope effect on the T 1 time indicates that the main mechanism of the vibrational relaxation of the free OH is reorientation of the topmost water molecules. The determined sub-picosecond T 1 time also suggests that the free OH reorients diffusively without the switching of the hydrogen-bond partner by the topmost water molecule. Water’s hydrogen-bond network is truncated at hydrophobic interfaces and the dynamics of the resulting free OH groups is not well understood. The authors experimentally show that the main vibrational relaxation mechanism for free OH at the air-water interface is a diffusive molecular reorientation, rather than intramolecular energy transfer.