Vibrational substructure in the OH stretching band of water Zhaohui Wang, Andrei Pakoulev, Yoonsoo Pang, Dana D. Dlott * School of Chemical Sciences, University of Illinois at Urbana-Champaign, Box 01-6 CLSL, 600 S. Mathews Avenue, Urbana, IL 61801, USA Received 10 June 2003; in final form 2 July 2003 Published online: Abstract Spectral diffusion in the OH stretching (m OH ) band of water is studied by ultrafast IR-Raman spectroscopy. The m OH transition consists of two overlapping inhomogeneously broadened subbands, a broader (500 cm 1 ) redshifted band and a smaller, narrower (200 cm 1 ) blueshifted band. The blueshifted band, which shows less spectral diffusion, has a longer lifetime (0.75 vs 0.55 ps) and a smaller vibrational frequency blueshift from the ground state (65 vs 90 cm 1 ), is tentatively assigned to water molecules where one hydrogen atom has a broken hydrogen bond. Ó 2003 Elsevier B.V. All rights reserved. 1. Introduction In this work, we use ultrafast IR-Raman spec- troscopy [1], a type of 3D vibrational spectroscopy [2], to investigate subband structures in the OH stretching band (m OH ) of water. In the past few years, ultrafast mid-IR hole-burning spectroscop- ies have been used to study spectral diffusion within m OH (3000–3600 cm 1 ) of the HOD solute in D 2 O solvent [3–9]. A combination of experiment and related theoretical work [10–17] has now demonstrated that spectral diffusion on the >50 fs time scale probed by experiments so far, is caused by the forming and breaking of hydrogen bonds. Only recently has ultrafast vibrational spec- troscopy been applied to m OH of water [9,18–22]. There are similarities and differences between HOD/D 2 O and water. The hydrogen-bonding dy- namics of the two are presumably quite similar [12]. The vibrational spectra are quite different. In HOD there are three separate bands, m OH , the OD stretch m OD , and the bend overtone 2d HOD . In water there is a single band, a mixture of symmetric m OH , antisymmetric m OH and 2d H 2 O . The vibrational relaxation (VR) pathways are also quite different, as a consequence of this different level structure [9,10,23,24]. Ultrafast mid-IR experiments in HOD probe an excitation localized mainly on the OH group [12], whereas water experiments probe a m OH that is delocalized over the entire molecule. Due to its high absorption coefficient [25], pump–probe experiments in water necessarily involve a bulk temperature jump (DT ¼ 30 K is typical [19,20] but the boiling point 373 K is easily reached), which is not a factor in dilute HOD/D 2 O. Chemical Physics Letters 378 (2003) 281–288 www.elsevier.com/locate/cplett * Corresponding author. Fax: +1-217-244-3186. E-mail address: dlott@scs.uiuc.edu (D.D. Dlott). 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0009-2614(03)01267-3