Vibrational Substructure in the OH Stretching Transition of Water and HOD Zhaohui Wang, Andrei Pakoulev, Yoonsoo Pang, and Dana D. Dlott* School of Chemical Sciences, UniVersity of Illinois at Urbana-Champaign, 600 South Mathews AVenue, Urbana, Illinois 61801 ReceiVed: April 2, 2004; In Final Form: August 13, 2004 Ultrafast nonlinear vibrational spectroscopy with mid-IR pumping and incoherent anti-Stokes Raman probing is used to study V) 1 excitations of OH stretching (ν OH ) of water and of HOD in D 2 O solvent (HOD/D 2 O). The parent ν OH decay and the appearance of daughter stretching and bending excitations are simultaneously monitored, which allows for characterization of the stretch decay pathways. At all times and with all pump frequencies within the ν OH band, the excited-state spectrum can be fit by two overlapping subbands, a broader red-shifted band ν OH R and a narrower blue-shifted band ν OH B . We show these subbands are dynamically distinguishable. They decay with different lifetimes and evidence characteristically different decay pathways. Excitations of the ν OH R subband generate bending vibrations that ν OH B does not. The shorter lifetime (∼0.5 ps) of the ν OH R subband compared to the ν OH B subband (0.8-0.9 ps) results primarily from enhanced stretch-to- bend anharmonic coupling. The subbands represent persistent structures in the excited state, in that interconversion between subbands (2-10 ps) is slower than excited-state decay. A tentative structural interpretation is proposed. The ν OH R subband, on the basis of simulations, its red shift ,and its shorter lifetime, is proposed to result from strongly hydrogen-bonded “ice-like” water. The ν OH R subband has a smaller amplitude in HOD/D 2 O than in water, possibly because HOD has a single localized OH-stretching vibration whereas water has two delocalized stretching vibrations. 1. Introduction In this work, we use ultrafast nonlinear vibrational spectros- copy to demonstrate that the OH-stretching (ν OH ) band of water consists of two distinct subbands that eVidence clearly distin- guishable dynamical behaVior. A subband is a spectrally contiguous region within the ν OH band that can be experimen- tally distinguished from other parts of the band. Here we measure the transient anti-Stokes Raman spectrum 1 of water and HOD solute in D 2 O (HOD/D 2 O) of ν OH excitations in V) 1 generated by a tunable mid-IR pulse and probed via the V) 1 f 0 Raman transition. We observe two overlapping subbands that are distinguishable by virtue of having different excited- state lifetimes and different vibrational relaxation (VR) path- ways. The blue-shifted subband undergoes slower VR with minimal generation of δ H2O or δ HOD bending excitations and the red-shifted subband undergoes faster VR with more efficient generation of bending excitations. Subbands of this type will be termed “dynamically distinguishable”. In the past, most discussions of water subbands 2-4 has focused on the idea that if subbands exist, they should be attributed to characteristic hydrogen-bonding environments. Subbands of this type will be termed “structurally distinguishable”. To the extent that we may associate different dynamics with different structures, our observations indicate that the two subbands represent water environments that are persistent in the sense that interconversion (estimated at 2-10 ps) is somewhat slower than the V) 1 excited-state lifetime of 0.5-1.0 ps. Thus our work suggests that a glass of water at ambient temperature consists of two distinct interconverting types of water molecules that behave in characteristically different ways. This dual structure is not at all what one expects on the basis of water simulations, 5 but a recent X-ray absorption and X-ray Raman study has been interpreted as indicating that water consists mainly of two hydrogen-bonded configurations. 5 The ease of fitting water vibrational spectra in the ν OH region by using a small number of overlapping Gaussian subbands has frequently been interpreted as an argument for the existence of interconverting structurally distinguishable subbands that rep- resent different hydrogen-bonded structures, 2 for instance “ice- like” or “bridged” structures. However, Gaussian fitting of ultrabroadened vibrational spectra in hydrogen-bonded liquids is a poor tool for discerning structurally distinct elements. Recently developed techniques in ultrafast nonlinear vibrational spectroscopy 6 allow a much more effective attack on this problem with use of line narrowing or hole-burning methods. 6,7 These methods ought to permit a straightforward determination of the existence of dynamically distinguishable subbands. Making the connection to underlying structures is more difficult owing to the rather complicated relationship between the vibrational spectrum and local structure. Usually simulations are needed to make this connection, although some insight into this problem is gained in this work by a comparison of results from both water and HOD/D 2 O. Most prior experimental work in this area used ultrafast IR 3,4,8-21 or IR-Raman 22,23 methods to study ν OH of HOD/D 2 O. There have been far fewer studies of water 15,21,22,24-28 itself. Most of these works agree that the rates of VR and orientational relaxation vary within the ν OH band. Some groups find that the HOD VR lifetime varies continuously across the band. 18 Our group 27 in a recent study of water found just two discrete lifetimes. The orientational relaxation rate in HOD is said to take on a small number of discrete values, 3,12 but if correct the * To whom correspondence should be addressed. E-mail dlott@ scs.uiuc.edu. 9054 J. Phys. Chem. A 2004, 108, 9054-9063 10.1021/jp048545t CCC: $27.50 © 2004 American Chemical Society Published on Web 09/25/2004