Reply to: Comment on ÔVibrational relaxation and spectral diffusion following ultrafast OH stretch excitation of waterÕ, by H.J. Bakker, A.J. Lock, D. Madsen Andrei Pakoulev, Zhaohui Wang, Yoonsoo Pang, Dana D. Dlott * School of Chemical Sciences, University of Illinois at Urbana-Champaign, P.O. Box 01-6 CLSL, 600 S. Mathews Avenue, Urbana, IL 61801, USA Received 7 October 2003; in final form 22 December 2003 Published online: 22 January 2004 Abstract Both IR and anti-Stokes Raman measurements on OH stretch vibrations m OH of water see two time constants, 0.3 and 0.6 ps. We attribute the former to spectral diffusion and the latter to the m OH lifetime. This interpretation is evident from our Raman spectra, which show the m OH spectrum changing shape and the m OH population decaying. The m OH decay is accompanied by the rise (0.6 ps) and subsequent fall (1.4 ps) of population in the d OH bending vibration. Bakker et al. [Chem. Phys. Lett. 385 (2004) 329] assert their IR data indicates an absence of (>100 fs) spectral diffusion, that the faster time constant is the m OH lifetime and that the slower is the lifetime of a conjectured intermediate state. The properties of this conjectured intermediate differ significantly from the bending vibration directly observed by us. The intensity analysis by Bakker et al. said to contradict our interpretation relies entirely on ad hoc assumptions about excited-state absorptions. We show these ad hoc assumptions are unfounded and some are unrea- sonable, and that a plausible explanation for the IR intensities exists that is consistent with our interpretation. Ó 2003 Elsevier B.V. All rights reserved. The Bakker group and the Dlott group have recently studied vibrational dynamics of water and have offered contradictory interpretations [1]. Both groups used mid- IR pulses to excite the m ¼ 0 ! 1 transition of the OH stretching band m OH , but the two groups used different probing techniques. We used incoherent anti-Stokes Raman scattering [2–5], and Bakker and coworkers [1,6,7] used IR transient absorption. Both groups see two time scales in their transients, 0.3 and 0.6 ps, but the interpretations of these two time scales contradict each other. We associate the faster time scale with ex- cited-state spectral diffusion and the slower with the excited-state lifetime. Bakker et al. [1] assert their data provides no evidence for spectral diffusion on the >100 fs time scale. They associate the faster time scale with excited-state decay to an intermediate state, and the slower time scale with decay of this intermediate state. The intermediate is suggested to be m ¼ 2 of the bending vibration d OH [1]. In our transient anti-Stokes Raman spectra [3–5], spectral diffusion and m OH decay processes are clearly evident. The m OH decay is seen to be accompanied by the simultaneous build up and subsequent decay of an in- termediate state, m ¼ 1 of d OH [4]. Bakker et al. [1] in their comment assert that our interpretation is incorrect because it is contradicted by their results, and they suggest that we were misled be- cause our pulse duration was too long. We reply that their analysis is superficial and flawed. In their IR probe experiments, fixed-wavenumber IR transients are mea- sured in the 3000–3620 cm 1 range [1,6,7]. All transients evidence both time scales to some extent. They display a complicated pattern, with the signals alternating various combinations of negative (absorption increase) and po- sitive (bleaching). The directions of these signals, espe- cially on the blue edge of the m OH transition, were used to argue the absence of spectral diffusion on the >100 fs time scale, leading to the assignment of the faster pro- cess to the m OH lifetime. An intermediate state that was not observed was conjectured to explain the slower process. * Corresponding author. Fax: +1-217-2443186. 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/j.cplett.2003.12.078 Chemical Physics Letters 385 (2004) 332–335 www.elsevier.com/locate/cplett