Nonlinear time-resolved vibrational spectroscopy is an indispensable tool for studying the structure and dynamics of liquid water. The stretching frequency of the hydroxyl bond at 3400 cm-1 is a highly sensitive probe of the molecule's local hydrogen bonding environment. By monitoring the time evolution after excitation of the OH stretch, it is possible to track the structure of the liquid on an ultrafast timescale. However, the inability to generate ultrafast broadband mid-IR pulses has made it difficult to simultaneously observe vibrational transitions at different frequencies. Typical femtosecond optical parametric amplifiers produce 100-400 cm-1 of bandwidth, which is narrow compared to the broad absorption spectrum of water. In this work, we use a recently developed source of ultrafast broadband mid-infrared (BBIR) pulses with detector limited bandwidth which spans from 1350 cm-1 to 4000 cm-1. We present experiments using BBIR to study the structure and dynamics of neat water and dilute HOD in D2O across the entire mid-IR. Using a combination of broadband transient absorption (TA) and two-dimensional infrared (2DIR) spectroscopy, we can correlate the OH stretching vibrations of H2O and HOD to other vibrational modes in the liquid. We show the nature of the molecular vibrations in these two systems is drastically different. In HOD, the hydroxyl stretch is coupled to both the OD stretch and the HOD bending mode; the coupling to the latter arises due to a hydrogen bond induced Fermi resonance which becomes stronger for excited vibrational states. On the other hand, we see that the hydroxyl vibrations in H2O are best described by excitons which are delocalized over several water molecules. We observe that the OH stretch in H2O is strongly mixed with the low frequency intermolecular modes which is evident from the strong response of the bath upon excitation of the OH stretch.

Ultrafast broadband infrared spectroscopy of neat and isotopically dilute water

246th National Meeting (2013)

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