Watching Hydrogen Bonds Break:  A Transient Absorption Study of Water

Abstract

Ultrafast infrared transient absorption measurements of the complete hydroxyl OD stretching mode spectrum of HOD in water, from 100 fs to tens of picoseconds, observe hydrogen bond breaking and monitor the equilibration of the hydrogen bond network in water. In addition, the vibrational lifetime, the time constant for hydrogen bond breaking, and the rate of orientational relaxation are determined. The reactant and photoproduct spectra of the hydrogen bond breaking process are identified by decomposing the transient spectra into two components, the initial spectrum associated with vibrational excited states (reactants) and the long-time spectrum associated with broken hydrogen bonds (photoproducts). By properly taking into account the perturbation of the reactant spectrum decay by the growth of the photoproduct spectrum, it is found that the vibrational relaxation (1.45 ps) and orientational relaxation (1.53 ps) are wavelength independent and, therefore, independent of the degree of hydrogen bonding. Energy deposited into water by vibrational relaxation does not immediately break a hydrogen bond by predissociation nor produce a thermally equilibrated hydrogen bond distribution at an elevated temperature. Following deposition of energy by vibrational relaxation, the hydrogen bond breaking time is 800 fs, and there is a transient period of several picoseconds during which the hydrogen bond distribution is not in thermal equilibrium.

Dynamics of Nanoscopic Water: Vibrational Echo and Infrared Pump−Probe Studies of Reverse Micelles

“…The data displayed two contributions to the time dependence: the decay of the excited vibrations, followed by a shift in the equilibrium distribution of hydrogen bonds, because of the deposition of energy into the liquid. 23 The shift to a new distribution of hydrogen bonds produces "photoproducts" with a wavelength-dependent spectrum that is different from the initial spectrum. Comparison to FTIR, temperature difference spectra demonstrated that, within ∼5 ps, the system reaches a new equilibrium distribution of hydrogen bonds, corresponding to a slightly elevated temperature.…”

“…At the absorption maximum, the shift to a new distribution of hydrogen bonds results in a long-lived bleach that is constant on a 100-ps time scale. 23 Separation of the lifetime decay and the growth of the photoproducts gave a wavelength-independent OD stretch vibrational lifetime of 1.5 ps. 23 The data have the appearance of a fast decay to a nonzero constant pump-probe signal that is ∼18% of the initial value of the signal.…”

“…23 Separation of the lifetime decay and the growth of the photoproducts gave a wavelength-independent OD stretch vibrational lifetime of 1.5 ps. 23 The data have the appearance of a fast decay to a nonzero constant pump-probe signal that is ∼18% of the initial value of the signal. The constant component should decay on a very long time scale as heat diffuses out of the laser excitation volume.…”

“…The vibrational lifetime determined for water is in agreement with recent work. 23 The lifetime measured in the same manner for the 6 M NaCl solution is also listed in Table 1. The NaCl solution will be discussed in connection with the vibrational echo data presented below.…”

“…Vibrational lifetimes, excitation transfer, dephasing and orientational correlation times are all observed to decay by ∼2 ps for bulk water. 20,[23][24][25] This paper focuses on the influence of confinement on the vibrational lifetimes and dephasing dynamics of water that is confined on a length scale of a few nanometers. The observations are related to the evolution of the hydrogen-bond network.…”