Ultrafast Energy Equilibration in Hydrogen-Bonded Liquids

Abstract

We have studied the equilibration dynamics of liquid water and alcohols following a local deposition of energy using time-resolved femtosecond mid-infrared pump-probe spectroscopy. The equilibration dynamics is monitored via the spectral response of the OH-stretch vibration. It is found that the equilibration leads to complicated changes of the absorption band of the OH-stretch vibration including a shift of the absorption band and a decrease of the absorption cross section. Interestingly, these spectral changes do not occur simultaneously, which indicates that they are associated with the equilibration dynamics of different lowfrequency modes. For water, we find an equilibration time constant of 0.55 ( 0.05 ps. We observe that the equilibration time strongly increases going from water to alcohols such as methanol, ethanol, and propanol which means that water molecules can adapt much faster to a local deposition of energy than other hydrogenbonding liquids.

Simulating Energy Relaxation in Pump–Probe Vibrational Spectroscopy of Hydrogen-Bonded Liquids

“…The timescale calculated for the relaxation of deuterated methanol, 8.5 ps, agrees well with the experimental thermal relaxation time τ eq ∼ 6.0, 7.0 ps in Ref. 37,42 The small difference may be ascribed to the approximate classical forcefield.…”

“…The hotspot thermostat has proved to be reliable yielding robust results while adopting a broad range of settings, in particular showing that the relaxation mechanism is independent on the T peak , which recalls the fact that a sample dynamics does not depend on the laser intensity. 42 We believe that this approach can be used to investigate vibrational relaxation on several systems, even more complex than those studied in this work.…”

“…The dependence on the isotopic dilution of the fast relaxation time, instead, is still debated. 37,38,42 4 Liquid water…”

OH and NH Stretching Vibrational Relaxation of Liquid Ethanolamine

“…3b. The observation of an induced absorption that is "blue"-shifted relative to the stationary absorption spectrum that is accompanied by a residual "red"-shifted bleach is highly indicative of a thermally heating of the sample volume [3,12,22]. Such a heating occurs when the initially deposited OH/NH stretching energy is canonically redistributed over all nuclear (intra and intermolecular) degrees of freedom of the liquid.…”

“…Associated molecular liquids have been particularly attractive research targets because the influence of hydrogen bonding (H-bonding) on the timescales and molecular mechanisms of the various vibrational dynamical phenomena remains until today only partially understood [1][2][3].…”