Ultrafast energy equilibration in hydrogen-bonded liquids

We have studied the equilibration dynamics of liquid water and alcohols fol lowing a local deposition of energy using time-resolved femtosecond mid-inf rared pump-probe spectroscopy. The equilibration dynamics is monitored via the spectral response of the OH-stretch vibration. It is found that the equ ilibration leads to complicated changes of the absorption band of the OH-st retch 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 equ ilibration dynamics of different low-frequency modes. For water, we find an equilibration time constant of 0.55 +/- 0.05 ps. We observe that the equil ibration time strongly increases going from water to alcohols such as metha nol, ethanol, and propanol which means that water molecules can adapt much faster to a local deposition of energy than other hydrogen-bonding liquids.