Resonant intermolecular transfer of vibrational energy in liquid water

Many biological, chemical and physical processes involve the transfer of en ergy. In the case of electronic excitations, transfer between molecules is rapid, whereas for vibrations in the condensed phase, resonant energy trans fer is an unlikely process because the typical timescale of vibrational rel axation (a few picoseconds) is much shorter than that of resonant intermole cular vibrational energy transfer(1,2). For the OH-stretch vibration in liq uid water, which is of particular importance due to its coupling to the hyd rogen bond, extensive investigations have shown that vibrational relaxation takes place with a time constant of 740 +/- 25 femtoseconds (ref. 7). So f or resonant intermolecular energy transfer to occur in liquid water, the in teraction between the OH-stretch modes of different water molecules needs t o be extremely strong. Here we report time-resolved pump-probe laser spectr oscopy measurements that reveal the occurrence of fast resonant intermolecu lar transfer of OH-stretch excitations over many water molecules before the excitation energy is dissipated. We find that the transfer process is medi ated by dipole-dipole interactions (the Forster transfer mechanism(9)) and additional mechanisms that are possibly based on intermolecular anharmonic interactions involving hydrogen bonds. Our findings suggest that liquid wat er may play an important role in transporting vibrational energy between OH groups located on either different biomolecules or along extended biologic al structures. OH groups in a hydrophobic environment should accordingly be able to remain in a vibrationally excited state longer than OH groups in a hydrophilic environment.