Hydrogen-Bond Disruption by Vibrational Excitations in Water

An excitation of the OH-stretch nu(OH) of water has unique disruptive effects on the local hydrogen bonding. The disruption is not an immediate vibrational predissociation, which is frequently the case with hydrogen-bonded clusters, but instead is a delayed disruption caused by a burst of energy from a vibrationally excited water molecule. The disruptive effects are the result of a fragile hydrogen-bonding network subjected to a large amount of vibrational energy released in a short time by the relaxation of nu(OH) stretching and delta(H2O) bending excitations. The energy of a single nu(OH) vibration distributed over one, two, or three (classical) water molecules would be enough to raise the local temperature to 1100, 700, or 570 K, respectively. Our understanding of the properties of the metastable water state having this excess energy in nearby hydrogen bonds, termed H2O*, has emerged as a result of experiments where a femtosecond IR pulse is used to pump nu(OH), which is probed by either Raman or IR spectroscopy. These experiments show that the H2O* spectrum is blue-shifted and narrowed, and the spectrum looks very much like supercritical water at approximately 600 K, which is consistent with the temperature estimates above. The H2O* is created within approximately 400 fs after nu(OH) excitation, and it relaxes with an 0.8 ps lifetime by re-formation of the disrupted hydrogen-bond network. Vibrationally excited H2O* with one quantum of excitation in the stretching mode has the same 0.8 ps lifetime, suggesting it also relaxes by hydrogen-bond re-formation.