What can be learned from femtochemistry: the H bond in water

Abstract

A non-empirical theory is presented showing how infrared pump–probe spectroscopy can be used to measure subpicosecond variations of the oxygen–oxygen distribution functions in liquid water, Δg(r, τ). It should be emphasised that the equations derived in the present contribution are exact up to the second order in the perturbation expansion. These equations are model independent and provide a route to predict time-dependent radial distribution functions that are measured by time-resolved X-ray when the out-of-equilibrium configuration is produced by an optical pump pulse. It is shown that the profiles of OH stretching bands of HDO/D₂O can be brought into a full coincidence with Δg(r, τ). We thus prove that femtosecond pump–probe experiments are indeed useful to measure the time dependency of a H bond length. The theoretical framework developed in the present contribution is applicable to any H bonded system and especially to the field of bio-molecules or bio-system where H bonds play a key role in the unfolding/folding mechanism.

Keywords:

• hydrogen bonding
• fs spectroscopy
• OH...O bond in liquid

Acknowledgements

The authors gratefully acknowledge the editor for the invitation to contribute to the special issue in honnor of Professor Pierre Turq.

Notes

1. The delay time corresponds to the elapsed time between the maximum of the pump pulse that is assumed to be Gaussian in shape and time at which we measure the distance that is supposed to be extremely fast compared to the optical excitation process, i.e. instantaneous. The transient MRDF may be calculated for negative delay time because a Gaussian pulse has an infinite time extension. The transient MRDF calculated is insensitive to the fraction of vibrator excited by the pump pulse. Moreover it is calculated for the first excited state whose population is evidently 0 before any excitation occurs.