S. Bratos

Anisotropy of pump-probe absorption of the hydrated electron: A statistical model

A statistical model is presented to interpret the pump-probe absorption anisotropy of the hydrated electron. Its basic ingredient is the assumption of cascade-type deactivation processes between its excited p-states. The theory uses the correlation function formalism of nonlinear statistical mechanics. A number of technical problems are discussed: construction of a relaxation operator describing the cascade, solution of the corresponding Heisenberg equation of motion, calculation of various tensorial quantities, etc. According to this model, the presence of a cascade reduces the pump-probe dichroism, particularly if the p-state splitting is small. Very short cascade relaxation times of the order of 10 fs are required for that purpose. As the upper state of an s↔p transition is quasi-degenerate, the magic angle theories do not apply here. The similarity between the deactivation processes in excited electronic states of polyatomic molecules and of the hydrated electron is emphasized.

MOLECULAR VIBRATIONS IN LIQUIDS

Experimental methods and basic theories permitting the study of molecular vibrations in liquids are briefly reviewed. Techniques such as IR absorption, spontaneous Raman scattering, ultrasonic absorption, Rayleigh-Brillouin scattering and stimulated Raman scattering are discussed in this context. The physical nature of different vibrational relaxation times measured by these experiments is discussed in detail.

XFEL experiments: jitter of pump–probe time delays and pulse intensities

Jitter of XFEL signals due to fluctuations in shot-to-shot time delays and intensities are explored in the frame of a statistical theory of X-ray diffraction from liquids. Deformed signals are calculated at different levels of pump-probe jitter. A new method is proposed to eliminate these distortions.

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

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.