J.-Cl. Leicknam

Femtosecond solvation dynamics of water: solvent response to vibrational excitation of the solute

Abstract A computer simulation study of a liquid H 2 O/D 2 O solution is presented to examine the solvent response to vibrational excitation of the solute. A mixed quantum-classical method is employed in which the quantum-mechanical intramolecular vibrational degrees of freedom of a dissolved H 2 O molecule are treated analytically, whereas the rotational-translational degrees of freedom are computed by classical molecular dynamics simulation. The three response function ( S α ( t ) correlating with the normal modes of H 2 O are found to be highly bimodal; the corresponding response times are of the order of 100 fs and 1 ps.

Ultrafast transient absorption spectroscopy of the hydrated electron: a theory

Abstract A theory is proposed to study ultrafast transient absorption of the hydrated electron. This theory is a statistical theory using a correlation function description of the nonlinear optical processes involved. Time- and frequency-resolved spectra are examined for both coherent and incoherent incident fields. The nonmonotonic behavior of spectra transients is studied in detail.

Pump-probe absorption anisotropy of the hydrated electron. A theory

Abstract Pump-probe absorption dichroism is studied in the case of the hydrated electron. This theory is a statistical theory using a correlation function description of the nonlinear optical processes involved. Its basic ingredient is the assumption of cascade-type interstate dynamics in the excited p -states of ( e rmaq − ). It is shown that if the latter are fast enough, the pump-probe dichroism disappears in most circumstances; it persists for a zero time delay and coinciding pump and probe frequencies. Cascade relaxation times of the order of 10 fs or less are required for that purpose.

Infra-Red Spectra of Hydrogen Bonded Systems: Theory and Experiment

Infra-red spectroscopy of hydrogen bonded systems is shortly reviewed. Both experimental and theoretical aspects of the problem are covered, but the accent is put on the theory. The first part of the paper refers to the classical, whereas its second part covers the time-resolved spectroscopy of the OH stretching band. Modern theories of vibrational band shapes are presented; they are all based on the density matrix approach of statistical mechanics and employ the correlation function formalism. Using this technique, molecular dynamics of hydrogen bonds can be monitored up to time scales of the order of a few tens of femtoseconds.

Raman study of rotational motions of hydrogen sulphide in the liquid state

Raman spectra of liquid H2S and D2S at various temperatures are analysed. The band profiles have a rotational origin and are compared with those calculated by the J extended diffusion model for an asymmetric rotor. The values obtained for the angular momentum correlation time correspond to a large angle rotational diffusion. The rotational motion is discussed in terms of the Chandler model for translational diffusion.

Inelastic neutron scattering from liquid water. Theory and simulation

Abstract A theory is described to interpret the recent data on incoherent inelastic neutron scattering from liquid water obtained with the help of spallation sources. An analytical theory, similar to those describing optical spectra of liquids, is presented first. The role of the librational motions and of the relaxation of the intramolecular vibrational motions is emphasized. Computer simulation results are discussed next. As fully classical simulations are not appropriate, the quantum degrees of freedom, associated with internal vibrations, are disentangled from the quasi-classical degrees of freedom, corresponding to the rotations and translations. Only the latter are simulated by molecular dynamics. Preliminary simulation results are presented and are compared with experimental data.

Real Time Visualization of Atomic Motions in Dense Phases

It has always been a dream of physicists and chemists to follow temporal variations of molecular geometry during a chemical reaction in real time, to “film” them in a way similar as in the everyday life. Unfortunately, chemical events take place on tiny time scales comprised between 10 fs and 100 ps, approximately. Visualizing atomic motions thus remained a dream over two centuries. This is no longer true today, consequence of an immense instrumental development the last decades. Two methods are particularly important. The first of them is ultrafast optical spectroscopy employing the recently developed laser technology. In his breakthrough work A. Zewail was able to show how can this method be used to follow the photoelectric dissociation of gaseous ICN in real time[1,2]. It has later been applied to several other problems, and particularly so to visualize OH..O motions in liquid water[3,4]. Unfortunately, visible light interacts predominantly with outer shell rather than with deeper lying core electrons that most directly indicate molecular geometry. It is thus difficult to convert spectral data into data on molecular geometry. The second method refers to time resolved x-ray diffraction and absorption. As x-rays interact predominantly with deeply lying core electrons which are tightly bonded to the nuclei, converting x-ray data into data relative to molecular geometry is, in principle at least, much more straightforward than in optical spectroscopy. Unfortunately, pulsed x-rays techniques are technically very demanding.