Reduced coupling of water molecules near the surface of reverse micelles

We report on vibrational dynamics of water near the surface of AOT reverse micelles studied by narrow-band excitation, mid-IR pump–probe spectroscopy. Evidence of OH-stretch frequency splitting into the symmetric and asymmetric modes is clearly observed for the interfacial H2O molecules. The polarization memory of interfacial waters is preserved over an exceptionally extended >10 ps timescale which is a factor of 100 longer than in bulk water. These observations point towards negligibly small intermolecular vibrational coupling between the water molecules as well as strongly reduced water rotational mobility within the interfacial water layer.

Relaxation dynamics of the hydrated electron studied with 5-fs pulses

Basically, the hydrated electron is an excess electron trapped in a potential well formed by surrounding water molecules, with an s-like ground state and three non-degenerate p-type excited states. The electron surrounded by the oriented water molecules is a chemical reactant with an unusually high electron donor capacity as its characteristic chemical feature. On the other hand, it seems to be one of the simplest physical systems to study solvation dynamics and to test mixed quantum classical theories experimentally. Yet, even after decades of intensive experimentation and calculations on the hydrated electron, understanding of its relaxation dynamics is far from being complete. One of the most important questions is the explanation of an ~1 ps relaxation rate of the photo-excited hydrated electron. This rate has been controversially attributed to the population lifetime of the p-state or cooling of the ground state after rapid relaxation from the p-state. We present the experimental study of the energy relaxation of the photo-excited hydrated electron. The results of frequency-resolved pump-probe with 5-fs pulses provide sufficient evidence in favor of the hot-ground-state model. The initial ultrafast energy relaxation of the photo-excited electron, controlled by the librations of the surrounding water molecules, takes place during the ~50 fs upon the excitation. We show that after the first 100 fs almost the entire population of the p-state is transferred to the hot ground state that subsequently cools down on a ps time scale

On the relation between the echo-peak shift and Brownian-oscillator correlation function

We show that for systems that exhibit bimodal dynamics in their system-bath correlation function the shift of the stimulated photon-echo maximum as a function of waiting time reflects fairly well the long time part of the correlation function. For early times this correspondence breaks down due to a fundamentally different behaviour of the echo-peak shift in this time domain and because of the effect of finite pulse duration on the echo-peak shift. The method is used to characterize the solvation dynamics in various dye solutions.

Nonlinear spectroscopy in the single optical cycle regime

In this contribution we present a theoretical analysis in which the frequency- and time-domain formalism of ultrafast nonlinear spectroscopy is thoroughly reexamined. The complete expressions valid even for single-cycle-pulse applications are derived for the nonlinear signal in the frequency and time domains. We also assert that the influence of geometrical delay smearing does not introduce a significant distortion of the observed traces provided that the geometry is carefully optimized. The derived formalism is applied to photon-echo spectroscopy on the hydrated electron with 5-fs pulses