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

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

Phase-amplitude retrieval:SHG FROG vs. SPIDER

Frequency resolved optical gating (FROG) and spectral phase interferometry for direct electric-field reconstruction (SPIDER) are nowadays leading techniques that provide access to phase-amplitude pulse retrieval. Each of these techniques has a number of outstanding features that establish the method applicability under certain experimental conditions. For instance, FROG is perfectly suited for pulse characterization precisely at the position of the sample in most spectroscopic applications because it utilizes similar excited-probe geometry. On the other hand, SPIDER has the advantage of real-time pulse measurement at high repetition rates. While in an ideal case both methods allow the precise amplitude-phase reconstruction of an ultrashort pulse, in practice specific experimental conditions such as phase-matching, detector noise etc. affect the reconstruction quality. For instance, experimental comparison of the techniques in the case characterization of a single pulse has shown some discrepancies in the retrieved parameters. We present a comparative study of SPIDER and second-harmonic generation (SHG) FROG techniques. Two main sources of errors are analyzed: the limited phase matching bandwidth of the nonlinear medium and the detector noise. We show that under similar experimental conditions SPIDER performs somewhat better than SHG FROG