Pump and probe technique and its application for study of the chemical reactions in the real-time domain

Spectroscopy of size-selected molecular clusters provides a unique approach to investigate solvation phenomena at the molecular level. For ions, rigorous mass-selection provides an opportunity to study the energetics and dynamics, with the solvent number of atoms or molecules being well defined. This mesoscopic solvation, which may involve energy redistribution, electron transfer and vibrational predissociation, has been examined in a variety of ionic clusters.

In A.Zewail laboratory in Pasadena, femtosecond time-resolved photoelectron spectroscopy (FPES) is utilized to investigate the evolution of ultrafast dynamics as a function of cluster size. The complexity of the reaction pathways for the ionic reaction is simplified by resolving, in time and energy, the photoelectron spectra of nascent and parent ions. A schematic representation of the molecular beam apparatus is illustrated in the following figure.

Anionic clusters are generated by secondary electron attachment after the supersonic expansion. The anionic clusters are extracted to the field- free time-of-flight region by a pulsed electric field. A particular size is selected by an interleaved-comb massgate before entering the interaction-with-light region.

Studies of such size-selected clusters provide valuable insights into stepwise solvation effects on the reaction dynamics and emergence of bulk-type behavior, with the hope of bridging the gap between the isolated gas-phase and condensed-phase dynamics.

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