Time-Resolved Phase-Sensitive Second Harmonic Generation Spectroscopy of the Hydrated Electron at the Water/Air Interface

Abstract

The hydrated electron attracts attention since its discovery over fifty years ago. Being one of the products of the ionization of water, hydrated electrons, which are free electrons in water, play significant roles in biological damage, atmospheric chemistry, nuclear chemistry, etc.. However, despite its importance and the large number of studies on the hydrated electron many aspects of it are still not resolved. One of these concern its existence and behaviour at the water surface, which is of great interest since many of the processes it is involved in take place at interfaces.

In this work, a technique is developed, that is based on the second harmonic generation (SHG) and enables the study dynamics of the hydrated electron at the water/air interface. By introducing a local oscillator, which interferes with obtained SHG from the water surface, a signal directly proportional to the sample concentration is obtained, in contrast to the quadratic dependence from conventional SHG. Moreover, the technique allows phase information to be obtained, which enables the determination of the real and imaginary parts of the 2nd order non-linear susceptibility. In addition to this, the technique uses a lock-in measurement, removing large constant offset from the interference. The technique yields high quality data on adsorbates with low surface concentration and has been extended to the time domain which provides insight into the dynamics of hydrated electrons at the water/air interface. In this experiment, the electron was generated using the charge-transfer-to-solvent transition of iodide and probed primarily over the first few picoseconds. This probes the initial solvation of the electron at the interface. Our results suggest that the dynamics are similar to the dynamics observed in the bulk, although the added phase-sensitivity provides new information about early solvation dynamics.