Picosecond Laser Photoionization in Polar Liquids and the Study of the Electron Solvation Process

Summary

The study of electrons in liquids and electron-solvent interactions has long been an important subject of theoretical and experimental work [l]. Recently the use of ultra rapid techniques for injecting electrons into liquids (relativistic electron beams in pulse radiolysis and laser photoionization of molecules and negative ions) coupled with rapid absorption spectroscopy has led to exciting new developments in the picosecond time range [2].

Photoionization of halide anions which possess charge transfer to solvent spectra (CTTS) must be very fast and the solvated electron (ē_s) formation kinetics must be controlled by the solvation process itself. Using their picosecond echelon technique, RENTZEPIS et al. [3] have discussed such a mechanism in the laser photoionization of the ferrocyanide anion in aqueous solution at 265 nm. Electron solvation in liquid alcohols has been investigated by picosecond pulse radiolysis and discussed in terms of the Stokes-Einstein-Debye representation [2], but temperature increases are to be expected in the spur after the deposition of radiation and the solvation process might be made faster. Picosecond laser photoionization seems convenient to avoid such effects but nothing has been done in alcohol solutions, as far as we know. Because in alcohol solution, the ferrocyanide anion is insoluble and the absorption spectra of the halide ions are blue-shifted with respect to the available 265 nm actinic light, we decided to study the photoionization of aromatic molecules and ions, for which a detailed description of the mechanism does not exist, even though the formation of a “semi-ionized”, intermediate CTTS state or a “solvated Rydberg state” has long been discussed [4].

In this paper we report photoionization of phenolate and phenol in alcohol solution, using a single exciting pulse of 27 ps duration at 265 nm. Solvated electron absorption was probed either with an analysing pulse at 630 nm obtained from the residual 530 nm pulse by stimulated Raman scattering (SRS) in a 2 cm cell containing dimethyl-sulfoxide [5] or with a continuum pulse generated by the 1064 nm fundamental in a 4 cm water cell (WC in Fig. 1).