Picosecond measurements of geminate charge pair recombination in photoionized liquids

Abstract

The recombination dynamics of geminate cation–electron pairs in liquid hexane is studied by combining picosecond photoionization and infrared-stimulated charge dissociation measurements. The observed yield of free charge is determined by the product of the charge pair survival probability and the probability for infrared-stimulated charge dissociation. Experimental results are compared with transient solutions of the Smoluchowski equation governing diffusive recombination in a Coulomb well. Reasonable agreement with experiment can be reached by allowing for a radial distribution of initial pair separations. However, the stimulated dissociation yield exhibits a functional dependence on pair separation which complicates the analysis. A simple model of geminate charge pair dissociation by infrared stimulation is presented and its implications for these experiments are discussed.

Conductivity of solvated electrons in hexane investigated with terahertz time-domain spectroscopy

“…46 -50 Early radiolysis and ͑time-resolved͒ photoionization experiments were largely concerned with the pathways and efficiency of the generation process of the excess electrons, [51][52][53][54] the mechanism of charge transport, [55][56][57][58] and the electron-ion recombination process. [59][60][61][62][63][64][65][66][67] Later experiments examined the variation of the electron mobility with temperature, 53,68 pressure, 57,68 -72 and molecular geometry, 48,56 the connection between electron mobility and free-ion yield, 51,53,54 and recombination rates of electrons with electron scavengers. 59,[73][74][75] From these studies, possible pathways and typical time scales for photogenerated electrons have emerged.…”

“…[81][82][83] Transient absorption spectroscopy has provided detailed information on the lifetime of electrons in liquids. [60][61][62][63][64]66,67,84 It has, however, provided less insight into the transport characteristics of these charge carriers, a property that is not manifested directly in the optical spectra. Radiolysis experiments, on the other hand, only provide data on the dc conductivity.…”

Charge Transport and Carrier Dynamics in Liquids Probed by THz Time-Domain Spectroscopy

“…Attention has been focused both on the characteristics of the solvated electrons themselves and on their generation and recombination with ions. Important experimental approaches have included radiolysis measurements [11][12][13], in which induced currents from irradiation of the sample are recorded, and laser-based probes [14][15][16], in which the infrared or visible transient absorption induced by an ionization step may be followed with high time resolution. From these investigations, a two-state model of solvated electrons has emerged that is capable of describing many of the experimental observations [12,13].…”

Ultraviolet and infrared laser excited two-color multiphoton ionization for determination of molecules in solution

“…The escape probability (P(r,E,T)) of an ion (electron) from the geminate pairs is given first order by (9) P(r,E,T) = exp(-rc/r)(l + ercE/2kT) (1) where e is the electron charge, E the electric field, k the Boltzmann constant, T the temperature, r the initial separation distance of the ion and the electron of the geminate pair, and rc (-e2/tkT, where f is the dielectric constant of the solvent) the Onsager radius. Most of the geminate pairs recombine very quickly (10)(11)(12)(13) and the quantum yield of free charge carries is extremely low (at most a few percent) in nonpolar solvents (9); a lower escape probability must give a smaller photocurrent signal. However, nonpolar solvents give extremely low leakage currents (3) and high electron mobility (14) as compared with polar solvents.…”