High-Mobility Electrons in Aqueous Iodide Solutions

Abstract

The photoexcitation of aqueous iodide solutions is a prototype for the generation of electrons in liquid water. Upon one-photon excitation, the precursors of the solvated electrons are localized states with a radius of a few angstroms. In contrast, with the aid of transient absorption spectroscopy at terahertz, near-infrared, and visible frequencies, we show that the two-photon absorption of ∼400 nm pulses can impulsively generate short-lived (∼250 fs), delocalized electrons that are released tens of angstroms away from the parent ion. We propose that these states can be ascribed to 5p → 6p transitions that, in turn, could be thought of as frustrated Rydberg orbitals or large radius excitons. By capitalizing on the unique capabilities of transient terahertz spectroscopy, we estimate that these delocalized states are characterized by an electronic mobility and diffusivity that are about 500 times greater than those of the fully relaxed electrons.

Figure 1

Optical-pump terahertz-probe results. a) THz pulse transmitted at equilibrium, with the optical pump off, by a free-flowing 10-μm thick aqueous solution containing 5 M of sodium iodide. The position of the THz peak is marked with E_max. b) Intensity (magnitude squared) of the Fourier transform of the THz field. c) Relative variation of the transmitted THz peak, ΔE/E_max, as a function of the pump–probe delay, t_PP. The pump pulses are centered at about 400 nm, are 50 fs long, and have an intensity of ∼100 GW/cm². The right axis refers to the corresponding change in the optical density, ΔOD(t_PP). d) Normalized transient OD as a function of the salt concentration: 1 M in blue, 5 M in red, and 7.5 M NaI in green. The decay time increases with salt concentration, from ∼150 fs for 1 M to ∼270 fs for 7.5M. e) The maximum change of the optical density is found at pump–probe overlap (t_PP = 0) and scales linearly with the salt concentration.

Figure 2

Transient absorption probed at infrared (a,b) and visible (c,d) wavelengths. The ∼10 μm thick liquid jet is irradiated with 400 nm, 100 fs long pulses with a peak intensity of ∼100 GW/cm². a) When the central probe wavelength is set to λ = 1000 nm, the signal increases within 100–300 fs and decays within about 1 ps. The dynamical processes are delayed for larger salt concentrations, from 2.5 M NaI (blue curve, empty circles) to 5 M NaI (red curve, empty circles). b) The maximum transient response scales with salt concentration, within the experimental uncertainty. c) The signal appears at longer delays when probed at 600 nm. It increases within ∼600 fs for the 2.5 M NaI aqueous sample (blue solid circles), and within ∼1 ps for the 5 M NaI solution (red solid circles). The signal is approximately constant at longer pump–probe delays, up to 20 ps. d) The signal at t_PP = 20 ps is proportional to the salt concentration.

Figure 3

Power dependence of the transient signals of a 5 M aqueous iodide solution. a) The maximum OPTP signal at pump–probe overlap (t_PP = 0) is plotted as red circles. This is assigned to the delocalized or “free” electrons (e_free). b) The blue squares correspond to the optical density measured at t_PP = 20 ps probed at a wavelength of 600 nm. We expect that this response depends on the amount of relaxed and solvated electrons (e_solv). The black solid lines are quadratic fit functions of the signals vs pump intensity.

Figure 4

Evolution of the photoionization of aqueous iodide with two-photon absorption at 400 nm. Cartoons of microscopic salt solutions are reported in panels a), b), c), and d). The experimental results are shown in panel e). a) Sketch of the sample before the pump arrival, i.e., at t_PP < 0. The unperturbed liquid water molecules are displayed with oxygen in red, hydrogen in gray, and hydrogen bonds with black dashed lines. An iodide anion is represented in yellow. b) At pump–probe delays close to zero, within about 0 and t_PP = 200 fs, a large transient signal is probed by THz radiation (red curve in panel e). This suggests the formation of delocalized or “free” charges (e_free). c) For t_PP between about 200 fs and 1.1 ps, the signal leaves the THz range and appears in the infrared (orange curve in panel e)). This can be associated to a localized precursor or “wet” electron (e_wet). d) At longer pump–probe delays, the signal is prominently found in the visible range and the electron is solvated (e_solv), see the blue curve in panel e). The solid black lines are obtained from the global fit procedure.