Time-resolved photoelectron spectroscopy of solvated electrons in aqueous NaI solution

Abstract

Time-resolved photoelectron spectroscopy was used to study the energetics and dynamics of solvated electrons in aqueous solution. Solvated electrons are generated by ultrafast photodetachment in a 100 mM aqueous NaI solution. Initially, an ensemble of strongly bound ("cold") solvated electrons and an ensemble of weakly bound ("hot") electrons in an unequilibrated solvent environment are observed. We report an ultrafast recombination channel for the "hot" electrons with a rate of (800 fs)(-1) which is in competition with thermalization occurring with a rate of (1.1 ps)(-1). The thermalized electrons recombine with the iodide radical with a rate of (22 ps)(-1). About 35% of the thermalized electrons escape geminate recombination and form free, solvated electrons. The vertical detachment energy for the solvated electron is determined to be 3.40 eV. No indication for a surface-bound electron at lower binding energies was observed.

Real-time observation of the charge transfer to solvent dynamics

“…At the other end of the reaction, the prevailing view of the solvated electron is that its s-like wave function resides in a nearly spherical cavity coordinated to 4-6 water molecules 37,38 , although recent work has shown that it may extend beyond the boundaries of this cavity 39 . The early time resemblance of the TA spectra 3,5,6 to that of the aqueous electron strongly suggests that at tB0.2 ps, the structure of its first solvation shell of e À (H 2 O) is already very similar to the equilibrium one. The ejection distance upon excitation in the CTTS state at low energy was estimated to be B0.5 nm 1,9 .…”

“…However, these methods turned out to be exclusively sensitive to the electron signal, that is, the end product of the photodetachment process, because of its very large oscillator strength over a broad spectral range and the fact that the excited CTTS absorption is close to that of the solvated electron 5,6 . These studies showed that in water the electron is detached from iodide in B0.2 ps.…”

“…The valence electrons of ground-state anions are bound by the nucleus, but the excited states are bound by solvent polarization 1 . The CTTS states of halides rapidly decay by ejection of an electron, which is then stabilized by solvation, leaving a neutral halogen atom behind [2][3][4][5][6][7][8][9][10][11][12][13][14] . Atomic anions are ideal for studies of the electron-transfer dynamics to the solvent, because the solute lacks internal (nuclear) degrees of freedom, such that the process of CTTSmediated electron ejection is entirely governed by the structure and motions of the solvation shell.…”

“…From the experimental point of view, there has been a huge effort aimed at observing the early-time dynamics of aqueous CTTS states using ultrafast transient absorption (TA) spectroscopy [1][2][3][4]15,22 and, more recently, ultrafast photoemission from liquid microjets 5,6 . However, these methods turned out to be exclusively sensitive to the electron signal, that is, the end product of the photodetachment process, because of its very large oscillator strength over a broad spectral range and the fact that the excited CTTS absorption is close to that of the solvated electron 5,6 .…”

Charge Transfer to Solvent Dynamics at the Ambient Water/Air Interface

“…A particularly useful means of initiating CTTS reactions is photoexcitation to CTTS absorption bands of certain aqueous anions. The most studied of these has been the CTTS of iodide [2][3][4][5][6][7][8][9][10][11][12][13] and a schematic of the solvation dynamics are shown in Figure 1. However, these studies have focussed on the bulk dynamics, while arguably, electron transfer reactions are of most interest at interfaces, be it on an aerosol particle, on an interstellar dust-grain, or at a water/biomolecule interface.…”