Time-resolved photoelectron spectroscopy of bulk liquids at ultra-low kinetic energy

Abstract

Time-resolved photoelectron spectroscopy (TR-PES) of ultrafast dynamics in solution is presented. To measure the photoelectron kinetic energy distribution (PKED) that is free from inelastic scattering in solution, photoelectrons were generated with ultra-low kinetic energies (ULKE: <5 eV). Time constants of the elementary processes in the charge-transfer-to-solvent (CTTS) reaction from Ito bulk water were in excellent agreement with those obtained by transient absorption spectroscopy, demonstrating the bulk-sensitivity of TR-PES-ULKE. The analysis suggests that the CTTS reaction proceeds via two intermediates, and that 30% of the first intermediate and 70 % of the second intermediate respectively are quenched by geminate recombination between the electron and the neutral iodine atom.

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 .…”

“…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 .…”

“…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.…”

“…These studies showed that in water the electron is detached from iodide in B0.2 ps. Thereafter, relaxation of the host solvent cavity forms (in B1 ps) a solvated electron [1][2][3][4][5] . Femtosecond X-ray absorption spectroscopy at the I L-edges detected the birth of the neutral halogen, but the time resolution was not sufficient to capture the early dynamics 7 .…”