Ultrafast solvation and recombination dynamics of electrons in fluids

E. MARCECA

Marseille

Seminario; PIIM-CNRS seminars; 2022

Laboratoire de Physique des Interactions Ioniques et Moléculaires - Aix Marseille Université

The seminar is focused on the role of the solvation environment of reactants during electron-transfer (ET) reactions. Subpicosecond solvation dynamics of electrons (principally in neat water) are studied since the pioneering pump?probe transient-absorption (TA) experiments performed years ago by the groups of Eisenthal and Gauduel, in which electrons were released by UV multiphoton ionization (MPI) of the solvent itself. Photoinduced injection of electronic charge, from the donor species to the solvent host, guarantees that an entire ensemble of ET reactions is simultaneously started. However, in order to exclusively ascribe any experimental spectral changes to the solvation dynamics, electrons must be generated by photodetachment of donors lacking internal modes, such as inorganic monoatomic halides (e.g. iodide). Once in the fluid, detached electrons are short-lived, thermalizing within a few hundred femtoseconds into a series of non-equilibrium species that can be monitored in the NIR. After that, unpaired electrons can recombine via electron-radical annihilation, or eventually become long-lasting solvated electrons. In contrast with MPI, the energy required to generate electrons in a fluid host doped with iodide is substantially lower. In particular, a single UV photon suffices to detach the 5p electron by excitation of a so-called charge-transfer-to-solvent (CTTS) band.TA experiments will be presented in the seminar that reveal the singular behavior of electrons originating from CTTS excitation of iodide solutions in low dielectric constant solvents, e.g. liquid and supercritical ammonia. The results evidence electron trapping by counterions leading to solvent-separated pairs of the type (K+, NH3, e) due to the tight proximity of K+ cations to the electron ejection point. A fast ( < 250 fs) increase of absorption, assigned to the development of electron species in the fluid, is followed by two decay components: one with a characteristic time around 10 ps, and a slower one that remains active for hundreds of picoseconds. While the first process is attributed to an early recombination of (I, e) pairs, the second decay and its asymptote reflects the effect of the K+ counterion on the geminate recombination dynamics, and on the electron yield. It was found that the formation of (K+, NH3, e) pairs brings the electron escape probability to very low values. The description of transient and equilibrium electron species in NH3 is complemented by static and dynamic scavenging experiments, and the reconstruction of the transient spectra of electrons as a function of time.