Direct observation of the collapse of the delocalized excess electron in water

Abstract

It is generally assumed that the hydrated electron occupies a quasi-spherical cavity surrounded by only a few water molecules in its equilibrated state. However, in the very moment of its generation, before water has had time to respond to the extra charge, it is expected to be significantly larger in size. According to a particle-in-a-box picture, the frequency of its absorption spectrum is a sensitive measure of the initial size of the electronic wavefunction. Here, using transient terahertz spectroscopy, we show that the excess electron initially absorbs in the far-infrared at a frequency for which accompanying ab initio molecular dynamics simulations estimate an initial delocalization length of 40 Å. The electron subsequently shrinks due to solvation and thereby leaves the terahertz observation window very quickly, within 200 fs.

Wavelength dependence of nanosecond infrared laser-induced breakdown in water: Evidence for multiphoton initiation via an intermediate state

“…This indicates that excitation into the CB becomes possible in an "autoionization" process, which involves reorganization of the excited water molecule into a form with lower ionization potential [30,31,41,45,48]. Above the threshold for vertical ionization, where nuclear position remain unchanged, the electron ejection length finally reaches a value of about 4 nm [30,49] [ Fig. 1(c)].…”

Dynamics of the Bulk Hydrated Electron from Many‐Body Wave‐Function Theory

“…[7] Although the conventional view of the cavity structure is supported by most theoretical studies,t he noncavity model has also been able to predict the observables close to the measured ones. [7a,f,g] Thet ime evolution as obtained from molecular dynamics (MD) simulation [8] enriches the picture:a fter generation of e À aq ,aregular cavity is being formed from abulk structure of water within several picoseconds via less regular intermediates.Consequently,the delocalized structures must exist, although their exact shapes, life times and relevance for chemistry are yet to be revealed. Another ongoing discussion has to do with the distinguishable surface and bulk structures of the hydrated electron.…”

“…Another ongoing discussion has to do with the distinguishable surface and bulk structures of the hydrated electron. [9] Much of the theoretical uncertainty results from the limitations of the computational approaches applied: [10] to name af ew,s tatic calculations of clusters with density functional theory DFT and correlated wave function methods, [11] MD of clusters with DFT [12] and correlated wave function methods, [13] condensed-phase and cluster molecular dynamics (MD) with one-electron pseudopotentials/Hamiltonians [7e,g, 9b,14] and with DFT,i ncluding the generalized gradient approximation (GGA), [15] GGA-based QM/MM, [8] and an ad hoc constructed dispersion-corrected hybrid functional. [16] DFT,t he work horse of ab initio MD,e xhibits an umber of well-known shortcomings.O ne of them is the inaccurate description of bulk liquid water, for which even modern functionals are not able to provide ab alanced description of structure,d ynamics and thermodynamics simultaneously.…”

“…Importantly,w eo bserve the regions of negative spin density,g iving rise to an egative isotropic hyperfine coupling and essential for explaining EPR spectra. [5] Negative spin density regions have not been reported in the bulk DFTbased QM/MM studies, [8] although they are predicted by most simulations. [1a, 11a] As it can be seen in Figure 1, our study based on MP2 correlated wave function MD confirms negative spin density regions in the condensed phase.…”

Focus: Phase-resolved nonlinear terahertz spectroscopy—From charge dynamics in solids to molecular excitations in liquids

“…57,58 The dynamics of excess electrons in water has been addressed in recent optical-pump-THz-probe experiments. 59 Excess electrons were generated with pump pulses at different wavelengths, inducing either multiphoton ionization of water molecules for pump wavelengths of 266 nm and 400 nm or field ionization with 800 nm pumping. The time evolution of the self-consistent electron potential as defined by the timedependent structure of the water environment was probed with THz pulses.…”