Importance of Electronic Relaxation for Inter-Coulombic Decay in Aqueous Systems

Abstract

Inspired by recent photoelectron spectroscopy experiments on hydroxide solutions, we have examined the conditions necessary for enhanced (and, in the case of solutions, detectable) inter-Coulombic decay (ICD)-Auger emission from an atomic site other than that originally excited. We present general guidelines, based on energetic and spatial overlap of molecular orbitals, for this enhancement of inter-Coulombic decay-based energy transfer in solutions. These guidelines indicate that this decay process should be exhibited by broad classes of biomolecules and suggest a design criterion for targeted radiooncology protocols. Our findings show that photoelectron spectroscopy cannot resolve the current hydroxide coordination controversy.An Auger process (see Fig. 1) involves the decay of a photo-excited electron-hole pair via annihilation of the hole by another electron, with simultaneous emission of an electron from a bound state to the continuum [1]. The rate is governed (in a Fermi golden rule framework) by both direct and exchange Coulomb integrals [2]. For photo-excited holes in inner-valence or core states, the associated orbitals are well localized on a given atom, such that Auger spectra are typically dominated by atom-specific transitions. Atomic and molecular phases comprise mainly localized electronic orbitals, and the decay rate is consequently quite small for processes involving electron emission from any atomic site other than that originally excited. In such systems, this type of ''off-site'' emission has been labeled as ICD, inter-Coulombic (atomic or molecular) decay [3].Although ICD has been studied primarily in the context of valence excitations [3][4][5], recent experiments have shown that core-excited systems may also decay in this fashion, as shown schematically in Fig. 1 [6]. Panel (a) depicts the instantaneous ground-state potential landscape and electron configuration for two atoms separated by some distance. The core electrons have energy substantially lower than the valence electrons and are tightly bound to the atomic nucleus, screening the nuclear charge Z such that the valence electrons are subject to an effective nuclear Coulomb potential proportional to Zeff ¼ Z _ 2, consistent with Gauss's law. (For the purpose of this discussion, we assume that A and B are atoms of first-row elements with 1s core electrons only.) These effective potentials, when added together, give rise to the multiple-Coulombwell landscape shown in Fig. 1(a), which will support some localized bound states and additional bonding states spanning multiple atomic centers. Core excitation of a given atom [ Fig. 1(b)] will increment the effective nuclear charge, steepening the local potential landscape at that site. This will cause a sudden downward shift in the energy of some electronic states, which in some cases may prevent coupling with states from neighboring atomic sites, or in others may lead to new electronic hybridization via tunneling through the resulting potential barrier. The key point for the present work is t...

Imaging local electronic corrugations and doped regions in graphene

“…For our supercell sizes, under periodic boundary conditions, the use of the excited electron and core-hole (XCH) approximation would place an additional electron in each supercell and would constitute an unphysically large doping concentration (that is, number of free carriers at the Fermi level), with a noticeable spurious impact on the core-excited density of states and associated X-ray absorption spectrum. This is contrary to the more common application of the XCH approximation to bound excitons in the X-ray excitations of molecules and molecular condensed phases 14,22,23 . The assignment of the area denoted as interlayer states and the distinctive pre-edge features noted for some of the domains in Figures 1c and 2b have been the topic of much recent contention for single-layered graphene (SLG) [16][17][18][19] .…”

On the nature and origin of dicationic, charge-separated species formed in liquid water on X-ray irradiation

“…The final product is a doubly charged species, but with the two positive charges located on different water units, i.e., a [H 2 O + ··H 2 O + ](aq) complex is formed. Computational work suggests that the ICD process is a rather general mechanism for electronic deactivation in hydrogen-bonded molecules in water such as hydrated biomolecules, 19 ammonia, [20][21] or water molecules interacting with a protein residue. 21 More recently ICD-like processes have also been observed for core-ionization and core-excitation of aqueous solutions.…”

Site- and energy-selective slow-electron production through intermolecular Coulombic decay

“…These properties of ICD and the fact that it appeared to be ubiquitious in hydrogenbonded systems [6][7][8] suggest its potential importance for radiation damage [9]. Low-energy electrons (LEEs) [10,11], as well as radical cations [12], the direct products of the ICD, are known to be effective in causing single-and double-strand breaks in DNA.…”