Multiple charge transfer driven complex reaction dynamics: covalent bonding meets van der Waals interactions

TL;DR

The paper addresses the challenge of capturing multiple charge-transfer (CT) dynamics at the single-molecule level in systems where covalent and van der Waals interactions coexist. It combines site-selective X-ray photoionization with COLTRIMS coincidence spectroscopy and high-level ab initio potential energy surface calculations on the N2Ar dimer to map SCT and DCT pathways. A key contribution is the experimental identification of SCT and DCT channels, including a heavy-ion transfer channel [NAr]^{+}, and the demonstration that two successive conical intersections enable electron shuttling between Ar and N2 during DCT. The work provides benchmark insights into nonadiabatic reaction mechanisms in complex natural systems and shows that simple mixed-interaction dimers can host rich CT dynamics.

Abstract

The details of multiple charge transfer within or among molecules (including the accompanying molecular structure evolution and energy distribution) are typically not accessible on a single molecule level in experiments targeting complex condensed chemical or biological systems. In order to gather such detailed insight, small prototype systems that cover the essence of such processes need to be identified and investigated. Here, we employ a small system consisting of a combination of covalent and van der Waals bonds for our studies, namely N2Ar dimers. We use synchrotron radiation to site-selectively enable the charge transfer processes and perform a coincidence measurement of the resulting electrons and ions. In combination with ab initio calculations, this approach enables a step-by-step tracking of the charge transfer and fragmentation dynamics. We find that ultrafast structural evolution of the dimer can trigger a second CT, thereby opening complex reaction pathways, in which electrons transfer back and forth between Ar and N2, and nonadiabatic transitions occur twice through conical intersections. These results demonstrate that multiple CT-induced transitions, particularly in such a simple dimer system, provide benchmark insights into the mechanisms of nonadiabatic reactions in complex natural systems.

Figures (4)

• Figure 1: Schematic illustration of charge transfer processes from calculated potential energy surfaces (PESs). Calculated PESs of the initial Auger final states, N$_2(\mathrm{X^1}\,\Sigma_g^+)$Ar$^{2+}$($\mathrm{^3P}$) and N$_2^{2+}$(a$^3\Pi_u$)Ar, produced following photoionization of the Ar 2p or N 1s shell, respectively. Ion pairs I (N$_2$$^{+}$, Ar$^{+}$) and II (N$^{+}$, Ar$^{+}$, N) are generated via conical intersections between these initial states and the N$_2^{+}$(D$^2\Pi_g$)Ar$^{+}$($\mathrm{^2P}$)) state. Subsequently, the ion pairs III (N$^{+}$, N$^{+}$, Ar) and IV (N$^{+}$, [NAr]$^{+}$) emerge through two additional conical intersections involving the N$_2^{+}$(D$^2\Pi_g$)Ar$^{+}$($\mathrm{^2P}$) and N$_2^{2+}$(a$^3\Pi_u$)Ar states. For clarity, the schematics highlight only the ionization pathways initiated at the Ar site.
• Figure 2: Measured three-body photoelectron-photoion-photoion coincidence spectrum. Time-of-flight correlations of two ions detected in coincidence with a photoelectron from (a) Ar 2p photoionization at 260 eV and (b) N 1s photoionization at 420 eV. In panel (a), two additional fragmentation channels, N$^{+}$ + [NAr]$^{+}$ and N$^{+}$ + N$^{+}$ + Ar, appear alongside the channels observed in both panels: N$_2^{+}$ + Ar$^{+}$, N$^{+}$ + Ar$^{+}$ + N, and N$^{+}$ + N$^{+}$. The N$^{+}$ + N$^{+}$ + Ar and N$^{+}$ + N$^{+}$ channels overlap in panel (a); their separation and analysis are discussed in Section \ref{['subsec2']}.
• Figure 3: Kinetic energy release distributions of the fragment ions. Measured KER spectra of two charged ions measured in coincidence with a photoelectron after breakup of the N$_2$Ar dimers into (a) N$_2^{+}$ + Ar$^{+}$ and (b) N$^{+}$ + Ar$^{+}$ + N following Ar 2p photoionization at 260 eV, and into (c) N$_2^{+}$ + Ar$^{+}$ and (d) N$^{+}$ + Ar$^{+}$ + N following N 1s photoionization at 420 eV. Error bars indicate statistical uncertainties.
• Figure 4: Kinetic energy distributions of the fragment ions and electrons. (a) Measured electron kinetic energy and (b) KER spectra for N$^{+}$ + N$^{+}$ pairs following Ar 2p or N$_2$ valence DI at 260 eV. Shown are total events (blue line) and partial events (red line) selected by applying the momentum-conservation filter 5 a.u. $< |\sum p^i_z| <$ 20 a.u., which effectively suppresses most background contributions from N$_2$ monomer dissociation. The left and right $y$ axes indicate event counts for total and partial electrons in (a) and ions in (b), respectively. (c) Measured electron kinetic energy and (d) KER spectra for N$_2$Ar dimer breakup into N$^{+}$ + [NAr]$^{+}$ following Ar 2p photoionization at 260 eV. The two peaks in (c) correspond to photoelectrons from the Ar 2p$_{1/2}$ and Ar 2p$_{3/2}$ shells, respectively. Error bars represent statistical uncertainties.