Intermolecular Charge Transfer Induced Fragmentation of Formic Acid Dimers

We investigate the intermolecular nonradiative charge transfer process in a double hydrogen-bonded formic acid (FA) dimer, initiated by electron-collision induced double ionization of one FA molecule. Through fragment ions and electron coincident momentum measurements and ab initio calculations, we obtain direct evidence that electron transfer from the neighboring FA molecule to fill one of the two vacancies occurs by a potential energy curve crossing of ${\mathrm{FA}}^{++}+\mathrm{FA}$ with ${\mathrm{FA}}^{+}+{\mathrm{FA}}^{+*}$ curves, forming an electronic excited state of dicationic dimers. This process causes the breaking of two hydrogen bonds and subsequently the cleavage of $\mathrm{C}─\mathrm{H}$ and $\mathrm{C}─\mathrm{O}$ covalent bonds in the dimers, which is expected to be a general phenomenon occurring in molecular complexes and can have important implications for radiation damage to biological matter.

Figure 1

Illustration of direct CT in FA dimers. (a) Two outer-valence (ov) vacancies are initially created on one site of the dimer upon electron impact, which forms a ${\mathrm{FA}}^{++}·\mathrm{FA}$ dicationic dimer. (b) One of the two vacancies is filled by an electron from the neighboring site. This process occurs by a PEC crossing of the ${\mathrm{FA}}^{++}+\mathrm{FA}$ curve with a ${\mathrm{FA}}^{+}+{\mathrm{FA}}^{+*}$ curve. (c) As a result, the dimer fragments with a Coulomb explosion pathway and subsequent neutral dissociation. (d) Calculated PECs of one-site dicationic ground state (${\mathrm{FA}}^{++}·\mathrm{FA}$) and two-site dicationic ground (${\mathrm{FA}}^{+}·{\mathrm{FA}}^{+}$) and various excited state dimers as a function of C-C distance using the CASSCF method [27].

Figure 2

Measured time-correlation map between two ions. The distributions at the solid, dashed, dotted, and dash-dotted diagonal lines show the breakup of the dicationic dimer into ${\mathrm{FA}}^{+}+{\mathrm{FA}}^{+}$, ${\mathrm{FA}}^{+}+{(\mathrm{FA}\text{−}\mathrm{H})}^{+}+\mathrm{H}$, ${(\mathrm{FA}\text{−}\mathrm{H})}^{+}+{(\mathrm{FA}\text{−}\mathrm{H})}^{+}+2\times \mathrm{H}$, and ${(\mathrm{FA}\text{−}\mathrm{H})}^{+}+\mathrm{H}+{(\mathrm{FA}\text{−}\mathrm{OH})}^{+}+\mathrm{OH}$ channels.

Figure 3

Projectile energy loss spectra. The distributions show the results for the double ionization of FA dimers leading to ${\mathrm{FA}}^{+}+{(\mathrm{FA}\text{−}\mathrm{H})}^{+}+\mathrm{H}$, ${(\mathrm{FA}\text{−}\mathrm{H})}^{+}+{(\mathrm{FA}\text{−}\mathrm{H})}^{+}+2\times \mathrm{H}$ and ${(\mathrm{FA}\text{−}\mathrm{H})}^{+}+\mathrm{H}+{(\mathrm{FA}\text{−}\mathrm{OH})}^{+}+\mathrm{OH}$ channels. Also included is the spectrum for the single ionization of helium atom (${\mathrm{He}}^{+}$). I.P., ionization potential. Vertical dashed and dash-dotted lines represent the double-ionization thresholds of the FA monomer and dimer, respectively.

Figure 4

Fragmentation dynamics of the dicationic dimer into ${\mathrm{FA}}^{+}+{(\mathrm{FA}\text{−}\mathrm{H})}^{+}+\mathrm{H}$ pathway. (a) Measured KER spectrum in comparison to AIMD calculation. (b)–(f) The AIMD calculated results as a function of propagation time for the KE of $\mathrm{C}─\mathrm{H}$ bonded H atoms (b); The center-of-mass distance $R_{\mathrm{c}.\mathrm{m}.}$ between (c) ${\mathrm{FA}}^{+}$ and ${(\mathrm{FA}\text{−}\mathrm{H})}^{+}+\mathrm{H}$ and between (d) ${(\mathrm{FA}\text{−}\mathrm{H})}^{+}$ and neutral H. The $E_{\mathrm{vib}}$ for the (e) nondissociative ${\mathrm{FA}}^{+}$ cation and the (f) dissociative ion ${\mathrm{FA}}^{+}\to {(\mathrm{FA}\text{−}\mathrm{H})}^{+}+\mathrm{H}$. The insets in (c) show the path-integral trajectories in the time range of 0-15 and 35–55 fs.

Figure 5

Fragmentation dynamics of the dicationic dimer into ${(\mathrm{FA}\text{−}\mathrm{H})}^{+}+{(\mathrm{FA}\text{−}\mathrm{H})}^{+}+2\times \mathrm{H}$ pathway. (a) Measured and AIMD calculated kinetic energy sum spectrum of ${(\mathrm{FA}\text{−}\mathrm{H})}^{+}+{(\mathrm{FA}\text{−}\mathrm{H})}^{+}$ ion pair. (b) The calculated KE of $\mathrm{C}─\mathrm{H}$ bonded H atom (dotted line) and the $E_{\mathrm{vib}}$ of ${(\mathrm{FA}\text{−}\mathrm{H})}^{+}$ group (solid line) for one typical trajectory of the second H-loss channel. The calculated $R_{\mathrm{c}.\mathrm{m}.}$ between ${(\mathrm{FA}\text{−}\mathrm{H})}^{+}$ and neutral H for the (c) first and (d) second H-loss channels.