Ultrafast Charge Transfer of a Valence Double Hole in Glycine Driven Exclusively by Nuclear Motion

We explore theoretically the ultrafast transfer of a double electron hole between the functional groups of glycine after $K$-shell ionization and subsequent Auger decay. Although a large energy gap of about 15 eV initially exists between the two electronic states involved and coherent electronic dynamics play no role in the hole transfer, we find that the double hole is transferred within 3 to 4 fs between both functional ends of the glycine molecule driven solely by specific nuclear displacements and non-Born-Oppenheimer effects. The nuclear displacements along specific vibrational modes are of the order of 15% of a typical chemical bond between carbon, oxygen, and nitrogen atoms and about 30% for bonds involving hydrogen atoms. The time required for the hole transfer corresponds to less than half a vibrational period of the involved nuclear modes. This finding challenges the common wisdom that nuclear dynamics of the molecular skeleton are unimportant for charge transfer processes at the few-femtosecond time scale and shows that they can even play a prominent role. It also indicates that in x-ray imaging experiments, in which ionization is unavoidable, valence electron redistribution caused by nuclear dynamics might be much faster than previously anticipated. Thus, non-Born-Oppenheimer effects may affect the apparent electron densities extracted from such measurements.

Figure 1

Auger intensity for valence doubly ionized singlet states of glycine (${\mathrm{NH}}_2{\mathrm{CH}}_2\mathrm{COOH}$) after decay of a $K$-shell core hole in (a) the nitrogen atom of the amino group and (b) one of the oxygen atoms in the carboxyl group. The energy axis refers to the double ionization potential (DIP) of the corresponding electronic state at the equilibrium geometry of the neutral system. A gray circle labels the atom where the primary $K$-shell photoionization is assumed to take place. Electronic eigenstate 118 is strongly dominated by a $[(\mathrm{N}2s{)}^{-2}]$ configuration. States 119 and 120 are dominated by $[(\mathrm{O}2s{)}^{-2}]$ configurations whereas state 117 corresponds to a $2s$ hole in each oxygen atom, $[(\mathrm{O}2s{)}^{-1},(\mathrm{O}2s{)}^{-1}]$.

Figure 2

Temporal evolution of double-hole charge density $Q(\mathbf{r},t)$ of a representative trajectory initiated from state $S_{119}[(\mathrm{O}2s{)}^{-2}]$.

Figure 3

Charge on heavy atoms of the glycine dication, when the dynamics are initiated from state $S_{119}[(\mathrm{O}2s{)}^{-2}]$.

Figure 4

(a) Potential energy surfaces of electronic states $S_{118}$, $S_{119}$, and $S_{120}$ along one representative trajectory started in state $S_{119}$. The evolution of the full set of 120 potential energy surfaces along this trajectory is shown in Fig. S1 [27]. The circle indicates a region of avoided crossing between the two potential functions. (b) Ensemble averaged adiabatic populations. (c) Ensemble averaged asymmetric COO stretch $R^{(\mathrm{a})}=|R_{\mathrm{C}\text{−}\mathrm{OH}}-R_{\mathrm{C}=\mathrm{O}}|$, symmetric COO stretch $R^{(\mathrm{s})}=(R_{\mathrm{C}\text{−}\mathrm{OH}}+R_{\mathrm{C}=\mathrm{O}})/2$, CN stretch $R^{(\mathrm{CN})}$, and OH stretch $R^{(\mathrm{OH})}$ vibrational coordinates.