Core Ionization Initiates Subfemtosecond Charge Migration in the Valence Shell of Molecules

After the ionization of a valence electron, the created hole can migrate ultrafast from one end of the molecule to another. Because of the advent of attosecond pulse techniques, the measuring and understanding of charge migration has become a central topic in attosecond science. Here, we pose the hitherto unconsidered question whether ionizing a core electron will also lead to charge migration. It is found that the created hole in the core stays put, but in response to this hole interesting electron dynamics takes place which can lead to intense charge migration in the valence shell. This migration is typically faster than that after the ionization of a valence electron and transpires on a shorter time scale than the natural decay of the core hole by the Auger process, making the subject very challenging to attosecond science.

Figure 1

Nitrogen $1s$ ionization spectrum of nitrosobenzene computed with the accurate ab initio Green’s function ADC(4) approach (blue bars). The spectral envelope (red curve) is obtained by convoluting each computed line with a Gaussian with a FWHM of 1.8 eV, which accounts for the natural linewidth, vibrational broadening, and experimental resolution. For comparison, the experimental $\mathrm{N}1s$ XPS spectrum of nitrosobenzene is shown in the inset (taken from Ref. [22]). The value of 1.8 eV used in the convolution was extracted from the main peak of that spectrum.

Figure 2

The four relevant valence molecular Hartree-Fock orbitals participating in the ionization spectrum of nitrosobenzene. Shown are the $\mathrm{HOMO}-1$ and $\mathrm{HOMO}-3$ orbitals occupied in the electronic ground state of this molecule and the unoccupied orbitals LUMO and $\mathrm{LUMO}+2$ (see the text).

Figure 3

Evolution of the hole density $Q(z,t)$ of nitrosobenzene after ionization out of a $\mathrm{N}1s$ orbital. The $z$ axis, denoted as the “molecular axis,” passes through the nitroso carbon (C1) and through the bond between C3 and C4 (see Fig. 1). The positions of the atoms along the molecular axis are also indicated. Note that the hole density is positive where electrons are deficient and negative where they are surplus.

Figure 4

The dominating hole-occupation numbers of nitrosobenzene after the ionization of a $\mathrm{N}1s$ electron are shown as a function of time. In addition, snapshots of the three-dimensional hole density $Q(\overrightarrow{r},t)$ at the times $t=0$, 0.3, and 0.7 fs (marked with blue dots on the upper axis) are depicted at the top of the figure. The positive hole density is shown in green, and the negative electron density is depicted in orange.