Attosecond electronic dynamics of core-excited states of ${\mathrm{N}}_2\mathrm{O}$ in the soft x-ray region

We present a combined experimental and theoretical study of the transient absorption spectroscopy of ${\mathrm{N}}_2\mathrm{O}$ at the N $K$ edge (400 eV) by employing an attosecond soft x-ray pulse and an overlapping intense femtosecond infrared pulse. Our experiment demonstrates that tunnel ionization of the core-excited states of neutral ${\mathrm{N}}_2\mathrm{O}$ causes prominent half-cycle oscillations in the absorption signal. In addition, we observe the appearance of the ${\mathrm{N}}_2{\mathrm{O}}^{+}$ signal with increasing delay as a result of the tunneling ionization of ${\mathrm{N}}_2\mathrm{O}$, thereby allowing us to follow the tunneling process in real time. We have developed theoretical models that allow us to interpret and reproduce the main trends of the experimental data.

Figure 1

(a) Schematic of the experimental setup. (b) Molecular geometry of ${\mathrm{N}}_2\mathrm{O}$. (c) Measured (green circles) and reference [18] (gray curve) absorption spectrum of ${\mathrm{N}}_2\mathrm{O}$ at the N $K$ edge; the blue and red lines represent the theoretical assignments [18] of the absorption peaks corresponding to the excitation of the ${\mathrm{N}}_{\mathrm{t}} 1s$ electron and the ${\mathrm{N}}_{\mathrm{c}} 1s$ electron, respectively. (d) Molecular orbital diagram of ${\mathrm{N}}_2\mathrm{O}$. HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital.

Figure 2

Measured transient absorption spectra of ${\mathrm{N}}_2\mathrm{O}$ at the N $K$ edge. The positive delay indicates that the IR pulses arrive earlier than the SX pulses. The transitions of corresponding absorption peaks are shown on the right side of the figure.

Figure 3

Computed excitation energies of the various core-excited states of ${\mathrm{N}}_2\mathrm{O}$ labeled with excitation from either the ${\mathrm{N}}_{\mathrm{t}}$ or ${\mathrm{N}}_{\mathrm{c}} 1s$ orbital. The thick black lines with shaded areas indicate the ${\mathrm{N}}_{\mathrm{t}}1s^{-1}$ and ${\mathrm{N}}_{\mathrm{c}}1s^{-1}$ thresholds.

Figure 4

(a) Schematic illustrations of the three theoretical processes considered in the TAS of ${\mathrm{N}}_2\mathrm{O}/{\mathrm{N}}_2{\mathrm{O}}^{+}$. Computed OD signal for linearly polarized light perpendicular to the molecular axis for (b) process I, (c) process I $+$ II, and (d) process III (see text for details). (e) Computed OD signal which considers all the processes in (a) for randomly oriented molecules.

Figure 5

Intuitive explanation of the mechanism of the half-cycle oscillations by the simple dipole model. (a) ADK ionization rate of the core-excited state used in the simple dipole model simulation. (b) Temporal evolution of the amplitude of the dipole between the ground state and the core-excited state. The red solid curve and the blue solid curve represent the cases when the IR pulse exists and the delay between the IR and the SX pulses is set to $+T_{\mathrm{IR}}/8$ and $-T_{\mathrm{IR}}/8$ ($T_{\mathrm{IR}}$ is the period of the IR field), respectively. The black dashed curve represents the case when the IR pulse does not exist. (c) The transient absorption spectra obtained from the simple dipole model. The origin of the vertical axis is adjusted such that the photon energy of the absorption peak becomes zero when the IR is not present.