Exploring Interatomic Coulombic Decay by Free Electron Lasers

To exploit the high intensity of laser radiation, we propose to select frequencies at which single-photon absorption is of too low energy and two or more photons are needed to produce states of an atom that can undergo interatomic Coulombic decay (ICD) with its neighbors. For ${\mathrm{Ne}}_2$ it is explicitly demonstrated that the proposed multiphoton absorption scheme is much more efficient than schemes used until now, which rely on single-photon absorption. Extensive calculations on ${\mathrm{Ne}}_2$ show how the low-energy ICD electrons and ${\mathrm{Ne}}^{+}$ pairs are produced for different laser intensities and pulse durations. At higher intensities the production of ${\mathrm{Ne}}^{+}$ pairs by successive ionization of the two atoms becomes competitive and the respective emitted electrons interfere with the ICD electrons. It is also shown that a measurement after a time delay can be used to determine the contribution of ICD even at high laser intensity.

Figure 1

Ab initio PECs of the states relevant to the process (1) of exposing the Ne dimer to an intense laser pulse. Lower panel: The ${\mathrm{Ne}}_2$ ground state. Middle panel: PECs of the ${\mathrm{Ne}}^{+}(2p^{-1})\mathrm{Ne}$ ionic OV states. Upper panel: PECs of the ${\mathrm{Ne}}^{+}(2s^{-1})\mathrm{Ne}$ ionic IV states which decay by ICD and of the dications ${\mathrm{Ne}}^{+}(2p^{-1})-{\mathrm{Ne}}^{+}(2p^{-1})$ which are the final states of ICD. The photon frequency of $\omega =26.888\mathrm{eV}$ used in the calculations is indicated in the figure. Note the different energy scales in all panels.

Figure 2

Integral yield of the ${\mathrm{Ne}}^{+}$ ion pairs produced by exposing Ne dimers to a Gaussian laser pulse of 60 fs duration as a function of the peak intensity of the pulse. Open circles: Yield computed for the process (1) for a pulse with a central frequency $\omega =26.888\mathrm{eV}$. At least two photons are required for the process. Open squares: Yield computed for a pulse with a central frequency of $\omega =53.776\mathrm{eV}$. The ICD is triggered directly by the absorption of one photon. At the intensities shown, the ICD initiated by process (1) via two-photon absorption is clearly the more efficient mechanism for the production of these ionic fragments.

Figure 3

Electron spectra after exposure of ${\mathrm{Ne}}_2$ to a 60 fs Gaussian pulse for three intensities (solid curves, “Total”). For comparison the individual contributions of the ICD process (broken curves, “ICD”) are also shown. For each intensity two numbers are reported in brackets: the fraction of neutral dimers which have survived the pulse and the fraction of dimers which have become singly ionized.

Figure 4

Electron spectrum after exposure of ${\mathrm{Ne}}_2$ to a 30 fs Gaussian pulse of intensity $1\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ (solid curve, “Total”). Also shown are the individual contributions of the ICD process (dashed curve, “ICD”), the spectrum measured 30 fs after the pulse maximum (dotted curve, “${\mathrm{Total}}_{30}$”), and the difference between the “Total” and the “${\mathrm{Total}}_{30}$” spectra (dash-dotted curve).

Figure 5

Electron spectrum after exposure of ${\mathrm{Ne}}_2$ to a 30 fs Gaussian pulse of intensity $5\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ (solid curve, “Total”). For analysis the individual contribution of the ICD process (dashed curve, “ICD”), the individual contribution of the direct ionization, i.e., excluding ICD from the calculations (dotted curve, “Direct”), and the results obtained excluding only the ionization of ${\mathrm{Ne}}^{+}(2s^{-1})\mathrm{Ne}$ to produce ${\mathrm{Ne}}^{+}(2s^{-1})-{\mathrm{Ne}}^{+}(2p^{-1})$ (dashed-dotted curve, “Subtotal”) are shown.

Figure 6

The strong FEL pulse ionizes an outervalence electron of an atom in the dimer producing a photoelectron. This pulse with resonant central frequency also excites an innervalence electron to fill the initial vacancy, thus producing an innervalence ionized state of the same atom. This ionic state of the dimer relaxes by ICD producing strongly repelling ionic fragments, which undergo a Coulomb explosion, and low-energy ICD electrons.