Resonantly Enhanced Difference-Frequency Generation in the Core X-ray Absorption of Molecules.

Carles Serrat

doi:10.1021/acs.jpca.1c06950

Abstract

We use real-time time-dependent density functional theory simulations to numerically demonstrate that resonantly enhanced difference-frequency generation (re-DFG) involving intense ultrashort coherent X-ray pulses can selectively excite core states of atoms in molecules. As a model case, we evaluate the spectral selectivity of re-DFG excitation of the oxygen K-edge by illumination of a single gas-phase water molecule with two-color X-ray pulses of different photon energies and durations. The re-DFG excitation is further probed by a small delayed pulse with central photon energy resonant with the oxygen K-edge peak absorption line. Based on these results, we anticipate that highly selective excitation by re-DFG X-ray nonlinear processes might be achieved in more complex molecular systems and bulk materials by using highly penetrating two-color hard X-ray pulses, with extensive applications.

Highlights

Selective photodissociation of molecules by resonant excitation near a chosen core ionization edge by means of nonlinear interactions in the X-ray range is becoming conceivable due to recent advances in the development of intense ultrashort X-ray coherent pulse sources, such as synchrotron, free-electron lasers (FELs), and in high-harmonic generation.[1−10]Several nonlinear interactions in the X-ray range are being investigated.[11]
In a previous work,[12] we showed how the phase-sensitivity cancellation of the antiStokes component previously described in two- and three-level systems in the infrared and optical regions can be extended to the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)[6] of chemical species by highly localized four-wave-mixing (FWM) nonlinear processes
We have numerically shown that two-color (3Ω, 4Ω) resonantly enhanced difference-frequency generation (re-difference-frequency generation (DFG)) can be implemented using synchronized femtosecond X-ray pulses for the selective excitation of atomic core states, considering a single gas-phase water molecule as a model system

Summary

■ INTRODUCTION

Selective photodissociation of molecules by resonant excitation near a chosen core ionization edge by means of nonlinear interactions in the X-ray range is becoming conceivable due to recent advances in the development of intense ultrashort X-ray coherent pulse sources, such as synchrotron, free-electron lasers (FELs), and in high-harmonic generation.[1−10]. It is worth noting that, in order to optimize the computational requirements of our rt-TDDFT simulations, the durations of the pulses that we have considered are remarkably short (1.67, 2.56, and 3.43 fs), and much narrower peaks in Figure 3 can be expected by considering longer incident pulses, which would result in a more selective re-DFG effect They can be most regularly produced in X-ray FEL experiments.

■ DISCUSSION AND CONCLUSIONS

■ REFERENCES