Valence excitation of NO$_2$ by impulsive stimulated x-ray Raman scattering

TL;DR

This study uses advanced quantum simulations to identify optimal conditions for valence excitation in NO₂ via impulsive x-ray Raman scattering, revealing maximum population transfer efficiencies below the Oxygen K-edge.

Contribution

It provides the first fully-correlated first-principles calculations of valence excitation in NO₂ driven by impulsive x-ray Raman scattering, including ionization effects and convergence testing.

Findings

Maximum population transfer of 0.7% at 3.16×10^{17} W/cm^2 intensity

Optimal excitation achieved with 6 eV red-detuned frequency from the second-order maximum

Behavior consistent with nonresonant Raman transition mechanism

Abstract

The global optimum for valence population transfer in the NO$_2$ molecule driven by impulsive x-ray stimulated Raman scattering of one-femtosecond x-ray pulses tuned below the Oxygen K-edge is determined with the Multiconfiguration Time-Dependent Hartree-Fock method, a fully-correlated first-principles treatment that allows for the ionization of every electron in the molecule. Final valence state populations computed in the fixed-nuclei, nonrelativistic approximation are reported as a function of central wavelength and intensity. The convergence of the calculations with respect to their adjustable parameters is fully tested. Fixing the 1fs duration but varying the central frequency and intensity of the pulse, without chirp, orientation-averaged maximum population transfer of 0.7\% to the valence B$_1$ state is obtained at an intensity of 3.16$\times$10$^{17}$ W cm$^{-2}$, with the central frequency substantially 6eV red-detuned from the 2nd order optimum; 2.39\% is obtained at one specific orientation. The behavior near the global optimum, below the Oxygen K-edge, is consistent with the mechanism of nonresonant Raman transitions driven by the near-edge fine structure oscillator strength.