Strong-field ionization and high-order-harmonic generation during polyatomic molecular dynamics of N${}_2$O${}_4$

We present state-of-the-art ab initio–type computations of strong-field ionization (SFI) and the single-molecule response contribution to high-order-harmonic generation (HHG) in the polyatomic molecule N${}_2$O${}_4$. The numerical method uses a mixed orbital- and grid-based approach to model the multielectron bound states and single-electron continuum. The effects of ionic-core attraction and Coulomb-mediated interchannel coupling are rigorously included. We show that full-dimensionality time-dependent multielectron computations of SFI and HHG in polyatomic molecules are now feasible. The computational results indicate that (a) SFI yields in N${}_2$O${}_4$ are dominated by a single ionic state (the $A_g$ state), and are strongly modulated by the N-N stretch coordinate; and (b) the HHG radiating dipole as a function of the N${}_2$O${}_4$ N-N stretch is dominated by the same, single state. The molecular-beam coincidence measurements presented here support the computational results. Due to the differences in the estimated vibrational amplitude, however, the computations are not in full agreement with previous HHG data.

Figure 1

Calculated probability of forming the $A_g$ and $B_{2g}$ cation states via SFI as a function of the N-N bond length. See the text for the parameters of the laser pulse used in the simulation. The vertical dashed line indicates the neutral equilibrium N-N bond length (1.76 Å). The vertical dash-dotted lines give the estimated amplitude of vibrational motion in our SFI experiment ($-0.14/+0.18$ Å; for details see the text).

Figure 2

Dyson orbitals (upper rows of each panel) and normalized angle-dependent ionization yields (bottom rows) as a function of the N-N bond length for the two ionic states considered in this work.

Figure 3

Bond-length-dependent HHG signal: HHG from the $A_g$ channel ($\times$), HHG from the $B_{2g}$ channel ($\circ$), and the coherent sum of the two channel contributions (+). Also see Fig. 1 caption.

Figure 4

HHG spectra calculated with (dashed curves) and without (solid curves) Coulomb interchannel coupling for a N-N distance of 2.6 Å. Shown are the HHG yields for the A${}_g$ channel ($\times$), the B${}_{2g}$ channel ($\circ$), and the coherent sum of the two channel contributions (+).

Figure 5

Coincident photoion and photoelectron yields for wave-packet dynamics in N${}_2$O${}_4$ as a function of pump-probe delay. For details, see the text.