Isolating strong-field dynamics in molecular systems

Strong-field ionization followed by recollision provides a unique pump-probe measurement which reveals a range of electronic processes, combining sub-Angstrom spatial and attosecond temporal resolution. A major limitation of this approach is imposed by the coupling between the spatial and temporal degrees of freedom. In this paper we focus on the study of high harmonic generation and demonstrate the ability to isolate the internal dynamics—decoupling the temporal information from the spatial one. By applying an in situ approach we reveal the universality of the intrinsic pump-probe measurement and establish its validity in molecular systems. When several orbitals are involved we identify the fingerprint of the transition from the single-channel case into the multiple-channel dynamics, where complex multielectron phenomena are expected to be observed.

Figure 1

(a) Sideband oscillations phase in a RABITT measurement in ${\mathrm{SF}}_6$ (red squares), compared with the expected linear harmonic phase difference for a single nonresonant channel (dashed gray). (b) Odd (solid lines) and even (dashed lines) harmonic oscillation phases in response to the two-color delay scan in ${\mathrm{SF}}_6$.

Figure 2

(a) The HHG power spectrum of ${\mathrm{N}}_2$ for $0^{\circ },{45}^{\circ }$, and ${90}^{\circ }$ alignment angles. (b) Odd (solid lines) and even (dashed lines) harmonic oscillation phases in response to the two-color delay scan for $0^{\circ }$, ${45}^{\circ }$, and ${90}^{\circ }$ alignment angles. Clearly, the rotation dramatically attenuates the power spectrum but introduces no significant change to the phases. The rapid change in the phase in the vicinity of the cutoff frequency is attributed to long trajectories.

Figure 3

(a) The HHG power spectrum of ${\mathrm{CO}}_2$ for various alignment angles from $0^{\circ }$ to ${90}^{\circ }$. (b) Odd (solid lines) and even (dashed lines) harmonic oscillation phases in response to the two-color delay scan for the same alignment angles. In ${\mathrm{CO}}_2$ the two-color phases vary monotonically with the alignment angle above harmonic H25.

Figure 4

The standard deviation of the two-color phase with respect to the alignment angle ${\varphi }_{\mathrm{w}}$ for odd (a, c) and even (b, d) harmonics for various driving laser intensities. In the upper panel, ${\varphi }_{\mathrm{w}}$ is presented against the harmonic axis, showing a divergence of the curves at different intensities. In the lower panel, the harmonic axis is replaced by the trajectory duration (in atomic units), $\tau =t_1^{st}-t_0^{st}$, resulting in coalescence of the measurements.