Attosecond Molecular Angular Streaking with All-Ionic Fragments Detection

Attosecond angular streaking (or “attoclock”) is an insightful technique for probing the ultrafast electron dynamics in strong laser fields. Up until recently, this technique relied solely on an accurate measurement of the photoelectron momentum distribution and has remained restricted to atomic targets. Here, we propose a novel attosecond angular streaking scheme applicable to molecules, for which the ionic fragments of dissociative ionization are detected in the polarization plane of a close-to-circular polarized laser light. Our ionic attoclock measurements are consistent with theoretical results from a numerical solution of the time-dependent Schrödinger equation and an upper bound of 10 as on the tunneling time from the attoclock readings in the ${\mathrm{H}}_2$ molecule has been given, which is significantly smaller than any definitions of tunneling time available in the literatures.

Figure 1

Graphical representation of the molecular attosecond angular streaking scheme. When subjected to an elliptically polarized laser pulse, the ${\mathrm{H}}_2$ molecule at its equilibrium interatomic separation is tunnel ionized and the ${\mathrm{H}}_2^{+}$ ion starts to expand. At the internuclear separation $R_{\mathrm{res}}$, when $E(1s{\sigma }_g)-E(2p{\sigma }_u)=\omega$, a transition to the ${\mathrm{H}}_2^{+}$ dissociative state takes place. The dissociation of ${\mathrm{H}}_2^{+}$ is strongly aligned with the electric field vector, i.e., the major polarization axis, providing a natural frame in which the attoclock offset angle of ${\mathrm{H}}_2^{+}$ ion can be read.

Figure 2

Experimental (top row) and simulated (bottom row) momentum distributions of the ${\mathrm{H}}_2^{+}$ (left) and ${\mathrm{H}}^{+}$ (right) in the polarization plane of an elliptically polarized few-cycle laser pulse with a center wavelength of 760 nm, a peak intensity of $4.6\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and an ellipticity of 0.70. The major axis of the polarization ellipse (electric field $E$) is horizontal whereas the minor axis (vector potential $A$) is vertical. The angles $\alpha$ and $\beta$ are defined between the emission directions of ${\mathrm{H}}_2^{+}$ ($\alpha$) and ${\mathrm{H}}^{+}$ ($\beta$) relative to the minor and major axes of the polarization ellipse, respectively, as indicated in panels (a) and (b).

Figure 3

Radially integrated momentum distributions of ${\mathrm{H}}_2^{+}$ (blue) and ${\mathrm{H}}^{+}$ (red) at the field intensities of $2.2\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ (top) and $7.0\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ (bottom).

Figure 4

The molecular attoclock offset angles ${\alpha }_{\max }$ in the ${\mathrm{H}}_2^{+}$ angular distribution and ${\beta }_{\max }$ in the ${\mathrm{H}}^{+}$ angular distribution from the TDSE calculations (theory) and the experiment. The dotted line visualizes the prediction of the KR model ${\alpha }_{\max }\propto I^{-0.5}$.