Comparison Study of Strong-Field Ionization of Molecules and Atoms by Bicircular Two-Color Femtosecond Laser Pulses

We experimentally investigate the single and double ionization of ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ molecules in bicircular two-color femtosecond laser pulses, and compare with their companion atoms of Ar and Xe with comparable ionization thresholds. Electron recollision assisted enhanced ionization is observed in ${\mathrm{N}}_2$ and Ar by controlling the helicity and field ratio between the two colors, whereas the enhanced ionization via the recollision is almost absent in ${\mathrm{O}}_2$ and Xe. Our $S$-matrix simulations clearly reveal the crucial role of the detailed electronic structures of ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ on the two-dimensional recollision of the electrons driven by the bicircular two-color laser fields. As compared to Ar, the resonant multiphoton excitation dominates the double ionization of Xe.

Figure 1

Experimental setup. DL: delay line; QWP: quarter wave plate; VP: view port; NF: neutral filter. Inset: time-dependent electric field of the counterrotating and corotating two-color fields.

Figure 2

Single and double ionization yields of Ar and ${\mathrm{N}}_2$ driven by bicircular two-color laser fields. Solid and hollow symbols represent the counter- and co-rotating cases, respectively. (a) and (b) are the measured ion yields, and (c)-(f) are the yield ratios of different channels. The solid curves are numerical fits of the experimental data.

Figure 3

Experimentally measured yield ratios between counterrotating and corotating cases of (a) Ar, (b) ${\mathrm{N}}_2$, (d) Xe, and (e) ${\mathrm{O}}_2$, respectively. The numerically simulated yield ratios of ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ molecules are correspondingly plotted in (c) and (f). The solid curves are numerical fits of the measured and simulated data.

Figure 4

KER spectra of (a), (b) (${\mathrm{N}}^{+}$, N) and (c),(d) (${\mathrm{N}}^{+}$, ${\mathrm{N}}^{+}$) channels at different field ratios of $E_{SH}/E_{FW}$ between the counterrotating (red solid curves) and corotating (blue dashed curves) bicircular two-color laser fields. The insets in (a), (b), and (d) are the electric fields of the counterrotating and corotating cases at corresponding field ratios. The inset of (c) is the shape of the HOMO of ${\mathrm{N}}_2$.

Figure 5

Same as Fig. 2 but for various ionization channels of Xe and ${\mathrm{O}}_2$. The solid curves are numerical fits of the experimental data.

Figure 6

Same as Fig. 4 but for(a),(b) $({\mathrm{O}}^{+},\mathrm{O})$ and (c),(d) $({\mathrm{O}}^{+},{\mathrm{O}}^{+}$) channels of ${\mathrm{O}}_2$.