Figure 1

Temporal evolution of the rotational wave packet in ${\mathrm{N}}_2$. Right axis: simulated $⟨{cos}^2\theta ⟩$ of the angular distribution of the molecular axis (dotted black curve). Left axis: normalized magnitudes of the observed high harmonic signals, integrated over the harmonics 21st-25th (thin red curve) and over the 35th-37th (thick blue curve). The common horizontal dashed line shows the normalized harmonic intensity ($=1$, left axis) and $⟨{cos}^2\theta ⟩$($=1/3$, right axis) for the randomly aligned case.Reuse & Permissions

Figure 2

The dipole acceleration is responsible for the harmonic signal. We illustrate the physical mechanism responsible for the dipole. The upper panels sketch the HOMO for ${\mathrm{N}}_2$ ${\sigma }_g$ (left) and ${\mathrm{O}}_2$ ${\pi }_g$ (right) aligned parallel to the laser electric field. The color represents the sign of the wave function. The middle panels show the real part of the coherent superposition of the molecular orbital ${\Psi }_g$ and the recolliding electron wave packet ${\Psi }_c$. The continuum electron is propagating in the direction parallel to the field (shown by the arrows denoted by k). The coherent addition of ${\Psi }_g$ and ${\Psi }_c$ induces a dipole. The bottom panels show the square of the wave function. The induced dipoles can be seen as the asymmetric localization of the electron densities, while the red (lower) arrows show the directions of the dipoles. For ${\mathrm{N}}_2$, the dipole oscillates along the electric field direction as the electron propagates, leading to harmonic emission. For ${\mathrm{O}}_2$, the dipole oscillates in the perpendicular direction, thus harmonic emission is forbidden; see text for explanation.Reuse & Permissions

Figure 3

Temporal evolution of a rotational wave packet in ${\mathrm{O}}_2$. (a) Two different representations of the rotational wave packet: $⟨{cos}^2\theta ⟩$ (thick gray curve, left axis); $⟨{sin}^{2}2\theta ⟩$ (thin black curve, right axis). The common horizontal dotted line shows the values for the randomly aligned case. (b) Right axis: $⟨{sin}^{2}2\theta ⟩$ (thin black curve) as shown in the gray square area of (a). Left axis: normalized magnitude of the spectrally-integrated harmonic signal (thick red curve). The common horizontal dashed line shows the values for the randomly aligned case.Reuse & Permissions