Figure 1

(a) Tunnel ionization creates wave packet pairs in the ionization continuum (${\Psi }_c$) and in the valence shell (${\Psi }_b$). Upon recombination of two wave packets, high harmonics are emitted. At each recollision time, we scan the recollision angle (${\theta }_c$) with respect to the molecule by scanning the phase delay between the 800 and 400 nm laser fields. The time of recollision is determined by the observed photon energy of the high-harmonic emission. We read the structure of the valence electron wave packet from the relationship between the polarization angle of the high harmonics and the recollision angle (see text). (b) (left) Schematic diagram of the calculated, selected valence orbitals of ${\mathrm{C}}_2{\mathrm{H}}_6$. (right) The calculated angular distribution of the tunnel ionization probability for the most loosely bound orbital ($E_g$ symmetry, magenta line) and the second most loosely bound orbital ($A_g$ symmetry, red line) of ${\mathrm{C}}_2{\mathrm{H}}_6$ at the laser intensity of $2.0\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. We show only one of the two degenerate $E_g$ orbitals. The dotted line is the incoherent sum of these two lines. The ionization probability of the other states is negligible.Reuse & Permissions

Figure 2

High-harmonic generation spectra from ${\mathrm{C}}_2{\mathrm{H}}_6$ as a function of the delay between the two laser pulses. We choose the zero-delay arbitrarily. The total harmonic intensity modulates with the period of 0.66 fs while the intensity of 14th and 16th harmonics modulates with the period of 0.33 fs.Reuse & Permissions

Figure 3

${\varphi }_{\mathrm{HHG}}$ (in degrees) as a function of the delay between the two laser pulses for (a) ${\mathrm{H}}_2$ and (b) ${\mathrm{C}}_2{\mathrm{H}}_6$. We interpolate ${\varphi }_{\mathrm{HHG}}$ along the two axes. The right axis shows the recollision time which is calculated from the harmonic number.Reuse & Permissions

Figure 4

Calculated ${\varphi }_{\mathrm{HHG}}$ (in degrees) as a function of the harmonic order and the phase delay for an initial phase [$\varphi (0)$] of (a) $-0.4\pi$ and (b) $0.4\pi$. We use the energy difference between the most and the second most loosely bound orbitals of ${\mathrm{C}}_2{\mathrm{H}}_6$, $\Delta E=2.0\mathrm{eV}$. The right panel shows the calculated $|{\Psi }_b{|}^2$ at selected recollision time.Reuse & Permissions