Role of orbital symmetry in high-order harmonic generation from aligned molecules

High-order harmonic generation has been explored theoretically and experimentally in ${\text{CO}}_2$, a linear molecule characterized by doubly antisymmetric highest-occupied molecular orbitals. For the first time to our knowledge, the high-harmonic yield is studied as a function of the angle between the molecular axis and the polarization direction. A minimum yield is found at $0°$ for all harmonics, which constitutes evidence for destructively interfering terms in the ionization probability amplitude playing an important role in strong-field phenomena in molecules. The maximum yield is always found at intermediate angles, but the detailed behavior is harmonic specific. Numerical simulations reproduce the main features observed in the experiment.

Figure 1

Potential and initial wave function used for the numerical simulation of harmonic generation in ${\text{CO}}_2$.

Figure 2

(a) Calculated orientation dependence of the $9\text{th}$ to $17\text{th}$ harmonics for a laser with $797\text{nm}$ wavelength and $2\times {10}^{14}\mathrm{W}∕{\text{cm}}^2$ intensity. (b) Experimental ratio of harmonic generation in ${\text{CO}}_2$ with and without the preparation beam present as a function of the relative polarizations of the preparation and the pump pulses in the range $0°–90°$.

Figure 3

Calculated orientation dependence of the ionization probability.

Figure 4

Example of a “harmonic spectrum difference image” (see text). Lighter colors indicate enhancement of harmonic yield caused by the presence of the preparation pulse; darker colors indicate suppression.