Orientation-dependent high-order harmonic generation in HCN: Insights from time-dependent density-functional-theory calculations

This study examines the orientation-dependent high-order harmonic spectra of HCN, using time-dependent density-functional-theory calculations. In HCN, which lacks inversion symmetry, the emergence of even harmonics is a notable feature. At a parallel orientation to the laser polarization, these even harmonics manifest with intensities surpassing those of odd harmonics. However, their magnitudes diminish as the orientation shifts away from parallel. Conversely, at a perpendicular orientation to the laser polarization, the spectrum predominantly displays odd harmonics, although an exclusively even harmonic spectrum is generated with the harmonic polarization perpendicular to the laser. The intensities of these even harmonics wane as the molecular orientation deviates from the perpendicular. The behavior of odd harmonics is more intricate, generally showing heightened intensities at perpendicular orientations. The influence of laser parameters on the orientation dependence is minimal. Across various orientations and laser settings, a consistent minimum is observed near 22--23 eV, underscoring a universal feature of the high-order harmonic spectra of HCN. Regardless of the orientation, the highest occupied molecular orbitals (HOMOs) contribute little to the high-order even harmonics. As far as the high-order odd harmonics are concerned, the HOMOs only dominate when the orientation angle is greater than ${30}^{\ensuremath{\circ}}$.