Role of nuclear symmetry in below-threshold harmonic generation of molecules

Below-threshold harmonic generation (BTHG) is one of few potential methods for the generation of novel vacuum-ultraviolet (VUV) light sources. However, most previous studies have focused on the atomic BTHG; a comprehensive understanding of the molecular BTHG is valuable for the advancement of this field. Here, we present an ab initio study of the role of nuclear symmetry on BTHG of both homonuclear and heteronuclear diatomic molecules. We reveal very distinctive electronic dynamical behaviors in molecular BTHG by identifying subtle multiple-channel contributions. In particular, it is found that the channels associated with the electron-nuclear coupling from light nuclear core to heavy nuclear core of the asymmetric molecules play an important role in the constructive interference of the harmonic emission, leading to a significant enhancement of the BTHG. Our results suggest that asymmetric molecular systems can be used for substantial improvement of the molecular BTHG-based VUV light sources.