High-order harmonic generation of the cyclo18carbon molecule irradiated by circularly polarized laser pulse

High-order harmonic generation of the cyclo18carbon (C ₁₈) molecule under few-cycle circularly polarized laser pulse is studied by time-dependent density functional theory. Compared with the harmonic emission of the ring molecule C ₆H ₆ having similar ionization potential, the C ₁₈ molecule has higher efficiency and cutoff energy than C ₆H ₆ with the same laser field parameters. Further researches indicate that the harmonic efficiency and cutoff energy of the C ₁₈ molecule increase gradually with the increase of the laser intensity of the driving laser or decrease of the wavelength, both are larger than those of the C ₆H ₆ molecule. Through the analysis of the time-dependent evolution of the electronic wave packets, it is also found that the higher efficiency of harmonic generation can be attributed to the larger spatial scale of the C ₁₈ molecule, which leads to a greater chance for the ionized electrons from one atom to recombine with others of the parent molecule. Selecting the suitable driving laser pulse, it is demonstrated that high-order harmonic generation in the C ₁₈ molecule has a wide range of applications in producing circularly polarized isolated attosecond pulse.