| With the development of science and technology,the knowledge on the motion of matter has arrived at the attosecond timescale.High-order harmonic generation(HHG),which can be used to produce attosecond pulses,provides a powerful tool in attosecond measurements.It has been shown that with the HHG,one can image the highest-occupied molecular orbital,track the stretching of molecular bond length in a photochemical reaction,probe the attosecond vibrational dynamics of the nuclear wave packet,reconstruct the rescattering trajectories of the electron,and trace the excited-state dynamics in the ultrafast timescale,etc.Previous studies on HHG mainly focus on harmonics with energy higher than the ionization potential of the target(i.e.,the so-called plateau harmonics),which can be well described by a classical or quantum three-step model:tunneling ionization of the electron from the laser-dressed potential,propagation of the freed electron in the laser field;as the laser filed changes its direction,the electron can return to the nuclei and recombine with its parent ions with the emission of a high-energy photon.For harmonics with energy lower than the ionization potential(i.e.,the so-called below-threshold harmonics),the generation mechanism goes beyond the description of the three-step model and includes both classical and quantum effects.Because of the potential application as the VUV light source,below-threshold harmonics(BTH)have attracted great attention in recent years.Present studies on BTH concentrate on atoms.For molecules with more degrees of freedom and having the orientation effect,the generation mechanism of BTH is more complex than atoms.Studies on molecular HHG have shown that the orientation dependence of plateau harmonics of H2+ in linearly polarized laser fields is closely associated with bound-continuum transition dipole of the molecule,but the orientation dependence of BTH is not very clear.In chapter 3,the orientation effect for BTH from H2+ was studied through numerical solution of the time-dependent Schrodinger equation(TDSE).With excluding the contributions of excited states successively,the orientation dependence of BTH was analyzed in detail.The research results show that the complex orientation dependence of BTH parallel or perpendicular to the laser polarization is closely related to the resonance effect between the ground state and the neighboring excited states,where the axis symmetry of these excited states play an important role.A simple model which contains the bound-bound transition dipole is used to explain these complex orientation-related phenomena.The main results are checked with using the first excited state as the initial state in TDSE simulations.Our results give suggestions on relevant experimental studies of orientation dependence of BTH.Furthermore,the generation mechanism of near-threshold harmonics(NTH,i.e.,harmonics with energy near the threshold)in strong and short-wavelength(λ≤ 800nm)laser fields has also been studied in chapter 4,.Our simulations show that the property of the Coulomb potential has an important influence on NTH.When the range of the Coulomb potential is short,the emission of NTH is suppressed remarkably.In particular,for a delta potential with only a bound state,the suppressed region of NTH is in agreement with the prediction of a developed classical three-step model that considers the tunneling position of the electron.When the range of the Coulomb potential is long,the emission of NTH does not show the suppression phenomenon.In this case,we anticipate the excited state of the system plays an important role in the generation of NTH.Our results shed light on the complex generation mechanism of NTH in strong laser fields with relatively high frequencies. |
