The measurement and control of ultrafast dynamics of electrons in atoms and molecules are always the goal of scientists. The electrons in molecules and atoms move with a timescale of attoseconds, so it is necessary to develop attosecond resolution technology to detect the motion of electrons accurately. Strong laser-induced ionization of atoms and molecules, as the footstone of laser-induced physical phenomena, is one of the frontiers of ultrafast topics. The attoclock technology, using circularly polarized femtosecond laser pulses, has become an important experimental tool for studying photoionization dynamics and has been widely used to study the tunneling time delay of atomic system. For molecular systems, due to the complexity of molecular orbitals, the study on molecules with attoclock is less explored and it is crucial to understand the ionization dynamical process of molecules.

The attoclock have pioneered the experimental study of photoionization time delays. By using a circularly polarized field to deflect ionized photoelectrons at different times to various angles in momentum space. Similar to the hands of a clock, a correlation is established between the ionization time and the final emission angle of electrons. Attoclock technology has been widely used to measure the tunneling time delay of atomic systems since it was proposed. Recently, the improved attoclock approach has been demonstrated, which was based on a two-color field (800 nm strong circularly polarized field for ionization, and 400 nm weak linear polarization field for marking the ionization instance) to investigate the tunneling time of atoms. Additionally, a double-hand attoclock is used to retrieve the phase structure of the photoelectron wave packet, which provide an important approach for studying the time delay of strong-field ionization of atoms. We will discuss the bi-circular attoclock configurations to study the ionization dynamics of H₂ and CO molecules.

In this paper, we summarized the recent study on ionization dynamics of the homonuclear diatomic molecule H₂ and the heteronuclear diatomic molecule CO by using a bi-wavelength circularly polarized laser field. We have also studied the tunneling dynamics information of electrons at different internuclear distances using a semi-classical quantum trajectory Monte Carlo. We show that the momentum angular distribution of photoelectrons is dependent on the internuclear distance and molecular orientation. We further disentangled the orientation and internuclear-distance dependent effect of the long-range Coulomb potential and the initial phase on molecular-frame photoelectron momentum distributions. Then the dependence between the initial phase structure of the tunneling electron wave packet and the internuclear distance and molecular orientation was obtained. We found that the initial phase structure was related to the Wigner time delay, which carried information about the transition of electrons from the bound state to the continuum in the molecular frame. The bi-circular attoclock can be further extended to the photoionization process of complex molecules.