Kai-Jun Yuan

Molecular orbital symmetry is shown to be an important factor in determining orders and helicities (polarizations) of high-order harmonic generation (HHG) by intense femtosecond counter-rotating bichromatic circularly polarized laser pulses. Numerical solutions of time-dependent Schrödinger equations (TDSE) for the one-electron molecular ions H₂⁺ and H₃²⁺ for different initial electronic states show that harmonic orders and helicities are dependent on orbital symmetries and of the net incident pulse electric field. The numerical results and properties of the harmonics are described by dynamical symmetry theory and time profile analysis of the high-order harmonics, thus confirming that orbital and laser pulse symmetry dependence are generic in HHG of molecules.

The ultrafast photoionization dynamics of N₂ molecules by x-ray/XUV laser pulses is investigated. The molecular frame photoelectron momentum distributions (MF-PMDs) and the molecular frame photoelectron angular distributions (MF-PADs) are obtained by numerically solving 2D time-dependent Schrödinger equations within the single-electron approximation (SEA) frame. The results show that the molecular photoionization diffraction appears in 5 nm laser fields. However, when the laser wavelength is 30 nm, the molecular photoionization diffraction disappears and the MF-PMDs show four-lobe pattern. The ultrafast photoionization model can be employed to describe the MF-PMDs and MF-PADs of N₂ molecules.

Molecular frame photoelectron angular distributions (MFPADs) are theoretically studied by intense two-color attosecond linearly polarized laser pulses. Simulations are performed on aligned H 2 + from numerical solutions of corresponding time-dependent Schrödinger equations. Asymmetric MFPADs are produced by a time-delayed soft x-ray attosecond pulse in the presence of an extreme ultraviolet (XUV) pump pulse. We present the dependence of the asymmetry ratio Γ of the MFPADs on the laser parameters of the pump-probe pulses. The asymmetry ratio Γ varies with the time delay, illustrating the coherent superposition of electronic states. Altering the phase, intensity and duration of the pump pulse gives rise to a modulation of the asymmetry ratio Γ as well. Moreover, the asymmetry of the MFPADs disappears in the case of long duration probe pulses, illustrating the coherent charge migration dynamics on the attosecond time scale.

Controlling coherence and interference of quantum states is one of the central goals in quantum science. Different from energetically discrete quantum states, however, it remains a demanding task to visualize coherent properties of degenerate states (e.g., magnetic sublevels). It becomes further inaccessible in the absence of an external perturbation (e.g., Zeeman effect). Here, we present a theoretical analysis of all-optical control of degenerate magnetic states in the molecular hydrogen ion, H 2 + , by using two time-delayed co- and counterrotating circularly polarized attosecond extreme-ultraviolet (XUV) pulses. We perform accurate simulations to examine this model by solving the three-dimensional time-dependent Schrödinger equation. A counterintuitive phenomenon of quantum interference between degenerate magnetic sublevels appears in the time-dependent electronic probability density, which is observable by using x-ray-induced transient angular and energy-resolved photoelectron spectra. This work provides an insight into quantum interference of electron dynamics inside molecules at the quantum degeneracy level.

We theoretically study ultrafast photoelectron diffraction in triatomic molecules with cyclic geometry by ultrafast circular soft X-ray attosecond pulses. Molecular frame photoelectron distributions show complex diffraction patterns, arising from molecular multiple center interference and circular charge migration. It is found that photoelectron diffraction patterns depend on the initial electronic state, encoding the information of molecular orbital symmetries. In a resonant coherent electron excitation process, time-resolved photoelectron diffraction patterns enables us to reconstruct the charge migration with highly spatiotemporal resolutions. We simulate and analyze the results from ab initio calculations of the single electron triangular hydrogen molecular ion H₃ ²⁺ which is used as a benchmark molecular system in combination with an ultrafast multi-center and multi-state photoionization model. This approach presents a general scheme which can be applied to explore circular charge migration from electron orbits and attosecond coherent electron dynamics in polyatomic systems by circular ultrafast laser pulses.