This study investigates the photoexcitation and ionization of a nitrogen molecule under ultrafast (femtosecond/attosecond) laser pulse irradiation. The real-time and real-space time-dependent density functional (TDDFT) is applied to describe the electron dynamics during the linear and nonlinear electron-photon interactions. The calculations describe well the behavior of the ionization process, and the results of ionization rates show good correspondence with the experimental results. In addition, the effects of near-infrared femtosecond laser pulse trains and the selected extreme ultraviolet attosecond laser pulse trains on electron dynamics are discussed. Theoretical results show that pulse number, laser frequency, and pulse delay are the key parameters for the control of electron dynamics including the electron excitation, energy absorption, electron density, and electron density oscillation.
The emission properties of double-pulse (DP) over single-pulse (SP) femtosecond laser breakdown spectroscopy (fs- LIBS) of polymethyl methacrylate (PMMA) were investigated. The signal enhancements in the DP fs-LIBS strongly depended on the DP delay and were influenced by the type of emission particles. Intensity enhancement of emission lines increased in the sequence of molecules, neutral atoms, and ions. Electron density and temperature were reported to characterize the plasmas. Both the electron density and plasma temperature exhibit similar variation trajectories with respect to the DP delay and feature a distinct increase at an optimal DP delay of ~80 ps, indicating reheating of preproduced plume is responsible for the emission enhancement. The dependence of the signal emission on laser energy was also studied, showing the emission intensity was linear to the pulse energy. However, the signal enhancement was nonlinear to the pulse energy, suggesting that the signal enhancement was related to the energy coupling efficiency of second pulse to the first pulse generated plume.