Resonance enhanced two-photon ionization process of hydrogen atom via the resonant laser pulse is studied by Bohmian mechanics (BM) method. By analyzing the trajectories and energies of Bohmian particles (BPs), we find that under the action of high frequency and low intensity multi-circle resonant laser pulses, the ionized BPs first absorb one photon completing the excitation, and then absorb another photon, completing the ionization after staying in the first excited state for a period of time. The analysis of work done by the forces shows that the electric field force and quantum force play a major role in the whole ionization process. At the excitation moment and in the excitation-ionization process, the effect of the quantum force is greater than that of the electric field force. Finally, we discuss the principle of work and energy for BPs, and find that the electric field force and quantum force are non-conservative forces whose work is equal to the increment of mechanical energy of the system. In addition, it is proved that the quantum potential energy actually comes from the kinetic energy of the system and the increment of kinetic energy is equal to that of the kinetic energy of the system.

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The ionization dynamics of two-electron atom in an intense laser field is studied by the Bohmian mechanics (BM) theory, and the xenon atomic potential function is used as a model. The single ionization process and double ionization process are calculated by the BM theory and their results are in good agreement with those calculated by numerically solving the time-dependent Schrödinger equation. The analyses of the types, trajectories, and forces of Bohmian particles (BPs) undergoing the single and double ionizations indicate that the re-collision process accounts for a considerable proportion in the singly ionized cases. Furthermore, the analysis of the work done by the external force acting on the BPs shows that the quantum force plays an important role in the re-collision process. This work is helpful in understanding the ionization of two-electron atom in an intense laser field.

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The near-infrared spectrum of NiCl has been recorded at high resolution in the 13,200-13,500 cm-1 and 13,600-13,750 cm-1 regions using Intracavity Laser Spectroscopy (ILS). The NiCl molecules were produced in the plasma discharge of a Ni-lined copper hollow cathode with 0.3-0.6 torr of argon as the sputter gas, and a trace amount of CCl 4. Several electronic transitions were observed, including three transitions involving two previously unreported excited electronic states. The (0,0) and (1,0) bands of the [13.5] 2 Φ 7/2-[0.16] A 2 Δ 5/2 transition were observed near 13,318 cm-1 and 13,709 cm-1 , respectively. The (0,0) band of the [13.8] 2 Π 1/2-[0.38] X 2 Π 1/2 transition was observed near 13,480 cm-1. With analysis of these transitions, molecular constants have been obtained for eleven of the twelve predicted NiCl doublets states with term energies less than 16,000 cm-1. Comparison of experimentally observed transitions with the theoretical work is presented.