Time-resolved laser spectroscopic studies of atoms, ions and Molecules

Sammanfattning: Time-resolved laser spectroscopy has been performed to study the radiative properties of various microscopic systems. Using Laser-Induced Fluorescence (LIF) techniques, natural radiative lifetimes have been studied not only in neutral B, S, Ge, Ce, Pb, and in singly ionized V, Fe, Zr, Mo, La, Ce, Eu, Gd and Yb, but also in doubly ionized La, Ce, Pr, Eu, Er, Gd, Tm, Lu, and W. Data are compared with theoretical calculations and astrophysical implications are discussed. Thermal evaporation or laser-produced plasma generation has been employed for free atom and ion production. The Stimulated Brillouin Scattering (SBS) technique was used to shorten the 8 nano-second commercial laser system pulses to 1 ns. To obtain even shorter-duration laser pulses, a further laser system (pulse-length of less then 100 pico-seconds), which includes a Q-switched and mode-locked Nd:YAG laser, a Distributed Feedback Dye Laser (DFDL) and a butterfly Ti:Sapphire amplifier, was utilized. For accessing the Ultra-Violet (UV) region, mainly nonlinear conversion in crystals was used. Further down into the Vacuum Ultra-Violet (VUV) region, Stimulated Raman Shifting (SRS) in hydrogen and resonance-enhanced sum-difference four-wave-mixing in krypton were employed. In the Extreme Ultra-Violet (EUV) region, high-order harmonics were produced in different noble gases. Time-resolved fluorescence signals were detected by a fast detection system, which includes a fast detector (e.g. a micro-channel-plate photo-multiplier tube) and a fast transient oscilloscope. Landé factors of levels in the even-parity Rydberg series of neutral lead were studied by analysis of the Zeeman quantum beats in the time-resolved fluorescence. With two-colour laser (1EVU + 1UV) pump-probe and time-of-flight ion detection techniques, photon ionisation cross-sections of He (2p states) were investigated. The pump-probe technique was also utilised to determine the radiative lifetimes of some short-lived N2 molecular states by changing the delay time between the pump and probe laser pulses.