Radiative Lifetime Measurments Of Highly Excited Odd-parity Levels Of Neutral Lanthanum

The radiative transition parameters of the atoms （such as natural radiativelifetimes, branching ratios, transition probabilities and oscillator strength） are akind of extremely important atomic spectroscopic data in atomic physics andastrophysics. At present, the application of rare earth elements has spread to manyfields, so the investigative of their radiative parameters is important andmeaningful. With the invention of lasers, a variety of new spectroscopic techniqueshave been developed owing to the high power and good monochromaticity of laser,which provides a powerful tool for the study of the atomic radiative properties. Thetime-resolved laser-induced fluorescence spectroscopy （TR-LIF） technique is oneof the most important and most reliable means to study the radiative transitioncharacteristics （such as natural radiative lifetimes）.Using the TR-LIF technique, we carried out experimental investigation on thehighly excited odd-parity levels of neutral lanthanum. To obtain desired tunableUV exciting laser, the wavelength range of dye laser needs to be expanded. A dyelaser （Sirah Cobra-Stretch） operating with different dye （Coumarin307andRhodamine6G） pumped by a Q-switched Nd:YAG355nm laser （Spectra-PhysicsQuanta-Ray） working with a10Hz repetition rate and about8ns pulse durationwas used for generating dye laser, and then the second harmonic of the dye laserwas produced by a nonlinear optic crystal （BBO crystal）. In order to obtain atomicbeam, laser ablation technique was employed. A532nm laser pulse with about8nsduration from a Q-switched Nd:YAG laser （Continuum Precision II） working at a10Hz repetition rate was moderately focused on a rotating La plate with99.99% purity. The delay time between the excitation and ablation pulses can be optionallymodified by a digital delay generator. In order to collect the fluorescence signal, agrating monochromator used for selecting the appropriate observation channel wasemployed. In addition, a photomultiplier tube and500MHz digital oscilloscopewere used to store and analyze the signal data. To improve the signal-to-noise ratio,the decay curves were acquired by averaging over1000shots. To avoid thepossible decay signal distortion from the quantum beats induced by the earthmagnetic field, an appropriate magnetic field of about100Gauss produced by apair of Helmholtz coils was employed. Furthermore, the applied magnetic field canalso effectively reduce the recombination background from the plasma. In theexperiments, various possible efects （radiation trapping, flight-out-of-view efects,collision-induced quenching, and saturation, etc.） which may lead to experimentalerrors have been carefully checked and eliminated. To eliminate these effects, thelifetime measurements were carried out by way of changing and optimizingexperimental conditions. For the lifetimes longer than80ns, the lifetime valueswere evaluated by a least-square exponential fit procedure. The determinations ofshort-lived states are through deconvolution fittings to the measured fluorescencedecay curves.In this paper, radiative lifetimes of24highly excited odd-parity levels of La I inthe range from34213.53to40910.11cm^–1have been determined. The lifetimevalues, measured for the first time to our best knowledge, are in the range from15.7to121ns. The uncertainties of lifetime results are not larger than±10%, andthey were composed of the statistical fluctuations and the systematic errors.The lifetimes investigated in this thesis not only are helpful for betterunderstanding the characteristic of energy level of La I, but also will supply a largenumber of important experimental reference data for theoretical calculations of theatomic structure. In addition, a combination of lifetimes with future experimentalor theoretical branching ratios may give transition probabilities and oscillator strengths which would provide essential atomic data for astrophysical analysis.