Radiative Lifetime Measurements Of Odd-parity Levels Of Tb I

For more than a century, it is great interesting to investigate atomic internal configuration for human. Recently, with the development of laser technique, it offer us a powerful tool for investigating atomic internal configuration in depth. And laser spectroscopy has been an indispensable technique in atomic and molecular physics. In atomic spectroscopy, the radiative transition parameters of atoms and ions for celestial spectroscopic analysis is very important spectral data, including natural radiative lifetime, brach ratio, transition probability, oscillator strength and so on. So we can get a deeper insight into the properties of atomic internal configuration and atomic state.This paper studies the Neutral terbium(Tb I, Z=65), which belongs to the sixth period elements. It is one of the rare earth elements. The ground state of Tb I is 4f96s26H(15)/2 0and the ionization limit is 47294cm-1. Because 4f shell is not filled,s electronic and f electronic may be excitated, it produces complex spectral properties. Hence, it is known that the spectral structure of Tb I is complex to some extent and many known levels have very large J-values even up to 21/2. Although an enormous amount of spectroscopic work on both the fine and the hyperfine structures in Tb I has been done, the understanding of atomic structure and spectroscopic characteristics is insufficient yet. For example, most known levels are still not designated by atomic state notations. So it is very significant to explore the primary characteristic of the levels of Tb I, such as radiative lifetimes. Lande factors etc.Based on the research background mentioned, in this paper, we combined the laser ablation atomic beam technology, the time-resolved laser-induced fluorescence spectroscopy technology and modern atomic structure theory, and used the one-step excitation methods to excitated odd-parity levels, so it could measure radiative lifetimes of 27 odd-parity levels in Tb I.The experiment was performed in a copper vacuum chamber. We use laser ablation atomic beam technology to get laser-induced plasma. A 532 nm laser pulse of about 8 ns duration from a Q-switched Nd:YAG laser (Continuum Precision II) was moderately focused on sheet of terbium, so it can produce laser-induced plasma beam. The pulse energy was generally 5-1 OmJ, we can get sufficient Tb atoms in metastable states. In order to obtain a tunable UV exciting laser from 362 to 378 nm, the second harmonic of the dye laser was produced by a BBO type-I crystal and then it was used to get the first-order Stokes-stimulated Raman scattering (SSRS) light in a H2 gas cell.And a dye laser (Sirah Cobra-Stretch) operating with DCM dye pumped by a Q switched Nd:YAG 355 nm laser.Tunable exciting laser was horizontally sent through the vacuum chamber where it intersected the vertical atomic beam. Because of spontaneous emission, atoms can not always stay in the excited state, the atomic beam in the excited state was populated. The fluorescence signal was focused into a grating monochromator by a convex fused-silica lens in a direction perpendicular to the laser and the atomic beams, and then it was detected by a photomultiplier tube(PMT).A digital oscilloscope connected to a computer for storing and analyzing signals was used to register the time-resolved fluorescence photocurrent signals from the PMT.The radiative lifetimes of 27 odd-parity levels of Tb I from 27220 to 33293 cm-1 are measured in this paper and they fall in the range from 6.5 to 43.9 ns. All the obtained lifetimes are not longer than 50 ns. To the best of our knowledge,25 lifetimes among the obtained results are reported for the first time. By comparing the previous literature, three levels of Tb II have been measured both by Den Hartog et al and by us with the TR-LIF technique, there are in good agreement. So these results show the correctness of this conclusion.The accuracy of lifetime measurements is effected by collision effect, flight-out-of-view effect, radiative trapping, the influence of the nonlinear response and so on, which were briefly described in thesis. And we analysed the corresponding fluorescent signal and matters needing attention.We believe that they would be helpful for insight into the atomic structure and radiative properties as well as for theoretical researches of neutral terbium. The data on the atomic structure and transition probabilities have a profound significance, the date of transition probabilities is very important to determine the value of element abundances using the spectral line intensity. So the structural parameters of atomic energy levels of Tb in astrophysics, analytical chemistry and other disciplines play very important roles.