Natural Radiative Lifetimes And Landé Factors Of The Levels Along Rydberg Series In SnⅠ

Electromagnetic radiation characteristics of atoms and molecules has been an important research topic in the field of atomic and molecular physics. The radiative lifetime is a kind of important parameter for people to understand atoms and molecules. Therefore, lifetime measurements are of great significance in fields of atomic physics, astrophysics, plasma physics, laser physics. Landéfactor is an indispensable parameter for characterization of level properties and improvement of theoretical atomic models. The study on atoms in Rydberg states which is one of the active frontal topic in the field of atom physics and laser spectroscopy is an important window for seeing atomic structure and dynamics since by studing disturbances between degenerated and perturbed Rydberg series one can get the configuration interaction information. Neutral tin (Sn I, Z=50), which belongs to the carbon group elements, is characterized by a 5s25p2 ground configuration. There are significant differences in electron orbit penetration and Coulomb interaction in Sn I compared with other carbon group elements because it has higher nuclear charge number and larger ion core. Hence, it is of great significance to study spectral characteristics of Sn.In this thesis, the radiative lifetimes of even-parity Rydberg series in Sn I have been measured using the laser-induced fluorescence method and time-resolved laser spectroscopy technique, while LandégJ factors have been determined by also the Zeeman quantum beat technique.Two linearly polarized dye lasers (Sirah Cobra-Stretch) pumped respectively by two Q-switched Nd:YAG lasers (Spectra-Physics Quanta-Ray Pro-Series and Continuum Precision II), wavelength at 355 nm and 532 nm, working at a 10 Hz repetition rate and with about 10 ns pulse duration were used as the light source. It's very necessary to calibrate the wavelength of dye laser by using a wavemeter at first because the wavelength of dye laser display on the computer is different from the true wavelength after used for a long time.To produce Zeeman splitting, two sets of Helmholtz coils, which were operated with two high-stability constant-current power supplies, were used for generating magnetic field. For testing the reliability of the system, we measure the lifetime of the odd-parity 6s6p 3P1(11992.007 cm-1)level of atom Yb. Experimental result is 864±6.5 ns which is in good agreement with 875 ns measured by others. So the accuracy of experimental system is out of question.In experiment, the atomic beam including adequate neutral tin atoms was generated in a vacuum chamber maintained at a background pressure of about 3.0×10-3 Pa by resistive heating of Sn metal kept in a high-temperature oven, in which the temperature can operate up to 1700 K. When the first-step exciting laser was sent through the vacuum chamber, where it intersected the atomic beam, the intermediate levels of Sn I(34914.28,38628.88,39257.05 or 44508.68 cm-1)were excited from the ground state. Then the second-step exciting laser, which was sent from opposite directions of the first one, intersected with the first-step laser and the vertical atomic beam at the vacuum chamber, was used to excite the atoms to the objective levels from the intermediate levels. Therefore, the population of high-excited levels could be accomplished. Following the excitation, 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.Using the two-color two-step exciting scheme, the lifetimes of nine levels from the 5p7p configuration and 40 lifetimes concerning 5pnp J = 1 (n = 10-13, 15-19) and J = 2 (n = 10-13, 15-19, 27, 31, 32), 5pnf J = 2 (n = 4, 5, 9-19, 22, 23) levels along Rydberg series as well as all 5p8p perturbing levels ranging from 52263.8 to 59099.9 cm-1 have been measured. Moveover, lifetime calculations by the multichannel quantum defect theory (MQDT) have been performed, and good agreements between the experimental and theoretical results have been achieved for most levels under study. In the present experiment the lifetime measurements were performed at room temperature, so the experimental results should be considered as the room-temperature lifetimes. In view of the effect of blackbody radiation on the lifetimes of Rydberg levels, the lifetimes at 0 K were calculated approximately and it is found that the effect of blackbody radiation for the short lifetimes was neglectable.In addition, the LandégJ factors belonging to even-parity J = 1 5pnp (n = 7, 11~13, 15~19), J = 2 5pnp (n = 7, 11~13, 15~19, 31, 32) and 5pnf (n = 4, 5, 9~19, 22, 23) Rydberg series as well as all the 5p7p and 5p8p perturbing levels have been measured using two-color two-step excitation scheme. The experimental results have also been compared with the MQDT results, and a good agreement has been achieved for most levels. Recurring to MQDT analyses and Lande factors results, two new levels 5p9f (1/2, 5/2)2 and 13f (1/2, 5/2)2 have been determined. In this thesis, the gJ values of some levels cannot be determined because their fluorescence intensities were extraordinarily weak. However, by using the MQDT theory, many gJ values have been predicted for the levels not measured.In measurements, the accuracy of the radiative lifetimes could be influenced by the effects of Zeeman quantum beat, flight-out-of-view, radiation trapping and so on. In order to obtain the reliable lifetime values, the experimental conditions were optimized carefully. For example, at the excitation region, for J≠0 levels, the fluorescence signals could be distorted by Zeeman quantum beats resulted from the weak earth magnetic field. To eliminate this effect, a strong magnetic field was applied to produce a very rapid quantum-beat oscillation which can overstep the time resolving power of instrument. Thus the decay of fluorescence signal will maintain the rule of exponent decay.The atomic structure parameters investigated in this thesis will be very useful in many fields. For example, the atomic transition probabilities and oscillation strength can be deduced from measured radiative lifetimes combined with precise branching fractions which will be the next work of our group in the future. In addition, the radiative lifetimes and LandégJ factors are useful for more insight into the atomic structure of excited levels. These experimental data are also important for improving the theoretical study of the neutral atoms with moderate atomic numbers. Finally, the results of this thesis will considerably cover the shortage of atomic structure parameters of Sn I, and enrich the correlated atomic database.