Atomic And Ionic Level Lifetime Measurements Of Several Rare-earth Elements

The radiative transition parameters of the atoms and ions (such as naturalradiative lifetimes, branching ratios, transition probabilities and oscillator strength)are a kind of extremely important atomic spectroscopic data in atomic physics,plasma physics and astrophysics. Due to the applications of the rare earth (RE)elements are very widespread and important in many fields, much attention hasbeen paid to the studies of the radiative parameters of the RE atoms and ions. Withthe invention of lasers, laser spectral technique has achieved a rapid development,which provides a powerful tool for the study of the radiative properties of atomsand ions.In this paper, using the time-resolved laser spectroscopy technique and laserablation plasma method, at the same time utilizing a tunable ultraviolet (UV) laseras exciting source, we carried out experimental investigation on radiative lifetimesof the four atoms and ions (Tb I,La I,Gd I and Gd II) of the RE elements.To obtain desired tunable UV exciting laser, the wavelength range of dye laserneeds to be expanded. A dye laser (Sirah Cobra-Stretch) operating with differentdye (DCM,Rhodamine 6G and Coumarin 307) pumped by a Q-switchedNd³⁺:YAG 355 nm laser (Spectra-Physics Quanta-Ray) working with about 8 nspulse duration was used for generating dye laser, and then the second harmonic ofthe dye laser was produced by a nonlinear optic crystal (BBO crystal). Sometimes,in order to further spread the ranges of laser wavelengths, we also used thestimulated Raman shift method. In order to obtain atomic and ionic beam, laserablation technique was employed. A 532 nm laser pulse with about 8 ns duration from a Q-switched Nd3+:YAG laser (Continuum Precision II) working at a 10 Hzrepetition rate was moderately focused on a rotating metal target. Then the plasmacontaining a certain number of neutral atoms and low valence ions was producedby the laser. The interval between the excitation and the ablation pulses can beadjusted by a high precision digital delay generator. The interaction between theUV laser and atom (ion) beam was performed in a copper vacuum chamber whichwas evacuated by mechanical pump and turbomolecular pump to a pressure below3×10–3Pa. An appropriate magnetic field of about 100 Gauss produced by a pair ofHelmholtz coils was used to wash out the possible decay signal distortion from thequantum beats induced by the earth magnetic field. Furthermore, the appliedmagnetic field can also effectively reduce the recombination background from theplasma. The atoms or ions on an excited upper level will execute a downwardtransition spontaneously while emitting fluorescence. In order to collect thefluorescence signal, a grating monochromator used for selecting the appropriateobservation channel was employed. In addition, a photomultiplier tube and 500MHz digital oscilloscope were used to store and analyze the signal data. To obtaina good signal-to-noise ratio, we usually choose the optimum fluorescence channel,which has much stronger line strengths and could avoid the effect of the stray lightfrom UV laser, as the observe channel. For the lifetimes longer than 80 ns, thelifetime values were evaluated by a least-square exponential fit procedure. Whilefor the short-lived levels, deconvolution of the fluorescence decay curves betweenthe recorded signal and excitation pulse is essential for considering the effects ofthe finite duration of the excitation pulse and the response time of the detectionsystem. For each measured level, several curves were recorded under differentexperimental conditions, and the lifetimes evaluated from them were averaged asthe final result.The energy levels of RE atoms and ions are considerably rich,however, the linewidth of dye laser is finite (0.08 cm^-1), to ensure only one level tobe excited at a time, the wavelength of excitation light for each level was carefully chosen from available excitation paths to ensure that it was proper and free fromblends. Therefore, before the measurements, we need to calculate the excitationwavelengths and fluorescence channels of all the atomic (ionic) energy levels andother ionic (atomic) levels which may affect the measurements. In the experiment,we shall choose a simple and accurate excitation scheme. Sometimes, to obtain anextra confidence of measurements, some levels were measured twice by using twodifferent excitation wavelengths.During measurements, attentions to various effects (collision effect,flight-out-of-view effect, radiation trapping effect and saturation effect etc.) whichcould bring errors to measured lifetimes were carefully paid. In this paper, adetailed analysis on these effects was discussed. To eliminate these effects, thelifetime measurements were carried out by way of changing and optimizingexperimental conditions. For example, collision effect could be eliminated byenhancing the vacuum or reducing the intensities of the ablation laser. In addition,increase of the delay time between the ablation and excitation pulses, which couldalter the atom density and velocity, is an effective method to eliminate the collisioneffect and the flight-out-of-view effect.The thesis includes three parts.1. The experimental lifetimes of 27 odd-parity levels of Tb I in the range from27220.46 to 33293.14 cm^-1 were measured, and 25 lifetimes were reported for thefirst time.2. Radiative lifetimes of 56 odd-parity levels of La I in the range from23874.95 to 40910.11 cm^-1 have been determined. And the most levels are above30000 cm-1. Among these lifetime values, 45 lifetimes were measured for the firsttime to our best knowledge.3. We carried out experimental investigation on radiative lifetimes ofhigh-lying even-parity states in Gd I and Gd II, and the results of 94 Gd I levelsfrom 29717.231 to 41692.155 cm 1and 10 Gd II levels from 38057.954 to 49291.082 cm 1were obtained, respectively. For Gd I, to our best knowledge, thelifetime values of 60 levels above 37000 cm-1were measured for the first time.And for Gd II, the lifetimes of levels above 42000 cm-1were reported for the firsttime.The uncertainties of lifetime results in this thesis do not exceed±10%, theywere composed of the systematic errors arising from the fit processes and thestatistical fluctuations from different recordings. The former were mainly due tosome unsteady factors such as the dither of fluorescence curve and excitation pulseshape caused by the instability of the excitation pulse and the transit time jitter ofthe PMT.The lifetimes investigated in this thesis not only supply a large number ofimportant experimental reference data for theoretical calculations of the atomicstructure and dynamics process, but also will be very useful for understanding thecharacteristics of the radiative characteristics of the atomic and ionic level of someelements. In addition, a combination of lifetimes with future experimental ortheoretical branching ratios may give transition probabilities and oscillatorstrengths which would provide a series of more valuable atomic data forastrophysical analysis, plasma diagnostics and laser fusion research.