Studies On Radiative Parameters Of Rare-earth Elements:Tm ?,Er? And Er?

Transition probability and oscillator strength of rare-earth(RE)atoms and ions are of particularly importance in many fields.With the development of new technologies,such as large aperture telescopes,new spectrometers,and advanced CCD detector arrays,people have been recorded spectra with high signal-to-noise ratios and high spectral resolving powers,which require accurate transition probabilities and oscillator strengths for the line identifications and chemical abundance determinations.RE elements characterized with open shells make the calculations of transition probability and oscillator strength very difficult.Thus,reliable experimental parameters will be valuable for benchmarking theoretical calculations.In addition,the rich emission spectra of neutral and singly ionized RE elements in the visible region make RE elements increasingly being used in many commercial metal-halide high-intensity-discharge lamps,resulting the lamps have excellent colour rendering indices.Transition probability data are needed for modeling and diagnosing of these lamps.Therefore,the research on transition probability and oscillator strength for RE elements has scientific significance and application value.A accurate way of obtain experimental transition probabilities and oscillator strengths is to combination of the measured lifetime and branching fraction results.In recent years,although many researchers have been worked on acquisition of the transition probabilities and oscillator strengths for RE atoms and ions,sufficient data are not available for Tm III,Er I and Er II.For this reason,by time-resolved laser-induced fluorescence method,radiative lifetimes for the levels of Tm III,Er I and Er II were measured.In the measurements,Free doubly-ionized Tm,neutral and singly-ionized Er in their ground and low-lying metastable levels were produced by laser-induced plasma.A Nd:YAG laser was used to pump a dye laser which was followed by different nonlinear processes to obtain tunable excitation source.The fluorescence decay signal emitted from the excited state under investigation was selected by a monochromator,equipped with a photomultiplier tube to convert the fluorescence into an electric signal.Then the signal was sent to a digital oscilloscope for recording.Finally,a exponential or convolutional fit can be performed to evaluate the lifetimes from the detected time-resolved fluorescence signals.Branching fraction determinations for this work were completed based on the emission spectra of hollow cathode lamps.The spectral lines characterize with Gaussian shape and its peak intensities obtain using Gaussian single-or multiple-peak fits.For the emission spectra,accurate calibration of spectral response is indispensable because the instrument system has different responses at different wavelengths.The peak intensity of invested line was divided by the spectrum response to obtain the calibrated intensity.The calibrated intensities for lines from a common upper level are used to determine branching fraction.By combining the measured lifetime branching fractions results,absolute transition probabilities and oscillator strengths were determined.Using the above-mentioned methods,two parts of results obtained in this work are as follows:1.Natural radiative lifetimes for five even-parity levels of Tm III in the energy range between 27547.25 and 47624.18 cm~-11 were measured by time-resolved laser-induced fluorescence method.To the best of our knowledge,three lifetimes determined in this paper are reported for the first time,while for the two levels whose lifetimes were reported by literature are found to be consistent with previous results.The measured lifetimes range from 43 to 256 ns with uncertainties within 10 percent.At present,the lifetimes for seven even-parity levels have been determined in this work and previous literatures.The branching fraction measurements were performed based on the emission spectra of a hollow cathode lamp.By combining the measured branching fractions and the lifetime values reported in this work and in literature,experimental transition probabilities and oscillator strengths for 11 transitions were derived for the first time.2.Radiative lifetimes for 104 levels of Er I in the energy range between31926.003 and 44525.705 cm~-11 and 51 levels of Er II from 31381.779 to 47840.962cm~-11 were measured by time-resolved laser-induced fluorescence method.To our best knowledge,the lifetimes for 101 out of 104 levels in Er I and for 45 out of 51 levels in Er II were measured for the first time.We see good agreements between our and previous results.The measured results are in range from 4.8 to 493 ns.Branching fraction determinations for 356 lines from 47 out of 104 Er I levels and 122 lines related to 19 out of 51 Er II levels were completed based on the emission spectra of hollow cathode lamps available from the digital library of National Solar Observatory(http://diglib.nso.edu/).By combining these branching fractions and the lifetime results measured in this work,absolute transition probabilities and oscillator strengths were determined for 352 lines of Er I and 92 lines of Er II for the first time,increasing the total number of lines with experimental transition probabilities to over 910 for Er I and over 540 for Er II.In conclusion,radiative lifetimes,branching fractions,and deduced transition probabilities and oscillator strengths for Tm III,Er I and Er II were determined in this paper.These data will well enrich current atomic database and be widely used in the fields,such as in astronomical spectral analysis,in the design of metal-halide high-intensity-discharge lamps,in the investigation of plasma in thermonuclear devices,and in the atomic theory research,etc.