Radiative Lifetime Measurements On Odd-parity Levels Of La Ⅰ By Time-resolved Laser Spectroscopy

Recently, the rapid development of laser technique offer us a powerful tool for investigating atomic internal configuration in depth. Laser spectroscopy have been an indispensable technique in atomic and molecular Physics. In particular with the help of the short intense laser pulse experiment made measuring the lifetime of short-lived excited states in possible, and promoted, therefore, relative researches and explorations in following basic areas: the interactions between laser and particles of matter, measurement of the energy level structures, etc.Neutral lanthanum (La I, Z=57), which is characterized by the 5d6s2 2D3/2 ground state, is the first element of the rare-earth series. Its ionization limit is 44981 cm-1. It is a product of the fusion reaction that occurs in the late stages of stellar evolution. Because the acquirement of accurate atomic data (such as hyperfine structure, oscillator strength, branching fraction, radiative lifetime, etc.) is helpful to determine the abundance of rare-earth elements in the Sun and peculiar stars, researchers continue to measure the atomic data in recent years. In addition, accurate experimental values are very helpful in optimizing the computation model in theoretical research.According to the NIST compilations, our knowledge of both the energy levels and radiative paraneters of La I is still very fragmentary and calls for many additional efforts. The main reasons why the data of energy levels and radiative parameters of are La I so rare are as follows: Firstly, the atomic beam of La I used in spectral measurement are difficultly produced; Secondly, for all levels, the excitations from the ground level need ultraviolet laser(UV)or vacuum ultraviolet(VUV)laser which is rather difficult to obtain.Based on the research background mentioned, in this paper, we combined laser induced plasma technology with the time-resolved laser-induced fluorescence spectroscopy technology, and used the single-photon one-step excitation methods to measure the natural radiative lifetime of odd-parity levels of La I. the laser system used in the experiment was consisted of two Nd:YAG lasers, one was used for pumping dye laser, the other was used for ablating plasma. The laser beams from the dye laser interacted with the plasma beam and excited the atom from the low-level to the high-levels to be studied. Then we use a grating monochromator and photomultiplier tube to detect radiation fluorescence signal, and finally we recorded the temporal electric signal by oscilloscope, stored it in a personal computer.One-step excitation process was used in experimental study of the natural radiative lifetime of odd-parity levels of La I. The plasma was produced by laser ablation of a 99.99% purity rotating lanthanum target. The experiment was performed in a copper vacuum chamber vacuated by a 450 L/s turbo molecular pump to a background pressure below 3×10?3 Pa. The energy levels of interest were excited by light from a dye laser. The dye laser light was frequency doubled in aβ-barium borate type-I crystal, and then it was focused into a H2 gas cell for getting the first or second-order Stokes components as the excitation pulse. This excitation pulse was horizontally sent through the plasma. A digital delay generator was used for adjusting the delay between the excitation and the ablation lasers. Laser-induced fluorescence emitting from the desired upper levels, focused on the entrance slit of the monochromator by a fused-silica lens and filtered by a monochromator, was detected by a photomultiplier tube (PMT). A digital oscilloscope triggered by the exciting laser with a fast photoelectric diode was used to register the time-resolved photocurrent signal from the PMT. A personal computer connected to the oscilloscope by a general purpose interface bus cable was used for storing and analyzing signal data.In this work, we measured 32 lifetimes in the range of 7.3 to 247 ns. For short-lived excited states (τ< 80 ns), a convolution of an exponential and a laser pulse recorded by the same detection system was fitted to the decay curve. For the long-lived excited states, the lifetimes were evaluated using a least-square exponential fit procedure to the recorded fluorescence curves. To our best knowledge, among the 32 measured lifetimes, 23 results were reported for the first time. The other nine lifetimes have been compared with previous experimental results. Except for the lifetime of the 4 04 f 5d 6s F5 /2level, the results of other levels are in good agreement with previously reported data within the experimental errors. The uncertainties of lifetime results reported in this paper are not larger than±1 0%. These uncertainties consist of both the statistical fluctuations from different recordings and the systematic uncertainties arising from the fit processes. In fact, the latter uncertainties reflect the systematic influences from the transittime jitter of the PMT (1.2 ns) and the randomicity of stray light of the exciting laser when recording excitation pulse shapes.The results of this study will help people further understand the energy level structures of La I, A combination of our work with future branching fraction data may give transition probabilities and oscillator strengths that are helpful for astrophysical analysis and for the calculations of atomic structure and radiative properties of La I.