Research On Atomic Emission Spectrum Characteristics Of Laser-Induced Plasma

The technology of laser induced breakdown spectroscopy is through collecting the atomic emission spectrum to detect the composition of substances. In this paper, the samples was ablated by a Nd: YAG nanosecond pulse laser with wavelength 1064 nm and the plasma was produced. The spectrum was recorded by grating spectrograph. Through studying the spectrum characteristics to find out the ways to enhance the spectral quality, improve the accuracy of quantitative analysis. The main subjects are as the following:1、Effect of sample temperature on the laser-induced soil plasma radiation characteristics The target prepared from the national standard soil samples was ablated in air by a nanosecond pulse laser with wavelength 1064 nm under different temperatures(TS￡350°C). The line intensity and signal-to-noise ratio of spectrum were measured, the detection limit of analytical spectroscopy and the accuracy of signal measurement were calculated. The experimental results show that, with the increase of sample temperature under the condition of laser energy for 200 m J, plasma radiation increases and reaches the maximum at the sample temperature of 300℃. Calculations show that compared with that of the room temperature, the spectrum line intensities of Al, Mg, Ba and Fe increased by 67%, 58%, 61% and 52% respectively, and the spectral signal-to- noise ratios increased by 41%, 51%, 28% and 38% respectively. The detection limit of analytical elements and the stability of spectral signal were improved. By shooting the plasma images and measurement of the parameters of the plasma at different conditions, we can properly explain the mechanism of the radiation characteristic of laser induced plasma with the transformation of specimen temperature.2、Influencing of specimen temperature on the laser-induced metal plasma radiation intensities The metal sample was heated and ablated by a nanosecond pulse laser in the air under the certain temperature. The plasma was produced. Then the line intensity and signal-to-noise ratio of spectrum were measured under the different sample temperature. The results show that, with the increase of sample temperature under the condition of laser energy for 200 mJ, plasma radiation increases and reached the maxmum at the sample temperature of 150℃. Calculations show that the spectral line intensities of Mo, Cr, Ni and Mn increased by 54.56%, 72.43%, 70.29% and 54.01% respectively, and the spectral signal-to- noise ratios increased by 37.44%, 40.74%, 38.6% and 37.06% respect ively compared with that of the room temperature. By observing the plasma images and measuring the temperature of the plasma, electron density and the sample evaporation, we discussed the cause of the radiation enhancement of laser-induced metal plasma.3、The study of self-absorption effect in nanosecond laser induced plasma emission spectrum A plane mirror device was used to constraint laser plasma in the experiment, and the change of the spectral lines with or without the plane mirror device was compared. Without the plane mirror device, the half-maximum line widths of spectral line of the sample element Al, Mg and Mn were 0.1612 nm、0.2389 nm and 0.0582 nm, respectively. But they were 0.1138 nm、0.1301 nm and 0.0471 nm with four plane mirrors device constraining plasma. It shows that the self-absorption effect of spectral lines decreased significantly by the use of plane mirror device to restrain plasma on the space,while the spectral line becomes sharper and the intensity improved significantly. By observing the plasma photos and measuring of plasma temperature and electron density, the reason to reducing the self-absorption effect in laser spectrum is discussed.4、Determination of Cr and Pb in soil by laser-induced breakdown spectroscopy Laser plasma was constrained by the plane mirror device in the experiment. The elements calibration curve was measured and drawn, and the content of element Cr and Pb in the soil samples was quantitatively analyzed by the method of exciting soil plasma emission spectrum. The soil plasma was induced by the laser. The detection results were compared with that without the plane mirror device. Calculation shows that, when the plane mirror device was added in the experiments, the relative standard deviation of the measurement results of element Cr and Pb from 5.22% and 8.12% decreased to 3.43% and 3.94% than that without mirror; while the detection limits from 4.13×10-3% and 62.54×10-3% decreased to 2.61×10-3% and 44.22×10-3%, respectively. It is thus that the plane mirror device can obviously improve the measurement accuracy of laser-induced breakdown spectroscopy.