1.College of Science, Jilin Institute of Chemical Technology, Jilin 132022, China 2.Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012, China 3.Jilin Provincial Key Laboratory of Applied Atomic and Molecular Spectroscopy (Jilin University), Changchun 130012, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11674128, 11674124) and the Jilin Province Scientific and Technological Development Program, China (Grant No. 20170101063JC).
Received Date:14 December 2018
Accepted Date:06 January 2019
Available Online:01 March 2019
Published Online:20 March 2019
Abstract:From previously published results of laser-induced breakdown spectroscopy, one can know that the change in the distance from the sample surface to the focusing lens has an important influence on the interaction between the sample and the laser, and increasing the sample temperature can enhance the coupling between the laser and the sample. However, almost no work has devoted to directly studying the influence of the distance between focusing lens and sample surface on the spectral intensity of laser-induced breakdown spectroscopy under different sample temperatures. In this paper, we investigate experimentally this subject. An Nd:YAG laser is used to excite the sample to produce the plasma. The detected spectral lines are Cu (I) 510.55 nm, Cu (I) 515.32 nm, and Cu (I) 521.82 nm. The focal length of focusing lens is 200 mm. The distance between focusing lens and sample surface ranges from 170 mm to 200 mm. The sample is heated from 25 ℃ to 270 ℃, and the laser energy is 26 mJ. In general, the spectral intensity of laser-induced breakdown spectroscopy can be effectively enhanced by increasing the sample temperature. At the sample temperatures of 25 ℃ and 100 ℃, the spectral intensity increases monotonically with the increase of the distance between focusing lens and sample surface; at higher sample temperatures (150, 200, 250, and 270 ℃), the spectral intensity first increases and then decreases with the increase of the distance between focusing lens and sample surface. In addition, near the focal point, with the increase of sample temperature, the increase of the spectral intensity is not obvious, and the spectral intensity decreases with the increase of sample temperature, which is particularly noteworthy in improving the spectral intensity of laser-induced breakdown spectroscopy by increasing sample temperature. In order to further understand the influences of these two conditions on laser-induced breakdown spectroscopy, we also calculate the plasma temperature and electron density, and find that the variation of plasma temperature and electron density are almost the same as that of spectral intensity. The plasma temperature and electron density at higher sample temperature are higher. Keywords:laser-induced breakdown spectroscopy/ sample temperature/ distance between focusing lens and sample surface/ plasma temperature and electron density
全文HTML
--> --> -->
3.结果与讨论首先了解升高样品温度对LIBS光谱强度的影响. 如图2所示, 激光的能量为26 mJ, 聚焦透镜到样品表面的距离为190 mm, 样品温度范围为25 ℃到270 ℃. 从图2可以观察到三条Cu (I)线, 波长分别为510.55, 515.23和521.82 nm. 随着样品温度的升高, 光谱强度逐渐增强, 这表明升高样品温度能有效地改善LIBS光谱的辐射强度. 在200 ℃时, 光谱强度达到了最大值. 继续升高样品温度, 光谱强度减弱. 光谱随温度的变化类似于增加激光能量, 随着激光能量的增加, 光谱出现饱和现象, 该现象归因于等离子体屏蔽效应[38-41]. 当一束激光脉冲照射靶材表面, 激光脉冲前沿将激发靶材产生高温、高密度的等离子体, 随后等离子体迅速膨胀. 在膨胀的过程中, 等离子体将继续吸收激光脉冲的能量. 最终, 快速膨胀的等离子体将阻止脉冲后沿的激光到达样品表面, 形成等离子体屏蔽效应. 例如, Haq等[42]研究了激光诱导镁等离子体光谱强度随着激光能量的变化, 结果表明, 随着激光能量的增加, 信号强度出现饱和, 并归因于等离子体屏蔽效应. 另外, 升高样品温度能增强激光与样品之间的能量耦合[20,22,26,43,44], 相对于较低温度, 高温下的样品将吸收更多的激光能量. 因为随着样品温度的升高, 相当于增加激光能量. 所以, 当样品温度升高到200 ℃时, 出现强的等离子体屏蔽, 同时光谱强度达到了最大值. 图 2 不同温度下LIBS辐射强度的比较, 其中图(b)来自于图(a), 聚焦透镜到样品表面的距离为190 mm、激光能量为26 mJ Figure2. Comparison of spectral lines of LIBS under different sample temperatures. Panel (b) is from panel (a). The distance between focusing lens and sample surface is 190 mm. Laser energy is 26 mJ.
显然, 等离子体辐射的光谱对初始的样品温度是非常敏感的. 同时, 聚焦透镜到样品表面的距离也能有效地影响光谱强度的变化. 通过移动聚焦透镜探测了不同样品温度下光谱强度的变化. 图3给出了100和200 ℃时等离子体光谱随着波长和聚焦透镜到样品表面距离的分布, 激光能量为26 mJ. 可以看出在510—525 nm波长范围内有较强的发射光谱, 能够明显观察到光谱的距离范围大约为30 mm. 高温样品的光谱强度明显高于低温样品的光谱强度, 同时, 对于不同的样品温度, 光谱强度随着距离的变化也是不同的. 在100 ℃时, 光谱强度随着距离的增加而增加; 而对于200 ℃的样品温度, 随着距离的增加, 光谱强度先增加而后降低. 实际上, 实验过程中的激光能量是不变的, 聚焦透镜与焦点之间距离的改变等价于激光能量密度的改变. 换句话说, 当移动聚焦透镜时, 相当于改变激光照射到样品表面光斑的尺寸. 在本实验中, 当透镜到样品表面距离增加时, 光斑直径变小; 当距离减小时, 光斑直径增大. 另外, 在焦点附近(大约200 mm的距离) 200 ℃的样品温度时, 光谱辐射并不是最强的. 因此, 不同样品温度下的激光诱导等离子体强烈地依赖于聚焦透镜到样品表面的距离. 图 3 不同温度下等离子体光谱随着波长和聚焦透镜到样品表面距离的分布(激光能量为26 mJ) (a)样品温度为100 ℃; (b) 样品温度为200 ℃ Figure3. Distribution of optical emission with the wavelength and the distance between focusing lens and sample surface under 100 ℃ (a) and 200 ℃ (b) sample temperatures. Laser energy is 26 mJ.
为了详细了解不同样品温度下聚焦透镜到样品表面距离对光谱强度的影响, 图4给出了Cu (I) 510.55 nm和Cu (I) 521.82 nm光谱峰强度的变化. 总体上来看, 升高样品温度能明显地提高LIBS的光谱强度. 这是由于样品温度增加, 样品的反射率降低[26], 样品吸收更多的激光能量, 将产生更强辐射的等离子体. 对于较低温度的样品(25和100 ℃), 随着聚焦距离的增加, 样品表面逐渐接近焦点位置, 等离子体光谱的辐射强度逐渐增加. 这是由于在该过程中, 光斑尺寸逐渐变小, 激光的能量密度增加, 产生更强的等离子体. 当样品温度增加到150 ℃时, 随着聚焦透镜到样品表面距离的增加, 光谱强度出现先增加而后降低的变化. 如前所述, 在距离增加的过程中, 光斑尺寸变得越来越小, 激光能量密度逐渐变大, 这个过程伴随等离子体屏蔽效应的加强. 在等离子体最初形成的过程中, 靶吸收激光能量而不会被等离子体羽的膨胀限制. 当光谱强度达到最大值时, 意味着出现更强的等离子体屏蔽效应, 随着距离的继续增加, 等离子体屏蔽效应变得更加明显, 并导致烧蚀质量的降低. 最终, 激光能量与靶材之间的能量耦合也变弱, 产生等离子体的辐射强度也降低了. 因此, 随着距离增加而光谱强度减弱归因于等离子体屏蔽效应的加强. 在样品为200, 250和270 ℃时, 也观察到了类似的变化趋势. 另一个值得注意的现象是当温度增加200 ℃时, 改变透镜到样品表面的距离获得的光谱辐射的最优化值达到了最大, 继续升高样品温度(250和270 ℃), 改变透镜到样品表面的距离获得的光谱辐射的最优化值与样品温度为200 ℃时的光谱辐射的最优化值几乎相等, 同时, 三个样品温度下的光谱随着距离的变化, 相当于升高样品温度后整体向左移动. 对于200, 250和270 ℃, 光谱强度最大值的位置分别为181, 183和187 mm. 因为样品温度的升高将吸收更多激光能量, 所以随着距离的增加, 较高温度(270 ℃)下在181 mm处的激光能量密度几乎等于较低的样品温度(200 ℃)在187 mm处样品吸收激光能量的密度. 这种情况下, 在样品温度较高时, 随着聚焦透镜到样品表面距离的增加, 更早地出现等离子体屏蔽的情况. 当出现等离子体屏蔽效应后, 光谱的辐射强度开始下降. 从上述分析可以看出, 在接近焦点位置附近(图4中195 mm到200 mm的距离), 随着样品温度的升高, 由于等离子体的屏蔽效应, 可能使得LIBS光谱的强度增加不明显, 还可能出现如图2中所示的温度升高光谱强度降低的情况发生, 这个现象非常值得注意. 图 4 不同样品温度下Cu (I) 510.55 nm (a)和Cu (I) 521.82 nm (b)光谱峰强度随着聚焦透镜到样品表面距离的变化(激光能量为26 mJ) Figure4. Evolution of spectral peak intensities at Cu (I) 510.55 nm (a) and Cu (I) 521.82 nm (b) with the distance between focusing lens and sample surface under different sample temperatures. Laser energy is 26 mJ.
图 8 不同样品温度下电子密度随着聚焦透镜到样品表面距离的变化(激光能量为26 mJ) Figure8. Evolution of electron density with the distance between focusing lens and sample surface under different sample temperatures. Laser energy is 26 mJ.