Influence of distance between focusing lens and sample surface on laser-induced breakdown spectroscopy of brass at different sample temperatures

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.